Author name code: peres ADS astronomy entries on 2022-09-14 author:"Peres, Giovanni" ------------------------------------------------------------------------ Title: A Spatially Resolved Study of Hard X-Ray Emission in Kepler's Supernova Remnant: Indications of Different Regimes of Particle Acceleration Authors: Sapienza, Vincenzo; Miceli, Marco; Bamba, Aya; Katsuda, Satoru; Nagayoshi, Tsutomu; Terada, Yukikatsu; Bocchino, Fabrizio; Orlando, Salvatore; Peres, Giovanni Bibcode: 2022ApJ...935..152S Altcode: 2022arXiv220706804S Synchrotron X-ray emission in young supernova remnants (SNRs) is a powerful diagnostic tool to study the population of high-energy electrons accelerated at the shock front and the acceleration process. We performed a spatially resolved spectral analysis of NuSTAR and XMM-Newton observations of the young Kepler's SNR, aiming to study in detail its nonthermal emission in hard X-rays. We selected a set of regions all around the rim of the shell and extracted the corresponding spectra. The spectra were analyzed by adopting a model of synchrotron radiation in the loss-limited regime, to constrain the dependence of the cutoff energy of the synchrotron radiation on the shock velocity. We identify two different regimes of particle acceleration, characterized by different Bohm factors. In the north, where the shock interacts with a dense circumstellar medium (CSM), we found a more efficient acceleration than in the south, where the shock velocity is higher and there are no signs of shock interaction with the dense CSM. Our results suggest an enhanced efficiency of the acceleration process in regions where the shock-CSM interaction generates an amplified and turbulent magnetic field. By combining hard X-ray spectra with radio and γ-ray observations of Kepler's SNR, we modeled the spectral energy distribution. In the light of our results we propose that the observed γ-ray emission is mainly hadronic and originates in the northern part of the shell. Title: The supernova remnant SN 1006 as a Galactic particle accelerator Authors: Giuffrida, Roberta; Miceli, Marco; Caprioli, Damiano; Decourchelle, Anne; Vink, Jacco; Orlando, Salvatore; Bocchino, Fabrizio; Greco, Emanuele; Peres, Giovanni Bibcode: 2022NatCo..13.5098G Altcode: 2022arXiv220814491G The origin of cosmic rays is a pivotal open issue of high-energy astrophysics. Supernova remnants are strong candidates to be the Galactic factory of cosmic rays, their blast waves being powerful particle accelerators. However, supernova remnants can power the observed flux of cosmic rays only if they transfer a significant fraction of their kinetic energy to the accelerated particles, but conclusive evidence for such efficient acceleration is still lacking. In this scenario, the shock energy channeled to cosmic rays should induce a higher post-shock density than that predicted by standard shock conditions. Here we show this effect, and probe its dependence on the orientation of the ambient magnetic field, by analyzing deep X-ray observations of the Galactic remnant of SN 1006. By comparing our results with state-of-the-art models, we conclude that SN 1006 is an efficient source of cosmic rays and obtain an observational support for the quasi-parallel acceleration mechanism. Title: Additional Evidence for a Pulsar Wind Nebula in the Heart of SN 1987A from Multiepoch X-Ray Data and MHD Modeling Authors: Greco, Emanuele; Miceli, Marco; Orlando, Salvatore; Olmi, Barbara; Bocchino, Fabrizio; Nagataki, Shigehiro; Sun, Lei; Vink, Jacco; Sapienza, Vincenzo; Ono, Masaomi; Dohi, Akira; Peres, Giovanni Bibcode: 2022ApJ...931..132G Altcode: 2022arXiv220406804G Since the day of its explosion, supernova (SN) 1987A has been closely monitored to study its evolution and to detect its central compact relic. In fact, the formation of a neutron star is strongly supported by the detection of neutrinos from the SN. However, besides the detection in the Atacama Large Millimeter/submillimeter Array (ALMA) data of a feature that is compatible with the emission arising from a protopulsar wind nebula (PWN), the only hint of the existence of such an elusive compact object is provided by the detection of hard emission in NuSTAR data up to ~20 keV. We report on the simultaneous analysis of multiepoch observations of SN 1987A performed with Chandra, XMM-Newton, and NuSTAR. We also compare the observations with a state-of-the-art three-dimensional magnetohydrodynamic simulation of SN 1987A. A heavily absorbed power law, consistent with the emission from a PWN embedded in the heart of SN 1987A, is needed to properly describe the high-energy part of the observed spectra. The spectral parameters of the best-fit power law are in agreement with the previous estimate, and exclude diffusive shock acceleration as a possible mechanism responsible for the observed nonthermal emission. The information extracted from our analysis is used to infer the physical characteristics of the pulsar and the broadband emission from its nebula, in agreement with the ALMA data. Analysis of the synthetic spectra also shows that, in the near future, the main contribution to the Fe K emission line will originate in the outermost shocked ejecta of SN 1987A. Title: Negative and positive feedback from a supernova remnant with SHREC: a detailed study of the shocked gas in IC443 Authors: Cosentino, G.; Jiménez-Serra, I.; Tan, J. C.; Henshaw, J. D.; Barnes, A. T.; Law, C. -Y.; Zeng, S.; Fontani, F.; Caselli, P.; Viti, S.; Zahorecz, S.; Rico-Villas, F.; Megías, A.; Miceli, M.; Orlando, S.; Ustamujic, S.; Greco, E.; Peres, G.; Bocchino, F.; Fedriani, R.; Gorai, P.; Testi, L.; Martín-Pintado, J. Bibcode: 2022MNRAS.511..953C Altcode: 2022MNRAS.tmp...90C; 2022arXiv220103008C Supernova remnants (SNRs) contribute to regulate the star formation efficiency and evolution of galaxies. As they expand into the interstellar medium (ISM), they transfer vast amounts of energy and momentum that displace, compress, and heat the surrounding material. Despite the extensive work in galaxy evolution models, it remains to be observationally validated to what extent the molecular ISM is affected by the interaction with SNRs. We use the first results of the ESO-ARO Public Spectroscopic Survey SHREC to investigate the shock interaction between the SNR IC443 and the nearby molecular clump G. We use high-sensitivity SiO(2-1) and H13CO+(1-0) maps obtained by SHREC together with SiO(1-0) observations obtained with the 40-m telescope at the Yebes Observatory. We find that the bulk of the SiO emission is arising from the ongoing shock interaction between IC443 and clump G. The shocked gas shows a well-ordered kinematic structure, with velocities blue-shifted with respect to the central velocity of the SNR, similar to what observed towards other SNR-cloud interaction sites. The shock compression enhances the molecular gas density, n(H2), up to >105 cm-3, a factor of >10 higher than the ambient gas density and similar to values required to ignite star formation. Finally, we estimate that up to 50 per cent of the momentum injected by IC443 is transferred to the interacting molecular material. Therefore, the molecular ISM may represent an important momentum carrier in sites of SNR-cloud interactions. Title: Evidence for past interaction with an asymmetric circumstellar shell in the young SNR Cassiopeia A Authors: Orlando, S.; Wongwathanarat, A.; Janka, H. -T.; Miceli, M.; Nagataki, S.; Ono, M.; Bocchino, F.; Vink, J.; Milisavljevic, D.; Patnaude, D. J.; Peres, G. Bibcode: 2022arXiv220201643O Altcode: Observations of the SNR Cassiopeia A (Cas A) show asymmetries in the reverse shock that cannot be explained by models describing a remnant expanding through a spherically symmetric wind of the progenitor star. We investigate whether a past interaction of Cas A with an asymmetric circumstellar shell can account for the observed asymmetries. We performed 3D MHD simulations that describe the remnant evolution from the SN to its interaction with a circumstellar shell. The initial conditions are provided by a 3D neutrino-driven SN model whose morphology resembles Cas A. We explored the parameter space of the shell, searching for a set of parameters able to produce reverse shock asymmetries at the age of 350 years analogous to those observed in Cas A. The interaction of the remnant with the shell can produce asymmetries resembling those observed in the reverse shock if the shell was asymmetric with the densest portion in the nearside to the northwest (NW). The reverse shock shows the following asymmetries at the age of Cas A: i) it moves inward in the observer frame in the NW region, while it moves outward in other regions; ii) the geometric center of the reverse shock is offset to the NW from the geometric center of the forward shock; iii) the reverse shock in the NW region has enhanced nonthermal emission because, there, the ejecta enter the reverse shock with a higher velocity (between 4000 and 7000 km/s) than in other regions (below 2000 km/s). The asymmetries observed in the reverse shock of Cas A can be interpreted as signatures of the interaction of the remnant with an asymmetric circumstellar shell that occurred between 180 and 240 years after the SN event. We suggest that the shell was, most likely, the result of a massive eruption from the progenitor star that occurred between $10^4$ and $10^5$ years prior to core-collapse. We estimate a total mass of the shell of the order 2 Msun. Title: Correcting the effect of stellar spots on ARIEL transmission spectra - II. The limb-darkening effect Authors: Cracchiolo, G.; Micela, G.; Morello, G.; Peres, G. Bibcode: 2021MNRAS.507.6118C Altcode: 2021arXiv210812526C; 2021MNRAS.tmp.2273C This paper is part of an effort to correct the transmission spectra of a transiting planet orbiting an active star. In Paper I (Cracchiolo, Micela & Peres 2020), we have demonstrated a methodology to minimize the potential bias induced by unocculted star spots on the transmission spectrum, assuming a spot model parametrized by filling factor and temperature. In this work, we introduce the limb-darkening effect, therefore the position of the spot in the stellar disc and the impact parameter of the transiting planet now play a key role. The method is tested on simulations of planetary transits of three representative kinds of planetary systems, at ARIEL resolution. We find that a realistic treatment of the limb darkening is required to reliably estimate both the spots parameters and the transmission spectrum of the transiting planet. Furthermore, we show that the influence of the spots on the retrieval of the planetary transmission spectrum is significant for spots close to the centre of the star, covering a fraction greater than 0.05 and with a temperature contrast greater than $500\, {\rm K}$, and that for these cases our method can confidently extract the transmission spectrum and the impact parameter of the transiting planet for both cases of occulted and not occulted spots, provided that we have an accurate characterization of the stellar parameters and a reliable simulator of the instrument performances. Title: Modeling the remnants of core-collapse supernovae from luminous blue variable stars Authors: Ustamujic, S.; Orlando, S.; Miceli, M.; Bocchino, F.; Limongi, M.; Chieffi, A.; Trigilio, C.; Umana, G.; Bufano, F.; Ingallinera, A.; Peres, G. Bibcode: 2021A&A...654A.167U Altcode: 2021arXiv210801951U Context. Luminous blue variable stars (LBVs) are massive evolved stars that suffer sporadic and violent mass-loss events. They have been proposed as the progenitors of some core-collapse supernovae (SNe), but this idea is still debated because of a lack of strong evidence. As supernova remnants (SNRs) can carry in their morphology the fingerprints of the progenitor stars as well as of the inhomogeneous circumstellar medium (CSM) sculpted by the progenitors, the study of SNRs from LBVs could help to place core-collapse SNe in context with the evolution of massive stars.
Aims: We investigate the physical, chemical, and morphological properties of the remnants of SNe originating from LBVs in order to search for signatures in the ejecta distribution and morphology of the remnants that could reveal the nature of the progenitors.
Methods: As a template of LBVs, we considered the LBV candidate Gal 026.47+0.02. We selected a grid of models that describe the evolution of a massive star with properties consistent with those of Gal 026.47+0.02 and its final fate as a core-collapse SN. We developed a three-dimensional hydrodynamic model that follows the post-explosion evolution of the ejecta from the breakout of the shock wave at the stellar surface to the interaction of the SNR with a CSM characterized by two dense nested toroidal shells, parametrized in agreement with multi-wavelength observations of Gal 026.47+0.02.
Results: Our models show a strong interaction of the blast wave with the CSM which determines an important slowdown of the expansion of the ejecta in the equatorial plane where the two shells lay, determining a high degree of asymmetry in the remnant. After ≈10 000 yr of evolution, the ejecta show an elongated shape forming a broad jet-like structure caused by the interaction with the shells and oriented along the axis of the toroidal shells. Models with high explosion energy show Fe-rich internal ejecta distributions surrounded by an elongated Si-rich structure with a more diffuse O-rich ejecta all around. Models with low explosion energy instead show a more homogeneous distribution of chemical elements with a very low presence of Fe-group elements.
Conclusions: The geometry and density distribution of the CSM where a LBV star goes SN are fundamental in determining the properties of the resulting SNR. For all the LBV-like progenitors explored here, we found that the remnants show a common morphology, namely elongated ejecta with an internal jet-like structure, which reflects the inhomogeneous and dense pre-SN CSM surrounding the star.

Movies are available at https://www.aanda.org Title: Modeling particle acceleration and non-thermal emission in supernova remnants Authors: Orlando, S.; Miceli, M.; Ustamujic, S.; Tutone, A.; Greco, E.; Petruk, O.; Bocchino, F.; Peres, G. Bibcode: 2021NewA...8601566O Altcode: 2020arXiv201213394O According to the most popular model for the origin of cosmic rays (CRs), supernova remnants (SNRs) are the site where CRs are accelerated. Observations across the electromagnetic spectrum support this picture through the detection of non-thermal emission that is compatible with being synchrotron or inverse Compton radiation from high energy electrons, or pion decay due to proton-proton interactions. These observations of growing quantity and quality promise to unveil many aspects of CRs acceleration and require more and more accurate tools for their interpretation. Here, we show how multi-dimensional MHD models of SNRs, including the effects on shock dynamics due to back-reaction of accelerated CRs and the synthesis of non-thermal emission, turned out to be very useful to investigate the signatures of CRs acceleration and to put constraints on the acceleration mechanism of high energy particles. These models have been used to interpret accurately observations of SNRs in various bands (radio, X-ray and γ-ray) and to extract from them key information about CRs acceleration. Title: X-ray emitting structures in the Vela SNR: ejecta anisotropies and progenitor stellar wind residuals Authors: Sapienza, V.; Miceli, M.; Peres, G.; Bocchino, F.; Orlando, S.; Greco, E.; Combi, J. A.; García, F.; Sasaki, M. Bibcode: 2021A&A...649A..56S Altcode: 2021arXiv210309637S Context. The Vela supernova remnant (SNR) shows several ejecta fragments (or shrapnel) protruding beyond the forward shock, which are most likely relics of anisotropies that developed during the supernova (SN) explosion. Recent studies have revealed high Si abundance in two shrapnel (shrapnel A and G), located in opposite directions with respect to the SNR center. This suggests the possible existence of a Si-rich jet-counterjet structure, similar to that observed in the SNR Cassiopea A.
Aims: We analyzed an XMM-Newton observation of a bright clump, behind shrapnel G, which lies along the direction connecting shrapnel A and G. The aim is to study the physical and chemical properties of this clump to ascertain whether it is part of this putative jet-like structure.
Methods: We produced background-corrected and adaptively-smoothed count-rate images and median photon energy maps, and performed a spatially resolved spectral analysis.
Results: We identified two structures with different physical properties. The first one is remarkably elongated along the direction connecting shrapnel A and G. Its X-ray spectrum is much softer than that of the other two shrapnel, to the point of hindering the determination of the Si abundance; however, its physical and chemical properties are consistent with those of shrapnel A and shrapnel G. The second structure, running along the southeast-northwest direction, has a higher temperature and appears similar to a thin filament. By analyzing the ROSAT data, we have found that this filament is part of a very large and coherent structure that we identified in the western rim of the shell.
Conclusions: We obtained a thorough description of the collimated, jet-like tail of shrapnel G in Vela SNR. In addition we discovered a coherent and very extended feature roughly perpendicular to the jet-like structure that we interpret as a signature of an earlier interaction of the remnant with the stellar wind of its progenitor star. The peculiar Ne/O ratio we found in the wind residual may be suggestive of a Wolf-Rayet progenitor for Vela SNR, though further analysis is required to address this point. Title: Modeling the mixed-morphology supernova remnant IC 443. Origins of its complex morphology and X-ray emission Authors: Ustamujic, S.; Orlando, S.; Greco, E.; Miceli, M.; Bocchino, F.; Tutone, A.; Peres, G. Bibcode: 2021A&A...649A..14U Altcode: 2020arXiv201208017U Context. The morphology and the distribution of material observed in supernova remnants (SNRs) reflect the interaction of the supernova (SN) blast wave with the ambient environment, the physical processes associated with the SN explosion, and the internal structure of the progenitor star. IC 443 is a mixed-morphology (MM) SNR located in a quite complex environment: it interacts with a molecular cloud in the northwestern and southeastern areas and with an atomic cloud in the northeast.
Aims: In this work, we aim to investigate the origin of the complex morphology and multi-thermal X-ray emission observed in SNR IC 443 through the study of the effect of the inhomogeneous ambient medium in shaping its observed structure and an exploration of the main parameters characterizing the remnant.
Methods: We developed a 3D hydrodynamic (HD) model for IC 443, which describes the interaction of the SNR with the environment, parametrized in agreement with the results of the multi-wavelength data analysis. We performed an ample exploration of the parameter space describing the initial blast wave and the environment, including the mass of the ejecta, the energy and position of the explosion, as well as the density, structure, and geometry of the surrounding clouds. From the simulations, we synthesized the X-ray emission maps and spectra and compared them with actual X-ray data collected by XMM-Newton.
Results: Our model explains the origin of the complex X-ray morphology of SNR IC 443 in a natural way, with the ability to reproduce, for the first time, most of the observed features, including the centrally-peaked X-ray morphology (characteristic of MM SNRs) when considering the origin of the explosion at the position where the pulsar wind nebula CXOU J061705.3+222127 was at the time of the explosion. In the model that best reproduces the observations, the mass of the ejecta and the energy of the explosion are ~7 M⊙ and ~1 × 1051 erg, respectively. From the exploration of the parameter space, we find that the density of the clouds is n > 300 cm−3 and that the age of SNR IC 443 is ~8000 yr.
Conclusions: The observed inhomogeneous ambient medium is the main property responsible for the complex structure and the X-ray morphology of SNR IC 443, resulting in a very asymmetric distribution of the ejecta due to the off-centered location of the explosion inside the cavity formed by the clouds. It can be argued that the centrally peaked morphology (typical of MM SNRs) is a natural consequence of the interaction with the complex environment. A combination of high resolution X-ray observations and accurate 3D HD modeling is needed to confirm whether this scenario is applicable to other MM SNRs.

Movies are available at https://www.aanda.org Title: Ariel: Enabling planetary science across light-years Authors: Tinetti, Giovanna; Eccleston, Paul; Haswell, Carole; Lagage, Pierre-Olivier; Leconte, Jérémy; Lüftinger, Theresa; Micela, Giusi; Min, Michel; Pilbratt, Göran; Puig, Ludovic; Swain, Mark; Testi, Leonardo; Turrini, Diego; Vandenbussche, Bart; Rosa Zapatero Osorio, Maria; Aret, Anna; Beaulieu, Jean-Philippe; Buchhave, Lars; Ferus, Martin; Griffin, Matt; Guedel, Manuel; Hartogh, Paul; Machado, Pedro; Malaguti, Giuseppe; Pallé, Enric; Rataj, Mirek; Ray, Tom; Ribas, Ignasi; Szabó, Robert; Tan, Jonathan; Werner, Stephanie; Ratti, Francesco; Scharmberg, Carsten; Salvignol, Jean-Christophe; Boudin, Nathalie; Halain, Jean-Philippe; Haag, Martin; Crouzet, Pierre-Elie; Kohley, Ralf; Symonds, Kate; Renk, Florian; Caldwell, Andrew; Abreu, Manuel; Alonso, Gustavo; Amiaux, Jerome; Berthé, Michel; Bishop, Georgia; Bowles, Neil; Carmona, Manuel; Coffey, Deirdre; Colomé, Josep; Crook, Martin; Désjonqueres, Lucile; Díaz, José J.; Drummond, Rachel; Focardi, Mauro; Gómez, Jose M.; Holmes, Warren; Krijger, Matthijs; Kovacs, Zsolt; Hunt, Tom; Machado, Richardo; Morgante, Gianluca; Ollivier, Marc; Ottensamer, Roland; Pace, Emanuele; Pagano, Teresa; Pascale, Enzo; Pearson, Chris; Møller Pedersen, Søren; Pniel, Moshe; Roose, Stéphane; Savini, Giorgio; Stamper, Richard; Szirovicza, Peter; Szoke, Janos; Tosh, Ian; Vilardell, Francesc; Barstow, Joanna; Borsato, Luca; Casewell, Sarah; Changeat, Quentin; Charnay, Benjamin; Civiš, Svatopluk; Coudé du Foresto, Vincent; Coustenis, Athena; Cowan, Nicolas; Danielski, Camilla; Demangeon, Olivier; Drossart, Pierre; Edwards, Billy N.; Gilli, Gabriella; Encrenaz, Therese; Kiss, Csaba; Kokori, Anastasia; Ikoma, Masahiro; Morales, Juan Carlos; Mendonça, João; Moneti, Andrea; Mugnai, Lorenzo; García Muñoz, Antonio; Helled, Ravit; Kama, Mihkel; Miguel, Yamila; Nikolaou, Nikos; Pagano, Isabella; Panic, Olja; Rengel, Miriam; Rickman, Hans; Rocchetto, Marco; Sarkar, Subhajit; Selsis, Franck; Tennyson, Jonathan; Tsiaras, Angelos; Venot, Olivia; Vida, Krisztián; Waldmann, Ingo P.; Yurchenko, Sergey; Szabó, Gyula; Zellem, Rob; Al-Refaie, Ahmed; Perez Alvarez, Javier; Anisman, Lara; Arhancet, Axel; Ateca, Jaume; Baeyens, Robin; Barnes, John R.; Bell, Taylor; Benatti, Serena; Biazzo, Katia; Błęcka, Maria; Bonomo, Aldo Stefano; Bosch, José; Bossini, Diego; Bourgalais, Jeremy; Brienza, Daniele; Brucalassi, Anna; Bruno, Giovanni; Caines, Hamish; Calcutt, Simon; Campante, Tiago; Canestrari, Rodolfo; Cann, Nick; Casali, Giada; Casas, Albert; Cassone, Giuseppe; Cara, Christophe; Carmona, Manuel; Carone, Ludmila; Carrasco, Nathalie; Changeat, Quentin; Chioetto, Paolo; Cortecchia, Fausto; Czupalla, Markus; Chubb, Katy L.; Ciaravella, Angela; Claret, Antonio; Claudi, Riccardo; Codella, Claudio; Garcia Comas, Maya; Cracchiolo, Gianluca; Cubillos, Patricio; Da Peppo, Vania; Decin, Leen; Dejabrun, Clemence; Delgado-Mena, Elisa; Di Giorgio, Anna; Diolaiti, Emiliano; Dorn, Caroline; Doublier, Vanessa; Doumayrou, Eric; Dransfield, Georgina; Dumaye, Luc; Dunford, Emma; Jimenez Escobar, Antonio; Van Eylen, Vincent; Farina, Maria; Fedele, Davide; Fernández, Alejandro; Fleury, Benjamin; Fonte, Sergio; Fontignie, Jean; Fossati, Luca; Funke, Bernd; Galy, Camille; Garai, Zoltán; García, Andrés; García-Rigo, Alberto; Garufi, Antonio; Germano Sacco, Giuseppe; Giacobbe, Paolo; Gómez, Alejandro; Gonzalez, Arturo; Gonzalez-Galindo, Francisco; Grassi, Davide; Griffith, Caitlin; Guarcello, Mario Giuseppe; Goujon, Audrey; Gressier, Amélie; Grzegorczyk, Aleksandra; Guillot, Tristan; Guilluy, Gloria; Hargrave, Peter; Hellin, Marie-Laure; Herrero, Enrique; Hills, Matt; Horeau, Benoit; Ito, Yuichi; Jessen, Niels Christian; Kabath, Petr; Kálmán, Szilárd; Kawashima, Yui; Kimura, Tadahiro; Knížek, Antonín; Kreidberg, Laura; Kruid, Ronald; Kruijssen, J. M. Diederik; Kubelík, Petr; Lara, Luisa; Lebonnois, Sebastien; Lee, David; Lefevre, Maxence; Lichtenberg, Tim; Locci, Daniele; Lombini, Matteo; Sanchez Lopez, Alejandro; Lorenzani, Andrea; MacDonald, Ryan; Magrini, Laura; Maldonado, Jesus; Marcq, Emmanuel; Migliorini, Alessandra; Modirrousta-Galian, Darius; Molaverdikhani, Karan; Molinari, Sergio; Mollière, Paul; Moreau, Vincent; Morello, Giuseppe; Morinaud, Gilles; Morvan, Mario; Moses, Julianne I.; Mouzali, Salima; Nakhjiri, Nariman; Naponiello, Luca; Narita, Norio; Nascimbeni, Valerio; Nikolaou, Athanasia; Noce, Vladimiro; Oliva, Fabrizio; Palladino, Pietro; Papageorgiou, Andreas; Parmentier, Vivien; Peres, Giovanni; Pérez, Javier; Perez-Hoyos, Santiago; Perger, Manuel; Cecchi Pestellini, Cesare; Petralia, Antonino; Philippon, Anne; Piccialli, Arianna; Pignatari, Marco; Piotto, Giampaolo; Podio, Linda; Polenta, Gianluca; Preti, Giampaolo; Pribulla, Theodor; Lopez Puertas, Manuel; Rainer, Monica; Reess, Jean-Michel; Rimmer, Paul; Robert, Séverine; Rosich, Albert; Rossi, Loic; Rust, Duncan; Saleh, Ayman; Sanna, Nicoletta; Schisano, Eugenio; Schreiber, Laura; Schwartz, Victor; Scippa, Antonio; Seli, Bálint; Shibata, Sho; Simpson, Caroline; Shorttle, Oliver; Skaf, N.; Skup, Konrad; Sobiecki, Mateusz; Sousa, Sergio; Sozzetti, Alessandro; Šponer, Judit; Steiger, Lukas; Tanga, Paolo; Tackley, Paul; Taylor, Jake; Tecza, Matthias; Terenzi, Luca; Tremblin, Pascal; Tozzi, Andrea; Triaud, Amaury; Trompet, Loïc; Tsai, Shang-Min; Tsantaki, Maria; Valencia, Diana; Carine Vandaele, Ann; Van der Swaelmen, Mathieu; Adibekyan, Vardan; Vasisht, Gautam; Vazan, Allona; Del Vecchio, Ciro; Waltham, Dave; Wawer, Piotr; Widemann, Thomas; Wolkenberg, Paulina; Hou Yip, Gordon; Yung, Yuk; Zilinskas, Mantas; Zingales, Tiziano; Zuppella, Paola Bibcode: 2021arXiv210404824T Altcode: Ariel, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey, was adopted as the fourth medium-class mission in ESA's Cosmic Vision programme to be launched in 2029. During its 4-year mission, Ariel will study what exoplanets are made of, how they formed and how they evolve, by surveying a diverse sample of about 1000 extrasolar planets, simultaneously in visible and infrared wavelengths. It is the first mission dedicated to measuring the chemical composition and thermal structures of hundreds of transiting exoplanets, enabling planetary science far beyond the boundaries of the Solar System. The payload consists of an off-axis Cassegrain telescope (primary mirror 1100 mm x 730 mm ellipse) and two separate instruments (FGS and AIRS) covering simultaneously 0.5-7.8 micron spectral range. The satellite is best placed into an L2 orbit to maximise the thermal stability and the field of regard. The payload module is passively cooled via a series of V-Groove radiators; the detectors for the AIRS are the only items that require active cooling via an active Ne JT cooler. The Ariel payload is developed by a consortium of more than 50 institutes from 16 ESA countries, which include the UK, France, Italy, Belgium, Poland, Spain, Austria, Denmark, Ireland, Portugal, Czech Republic, Hungary, the Netherlands, Sweden, Norway, Estonia, and a NASA contribution. Title: Correcting the effect of stellar spots on ARIEL transmission spectra Authors: Cracchiolo, G.; Micela, G.; Peres, G. Bibcode: 2021MNRAS.501.1733C Altcode: 2020MNRAS.tmp.3412C; 2020arXiv201110085C The goal of this study is to assess the impact of the stellar spots on the extraction of the planetary transmission spectra observed by ARIEL. We develop a method to model the stellar spectrum of a star in the presence of spots by using the out-of-transit observations. It is based on a chi squared minimization procedure of the out-of-transit spectrum on a grid of stellar spectra with different sizes and temperatures of the spots. The approach allows us also to study the temporal evolution of the spots when comparing stellar spectra observed at different epochs. We also present a method to correct the transit depth variations due to non-occulted stellar spots and estimate the error we introduce if we apply the same correction to crossings over the stellar spots. The method is tested on three types of stellar targets that ARIEL will observe in its 4-yr mission lifetime. In all the explored cases, the approach allows us to reliably recover the spot parameters (size and temperature) from out-of-transit observations and, for non-occulted spots, to confidently recover the planetary atmosphere transmission spectrum within the noise level (with average uncertainty of at most $3.3{{\ \rm per\ cent}}$ of the planetary signal). Conversely, we find systematic biases in the inferred planetary spectra due to the occulted spots, with measurable effects for the brightest targets especially for more contrasted spots. Title: Indication of a Pulsar Wind Nebula in the Hard X-Ray Emission from SN 1987A Authors: Greco, Emanuele; Miceli, Marco; Orlando, Salvatore; Olmi, Barbara; Bocchino, Fabrizio; Nagataki, Shigehiro; Ono, Masaomi; Dohi, Akira; Peres, Giovanni Bibcode: 2021ApJ...908L..45G Altcode: 2021arXiv210109029G Since the day of its explosion, SN 1987A (SN87A) was closely monitored with the aim to study its evolution and to detect its central compact relic. The detection of neutrinos from the supernova strongly supports the formation of a neutron star (NS). However, the constant and fruitless search for this object has led to different hypotheses on its nature. To date, the detection in the Atacama Large Millimeter/submillimeter Array data of a feature that is somehow compatible with the emission arising from a proto-pulsar wind nebula (PWN) is the only hint of the existence of such elusive compact object. Here we tackle this 33 yr old issue by analyzing archived observations of SN87A performed by Chandra and NuSTAR in different years. We firmly detect nonthermal emission in the 10-20 kev energy band, due to synchrotron radiation. The possible physical mechanism powering such emission is twofold: diffusive shock acceleration (DSA) or emission arising from an absorbed PWN. By relating a state-of-the-art magnetohydrodynamic simulation of SN87A to the actual data, we reconstruct the absorption pattern of the PWN embedded in the remnant and surrounded by cold ejecta. We found that, even though the DSA scenario cannot be firmly excluded, the most likely scenario that well explains the data is that of PWN emission. Title: The fully developed remnant of a neutrino-driven supernova. Evolution of ejecta structure and asymmetries in SNR Cassiopeia A Authors: Orlando, S.; Wongwathanarat, A.; Janka, H. -T.; Miceli, M.; Ono, M.; Nagataki, S.; Bocchino, F.; Peres, G. Bibcode: 2021A&A...645A..66O Altcode: 2020A&A...645A..66O; 2020arXiv200901789O Context. The remnants of core-collapse supernovae (SNe) are probes of the physical processes associated with their parent SNe.
Aims: Here we aim to explore to which extent the remnant keeps memory of the asymmetries that develop stochastically in the neutrino-heating layer due to hydrodynamic instabilities (e.g., convective overturn and the standing accretion shock instability; SASI) during the first second after core bounce.
Methods: We coupled a three-dimensional (3D) hydrodynamic model of a neutrino-driven SN explosion, which has the potential to reproduce the observed morphology of the Cassiopeia A (Cas A) remnant, with 3D (magneto)-hydrodynamic simulations of the remnant formation. The simulations cover ≈2000 yr of expansion and include all physical processes relevant to describe the complexities in the SN evolution and the subsequent interaction of the stellar debris with the wind of the progenitor star.
Results: The interaction of large-scale asymmetries left from the earliest phases of the explosion with the reverse shock produces, at the age of ≈350 yr, an ejecta structure and a remnant morphology which are remarkably similar to those observed in Cas A. Small-scale structures in the large-scale Fe-rich plumes that were created during the initial stages of the SN, combined with hydrodynamic instabilities that develop after the passage of the reverse shock, naturally produce a pattern of ring- and crown-like structures of shocked ejecta. The consequence is a spatial inversion of the ejecta layers with Si-rich ejecta being physically interior to Fe-rich ejecta. The full-fledged remnant shows voids and cavities in the innermost unshocked ejecta, which are physically connected with ring-like features of shocked ejecta in the main shell in most cases, resulting from the expansion of Fe-rich plumes and their inflation due to the decay of radioactive species. The asymmetric distributions of 44Ti and 56Fe, which are mostly concentrated in the northern hemisphere, and pointing opposite to the kick velocity of the neutron star, as well as their abundance ratio are both compatible with those inferred from high-energy observations of Chandra and NuSTAR. Finally, the simulations show that the fingerprints of the SN can still be visible ≈2000 yr after the explosion.
Conclusions: The main asymmetries and features observed in the ejecta distribution of Cas A can be explained by the interaction of the reverse shock with the initial large-scale asymmetries that developed from stochastic processes (e.g., convective overturn and SASI activity) that originate during the first seconds of the SN blast.

Movies associated to Figs. 7, 8, 12, 15 are available at https://www.aanda.org Title: Three-dimensional modeling from the onset of the SN to the full-fledged SNR. Role of an initial ejecta anisotropy on matter mixing Authors: Tutone, A.; Orlando, S.; Miceli, M.; Ustamujic, S.; Ono, M.; Nagataki, S.; Ferrand, G.; Greco, E.; Peres, G.; Warren, D. C.; Bocchino, F. Bibcode: 2020A&A...642A..67T Altcode: 2020arXiv200901157T Context. The manifold phases in the evolution of a core-collapse (CC) supernova (SN) play an important role in determining the physical properties and morphology of the resulting supernova remnant (SNR). Thus, the complex morphology of SNRs is expected to reflect possible asymmetries and structures developed during and soon after the SN explosion.
Aims: The aim of this work is to bridge the gap between CC SNe and their remnants by investigating how post-explosion anisotropies in the ejecta influence the structure and chemical properties of the remnant at later times.
Methods: We performed three-dimensional magneto-hydrodynamical simulations starting soon after the SN event and following the evolution of the system in the circumstellar medium, which includes the wind of the stellar progenitor, for 5000 yr, obtaining the physical scenario of a SNR. Here we focused the analysis on the case of a progenitor red supergiant of 19.8 M⊙. We also investigated how a post-explosion large-scale anisotropy in the SN affects the ejecta distribution and the matter mixing of heavy elements in the remnant during the first 5000 yr of evolution.
Results: In the case of a spherically symmetric SN explosion without large-scale anisotropies, the remnant roughly keeps memory of the original onion-like layering of ejecta soon after the SN event. Nevertheless, as the reverse shock hits the ejecta, the element distribution departs from a homologous expansion because of the slowing down of the outermost ejecta layers due to interaction with the reverse shock. In the case of a large-scale anisotropy developed after the SN, we found that the chemical stratification in the ejecta can be strongly modified and the original onion-like layering is not preserved. The anisotropy may cause spatial inversion of ejecta layers, for instance leading to Fe/Si-rich ejecta outside the O shell, and may determine the formation of Fe/Si-rich jet-like features that may protrude the remnant outline. The level of matter mixing and the properties of the jet-like feature are sensitive to the initial physical (density and velocity) and geometrical (size and position) initial characteristics of the anisotropy.

Movies associated to Figs. 3 and 6 are available at https://www.aanda.org Title: Unveiling pure-metal ejecta X-ray emission in supernova remnants through their radiative recombination continuum Authors: Greco, Emanuele; Vink, Jacco; Miceli, Marco; Orlando, Salvatore; Domček, Vladimir; Zhou, Ping; Bocchino, Fabrizio; Peres, Giovanni Bibcode: 2020A&A...638A.101G Altcode: 2020arXiv200412924G Context. Spectral analysis of X-ray emission from ejecta in supernova remnants (SNRs) is hampered by the low spectral resolution of CCD detectors, which typically creates a degeneracy between the best-fit values of chemical abundances and the plasma emission measure. The combined contribution of shocked ambient medium and ejecta to the emerging X-ray emission further complicates the determination of the ejecta mass and chemical composition. This degeneracy leads to big uncertainties in mass estimates and can introduce a bias in the comparison between the ejecta chemical composition derived from the observations and the yields predicted by explosive nucleosynthesis models.
Aims: We explore the capabilities of present and future spectral instruments with the aim of identifying a spectral feature that may allow us to discriminate between metal-rich and pure-metal plasmas in X-ray spectra of SNRs.
Methods: We studied the behavior of the most common X-ray emission processes of an optically thin plasma in the high-abundance regime. We investigated spectral features of bremsstrahlung, radiative recombination continua (RRC), and line emission, by exploring a wide range of chemical abundances, plasma temperatures, and ionization parameters. We then synthesized X-ray spectra from a state-of-the-art 3D hydrodynamic simulation of Cas A, by using the response matrix from the Chandra ACIS-S charged-coupled device detector and that of the XRISM/Resolve X-ray calorimeter spectrometer.
Results: We found that a bright RRC shows up when the plasma is made of pure-metal ejecta, and a high spectral resolution is needed to actually identify this ejecta signature. We tested and verified the applicability of our novel diagnostic tool and we propose a promising target for the future detection of such spectral feature: the southeastern Fe-rich clump of Cas A.
Conclusions: While there is no way to unambiguously reveal pure-metal ejecta emission with CCD detectors, X-ray calorimeters will be able to pinpoint the presence of pure-metal RRC and to recover correctly absolute mass and the chemical composition of the ejecta, opening a new window on the link between progenitor star, supernova and SNRs. Title: Hydrodynamic simulations unravel the progenitor-supernova-remnant connection in SN 1987A Authors: Orlando, S.; Ono, M.; Nagataki, S.; Miceli, M.; Umeda, H.; Ferrand, G.; Bocchino, F.; Petruk, O.; Peres, G.; Takahashi, K.; Yoshida, T. Bibcode: 2020A&A...636A..22O Altcode: 2019arXiv191203070O Context. Massive stars end their lives in catastrophic supernova (SN) explosions. Key information on the explosion processes and on the progenitor stars can be extracted from observations of supernova remnants (SNRs), which are the outcome of SNe. Deciphering these observations, however, is challenging because of the complex morphology of SNRs.
Aims: We aim to link the dynamical and radiative properties of the remnant of SN 1987A to the geometrical and physical characteristics of the parent aspherical SN explosion and to the internal structure of its progenitor star.
Methods: We performed comprehensive three-dimensional hydrodynamic simulations which describe the long-term evolution of SN 1987A from the onset of the SN to the full-fledged remnant at the age of 50 years, accounting for the pre-SN structure of the progenitor star. The simulations include all physical processes relevant for the complex phases of SN evolution and for the interaction of the SNR with the highly inhomogeneous ambient environment around SN 1987A. Furthermore, the simulations follow the life cycle of elements from the synthesis in the progenitor star through the nuclear reaction network of the SN to the enrichment of the circumstellar medium as a result of the mixing of chemically homogeneous layers of ejecta. From the simulations, we synthesize observables that are to be compared with observations.
Results: By comparing the model results with observations, we constrained the initial SN anisotropy causing Doppler shifts, observed in the emission lines of heavy elements from ejecta, and leading to the remnant evolution observed in the X-ray band in the last thirty years. In particular, we found that the high mixing of ejecta unveiled by high redshifts and broadenings of [Fe II] and 44Ti lines require a highly asymmetric SN explosion channeling a significant fraction of energy along an axis that is almost lying in the plane of the central equatorial ring around SN 1987A, roughly along the line-of-sight, but with an offset of 40°, with the lobe propagating away from the observer slightly more energetic than the other. Furthermore, we found unambiguously that the observed distribution of ejecta and the dynamical and radiative properties of the SNR can be best reproduced if the structure of the progenitor star was that of a blue supergiant which had resulted from the merging of two massive stars.

Movies associated to Figs. 4, 7, 9, and 10 are available at https://www.aanda.org Title: Effects of intense flaring activity on accretion disk of Classical T Tauri Stars Authors: Colombo, S.; Orlando, S.; Peres, G.; Reale, F.; Argiroffi, C.; Bonito, R.; Ibgui, L.; Stehle, C. Bibcode: 2020MmSAI..91..232C Altcode: Classical T Tauri stars (CTTSs) are young low-mass stellar objects that accrete mass from their circumstellar disks characterized by high levels of coronal activity. Here, we investigate, using a 3D MHD model, if an intense coronal activity due to flares that occur close to the accretion disk may perturb the stability of the inner disk triggering accretion episodes. We observe the formation of several loops that link the star to the disk; all these loops build up a hot extended corona with an X-ray luminosity comparable with typical values observed in CTTSs. We found that the intense flaring activity close to the disk can strongly perturb the disk stability, in fact accretion funnels may be triggered by the flaring activity and thus contribute to the mass accretion rate of the star. Title: Linking core-collapse supernova explosions to supernova remnants through 3D MHD modeling Authors: Orlando, S.; Wongwathanarat, A.; Janka, H. -T.; Miceli, M.; Ono, M.; Nagataki, S.; Bocchino, F.; Peres, G. Bibcode: 2020MmSAI..91..325O Altcode: 2020arXiv201213630O The structure and morphology of supernova remnants (SNRs) reflect the properties of the parent supernovae (SNe) and the characteristics of the inhomogeneous environments through which the remnants expand. Linking the morphology of SNRs to anisotropies developed in their parent SNe can be essential to obtain key information on many aspects of the explosion processes associated with SNe. Nowadays, our capability to study the SN-SNR connection has been largely improved thanks to multi-dimensional models describing the long-term evolution from the SN to the SNR as well as to observational data of growing quality and quantity across the electromagnetic spectrum which allow to constrain the models. Here we used the numerical resources obtained in the framework of the ``Accordo Quadro INAF-CINECA (2017)'' together with a CINECA ISCRA Award N.HP10BARP6Y to describe the full evolution of a SNR from the core-collapse to the full-fledged SNR at the age of 2000 years. Our simulations were compared with observations of SNR Cassiopeia A (Cas A) at the age of ∼ 350 years. Thanks to these simulations we were able to link the physical, chemical and morphological properties of a SNR to the physical processes governing the complex phases of the SN explosion. Title: Non-LTE radiation hydrodynamics in PLUTO Authors: Colombo, S.; Ibgui, L.; Orlando, S.; Rodríguez, R.; Espinosa, G.; González, M.; Stehlé, C.; Peres, G. Bibcode: 2019A&A...631A..41C Altcode: 2019arXiv190704591C Context. Modeling the dynamics of most astrophysical structures requires an adequate description of the interaction of radiation and matter. Several numerical (magneto-) hydrodynamics codes were upgraded with a radiation module to fulfill this request. However, those that used either the flux-limited diffusion (FLD) or the M1 radiation moment approaches are restricted to local thermodynamic equilibrium (LTE). This assumption may not be valid in some astrophysical cases.
Aims: We present an upgraded version of the LTE radiation-hydrodynamics (RHD) module implemented in the PLUTO code, which we have extended to handle non-LTE regimes.
Methods: Starting from the general frequency-integrated comoving-frame equations of RHD, we have justified all the assumptions that were made to obtain the non-LTE equations that are implemented in the module under the FLD approximation. An operator-split method with two substeps was employed: the hydrodynamics part was solved with an explicit method by the solvers that are currently available in PLUTO, and the non-LTE radiation diffusion and energy exchange part was solved with an implicit method. The module was implemented in the PLUTO environment. It uses databases of radiative quantities that can be provided independently by the user: the radiative power loss, and the Planck and Rosseland mean opacities. In our case, these quantities were determined from a collisional-radiative steady-state model, and they are tabulated as functions of temperature and density.
Results: Our implementation has been validated through different tests, in particular, radiative shock tests. The agreement with the semi-analytical solutions (when available) is good, with a maximum error of 7%. Moreover, we have proved that a non-LTE approach is of paramount importance to properly model accretion shock structures. Conclusion. Our radiation FLD module represents a step toward a general non-LTE RHD modeling.

The module is available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/631/A41 and upon request to the first author. Title: The Gaia-ESO survey: star formation history in Lambda Ori star forming region Authors: Prisinzano, Loredana; Trentacoste, Antonino; Peres, Giovanni; Damiani, Francesco; Stelzer, Beate Bibcode: 2019lges.confE..36P Altcode: Stellar age distributions of young stellar clusters are crucial to assess the duration of the star formation process and how it depends on the environment.

The Lambda Ori star-forming region includes three different clouds hosting the three young stellar clusters Collinder 69, Barnard 30 and Barnard 35, that have been observed within the Gaia-ESO survey (GES). Taking advantage of GES, Gaia DR2 and literature data available for these subclusters, we derived stellar membership and stellar ages from the HR diagram. We will present the comparison of the star formation history of the entire Lambda Ori star forming region in order to understand the effects of environment on the star formation process. Title: VizieR Online Data Catalog: Radiation module for the code PLUTO (Colombo+, 2019) Authors: Colombo, S.; Ibgui, L.; Orlando, S.; Rodriguez, R.; Espinosa, G.; Gonzalez, M.; Stehle, C.; Peres, G. Bibcode: 2019yCat..36310041C Altcode: PLUTO is a freely-distributed software for the numerical solution of mixed hyperbolic/parabolic systems of partial differential equations (conservation laws) targeting high Mach number flows in astrophysical fluid dynamics. The code is designed with a modular and flexible structure whereby different numerical algorithms can be separately combined to solve systems of conservation laws using the finite volume or finite difference approach based on Godunov-type schemes.

See http://plutocode.ph.unito.it/ and http://cerere.astropa.unipa.it/progetti_ricerca/HPC/resources.htm for more details.

(1 data file). Title: The Cherenkov Telescope Array Authors: Williams, David; Balmaverde, Barbara; Benbow, Wystan; Bucciantini, Niccolò; Buckley, James; Burton, Michael; Capuzzo-Dolcetta, Roberto; Chaty, Sylvain; Coppi, Paolo; Costa, Alessandro; D'Ammando, Filippo; De Cesare, Giovanni; Di Girolamo, Tristano; Doro, Michele; Giglietto, Nicola; Hnatyk, Bohdan; Holder, Jamie; Humensky, Brian; Kaaret, Phil; Kieda, David; Kohri, Kazunori; Martinez, Manel; Minaya Flores, Ignacio A.; Mirzoyan, Razmik; Morselli, Aldo; Mukherjee, Reshmi; Nieto Castaño, Daniel; Nikolajuk, Marek; Ong, Rene; Otte, A. Nepomuk; Pareschi, Giovanni; Peres, Giovanni; Reisenegger, Andreas; Romeo, Giuseppe; Santander, Marcos; Sciacca, Eva; Schüssler, Fabian; Sergijenko, Olga; Vandenbroucke, Justin; Vassiliev, Vladimir; Vorobiov, Serguei Bibcode: 2019BAAS...51g.291W Altcode: 2019astro2020U.291W The Cherenkov Telescope Array (CTA), a unified worldwide effort and the sole proposed implementation of the "Advanced Cherenkov Telescope Array" recommended by the Astro2010 survey, is a ground-based instrument for studies of very-high-energy gamma rays from 20 GeV to 300 TeV. This white paper summarizes the CTA project and its science goals. Title: Effects of radiation in accretion regions of classical T Tauri stars. Pre-heating of accretion column in non-LTE regime Authors: Colombo, S.; Ibgui, L.; Orlando, S.; Rodriguez, R.; Espinosa, G.; González, M.; Stehlé, C.; de Sá, L.; Argiroffi, C.; Bonito, R.; Peres, G. Bibcode: 2019A&A...629L...9C Altcode: 2019arXiv190806799C Context. Models and observations indicate that the impact of matter accreting onto the surface of young stars produces regions at the base of accretion columns where optically thin and thick plasma components coexist. Thus, an accurate description of these impacts is necessary to account for the effects of absorption and emission of radiation.
Aims: We study the effects of radiation emerging from shock-heated plasma in impact regions on the structure of the pre-shock down-falling material. We investigate whether a significant absorption of radiation occurs and if it leads to a pre-shock heating of the accreting gas.
Methods: We developed a radiation hydrodynamics model describing an accretion column impacting onto the surface of a classical T Tauri star. The model takes into account the stellar gravity, the thermal conduction, and the effects of radiative losses and of absorption of radiation by matter in the nonlocal thermodynamic equilibrium regime.
Results: After the impact, a hot slab of post-shock plasma develops at the base of the accretion column. Part of the radiation emerging from the slab is absorbed by the pre-shock accreting material. As a result, the pre-shock accretion column gradually heats up to temperatures of 105 K, forming a radiative precursor of the shock. The precursor has a thermal structure with the hottest part at T ≈ 105 K, with a size comparable to that of the hot slab, above the post-shock region. At larger distances the temperature gradually decreases to T ≈ 104 K.
Conclusions: Our model predicts that ≈70% of the radiation emitted by the post-shock plasma is absorbed by the pre-shock accretion column immediately above the slab and is re-emitted in the UV band. This may explain why accretion rates derived from UV observations are systematically higher than rates inferred from X-ray observations. Title: Discovery of a jet-like structure with overionized plasma in the SNR IC 443 Authors: Greco, E.; Miceli, M.; Orlando, S.; Peres, G.; Troja, E.; Bocchino, F. Bibcode: 2019sros.confE.198G Altcode: IC 443 is a supernova remnant (SNR) located in a complex environment and interacting with nearby clouds. Indications for the presence of overionized plasma have been found though the possible physical causes of overionization are still debated. Moreover, because of its peculiar position and proper motion, it is not clear if the pulsar wind nebula (PWN) within the remnant is the relic of the IC 443 progenitor star or just a rambling one seen in projection on the remnant. We addressed the study of the IC 443 X-ray emission in order to investigate the relationship between the PWN and the remnant, the presence of overionization and its origin. We identified an elongated (jet-like) structure with Mg-rich plasma in overionization. The head of the jet is interacting with a molecular cloud and the jet is aligned with the position of the PWN at the instant of the supernova explosion. Interestingly, the direction of the jet of ejecta is somehow consistent with the direction of the PWN jet. Our discovery of a jet of ejecta in IC 443 enlarge the sample of core-collapse SNRs with collimated ejecta structures. IC 443's jet is the first one which shows overionized plasma, possibly associated with the adiabatic expansion of ejecta. The match between the jet's direction and the original position of the PWN strongly supports the association between the neutron star and IC 443. Title: Studying the radiative recombination continua in the X-ray spectra of pure ejecta and of overionized plasmas Authors: Greco, E.; Vink, J.; Miceli, M.; Domcek, V.; Zhou, P.; Orlando, S.; Peres, G.; Bocchino, F. Bibcode: 2019sros.confE.144G Altcode: Several Supernova Remnants (SNRs) show signatures of over ionization. A plasma is over ionized when its ionization state is higher than that expected from its electron temperature, namely when it is non-equilibrium of ionization and in recombining phase. The physical origin of over ionized plasma in SNRs is largely debated in the literature. The characteristic feature of a recombining plasma in the X-rays is the presence of prominent Radiative Recombination Continua (RRC), associated with electron recombining with ions. However, the intensity of the RRCs in the X-rays also increases with the plasma metallicity. We here present a thorough spectral study to investigate whether the bright RRC originating from a pure ejecta plasma in collisional ionization equilibrium can mimic that of an over ionized plasma in non-equilibrium of ionization. We performed series of spectral simulations in the X-rays, by exploring different values of temperature, emission measure, and chemical abundances. We also considered spectra emerging from a combination of interstellar medium and pure ejecta and compared them against actual observations of recombining SNRs. We present here some preliminary results and discuss in which conditions pure ejecta plasma in collisional ionization equilibrium might mimic features usually attributed to over ionized plasma in non-equilibrium of ionization. Title: A Virtual Reality Environment for Scientific Exploitation of 3D MHD Astrophysical Simulations Authors: Bocchino, Fabrizio; Orlando, S.; Pillitteri, I.; Miceli, M.; Peres, G. Bibcode: 2019sros.confE..76B Altcode: Virtual Reality (VR) hardware and software are now routinely used in several fields for public outreach and education with excellent feedback. For this purpose, YouTube and online multimedia digital stores have several VR titles of great impact in the Astro physics and Space categories of their catalogues. However, scientific use of VR environments are still at their beginning. Fully 3D Magneto-hydrodynamical simulations of astrophysical phenomena represent a challenge in standard data visualization for scientific purposes, for the amount of processed data and the wealth of scientific information they contain. In this poster, we present a VR platform based on Oculus Rift for data visualization of 3D MHD models of Supernova Remnants and Young Stellar Objects developed at INAF-Osservatorio Astronomico di Palermo. We show that fully immersive space-walks inside properly configured data cubes may be of great importance to study the distribution of hot plasmas, cold materials and the magnetic fields of these environments. Title: Measuring the post-shock temperatures of heavy ions in SN 1987A Authors: Miceli, M.; Orlando, S.; Bocchino, F.; Park, S.; Pazhayath Ravi, A.; Burrows, D.; Frank, K.; Argiroffi, C.; Peres, G.; Greco, E.; Petruk, O. Bibcode: 2019sros.confE.108M Altcode: The shock waves originated from supernova explosions provide crucial information on the physics of shock heating. Astrophysical shocks are typically collisionless and electrons, protons, and ions are expected to be heated at different temperatures, but the actual dependence of the post-shock temperature on the particle mass is still debated. We tackle this long standing issue through the analysis of high-resolution observations of SN 1987A, made with the Chandra X-ray telescope in 2007 and 2011, and we also include the latest 2018 observation in our analysis. We study the observed spectra in close comparison with a dedicated full 3-D hydrodynamic simulation. The simulation is able to reproduce self-consistently the whole broadening of the spectral lines of many ions altogether. We could therefore measure the post shock temperature of protons and selected ions. We found that the ion to proton temperature ratio is always significantly higher than one and increases linearly with the ion mass for a wide range of masses. Thanks to the new 2018 deep observation we are also able to further extend our results for a wide range of shock parameters. Title: Three-dimensional MHD modeling of SNR IC 443: effects of the inhomogeneous medium in shaping the remnant morphology Authors: Ustamujic, S.; Orlando, S.; Greco, E.; Miceli, M.; Bocchino, F.; Peres, G. Bibcode: 2019sros.confE.202U Altcode: In this study we aim at investigating the effects of the inhomogeneous medium in shaping the remnant morphology of IC 443 after the supernova (SN) explosion. The distribution of the interstellar medium in the vicinity of the supernova remnant (SNR) plays a fundamental role for our understanding of the morphology and later evolution. In particular, IC443 is a SNR located in a quite complex environment: it interacts with a molecular cloud in the northwestern and southeastern areas and with an atomic cloud in the northeast. We have developed a 3D MHD model for SNR IC~443 describing the interaction of the SNR with the environment, parametrized in agreement with the results of the multiwavelength data analysis. In this poster we present our preliminary results. Title: 3D MHD Simulations from the Onset of the SN to the Full-fledged SNR: Role of Ejecta Clumps on Matter Mixing Authors: Tutone, Antonio; Orlando, Salvatore; Miceli, Marco; Peres, Giovanni; Ono, Masaomi; Nagataki, Shigehiro; Ferrand, Gilles Bibcode: 2019sros.confE..91T Altcode: The physical properties and the morphology of supernova remnants (SNRs) partially reflect the structure of the stellar progenitor and possible asymmetries developed soon after their parent supernova (SN) explosions. The aim of this work is to bridge the gap between SNe and their remnants by investigating how the various chemically homogeneous layers at the time of the explosion map into the resulting abundance pattern observed when the remnant is fully developed. To this end, we have performed three-dimensional magneto-hydrodynamical simulations starting soon after the SN and following the interaction of the SN ejecta with the CSM (consisting in the wind of the stellar progenitor), obtaining the physical scenario of a SNR. As stellar progenitor, we have considered the case of a 19 M⊙ red supergiant. We investigated how the evolution of a post-explosion large-scale anisotropy in the ejecta and the role of its initial parameters (position, dimension, density and velocity contrast) can affect the ejecta distribution and the matter mixing of heavy elements in the remnant, covering 5000 years of evolution. Title: A stellar flare-coronal mass ejection event revealed by X-ray plasma motions Authors: Argiroffi, C.; Reale, F.; Drake, J. J.; Ciaravella, A.; Testa, P.; Bonito, R.; Miceli, M.; Orlando, S.; Peres, G. Bibcode: 2019NatAs...3..742A Altcode: 2019NatAs.tmp..328A; 2019arXiv190511325A Coronal mass ejections (CMEs), often associated with flares1-3, are the most powerful magnetic phenomena occurring on the Sun. Stars show magnetic activity levels up to ten thousand times higher4, and CME effects on stellar physics and circumstellar environments are predicted to be substantial5-9. However, stellar CMEs remain observationally unexplored. Using time-resolved high-resolution X-ray spectroscopy of a stellar flare on the active star HR 9024 observed with the High Energy Transmission Grating Spectrometer onboard the Chandra X-ray Observatory space telescope, we distinctly detected Doppler shifts in S xvi, Si xiv and Mg xii lines that indicate upward and downward motions of hot plasmas (around 10-25 MK) within the flaring loop, with velocities of 100-400 km s-1, in agreement with a model of a flaring magnetic tube. Most notably, we also detected a later blueshift in the O viii line that reveals an upward motion, with velocity 90 ± 30 km s-1, of cool plasma (about 4 MK), that we ascribe to a CME coupled to the flare. From this evidence we were able to derive a CME mass of 1 .2-0.8+2.6×1 021 g and a CME kinetic energy of 5 .2-3.6+27.7×1 034 erg. These values provide clues in the extrapolation of the solar case to higher activity levels in other stars, suggesting that CMEs could indeed be a major cause of mass and angular momentum loss. Title: New view of the corona of classical T Tauri stars: Effects of flaring activity in circumstellar disks Authors: Colombo, S.; Orlando, S.; Peres, G.; Reale, F.; Argiroffi, C.; Bonito, R.; Ibgui, L.; Stehlé, C. Bibcode: 2019A&A...624A..50C Altcode: 2019arXiv190207048C Context. Classical T Tauri stars (CTTSs) are young low-mass stellar objects that accrete mass from their circumstellar disks. They are characterized by high levels of coronal activity, as revealed by X-ray observations. This activity may affect the disk stability and the circumstellar environment.
Aims: Here we investigate if an intense coronal activity due to flares that occur close to the accretion disk may perturb the stability of the inner disk, disrupt the inner part of the disk, and might even trigger accretion phenomena with rates comparable with those observed.
Methods: We modeled a magnetized protostar surrounded by an accretion disk through 3D magnetohydrodinamic simulations. The model takes into account the gravity from the central star, the effects of viscosity in the disk, the thermal conduction (including the effects of heat flux saturation), the radiative losses from optically thin plasma, and a parameterized heating function to trigger the flares. We explored cases characterized by a dipole plus an octupole stellar magnetic field configuration and different density of the disk or by different levels of flaring activity.
Results: As a result of the simulated intense flaring activity, we observe the formation of several loops that link the star to the disk; all these loops build up a hot extended corona with an X-ray luminosity comparable with typical values observed in CTTSs. The intense flaring activity close to the disk can strongly perturb the disk stability. The flares trigger overpressure waves that travel through the disk and modify its configuration. Accretion funnels may be triggered by the flaring activity and thus contribute to the mass accretion rate of the star. Accretion rates synthesized from the simulations are in a range between 10-10 and 10-9 M⊙ yr-1. The accretion columns can be perturbed by the flares, and they can interact with each other; they might merge into larger streams. As a result, the accretion pattern can be rather complex: the streams are highly inhomogeneous, with a complex density structure, and clumped.

Movies are available at https://www.aanda.org Title: 3D MHD modeling of the expanding remnant of SN 1987A. Role of magnetic field and non-thermal radio emission Authors: Orlando, S.; Miceli, M.; Petruk, O.; Ono, M.; Nagataki, S.; Aloy, M. A.; Mimica, P.; Lee, S. -H.; Bocchino, F.; Peres, G.; Guarrasi, M. Bibcode: 2019A&A...622A..73O Altcode: 2018arXiv181200021O
Aims: We investigate the role played by a pre-supernova (SN) ambient magnetic field in the dynamics of the expanding remnant of SN 1987A, and the origin and evolution of the radio emission from the remnant, in particular during the interaction of the blast wave with the nebula surrounding the SN.
Methods: We modeled the evolution of SN 1987A from the breakout of the shock wave at the stellar surface to the expansion of its remnant through the surrounding nebula using three-dimensional magnetohydrodynamic simulations. The model considers the radiative cooling, the deviations from equilibrium of ionization, the deviation from temperature-equilibration between electrons and ions, and a plausible configuration of the pre-SN ambient magnetic field. We explore the strengths of the pre-SN magnetic field ranging between 1 and 100 μG at the inner edge of the nebula and we assume an average field strength at the stellar surface B0 ≈ 3 kG. From the simulations, we synthesize the thermal X-ray and the non-thermal radio emission and compare the model results with observations.
Results: The presence of an ambient magnetic field with strength in the range considered does not change significantly the overall evolution of the remnant. Nevertheless, the magnetic field reduces the erosion and fragmentation of the dense equatorial ring after the impact of the SN blast wave. As a result, the ring survives the passage of the blast, at least during the time covered by the simulations (40 yr). Our model is able to reproduce the morphology and lightcurves of SN 1987A in both X-ray and radio bands. The model reproduces the observed radio emission if the flux originating from the reverse shock is heavily suppressed. In this case, the radio emission originates mostly from the forward shock traveling through the H II region and this may explain why the radio emission seems to be insensitive to the interaction of the blast with the ring. Possible mechanisms for the suppression of emission from the reverse shock are investigated. We find that synchrotron self-absorption and free-free absorption have negligible effects on the emission during the interaction with the nebula. We suggest that the emission from the reverse shock at radio frequencies might be limited by highly magnetized ejecta.

Movies associated to Figs. 2 and 4 are available at http://www.aanda.org Title: Mass Accretion Impacts in Classical T Tauri Stars: A Multi-disciplinary Approach Authors: Orlando, S.; Argiroffi, C.; Bonito, R.; Colombo, S.; Peres, G.; Reale, F.; Miceli, M.; Ibgui, L.; Stehlé, C.; Matsakos, T. Bibcode: 2019ASSP...55...43O Altcode: No abstract at ADS Title: Collisionless shock heating of heavy ions in SN 1987A Authors: Miceli, Marco; Orlando, Salvatore; Burrows, David N.; Frank, Kari A.; Argiroffi, Costanza; Reale, Fabio; Peres, Giovanni; Petruk, Oleh; Bocchino, Fabrizio Bibcode: 2019NatAs...3..236M Altcode: 2019NatAs.tmp..194M; 2019arXiv190110336M Astrophysical shocks at all scales, from those in the heliosphere up to cosmological shock waves, are typically `collisionless', because the thickness of their jump region is much shorter than the collisional mean free path. Across these jumps, electrons, protons and ions are expected to be heated at different temperatures. Supernova remnants (SNRs) are ideal targets to study collisionless processes because of their bright post-shock emission and fast shocks, but the actual dependence of the post-shock temperature on the particle mass is still widely debated1. We tackle this longstanding issue through the analysis of deep multi-epoch and high-resolution observations, made with the Chandra X-ray telescope, of the youngest nearby supernova remnant, SN 1987A. We introduce a data analysis method by studying the observed spectra in close comparison with a dedicated full three-dimensional hydrodynamic simulation that self-consistently reproduces the broadening of the spectral lines of many ions together. We measure the post-shock temperature of protons and ions through comparison of the model with observations. Our results show that the ratio of ion temperature to proton temperature is always significantly higher than one and increases linearly with the ion mass for a wide range of masses and shock parameters. Title: Flaring activity on the disk of Classical T Tauri Stars: effects on disk stability Authors: Colombo, S.; Orlando, S.; Peres, G.; Reale, F.; Argiroffi, C.; Bonito, R.; Ibgui, L.; Stehlé, C. Bibcode: 2018sf2a.conf..121C Altcode: Classical T Tauri Stars (CTTSs) are young stellar objects surrounded by a circumstellar disk with which they exchange mass and angular momentum through accretion. Despite this process is a crucial aspect of star formation, some issues are still not clear; in particular how the material loses angular momentum and falls into the star. CTTSs are also characterized by strong X-ray emission. Part of this X-ray emission comes from the heated plasma in the external regions of the stellar corona with temperature between 1 and 100 MK. The plasma heating is presumably due to the strong magnetic field (Feigelson and Montmerle, 1999) in the form of high energetic flares in proximity of the stellar surface. This energetic phenomena may influence the circumstellar environment. Recently, Reale et al. (2018) proved that long flares may connect the disk to the stellar surface. Moreover a study of Orlando et al. (2011) has shown that an intense flare close to the disk may strongly perturb its stability, inducing accretion episodes. Starting from these lines of evidence, here we investigate the effects of multiple flares with low-to-medium intensity on the disk stability, and check if they may be responsible for triggering accretion episodes. To this end, we developed a 3D magnetohydrodynamics model describing a CTTS surrounded by an accretion disk subject to intense flaring activity. The flares occur randomly in proximity of a thick disk. We found that the flaring activity determines the formation of a hot extended corona that links the disk to the stellar surface. In addition, the flares strongly perturb the disk and trigger accretion phenomena with a mass accretion rate comparable with those inferred by X-ray observations. Title: Discovery of a jet-like structure with overionized plasma in the SNR IC 443 Authors: Greco, Emanuele; Miceli, Marco; Orlando, Salvatore; Peres, Giovanni; Troja, Eleonora; Bocchino, Fabrizio Bibcode: 2018A&A...615A.157G Altcode: 2018arXiv180406714G Context. IC 443 is a supernova remnant (SNR) located in a quite complex environment since it interacts with nearby clouds. Indications for the presence of overionized plasma have been found though the possible physical causes of overionization are still debated. Moreover, because of its peculiar position and proper motion, it is not clear if the pulsar wind nebula (PWN) within the remnant is the relic of the IC 443 progenitor star or just a rambling one seen in projection on the remnant.
Aims: Here we address the study of IC 443 plasma in order to clarify the relationship PWN-remnant, the presence of overionization and the origin of the latter.
Methods: We analyzed two XMM-Newton observations producing background-subtracted, vignetting-corrected and mosaicked images in two different energy bands and we performed a spatially resolved spectral analysis of the X-ray emission.
Results: We identified an elongated (jet-like) structure with Mg-rich plasma in overionization. The head of the jet is interacting with a molecular cloud and the jet is aligned with the position of the PWN at the instant of the supernova explosion. Interestingly, the direction of the jet of ejecta is somehow consistent with the direction of the PWN jet.
Conclusions: Our discovery of a jet of ejecta in IC 443 enlarge the sample of core-collapse SNRs with collimated ejecta structures. IC 443's jet is the first one which shows overionized plasma, possibly associated with the adiabatic expansion of ejecta. The match between the jet's direction and the original position of the PWN strongly supports the association between the neutron star and IC 443. Title: Chemical Evolution of Interstellar Methanol Ice Analogs upon Ultraviolet Irradiation: The Role of the Substrate Authors: Ciaravella, A.; Jiménez-Escobar, A.; Cosentino, G.; Cecchi-Pestellini, C.; Peres, G.; Candia, R.; Collura, A.; Barbera, M.; Di Cicca, G.; Varisco, S.; Venezia, A. M. Bibcode: 2018ApJ...858...35C Altcode: An important issue in the chemistry of interstellar ices is the role of dust materials. In this work, we study the effect of an amorphous water-rich magnesium silicate deposited onto ZnSe windows on the chemical evolution of ultraviolet-irradiated methanol ices. For comparison, we also irradiate similar ices deposited onto bare ZnSe windows. Silicates are produced at relatively low temperatures exploiting a sol-gel technique. The chemical composition of the synthesized material reflects the forsterite stoichiometry. Si-OH groups and magnesium carbonates are incorporated during the process. The results show that the substrate material does affect the chemical evolution of the ice. In particular, the CO2/CO ratio within the ice is larger for methanol ices deposited onto the silicate substrate as a result of concurrent effects: the photolysis of carbonates present in the adopted substrate as a source of CO2, CO, and carbon and oxygen atoms; reactions of water molecules and hydroxyl radicals released from the substrate with the CO formed in the ice by the photolysis of the methanol ice; and changes in the structure and energy of the silicate surface by ultraviolet irradiation, leading to more favorable conditions for chemical reactions or catalysis at the grain surface. The results of our experiments allow such chemical effects contributed by the various substrate material components to be disentangled. Title: Analysis of the XMM-Newton observations of IC443 Authors: Greco, E.; Miceli, M.; Peres, G.; Orlando, S.; Troja, E.; Bocchino, F. Bibcode: 2017xru..conf..276G Altcode: We analyze for the first time the full set of archive XMM-Newton EPIC observations of the Galactic Supernova Remnant IC 443. We aim at identifying the contribution of the shocked ejecta and interstellar medium and at the describing the physical and chemical properties of the shocked plasma. We also aim at addressing the presence of overionized plasma and its physical origin. We trace the morphology of Si- and S-rich ejecta with unprecedented spatial resolution, by adopting a novel method to produce maps of equivalent width. We describe in detail the method adopted and the results obtained and present preliminary results of a spatially resolved spectral analysis performed on selected regions, chosen on the basis of our image analysis. Title: Redshifted X-rays from the material accreting onto TW Hydrae: Evidence of a low-latitude accretion spot Authors: Argiroffi, C.; Drake, J. J.; Bonito, R.; Orlando, S.; Peres, G.; Miceli, M. Bibcode: 2017A&A...607A..14A Altcode: 2017arXiv170803158A Context. High resolution spectroscopy, providing constraints on plasma motions and temperatures, is a powerful means to investigate the structure of accretion streams in classical T Tauri stars (CTTS). In particular, the accretion shock region, where the accreting material is heated to temperatures of a few million degrees as it continues its inward bulk motion, can be probed by X-ray spectroscopy.
Aims: In an attempt to detect for the first time the motion of this X-ray-emitting post-shock material, we searched for a Doppler shift in the deep Chandra High Energy Transmission Grating observation of the CTTS TW Hya. This test should unveil the nature of this X-ray emitting plasma component in CTTS and constrain the accretion stream geometry.
Methods: We searched for a Doppler shift in the X-ray emission from TW Hya with two different methods: by measuring the position of a selected sample of emission lines and by fitting the whole TW Hya X-ray spectrum, allowing the line-of-sight velocity to vary.
Results: We found that the plasma at T 2 - 4 MK has a line-of-sight velocity of 38.3 ± 5.1 km s-1 with respect to the stellar photosphere. This result definitively confirms that this X-ray-emitting material originates in the post-shock region, at the base of the accretion stream, and not in coronal structures. The comparison of the observed velocity along the line of sight, 38.3 ± 5.1 km s-1, with the inferred intrinsic velocity of the post shock of TW Hya, vpost ≈ 110 - 120 km s-1, indicates that the footpoints of the accretion streams on TW Hya are located at low latitudes on the stellar surface.
Conclusions: Our results indicate that complex magnetic field geometries, such as those of TW Hya, permit low-latitude accretion spots. Moreover, since on TW Hya the redshift of the soft X-ray emission is very similar to that of the narrow component of the C iv resonance doublet at 1550 Å, then the plasma at 2 - 4 MK and that at 0.1 MK likely originate in the same post-shock regions. Title: Accretion disk coronae of intermediate polar cataclysmic variables. 3D magnetohydrodynamic modelling and thermal X-ray emission Authors: Barbera, E.; Orlando, S.; Peres, G. Bibcode: 2017A&A...600A.105B Altcode: 2017arXiv170204702B Context. Intermediate polar cataclysmic variables (IPCV) contain a magnetic, rotating white dwarf surrounded by a magnetically truncated accretion disk. To explain their strong flickering X-ray emission, accretion has been successfully taken into account. Nevertheless, observations suggest that accretion phenomena might not be the only process behind it. An intense flaring activity occurring on the surface of the disk may generate a corona, contribute to the thermal X-ray emission, and influence the system stability.
Aims: Our purposes are: investigating the formation of an extended corona above the accretion disk, due to an intense flaring activity occurring on the disk surface; studying the effects of flares on the disk and stellar magnetosphere; assessing its contribution to the observed thermal X-ray flux.
Methods: We have developed a 3D magnetohydrodynamic (MHD) model of a IPCV system. The model takes into account gravity, disk viscosity, thermal conduction, radiative losses, and coronal flare heating through heat injection at randomly chosen locations on the disk surface. To perform a parameter space exploration, several system conditions have been considered, with different magnetic field intensity and disk density values. From the results of the evolution of the model, we have synthesized the thermal X-ray emission.
Results: The simulations show the formation of an extended corona, linking disk and star. The flaring activity is capable of strongly influencing the disk configuration and possibly its stability, effectively deforming the magnetic field lines. Hot plasma evaporation phenomena occur in the layer immediately above the disk. The flaring activity gives rise to a thermal X-ray emission in both the [ 0.1-2.0 ] keV and the [ 2.0-10 ] keV X-ray bands.
Conclusions: An intense coronal activity occurring on the disk surface of an IPCV can affect the structure of the disk depending noticeably on the density of the disk and the magnetic field of the central object. Moreover, the synthesis of the thermal X-ray fluxes shows that this flaring activity may contribute to the observed flickering thermal X-ray emission. Title: Hydrodynamic modelling of accretion impacts in classical T Tauri stars: radiative heating of the pre-shock plasma Authors: Costa, G.; Orlando, S.; Peres, G.; Argiroffi, C.; Bonito, R. Bibcode: 2017A&A...597A...1C Altcode: 2016arXiv160901059C; 2016A&A...597A...1C Context. It is generally accepted that, in classical T Tauri stars, the plasma from the circumstellar disc accretes onto the stellar surface with free-fall velocity and the impact generates a shock. The impact region is expected to contribute to emission in different spectral bands; many studies have confirmed that the X-rays arise from the post-shock plasma but, otherwise, there are no studies in the literature investigating the origin of the observed UV emission which is apparently correlated to accretion.
Aims: We investigated the effect of radiative heating of the infalling material by the post-shock plasma at the base of the accretion stream, with the aim to identify in which region a significant part of the UV emission originates.
Methods: We developed a one-dimensional hydrodynamic model describing the impact of an accretion stream onto the stellar surface; the model takes into account the gravity, the radiative cooling of an optically thin plasma, the thermal conduction, and the heating due to absorption of X-ray radiation. The latter term represents the heating of the infalling plasma due to the absorption of X-rays emitted from the post-shock region.
Results: We found that the radiative heating of the pre-shock plasma plays a non-negligible role in the accretion phenomenon. In particular, the dense and cold plasma of the pre-shock accretion column is gradually heated up to a few 105K due to irradiation of X-rays arising from the shocked plasma at the impact region. This heating mechanism does not affect significantly the dynamics of the post-shock plasma. On the other hand, a region of radiatively heated gas (that we consider a precursor) forms in the unshocked accretion column and contributes significantly to UV emission. Our model naturally reproduces the luminosity of UV emission lines correlated to accretion and shows that most of the UV emission originates from the precursor. Title: 3D MHD modeling of twisted coronal loops Authors: Reale, F.; Orlando, S.; Guarrasi, M.; Mignone, A.; Peres, G.; Hood, A. W.; Priest, E. R. Bibcode: 2016ApJ...830...21R Altcode: 2016arXiv160705500R We perform MHD modeling of a single bright coronal loop to include the interaction with a non-uniform magnetic field. The field is stressed by random footpoint rotation in the central region and its energy is dissipated into heating by growing currents through anomalous magnetic diffusivity that switches on in the corona above a current density threshold. We model an entire single magnetic flux tube in the solar atmosphere extending from the high-β chromosphere to the low-β corona through the steep transition region. The magnetic field expands from the chromosphere to the corona. The maximum resolution is ∼30 km. We obtain an overall evolution typical of loop models and realistic loop emission in the EUV and X-ray bands. The plasma confined in the flux tube is heated to active region temperatures (∼3 MK) after ∼2/3 hr. Upflows from the chromosphere up to ∼100 km s-1 fill the core of the flux tube to densities above 109 cm-3. More heating is released in the low corona than the high corona and is finely structured both in space and time. Title: X-Raying the Dark Side of Venus—Scatter from Venus’ Magnetotail? Authors: Afshari, M.; Peres, G.; Jibben, P. R.; Petralia, A.; Reale, F.; Weber, M. Bibcode: 2016AJ....152..107A Altcode: 2016arXiv160706697A We analyze significant X-ray, EUV, and UV emission coming from the dark side of Venus observed with Hinode/XRT and Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) during a transit across the solar disk that occurred in 2012. As a check we have analyzed an analogous Mercury transit that occurred in 2006. We have used the latest version of the Hinode/XRT point spread function to deconvolve Venus and Mercury X-ray images, to remove instrumental scattering. After deconvolution, the flux from Venus’ shadow remains significant while that of Mercury becomes negligible. Since stray light contamination affects the XRT Ti-poly filter data we use, we performed the same analysis with XRT Al-mesh filter data, not affected by the light leak. Even the latter data show residual flux. We have also found significant EUV (304 Å, 193 Å, 335 Å) and UV (1700 Å) flux in Venus’ shadow, measured with SDO/AIA. The EUV emission from Venus’ dark side is reduced, but still significant, when deconvolution is applied. The light curves of the average flux of the shadow in the X-ray, EUV, and UV bands appear different as Venus crosses the solar disk, but in any of them the flux is, at any time, approximately proportional to the average flux in a ring surrounding Venus, and therefore proportional to that of the solar regions around Venus’ obscuring disk line of sight. The proportionality factor depends on the band. This phenomenon has no clear origin; we suggest that it may be due to scatter occurring in the very long magnetotail of Venus. Title: Impacts of fragmented accretion streams onto classical T Tauri stars: UV and X-ray emission lines Authors: Colombo, S.; Orlando, S.; Peres, G.; Argiroffi, C.; Reale, F. Bibcode: 2016A&A...594A..93C Altcode: 2016arXiv160703009C Context. The accretion process in classical T Tauri stars (CTTSs) can be studied through the analysis of some UV and X-ray emission lines which trace hot gas flows and act as diagnostics of the post-shock downfalling plasma. In the UV-band, where higher spectral resolution is available, these lines are characterized by rather complex profiles whose origin is still not clear.
Aims: We investigate the origin of UV and X-ray emission at impact regions of density structured (fragmented) accretion streams. We study if and how the stream fragmentation and the resulting structure of the post-shock region determine the observed profiles of UV and X-ray emission lines.
Methods: We modeled the impact of an accretion stream consisting of a series of dense blobs onto the chromosphere of a CTTS through two-dimensional (2D) magnetohydrodynamic (MHD) simulations. We explored different levels of stream fragmentation and accretion rates. From the model results, we synthesize C IV (1550 Å) and O VIII (18.97 Å) line profiles.
Results: The impacts of accreting blobs onto the stellar chromosphere produce reverse shocks propagating through the blobs and shocked upflows. These upflows, in turn, hit and shock the subsequent downfalling fragments. As a result, several plasma components differing for the downfalling velocity, density, and temperature are present altoghether. The profiles of C IV doublet are characterized by two main components: one narrow and redshifted to speed ≈ 50 km s-1 and the other broader and consisting of subcomponents with redshift to speed in the range 200-400 km s-1. The profiles of O VIII lines appear more symmetric than C IV and are redshifted to speed ≈ 150 km s-1.
Conclusions: Our model predicts profiles of C IV line remarkably similar to those observed and explains their origin in a natural way as due to stream fragmentation.

Movies are available at http://www.aanda.org Title: Modeling the shock-cloud interaction in SN 1006: Unveiling the origin of nonthermal X-ray and γ-ray emission Authors: Miceli, M.; Orlando, S.; Pereira, V.; Acero, F.; Katsuda, S.; Decourchelle, A.; Winkler, F. P.; Bonito, R.; Reale, F.; Peres, G.; Li, J.; Dubner, G. Bibcode: 2016A&A...593A..26M Altcode: 2016arXiv160608748M Context. The supernova remnant SN 1006 is a source of high-energy particles and its southwestern limb is interacting with a dense ambient cloud, thus is a promising region for γ-ray hadronic emission.
Aims: We aim at describing the physics and the nonthermal emission associated with the shock-cloud interaction to derive the physical parameters of the cloud (poorly constrained by the data analysis), to ascertain the origin of the observed spatial variations in the spectral properties of the X-ray synchrotron emission, and to predict spectral and morphological features of the resulting γ-ray emission.
Methods: We performed 3D magnetohydrodynamic simulations modeling the evolution of SN 1006 and its interaction with the ambient cloud, and explored different model setups. By applying the REMLIGHT code on the model results, we synthesized the synchrotron X-ray emission and compared it with actual observations to constrain the parameters of the model. We also synthesized the leptonic and hadronic γ-ray emission from the models, deriving constraints on the energy content of the hadrons accelerated at the southwestern limb.
Results: We found that the impact of the SN 1006 shock front with a uniform cloud with density 0.5 cm-3 can explain the observed morphology, the azimuthal variations of the cutoff frequency of the X-ray synchrotron emission, and the shock proper motion in the interaction region. Our results show that the current upper limit for the total hadronic energy in the southwestern limb is 2.5 × 1049 erg. Title: UV And X-Ray Emission from Impacts of Fragmented Accretion Streams on Classical T Tauri Stars Authors: Colombo, Salvatore; Orlando, Salvatore; Peres, Giovanni; Argiroffi, Costanza; Reale, Fabio Bibcode: 2016csss.confE..64C Altcode: According to the magnetoshperic accretion scenario, during their evo-
lution, Classical T Tauri stars accrete material from their circumstellar
disk. The accretion process is regulated by the stellar magnetic eld and
produces hot and dense post-shocks on the stellar surface as a result of
impacts of the downfalling material. The impact regions are expected
to strongly radiate in UV and X-rays. Several lines of evidence support
the magnetospheric accretion scenario, especially in optical and infrared
bands. However several points still remain unclear as, for instance,where
the complex-pro le UV lines originate, or whether and how UV and X-ray
emission is produced in the same shock region. The analysis of a large
solar eruption has shown that EUV excesses might be e ectively produced
by the impact of dense fragments onto the stellar surface. Since a steady
accretion stream does not reprouce observations, in this work we investi-
gate the e ects of a fragmented accretion stream on the
uxes and pro les
of C IV and O VIII emission lines. To this end we model the impact of a
fragmented accretion stream onto the chromosphere of a CTTS with 2D
axysimmetric magneto-hydrodynamic simulations. Our model takes into
account of the gravity, the stellar magnetic eld, the thermal conduction
and the radiative cooling from an optically thin plasma.
From the model results, we synthesize the UV and X-ray emission
including the e ect of Doppler shift along the line of sight. We nd that
a fragmented accretion stream produces complex pro les of UV emission
lines which consists of multiple components with di erent Doppler shifts.
Our model predicts line pro les that are consistent with those observed
and explain their origin as due to the stream fragmentation. Title: EUV Flickering of Solar Coronal Loops: A New Diagnostic of Coronal Heating Authors: Tajfirouze, E.; Reale, F.; Peres, G.; Testa, P. Bibcode: 2016ApJ...817L..11T Altcode: 2016arXiv160103935T A previous work of ours found the best agreement between EUV light curves observed in an active region core (with evidence of super-hot plasma) and those predicted from a model with a random combination of many pulse-heated strands with a power-law energy distribution. We extend that work by including spatially resolved strand modeling and by studying the evolution of emission along the loops in the EUV 94 Å and 335 Å channels of the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. Using the best parameters of the previous work as the input of the present one, we find that the amplitude of the random fluctuations driven by the random heat pulses increases from the bottom to the top of the loop in the 94 Å channel and from the top to the bottom in the 335 Å channel. This prediction is confirmed by the observation of a set of aligned neighboring pixels along a bright arc of an active region core. Maps of pixel fluctuations may therefore provide easy diagnostics of nanoflaring regions. Title: ADAHELI: exploring the fast, dynamic Sun in the x-ray, optical, and near-infrared Authors: Berrilli, Francesco; Soffitta, Paolo; Velli, Marco; Sabatini, Paolo; Bigazzi, Alberto; Bellazzini, Ronaldo; Bellot Rubio, Luis Ramon; Brez, Alessandro; Carbone, Vincenzo; Cauzzi, Gianna; Cavallini, Fabio; Consolini, Giuseppe; Curti, Fabio; Del Moro, Dario; Di Giorgio, Anna Maria; Ermolli, Ilaria; Fabiani, Sergio; Faurobert, Marianne; Feller, Alex; Galsgaard, Klaus; Gburek, Szymon; Giannattasio, Fabio; Giovannelli, Luca; Hirzberger, Johann; Jefferies, Stuart M.; Madjarska, Maria S.; Manni, Fabio; Mazzoni, Alessandro; Muleri, Fabio; Penza, Valentina; Peres, Giovanni; Piazzesi, Roberto; Pieralli, Francesca; Pietropaolo, Ermanno; Martinez Pillet, Valentin; Pinchera, Michele; Reale, Fabio; Romano, Paolo; Romoli, Andrea; Romoli, Marco; Rubini, Alda; Rudawy, Pawel; Sandri, Paolo; Scardigli, Stefano; Spandre, Gloria; Solanki, Sami K.; Stangalini, Marco; Vecchio, Antonio; Zuccarello, Francesca Bibcode: 2015JATIS...1d4006B Altcode: Advanced Astronomy for Heliophysics Plus (ADAHELI) is a project concept for a small solar and space weather mission with a budget compatible with an European Space Agency (ESA) S-class mission, including launch, and a fast development cycle. ADAHELI was submitted to the European Space Agency by a European-wide consortium of solar physics research institutes in response to the "Call for a small mission opportunity for a launch in 2017," of March 9, 2012. The ADAHELI project builds on the heritage of the former ADAHELI mission, which had successfully completed its phase-A study under the Italian Space Agency 2007 Small Mission Programme, thus proving the soundness and feasibility of its innovative low-budget design. ADAHELI is a solar space mission with two main instruments: ISODY: an imager, based on Fabry-Pérot interferometers, whose design is optimized to the acquisition of highest cadence, long-duration, multiline spectropolarimetric images in the visible/near-infrared region of the solar spectrum. XSPO: an x-ray polarimeter for solar flares in x-rays with energies in the 15 to 35 keV range. ADAHELI is capable of performing observations that cannot be addressed by other currently planned solar space missions, due to their limited telemetry, or by ground-based facilities, due to the problematic effect of the terrestrial atmosphere. Title: Accretion in young stars: measure of the stream velocity of TW Hya from the X-ray Doppler shift Authors: Argiroffi, Costanza; Bonito, Rosaria; Orlando, Salvatore; Miceli, Marco; Peres, Giovanni Bibcode: 2015eheu.conf...57A Altcode: High-resolution X-ray spectra are a unique tool to investigate the accretion process in young stars. In fact X-rays allow to investigate the accretion-shock region, where the infalling material is heated by strong shocks due to the impact with the denser stellar atmosphere. Here we show for the first time that it is possible to constrain the velocity of the accretion stream by measuring the Doppler shift of the emitted X-rays. To this aim we analyzed the deep Chandra/HETGS observation of the accreting young star TW Hya. We selected a sample of emission lines free from significant blends, fitted them with gaussian profiles, computed the radial velocity corresponding to each line, and averaged these velocities to obtain an accurate estimate of the global velocity of the X-ray emitting plasma. After correcting for Earth's motion, we compared this observed velocity with the photospheric radial velocity. In order to check this procedure we applied the same technique to other Chandra/HETGS spectra of single stars, whose X-rays are due only to coronal plasma. While spectra of pure coronal sources provide Doppler shifts in agreement with the known stellar radial velocity, we found that the X-ray spectrum of TW Hya is red-shifted by ~30-40 km/s with respect to the stellar photosphere. This proves that the X-ray emitting plasma on TW Hya is moving with respect to the stellar surface, definitively confirming that it originates in the accretion-shock region. The observed velocity suggests that the base of the accretion region is located at low latitudes of the stellar surface. Title: VizieR Online Data Catalog: UV variability and accretion in NGC 2264 (Venuti+, 2015) Authors: Venuti, L.; Bouvier, J.; Irwin, J.; Stauffer, J.; Hillenbrand, L. A.; Rebull, L.; Cody, A. M.; Alencar, S. H. P.; Micela, G.; Flaccomio, E.; Peres, G. Bibcode: 2015yCat..35810066V Altcode: Our study aims at characterizing the variability signatures at short wavelengths for several hundred accreting and non-accreting members of the star-forming region NGC 2264 (~3Myr). We performed simultaneous, 2-week-long UV (u-band) and optical (r-band) monitoring of the cluster with CFHT/MegaCam, with several flux measurements per observing night to retrace the variability features on timescales from hours to a couple of weeks. Photometric amplitudes and color behaviors are explored to investigate the physical origin of the observed variability in the different cases.

In table2, we present the average u and r photometry measured for individual sources in our sample, their amplitudes of variability, the amount of variability measured via the light curve rms and Stetson's (1996PASP..108..851S) J-index indicator, the r-to-(u-r) amplitude ratio and the uncertainty associated with the latter.

(1 data file). Title: UV variability and accretion dynamics in the young open cluster NGC 2264 Authors: Venuti, L.; Bouvier, J.; Irwin, J.; Stauffer, J. R.; Hillenbrand, L. A.; Rebull, L. M.; Cody, A. M.; Alencar, S. H. P.; Micela, G.; Flaccomio, E.; Peres, G. Bibcode: 2015A&A...581A..66V Altcode: 2015arXiv150606858V Context. Photometric variability is a distinctive feature of young stellar objects; exploring variability signatures at different wavelengths provides insight into the physical processes at work in these sources.
Aims: We explore the variability signatures at ultraviolet (UV) and optical wavelengths for several hundred accreting and non-accreting members of the star-forming region NGC 2264 (~3 Myr).
Methods: We performed simultaneous monitoring of u- and r-band variability for the cluster population with CFHT/MegaCam. The survey extended over two full weeks, with several flux measurements per observing night. A sample of about 750 young stars is probed in our study, homogeneously calibrated and reduced, with internally consistently derived stellar parameters. Objects span the mass range 0.1-2 M⊙; about 40% of them show evidence for active accretion based on various diagnostics (Hα, UV, and IR excesses).
Results: Statistically distinct variability properties are observed for accreting and non-accreting cluster members. The accretors exhibit a significantly higher level of variability than the non-accretors, in the optical and especially in the UV. The amount of u-band variability is found to correlate statistically with the median amount of UV excess in disk-bearing objects, which suggests that mass accretion and star-disk interaction are the main sources of variability in the u band. Spot models are applied to account for the amplitudes of variability of accreting and non-accreting members, which yields different results for each group. Cool magnetic spots, several hundred degrees colder than the stellar photosphere and covering from 5 to 30% of the stellar surface, appear to be the leading factor of variability for the non-accreting stars. In contrast, accretion spots with a temperature a few thousand degrees higher than the photospheric temperature and that extend over a few percent of the stellar surface best reproduce the variability of accreting objects. The color behavior is also found to be different between accreting and non-accreting stars. While objects commonly become redder when fainter, typical amplitudes of variability for accreting members rapidly increase from the r to the u band, which indicates a much stronger contrast at short wavelengths; a lower color dependence in the photometric amplitudes is instead measured for diskless stars. Finally, we compare the u-band variability monitored here on two-week timescales with that measured on both shorter (hours) and longer (years) timescales. We find that variability on timescales of hours is typically ~10% of the peak-to-peak variability on day timescales, while longer term variability on a timescale of years is consistent with amplitudes measured over weeks.
Conclusions: We conclude that for both accreting and non-accreting stars, the mid-term rotational modulation by hot and cold spots is the leading timescale for a variability of up to several years. In turn, this suggests that the accretion process is essentially stable over years, although it exhibits low-level shorter term variations in single accretion events.

Based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii.Full Table 2 is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/581/A66 Title: Accretion Shocks in Young Stars: the Role of Local Absorption on the X-ray Emission Authors: Bonito, R.; Argiroffi, C.; Orlando, S.; Miceli, M.; Peres, G.; Matsakos, T.; Stehle, C.; Ibgui, L. Bibcode: 2015csss...18..199B Altcode: We investigate the X-ray emission from accretion shocks in classical T Tauri stars, due to the infalling material impacting the stellar surface. Several aspects in both observations and models of the accretion process are still unclear: the observed X-ray luminosity of the post-shock plasma is below the predicted value, the density vs temperature structure of the shocked plasma, with increasing densities at higher temperature, is opposite of what expected from simple accretion shock models. To address these issues we performed numerical magnetohydrodynamic simulations describing the impact of an accretion stream onto the stellar surface and considered the local absorption due to the surrounding medium. We explored the effects of absorption for different viewing angles and for the He-like line triplets commonly used for density diagnostic. From the model results we synthesize the X-ray emission from the accretion shock, producing maps and spectra. We perform density and temperature diagnostics on the synthetic spectra, and we directly compare our results with the observations. Our model shows that the X-ray fluxes detected are lower than expected because of the local absorption. The emerging spectra suggest a complex density vs temperature distribution proving that a detailed model accounting for a realistic treatment of the local absorption is needed to interpret the observations of X-ray emitting accretion shocks. Title: Mass Accretion Processes in Young Stellar Objects: Role of Intense Flaring Activity Authors: Orlando, Salvatore; Reale, Fabio; Peres, Giovanni; Mignone, Andrea Bibcode: 2014mbhe.conf..108O Altcode: According to the magnetospheric accretion scenario, young low-mass stars are surrounded by circumstellar disks which they interact with through accretion of mass. The accretion builds up the star to its final mass and is also believed to power the mass outflows, which may in turn have a signicant role in removing the excess angular momentum from the star-disk system. Although the process of mass accretion is a critical aspect of star formation, some of its mechanisms are still to be fully understood. On the other hand, strong flaring activity is a common feature of young stellar objects (YSOs). In the Sun, such events give rise to perturbations of the interplanetary medium. Similar but more energetic phenomena occur in YSOs and may influence the circumstellar environment. In fact, a recent study has shown that an intense flaring activity close to the disk may strongly perturb the stability of circumstellar disks, thus inducing mass accretion episodes (Orlando et al. 2011). Here we review the main results obtained in the field and the future perspectives. Title: Magnetohydrodynamic Modeling of the Accretion Shocks in Classical T Tauri Stars: The Role of Local Absorption in the X-Ray Emission Authors: Bonito, R.; Orlando, S.; Argiroffi, C.; Miceli, M.; Peres, G.; Matsakos, T.; Stehle, C.; Ibgui, L. Bibcode: 2014ApJ...795L..34B Altcode: 2014arXiv1410.2775B We investigate the properties of X-ray emission from accretion shocks in classical T Tauri stars (CTTSs), generated where the infalling material impacts the stellar surface. Both observations and models of the accretion process reveal several aspects that are still unclear: the observed X-ray luminosity in accretion shocks is below the predicted value, and the density versus temperature structure of the shocked plasma, with increasing densities at higher temperature, deduced from the observations, is at odds with that proposed in the current picture of accretion shocks. To address these open issues, we investigate whether a correct treatment of the local absorption by the surrounding medium is crucial to explain the observations. To this end, we describe the impact of an accretion stream on a CTTS by considering a magnetohydrodynamic model. From the model results, we synthesize the X-ray emission from the accretion shock by producing maps and spectra. We perform density and temperature diagnostics on the synthetic spectra, and we directly compare the results with observations. Our model shows that the X-ray fluxes inferred from the emerging spectra are lower than expected because of the complex local absorption by the optically thick material of the chromosphere and of the unperturbed stream. Moreover, our model, including the effects of local absorption, explains in a natural way the apparently puzzling pattern of density versus temperature observed in the X-ray emission from accretion shocks. Title: Mapping accretion and its variability in the young open cluster NGC 2264: a study based on u-band photometry Authors: Venuti, L.; Bouvier, J.; Flaccomio, E.; Alencar, S. H. P.; Irwin, J.; Stauffer, J. R.; Cody, A. M.; Teixeira, P. S.; Sousa, A. P.; Micela, G.; Cuillandre, J. -C.; Peres, G. Bibcode: 2014A&A...570A..82V Altcode: 2014arXiv1408.0432V Context. The accretion process has a central role in the formation of stars and planets.
Aims: We aim at characterizing the accretion properties of several hundred members of the star-forming cluster NGC 2264 (3 Myr).
Methods: We performed a deep ugri mapping as well as a simultaneous u-band+r-band monitoring of the star-forming region with CFHT/MegaCam in order to directly probe the accretion process onto the star from UV excess measurements. Photometric properties and stellar parameters are determined homogeneously for about 750 monitored young objects, spanning the mass range ~0.1-2 M⊙. About 40% of the sample are classical (accreting) T Tauri stars, based on various diagnostics (Hα, UV and IR excesses). The remaining non-accreting members define the (photospheric + chromospheric) reference UV emission level over which flux excess is detected and measured.
Results: We revise the membership status of cluster members based on UV accretion signatures, and report a new population of 50 classical T Tauri star (CTTS) candidates. A large range of UV excess is measured for the CTTS population, varying from a few times 0.1 to ~3 mag. We convert these values to accretion luminosities and accretion rates, via a phenomenological description of the accretion shock emission. We thus obtain mass accretion rates ranging from a few 10-10 to ~10-7 M⊙/yr. Taking into account a mass-dependent detection threshold for weakly accreting objects, we find a >6σ correlation between mass accretion rate and stellar mass. A power-law fit, properly accounting for censored data (upper limits), yields Ṁacc ∝ M*1.4±0.3. At any given stellar mass, we find a large spread of accretion rates, extending over about 2 orders of magnitude. The monitoring of the UV excess on a timescale of a couple of weeks indicates that its variability typically amounts to 0.5 dex, i.e., much smaller than the observed spread in accretion rates. We suggest that a non-negligible age spread across the star-forming region may effectively contribute to the observed spread in accretion rates at a given mass. In addition, different accretion mechanisms (like, e.g., short-lived accretion bursts vs. more stable funnel-flow accretion) may be associated to different Ṁacc regimes.
Conclusions: A huge variety of accretion properties is observed for young stellar objects in the NGC 2264 cluster. While a definite correlation seems to hold between mass accretion rate and stellar mass over the mass range probed here, the origin of the large intrinsic spread observed in mass accretion rates at any given mass remains to be explored.

Based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii.Full Tables 2-4 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/570/A82 Title: 3D YSO accretion shock simulations: a study of the magnetic, chromospheric and stochastic flow effects Authors: Matsakos, T.; Chièze, J. -P.; Stehlé, C.; González, M.; Ibgui, L.; de Sá, L.; Lanz, T.; Orlando, S.; Bonito, R.; Argiroffi, C.; Reale, F.; Peres, G. Bibcode: 2014IAUS..302...66M Altcode: The structure and dynamics of young stellar object (YSO) accretion shocks depend strongly on the local magnetic field strength and configuration, as well as on the radiative transfer effects responsible for the energy losses. We present the first 3D YSO shock simulations of the interior of the stream, assuming a uniform background magnetic field, a clumpy infalling gas, and an acoustic energy flux flowing at the base of the chromosphere. We study the dynamical evolution and the post-shock structure as a function of the plasma-beta (thermal pressure over magnetic pressure). We find that a strong magnetic field (~hundreds of Gauss) leads to the formation of fibrils in the shocked gas due to the plasma confinement within flux tubes. The corresponding emission is smooth and fully distinguishable from the case of a weak magnetic field (~tenths of Gauss) where the hot slab demonstrates chaotic motion and oscillates periodically. Title: VizieR Online Data Catalog: Mapping accretion variability in NGC 2264 (Venuti+, 2014) Authors: Venuti, L.; Bouvier, J.; Flaccomio, E.; Alencar, S. H. P.; Irwin, J.; Stauffer, J. R.; Cody, A. M.; Teixeira, P. S.; Sousa, A. P.; Micela, G.; Cuillandre, J. -C.; Peres, G. Bibcode: 2014yCat..35700082V Altcode: 2014yCat..35709082V We present homogeneous multi-band (optical+UV) photometry (table2) and stellar parameters (table3: accreting/non-accreting status, spectral type, Av, bolometric luminosity, stellar mass, stellar radius and age estimate) for 757 young stars in the cluster (706 confirmed members + 51 newly identified candidates from our survey). In table4, we present the UV excess measurements, u-band excess luminosity, median accretion rates and their measured variability ranges for 237 accreting objects in our sample.

(3 data files). Title: X-rays from accretion shocks in classical T Tauri stars: 2D MHD modeling and the role of local absorption Authors: Argiroffi, C.; Bonito, R.; Orlando, S.; Miceli, M.; Reale, F.; Peres, G.; Matsakos, T.; Sthelé, C.; Ibgui, L. Bibcode: 2014IAUS..302...48A Altcode: In classical T Tauri stars (CTTS) strong shocks are formed where the accretion funnel impacts with the denser stellar chromosphere. Although current models of accretion provide a plausible global picture of this process, some fundamental aspects are still unclear: the observed X-ray luminosity in accretion shocks is order of magnitudes lower than predicted; the observed density and temperature structures of the hot post-shock region are puzzling and still unexplained by models.

To address these issues we performed 2D MHD simulations describing an accretion stream impacting onto the chromosphere of a CTTS, exploring different configurations and strengths of the magnetic field. From the model results we then synthesized the X-ray emission emerging from the hot post-shock, taking into account the local absorption due to the pre-shock stream and surrounding atmosphere.

We find that the different configurations and strengths of the magnetic field profoundly affect the hot post-shock properties. Moreover the emerging X-ray emission strongly depends also on the viewing angle under which accretion is observed. Some of the explored configuration are able to reproduce the observed features of X-ray spectra of CTTS. Title: Accretion impacts studied on the Sun Authors: Reale, F.; Orlando, S.; Testa, P.; Peres, G.; Landi, E.; Schrijver, C. Bibcode: 2014xru..confE.169R Altcode: Accretion in star-forming regions is a hot topic. The Sun has recently offered an interesting opportunity to study accretion impacts observed in great detail at high energies (Reale et al. 2013, Science, 341, 6143, 251). After the eruption of a dense filament triggered by an energetic flare on June 7, 2011 part of the ejected material falls back onto the solar surface. The impact of the downfalling plasma is similar to that of accretion flows on young stellar objects, and was imaged in the EUV by the Atmospheric Imaging Assembly (AIA) on-board the Solar Dynamics Observatory (SDO). Hydrodynamic simulations confirm that the high energy emission is produced by the impact of high-density plasma at the highest free-fall speeds and show the importance of the absorption in reducing the X-ray emission and of fragmentation in explaining the line broadenings. Impacts such as these present a laboratory for stellar astronomers to study the impact of dense (accreting) circumstellar material in unique detail. Title: Filling the gap between supernova explosions and their remnants: the Cassiopeia A laboratory Authors: Orlando, S.; Miceli, M.; Pumo, M.; Bocchino, F.; Reale, F.; Peres, G. Bibcode: 2014xru..confE.298O Altcode: Supernova remnats (SNRs) show a complex morphology characterized by an inhomogeneous spatial distribution of ejecta, believed to reflect pristine structures and features of the progenitor supernova (SN) explosion. Filling the gap between SN explosions and their remnants is very important for a comprehension of the origin of present-day structure of ejecta in SNRs and to probe and constraint current models of SN explosions. The SNR Cassiopeia A (Cas A) is an attractive laboratory for studying the SNe-SNRs connection, being one of the best studied SNRs for which its 3D structure is known. We present a three-dimensional hydrodynamic model describing the evolution of Cas A from the immediate aftermath of the SN explosion to its expansion through the interstellar medium, taking into account the distribution of element abundances of the ejecta, the backreaction of accelerated cosmic rays at the shock front, and the deviations from equilibrium of ionizazion for the most important elements. We use the model to derive the physical parameters characterizing the SN explosion and reproducing the today morphology of Cas A. Title: A New Look at Spitzer Primary Transit Observations of the Exoplanet HD 189733b Authors: Morello, G.; Waldmann, I. P.; Tinetti, G.; Peres, G.; Micela, G.; Howarth, I. D. Bibcode: 2014ApJ...786...22M Altcode: 2014arXiv1403.2874M Blind source separation techniques are used to reanalyze two exoplanetary transit light curves of the exoplanet HD 189733b recorded with the IR camera IRAC on board the Spitzer Space Telescope at 3.6 μm during the "cold" era. These observations, together with observations at other IR wavelengths, are crucial to characterize the atmosphere of the planet HD 189733b. Previous analyses of the same data sets reported discrepant results, hence the necessity of the reanalyses. The method we used here is based on the Independent Component Analysis (ICA) statistical technique, which ensures a high degree of objectivity. The use of ICA to detrend single photometric observations in a self-consistent way is novel in the literature. The advantage of our reanalyses over previous work is that we do not have to make any assumptions on the structure of the unknown instrumental systematics. Such "admission of ignorance" may result in larger error bars than reported in the literature, up to a factor 1.6. This is a worthwhile tradeoff for much higher objectivity, necessary for trustworthy claims. Our main results are (1) improved and robust values of orbital and stellar parameters, (2) new measurements of the transit depths at 3.6 μm, (3) consistency between the parameters estimated from the two observations, (4) repeatability of the measurement within the photometric level of ~2 × 10-4 in the IR, and (5) no evidence of stellar variability at the same photometric level within one year. Title: A new look at Spitzer primary transit observations of the exoplanet HD189733b Authors: Morello, G.; Waldmann, I. P.; Tinetti, G.; Peres, G.; Micela, G.; Howarth, I. D. Bibcode: 2014EPSC....9..462M Altcode: Blind source separation techniques are used to reanalyse two exoplanetary transit lightcurves of the exoplanet HD189733b recorded with the IR camera IRAC on board the Spitzer Space Telescope at 3.6μm during the "cold" era. These observations, together with observations at other IR wavelengths, are crucial to characterise the atmosphere of the planet HD189733b. Previous analyses of the same datasets reported discrepant results, hence the necessity of the reanalyses. The method we used here is based on the Independent Component Analysis (ICA) statistical technique, which ensures a high degree of objectivity. The use of ICA to detrend single photometric observations in a self-consistent way is novel in the literature. The advantage of our reanalyses over previous work is that we do not have to make any assumptions on the structure of the unknown instrumental systematics. Such "admission of ignorance" may result in larger error bars than reported in the literature, up to a factor 1.6. This is a worthwhile trade-off for much higher objectivity, necessary for trustworthy claims. Our main results are (1) improved and robust values of orbital and stellar parameters, (2) new measurements of the transit depths at 3.6μm, (3) consistency between the parameters estimated from the two observations, (4) repeatability of the measurement within the photometric level of ~ 2 × 10-4 in the IR, (5) no evidence of stellar variability at the same photometric level within 1 year. Title: Role of local absorption on the X-ray emission from MHD accretion shocks in classical T Tauri stars Authors: Bonito, R.; Orlando, S.; Argiroffi, C.; Miceli, M.; Reale, F.; Peres, G.; Matsakos, T.; Stehle, H. C.; Ibgui, L. Bibcode: 2014EPJWC..6405004B Altcode: Accretion processes onto classical T Tauri stars (CTTSs) are believed to generate shocks at the stellar surface due to the impact of supersonic downflowing plasma. Although current models of accretion streams provide a plausible global picture of this process, several aspects are still unclear. For example, the observed X-ray luminosity in accretion shocks is, in general, well below the predicted value. A possible explanation discussed in the literature is in terms of significant absorption of the emission due to the thick surrounding medium. Here we consider a 2D MHD model describing an accretion stream propagating through the atmosphere of a CTTS and impacting onto its chromosphere. The model includes all the relevant physics, namely the gravity, the thermal conduction, and the radiative cooling, and a realistic description of the unperturbed stellar atmosphere (from the chromosphere to the corona). From the model results, we synthesize the X-ray emission emerging from the hot slab produced by the accretion shock, exploring different configurations and strengths of the stellar magnetic field. The synthesis includes the local absorption by the thick surrounding medium and the Doppler shift of lines due to the component of plasma velocity along the line-of-sight. We explore the effects of absorption on the emerging X-ray spectrum, considering different inclinations of the accretion stream with respect to the observer. Finally we compare our results with the observations. Title: Mass Accretion Processes in Young Stellar Objects: Role of Intense Flaring Activity Authors: Orlando, S.; Reale, F.; Peres, G.; Mignone, A. Bibcode: 2014arXiv1401.0408O Altcode: According to the magnetospheric accretion scenario, young low-mass stars are surrounded by circumstellar disks which they interact with through accretion of mass. The accretion builds up the star to its final mass and is also believed to power the mass outflows, which may in turn have a significant role in removing the excess angular momentum from the star-disk system. Although the process of mass accretion is a critical aspect of star formation, some of its mechanisms are still to be fully understood. On the other hand, strong flaring activity is a common feature of young stellar objects (YSOs). In the Sun, such events give rise to perturbations of the interplanetary medium. Similar but more energetic phenomena occur in YSOs and may influence the circumstellar environment. In fact, a recent study has shown that an intense flaring activity close to the disk may strongly perturb the stability of circumstellar disks, thus inducing mass accretion episodes (Orlando et al. 2011). Here we review the main results obtained in the field and the future perspectives. Title: 3D numerical modeling of YSO accretion shocks Authors: Matsakos, T.; Chièze, J. -P.; Stehlé, C.; González, M.; Ibgui, L.; de Sá, L.; Lanz, T.; Orlando, S.; Bonito, R.; Argiroffi, C.; Reale, F.; Peres, G. Bibcode: 2014EPJWC..6404003M Altcode: The dynamics of YSO accretion shocks is determined by radiative processes as well as the strength and structure of the magnetic field. A quasi-periodic emission signature is theoretically expected to be observed, but observations do not confirm any such pattern. In this work, we assume a uniform background field, in the regime of optically thin energy losses, and we study the multi-dimensional shock evolution in the presence of perturbations, i.e. clumps in the stream and an acoustic energy flux flowing at the base of the chromosphere. We perform 3D MHD simulations using the PLUTO code, modelling locally the impact of the infalling gas onto the chromosphere. We find that the structure and dynamics of the post-shock region is strongly dependent on the plasma-beta (thermal over magnetic pressure), different values of which may give distinguishable emission signatures, relevant for observations. In particular, a strong magnetic field effectively confines the plasma inside its flux tubes and leads to the formation of quasi-independent fibrils. The fibrils may oscillate out of phase and hence the sum of their contributions in the emission results in a smooth overall profile. On the contrary, a weak magnetic field is not found to have any significant effect on the shocked plasma and the turbulent hot slab that forms is found to retain its periodic signature. Title: XIPE: the X-ray imaging polarimetry explorer Authors: Soffitta, Paolo; Barcons, Xavier; Bellazzini, Ronaldo; Braga, João; Costa, Enrico; Fraser, George W.; Gburek, Szymon; Huovelin, Juhani; Matt, Giorgio; Pearce, Mark; Poutanen, Juri; Reglero, Victor; Santangelo, Andrea; Sunyaev, Rashid A.; Tagliaferri, Gianpiero; Weisskopf, Martin; Aloisio, Roberto; Amato, Elena; Attiná, Primo; Axelsson, Magnus; Baldini, Luca; Basso, Stefano; Bianchi, Stefano; Blasi, Pasquale; Bregeon, Johan; Brez, Alessandro; Bucciantini, Niccoló; Burderi, Luciano; Burwitz, Vadim; Casella, Piergiorgio; Churazov, Eugene; Civitani, Marta; Covino, Stefano; Curado da Silva, Rui Miguel; Cusumano, Giancarlo; Dadina, Mauro; D'Amico, Flavio; De Rosa, Alessandra; Di Cosimo, Sergio; Di Persio, Giuseppe; Di Salvo, Tiziana; Dovciak, Michal; Elsner, Ronald; Eyles, Chris J.; Fabian, Andrew C.; Fabiani, Sergio; Feng, Hua; Giarrusso, Salvatore; Goosmann, René W.; Grandi, Paola; Grosso, Nicolas; Israel, Gianluca; Jackson, Miranda; Kaaret, Philip; Karas, Vladimir; Kuss, Michael; Lai, Dong; La Rosa, Giovanni; Larsson, Josefin; Larsson, Stefan; Latronico, Luca; Maggio, Antonio; Maia, Jorge; Marin, Frédéric; Massai, Marco Maria; Mineo, Teresa; Minuti, Massimo; Moretti, Elena; Muleri, Fabio; O'Dell, Stephen L.; Pareschi, Giovanni; Peres, Giovanni; Pesce, Melissa; Petrucci, Pierre-Olivier; Pinchera, Michele; Porquet, Delphine; Ramsey, Brian; Rea, Nanda; Reale, Fabio; Rodrigo, Juana Maria; Różańska, Agata; Rubini, Alda; Rudawy, Pawel; Ryde, Felix; Salvati, Marco; de Santiago, Valdivino Alexandre; Sazonov, Sergey; Sgró, Carmelo; Silver, Eric; Spandre, Gloria; Spiga, Daniele; Stella, Luigi; Tamagawa, Toru; Tamborra, Francesco; Tavecchio, Fabrizio; Teixeira Dias, Teresa; van Adelsberg, Matthew; Wu, Kinwah; Zane, Silvia Bibcode: 2013ExA....36..523S Altcode: 2013arXiv1309.6995S; 2013ExA...tmp...32S Abstract X-ray polarimetry, sometimes alone, and sometimes coupled to spectral and temporal variability measurements and to imaging, allows a wealth of physical phenomena in astrophysics to be studied. X-ray polarimetry investigates the acceleration process, for example, including those typical of magnetic reconnection in solar flares, but also emission in the strong magnetic fields of neutron stars and white dwarfs. It detects scattering in asymmetric structures such as accretion disks and columns, and in the so-called molecular torus and ionization cones. In addition, it allows fundamental physics in regimes of gravity and of magnetic field intensity not accessible to experiments on the Earth to be probed. Finally, models that describe fundamental interactions (e.g. quantum gravity and the extension of the Standard Model) can be tested. We describe in this paper the X-ray Imaging Polarimetry Explorer (XIPE), proposed in June 2012 to the first ESA call for a small mission with a launch in 2017. The proposal was, unfortunately, not selected. To be compliant with this schedule, we designed the payload mostly with existing items. The XIPE proposal takes advantage of the completed phase A of POLARIX for an ASI small mission program that was cancelled, but is different in many aspects: the detectors, the presence of a solar flare polarimeter and photometer and the use of a light platform derived by a mass production for a cluster of satellites. XIPE is composed of two out of the three existing JET-X telescopes with two Gas Pixel Detectors (GPD) filled with a He-DME mixture at their focus. Two additional GPDs filled with a 3-bar Ar-DME mixture always face the Sun to detect polarization from solar flares. The Minimum Detectable Polarization of a 1 mCrab source reaches 14 % in the 2-10 keV band in 105 s for pointed observations, and 0.6 % for an X10 class solar flare in the 15-35 keV energy band. The imaging capability is 24 arcsec Half Energy Width (HEW) in a Field of View of 14.7 arcmin × 14.7 arcmin. The spectral resolution is 20 % at 6 keV and the time resolution is 8 μs. The imaging capabilities of the JET-X optics and of the GPD have been demonstrated by a recent calibration campaign at PANTER X-ray test facility of the Max-Planck-Institut für extraterrestrische Physik (MPE, Germany). XIPE takes advantage of a low-earth equatorial orbit with Malindi as down-link station and of a Mission Operation Center (MOC) at INPE (Brazil). The data policy is organized with a Core Program that comprises three months of Science Verification Phase and 25 % of net observing time in the following 2 years. A competitive Guest Observer program covers the remaining 75 % of the net observing time. Title: Radiative accretion shocks along nonuniform stellar magnetic fields in classical T Tauri stars Authors: Orlando, S.; Bonito, R.; Argiroffi, C.; Reale, F.; Peres, G.; Miceli, M.; Matsakos, T.; Stehlé, C.; Ibgui, L.; de Sa, L.; Chièze, J. P.; Lanz, T. Bibcode: 2013A&A...559A.127O Altcode: 2013arXiv1309.5038O Context. According to the magnetospheric accretion model, hot spots form on the surface of classical T Tauri stars (CTTSs) in regions where accreting disk material impacts the stellar surface at supersonic velocity, generating a shock.
Aims: We investigate the dynamics and stability of postshock plasma that streams along nonuniform stellar magnetic fields at the impact region of accretion columns. We study how the magnetic field configuration and strength determine the structure, geometry, and location of the shock-heated plasma.
Methods: We model the impact of an accretion stream onto the chromosphere of a CTTS by 2D axisymmetric magnetohydrodynamic simulations. Our model considers the gravity, the radiative cooling, and the magnetic-field-oriented thermal conduction (including the effects of heat flux saturation). We explore different configurations and strengths of the magnetic field.
Results: The structure, stability, and location of the shocked plasma strongly depend on the configuration and strength of the magnetic field. In the case of weak magnetic fields (plasma β ≳ 1 in the postshock region), a large component of B may develop perpendicular to the stream at the base of the accretion column, which limits the sinking of the shocked plasma into the chromosphere and perturbs the overstable shock oscillations induced by radiative cooling. An envelope of dense and cold chromospheric material may also develop around the shocked column. For strong magnetic fields (β < 1 in the postshock region close to the chromosphere), the field configuration determines the position of the shock and its stand-off height. If the field is strongly tapered close to the chromosphere, an oblique shock may form well above the stellar surface at the height where the plasma β ≈ 1. In general, we find that a nonuniform magnetic field makes the distribution of emission measure vs. temperature of the postshock plasma at T > 106 K lower than when there is uniform magnetic field.
Conclusions: The initial magnetic field strength and configuration in the region of impact of the stream are expected to influence the chromospheric absorption and, therefore, the observability of the shock-heated plasma in the X-ray band. In addition, the field strength and configuration also influence the energy balance of the shocked plasma with its emission measure at T > 106 K, which is lower than expected for a uniform field. The above effects contribute to underestimating the mass accretion rates derived in the X-ray band.

Movies are available in electronic form at http://www.aanda.org Title: YSO accretion shocks: magnetic, chromospheric or stochastic flow effects can suppress fluctuations of X-ray emission Authors: Matsakos, T.; Chièze, J. -P.; Stehlé, C.; González, M.; Ibgui, L.; de Sá, L.; Lanz, T.; Orlando, S.; Bonito, R.; Argiroffi, C.; Reale, F.; Peres, G. Bibcode: 2013A&A...557A..69M Altcode: 2013arXiv1307.5389M Context. Theoretical arguments and numerical simulations of radiative shocks produced by the impact of the accreting gas onto young stars predict quasi-periodic oscillations in the emitted radiation. However, observational data do not show evidence of such periodicity.
Aims: We investigate whether physically plausible perturbations in the accretion column or in the chromosphere could disrupt the shock structure influencing the observability of the oscillatory behavior.
Methods: We performed local 2D magneto-hydrodynamical simulations of an accretion shock impacting a chromosphere, taking optically thin radiation losses and thermal conduction into account. We investigated the effects of several perturbation types, such as clumps in the accretion stream or chromospheric fluctuations, and also explored a wide range of plasma-β values.
Results: In the case of a weak magnetic field, the post-shock region shows chaotic motion and mixing, smoothing out the perturbations and retaining a global periodic signature. On the other hand, a strong magnetic field confines the plasma in flux tubes, which leads to the formation of fibrils that oscillate independently. Realistic values for the amplitude, length, and time scales of the perturbation are capable of bringing the fibril oscillations out of phase, suppressing the periodicity of the emission.
Conclusions: The strength of a locally uniform magnetic field in YSO accretion shocks determines the structure of the post-shock region, namely, whether it will be somewhat homogeneous or if it will split up to form a collection of fibrils. In the second case, the size and shape of the fibrils is found to depend strongly on the plasma-β value but not on the perturbation type. Therefore, the actual value of the protostellar magnetic field is expected to play a critical role in the time dependence of the observable emission. Title: A new look at the Spitzer primary transit observations of the exoplanet HD189733b Authors: Morello, G.; Peres, G.; Micela, G.; Waldmann, I. P.; Tinetti, G.; Howarth, I. D. Bibcode: 2013EPSC....8..997M Altcode: New blind source separation techniques are used to analyse uniformly eight primary transit lightcurves of the exoplanet HD189733b recorded with the infrared camera IRAC on board the Spitzer Space Telescope at 3.6μm. The observations were performed between 2006 and 2011: two of them were obtained in the"Cold Spitzer" regime, the others were taken in the "Warm Spitzer" period. The techniques we used to process the data are based on an Independent Component Analysis (ICA) approach, i.e. a computational method to disentangle the 'original source signals' from a set of observations/ recordings in which they are mixed. ICA assumes only the mutual statistical independence and the non-gaussianity of the source signals. Our objective was to extract the transit components by removing instrumental systematic effects and possibly other sources of astrophysical noise, such as background and stellar activity. The novelty of the algorithms used is their ability to extract the exoplanet signal in a single observation. In this presentation we will present the results obtained, detail the methods adopted and critically discuss the conclusions of our work by comparing said results to the ones obtained in the literature. Title: Bright Hot Impacts by Erupted Fragments Falling Back on the Sun: A Template for Stellar Accretion Authors: Reale, Fabio; Orlando, Salvatore; Testa, Paola; Peres, Giovanni; Landi, Enrico; Schrijver, Carolus J. Bibcode: 2013Sci...341..251R Altcode: Impacts of falling fragments observed after the eruption of a filament in a solar flare on 7 June 2011 are similar to those inferred for accretion flows on young stellar objects. As imaged in the ultraviolet (UV)-extreme UV range by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory, many impacts of dark, dense matter display uncommonly intense, compact brightenings. High-resolution hydrodynamic simulations show that such bright spots, with plasma temperatures increasing from ~104 to ~106 kelvin, occur when high-density plasma (>>1010 particles per cubic centimeter) hits the solar surface at several hundred kilometers per second, producing high-energy emission as in stellar accretion. The high-energy emission comes from the original fragment material and is heavily absorbed by optically thick plasma, possibly explaining the lower mass accretion rates inferred from x-rays relative to UV-optical-near infrared observations of young stars. Title: Hydrodynamic modelling of ejecta shrapnel in the Vela supernova remnant Authors: Miceli, M.; Orlando, S.; Reale, F.; Bocchino, F.; Peres, G. Bibcode: 2013MNRAS.430.2864M Altcode: 2013arXiv1301.3085M; 2013MNRAS.tmp..743M Many supernova remnants (SNRs) are characterized by a knotty ejecta structure. The Vela SNR is an excellent example of remnant in which detached clumps of ejecta are visible as X-ray emitting bullets that have been observed and studied in great detail. We aim at modelling the evolution of ejecta shrapnel in the Vela SNR, investigating the role of their initial parameters (position and density) and addressing the effects of thermal conduction and radiative losses. We performed a set of 2D hydrodynamic simulations describing the evolution of a density inhomogeneity in the ejecta profile. We explored different initial setups. We found that the final position of the shrapnel is very sensitive to its initial position within the ejecta, while the dependence on the initial density contrast is weaker. Our model also shows that moderately overdense knots can reproduce the detached features observed in the Vela SNR. Efficient thermal conduction produces detectable effects by determining an efficient mixing of the ejecta knot with the surrounding medium and shaping a characteristic elongated morphology in the clump. Title: Hot Plasma Detected in Active Regions by HINODE/XRT and SDO/AIA Authors: Reale, F.; Testa, P.; Guarrasi, M.; DeLuca, E.; Peres, G.; Golub, L. Bibcode: 2012ASPC..456..129R Altcode: Multiple ratios of Hinode/XRT filters showed evidence of a minor very hot emission measure component in active regions. Recently also SDO/AIA detected hot plasma in the core of an active region. Here we provide estimates showing that the amount of emission measure of the hot component detected with SDO is consistent with that detected with Hinode/XRT. Title: Generation of Knots in a Randomly Pulsed Protostellar Jet: Synthesis of the X-ray Emission Authors: Bonito, R.; Orlando, S.; Miceli, M.; Eislöffel, J.; Peres, G.; Favata, F. Bibcode: 2011ASPC..448..559B Altcode: 2011csss...16..559B X-rays from protostellar jets have been discovered first in HH 2 and HH 154 and are now considered as a general feature of jets. HH 154 is among the best studied X-ray emitting jet: up to now it is the only jet whose X-ray source proper motion has been measured. By observing this jet in X-rays, a complex morphology of the detected source has been revealed. Here we discuss the results of modeling a randomly ejected pulsed jet traveling into an initially homogeneous medium. Our model allows us to directly compare the model predictions and the observations. Title: X-ray emission from protostellar jet HH 154: first evidence of a diamond shock? Authors: Bonito, Rosaria; Orlando, Salvatore; Miceli, Marco; Peres, Giovanni; Micela, Giusi; Favata, Fabio Bibcode: 2011xru..conf..190B Altcode: No abstract at ADS Title: X-Ray Emission from Protostellar Jet HH 154: The First Evidence of a Diamond Shock? Authors: Bonito, R.; Orlando, S.; Miceli, M.; Peres, G.; Micela, G.; Favata, F. Bibcode: 2011ApJ...737...54B Altcode: 2011arXiv1105.4081B X-ray emission from about 10 protostellar jets has been discovered and it appears as a feature common to the most energetic jets. Although X-ray emission seems to originate from shocks internal to jets, the mechanism forming these shocks remains controversial. One of the best-studied X-ray jets is HH 154, which has been observed by Chandra over a time base of about 10 years. We analyze the Chandra observations of HH 154 by investigating the evolution of its X-ray source. We show that the X-ray emission consists of a bright stationary component and a faint elongated component. We interpret the observations by developing a hydrodynamic model describing a protostellar jet originating from a nozzle and compare the X-ray emission synthesized from the model with the X-ray observations. The model takes into account the thermal conduction and radiative losses and shows that the jet/nozzle leads to the formation of a diamond shock at the nozzle exit. The shock is stationary over the period covered by our simulations and generates an X-ray source with luminosity and spectral characteristics in excellent agreement with the observations. We conclude that the X-ray emission from HH 154 is consistent with a diamond shock originating from a nozzle through which the jet is launched into the ambient medium. We suggest that the physical origin of the nozzle could be related to the dense gas in which the HH 154 driving source is embedded and/or to the magnetic field at the jet launching/collimation region. Title: Mass accretion to young stars triggered by flaring activity in circumstellar discs Authors: Orlando, Salvatore; Reale, Fabio; Peres, Giovanni; Mignone, Andrea Bibcode: 2011MNRAS.415.3380O Altcode: 2011arXiv1104.5107O; 2011MNRAS.tmp..846O Young low-mass stars are characterized by ejection of collimated outflows and by circumstellar discs which they interact with through accretion of mass. The accretion builds up the star to its final mass and is also believed to power the mass outflows, which may in turn remove the excess angular momentum from the star-disc system. However, although the process of mass accretion is a critical aspect of star formation, some of its mechanisms are still to be fully understood. A point not considered to date and relevant for the accretion process is the evidence of very energetic and frequent flaring events in these stars. Flares may easily perturb the stability of the discs, thus influencing the transport of mass and angular momentum. Here we report on 3D magnetohydrodynamic modelling of the evolution of a flare with an idealized non-equilibrium initial condition occurring near the disc around a rotating magnetized star. The model takes into account the stellar magnetic field, the gravitational force, the viscosity of the disc, the magnetic-field-oriented thermal conduction (including the effects of heat flux saturation), the radiative losses from optically thin plasma and the coronal heating. We show that, during its first stage of evolution, the flare gives rise to a hot magnetic loop linking the disc to the star. The disc is strongly perturbed by the flare: disc material evaporates under the effect of the thermal conduction and an overpressure wave propagates through the disc. When the overpressure reaches the opposite side of the disc, a funnel flow starts to develop there, accreting substantial disc material on to the young star from the side of the disc opposite to the flare. Title: Solar Dynamics Observatory Discovers Thin High Temperature Strands in Coronal Active Regions Authors: Reale, Fabio; Guarrasi, Massimiliano; Testa, Paola; DeLuca, Edward E.; Peres, Giovanni; Golub, Leon Bibcode: 2011ApJ...736L..16R Altcode: 2011arXiv1106.1591R One scenario proposed to explain the million degree solar corona is a finely stranded corona where each strand is heated by a rapid pulse. However, such fine structure has neither been resolved through direct imaging observations nor conclusively shown through indirect observations of extended superhot plasma. Recently, it has been shown that the observed difference in the appearance of cool and warm coronal loops (~1 MK and ~2-3 MK, respectively)—warm loops appearing "fuzzier" than cool loops—can be explained by models of loops composed of subarcsecond strands, which are impulsively heated up to ~10 MK. That work predicts that images of hot coronal loops (gsim 6 MK) should again show fine structure. Here we show that the predicted effect is indeed widely observed in an active region with the Solar Dynamics Observatory, thus supporting a scenario where impulsive heating of fine loop strands plays an important role in powering the active corona. Title: Multiwavelength diagnostics of accretion in an X-ray selected sample of CTTSs Authors: Curran, R. L.; Argiroffi, C.; Sacco, G. G.; Orlando, S.; Peres, G.; Reale, F.; Maggio, A. Bibcode: 2011A&A...526A.104C Altcode: 2010arXiv1011.5915C Context. High resolution X-ray spectroscopy has revealed soft X-rays from high density plasma in classical T Tauri stars (CTTSs), probably arising from the accretion shock region. However, the mass accretion rates derived from the X-ray observations are consistently lower than those derived from UV/optical/NIR studies.
Aims: We aim to test the hypothesis that the high density soft X-ray emission originates from accretion by analysing, in a homogeneous manner, optical accretion indicators for an X-ray selected sample of CTTSs.
Methods: We analyse optical spectra of the X-ray selected sample of CTTSs and calculate the accretion rates based on measuring the Hα, Hβ, Hγ, He ii 4686 Å, He i 5016 Å, He i 5876 Å, O i 6300 Å, and He i 6678 Å equivalent widths. In addition, we also calculate the accretion rates based on the full width at 10% maximum of the Hα line. The different optical tracers of accretion are compared and discussed. The derived accretion rates are then compared to the accretion rates derived from the X-ray spectroscopy.
Results: We find that, for each CTTS in our sample, the different optical tracers predict mass-accretion rates that agree within the errors, albeit with a spread of ≈ 1 order of magnitude. Typically, mass-accretion rates derived from Hα and He i 5876 Å are larger than those derived from Hβ, Hγ, and O i. In addition, the Hα full width at 10%, whilst a good indicator of accretion, may not accurately measure the mass-accretion rate. When the optical mass-accretion rates are compared to the X-ray derived mass-accretion rates, we find that: a) the latter are always lower (but by varying amounts); b) the latter range within a factor of ≈ 2 around 2 × 10-10 M⊙ yr-1, despite the former spanning a range of ≈ 3 orders of magnitude. We suggest that the systematic underestimate of the X-ray derived mass-accretion rates could depend on the density distribution inside the accretion streams, where the densest part of the stream is not visible in the X-ray band because of the absorption by the stellar atmosphere. We also suggest that a non-negligible optical depth of X-ray emission lines produced by post-shock accreting plasma may explain the almost constant mass-accretion rates derived in X-rays if the effect is larger in stars with higher optical mass-accretion rates. Title: Science Objectives for an X-Ray Microcalorimeter Observing the Sun Authors: Laming, J. Martin; Adams, J.; Alexander, D.; Aschwanden, M; Bailey, C.; Bandler, S.; Bookbinder, J.; Bradshaw, S.; Brickhouse, N.; Chervenak, J.; Christe, S.; Cirtain, J.; Cranmer, S.; Deiker, S.; DeLuca, E.; Del Zanna, G.; Dennis, B.; Doschek, G.; Eckart, M.; Fludra, A.; Finkbeiner, F.; Grigis, P.; Harrison, R.; Ji, L.; Kankelborg, C.; Kashyap, V.; Kelly, D.; Kelley, R.; Kilbourne, C.; Klimchuk, J.; Ko, Y. -K.; Landi, E.; Linton, M.; Longcope, D.; Lukin, V.; Mariska, J.; Martinez-Galarce, D.; Mason, H.; McKenzie, D.; Osten, R.; Peres, G.; Pevtsov, A.; Porter, K. Phillips F. S.; Rabin, D.; Rakowski, C.; Raymond, J.; Reale, F.; Reeves, K.; Sadleir, J.; Savin, D.; Schmelz, J.; Smith, R. K.; Smith, S.; Stern, R.; Sylwester, J.; Tripathi, D.; Ugarte-Urra, I.; Young, P.; Warren, H.; Wood, B. Bibcode: 2010arXiv1011.4052L Altcode: We present the science case for a broadband X-ray imager with high-resolution spectroscopy, including simulations of X-ray spectral diagnostics of both active regions and solar flares. This is part of a trilogy of white papers discussing science, instrument (Bandler et al. 2010), and missions (Bookbinder et al. 2010) to exploit major advances recently made in transition-edge sensor (TES) detector technology that enable resolution better than 2 eV in an array that can handle high count rates. Combined with a modest X-ray mirror, this instrument would combine arcsecondscale imaging with high-resolution spectra over a field of view sufficiently large for the study of active regions and flares, enabling a wide range of studies such as the detection of microheating in active regions, ion-resolved velocity flows, and the presence of non-thermal electrons in hot plasmas. It would also enable more direct comparisons between solar and stellar soft X-ray spectra, a waveband in which (unusually) we currently have much better stellar data than we do of the Sun. Title: On the observability of T Tauri accretion shocks in the X-ray band Authors: Sacco, G. G.; Orlando, S.; Argiroffi, C.; Maggio, A.; Peres, G.; Reale, F.; Curran, R. L. Bibcode: 2010A&A...522A..55S Altcode: 2010arXiv1007.2423S Context. High resolution X-ray observations of classical T Tauri stars (CTTSs) show a soft X-ray excess due to high density plasma (ne = 1011-113 cm-3). This emission has been attributed to shock-heated accreting material impacting onto the stellar surface.
Aims: We investigate the observability of the shock-heated accreting material in the X-ray band as a function of the accretion stream properties (velocity, density, and metal abundance) in the case of plasma-β ≪ 1 (thermal pressure ≪ magnetic pressure) in the post-shock zone.
Methods: We use a 1-D hydrodynamic model describing the impact of an accretion stream onto the chromosphere of a CTTS, including the effects of radiative cooling, gravity stratification and thermal conduction. We explore the space of relevant parameters and synthesize from the model results the X-ray emission in the [0.5-8.0] keV band and in the resonance lines of O vii (21.60 Å) and Ne ix (13.45 Å), taking into account the absorption from the chromosphere.
Results: The accretion stream properties largely influence the temperature and the stand-off height of the shocked slab and its sinking in the chromosphere, determining the observability of the shocked plasma affected by chromospheric absorption. Our model predicts that X-ray observations preferentially detect emission from low density and high velocity shocked accretion streams due to the large absorption of dense post-shock plasma. In all the cases examined, the post-shock zone exhibits quasi-periodic oscillations due to thermal instabilities with periods ranging from 3×10-2 to 4×103 s. In the case of inhomogeneous streams and β ≪ 1, the shock oscillations are hardly detectable.
Conclusions: We suggest that, if accretion streams are inhomogeneous, the selection effect introduced by the absorption on observable plasma components may easily explain the discrepancy between the accretion rate measured by optical and X-ray data as well as the different densities measured using different He-like triplets in the X-ray band. Title: Chronology of star formation and disk evolution in the Eagle Nebula Authors: Guarcello, M. G.; Micela, G.; Peres, G.; Prisinzano, L.; Sciortino, S. Bibcode: 2010A&A...521A..61G Altcode: 2010arXiv1008.0422G Context. Massive star-forming regions are characterized by intense ionizing fluxes, strong stellar winds and, occasionally, supernovae explosions, all of which have important effects on the surrounding media, on the star-formation process and on the evolution of young stars and their circumstellar disks. We present a multiband study of the massive young cluster NGC 6611 and its parental cloud (the Eagle Nebula) with the aim of studying how OB stars affect the early stellar evolution and the formation of other stars.
Aims: We search for evidence of triggering of star formation by the massive stars inside NGC 6611 on a large spatial scale (~10 parsec) and ongoing disk photoevaporation in NGC 6611 and how its efficiency depends on the mass of the central stars.
Methods: We assemble a multiband catalog of the Eagle Nebula with photometric data, ranging from B band to 8.0 μm, and X-ray data obtained with two new and one archival Chandra/ACIS-I observation. We select the stars with disks from infrared photometry and disk-less ones from X-ray emission, which are associated both with NGC 6611 and the outer region of the Eagle Nebula. We study induced photoevaporation searching for the spatial variation of disk frequency for distinct stellar mass ranges. The triggering of star formation by OB stars has been investigated by deriving the history of star formation across the nebula.
Results: We find evidence of sequential star formation in the Eagle Nebula going from the southeast (2.6 Myears) to the northwest (0.3 Myears), with the median age of NGC 6611 members ~1 Myear. In NGC 6611, we observe a drop of the disk frequency close to massive stars (up to an average distance of 1 parsec), without observable effects at larger distances. Furthermore, disks are more frequent around low-mass stars (≤ 1 M⊙) than around high-mass stars, regardless of the distance from OB stars.
Conclusions: The star-formation chronology we find in the Eagle Nebula does not support the hypothesis of a large-scale process triggered by OB stars in NGC 6611. Instead, we speculate that it was triggered by the encounter (about 3 Myears ago) with a giant molecular shell created by supernovae explosions about 6 Myears ago. We find evidence of disk photoevaporation close to OB stars, where disks are heated by incident extreme ultraviolet (EUV) radiation. No effects are observed at large distances from OB stars, where photoevaporation is induced by the far ultraviolet (FUV) radiation, and long timescales are usually required to completely dissipate the disks. Title: Pre-main sequence stars with disks in the Eagle Nebula observed in scattered light Authors: Guarcello, M. G.; Damiani, F.; Micela, G.; Peres, G.; Prisinzano, L.; Sciortino, S. Bibcode: 2010A&A...521A..18G Altcode: 2010arXiv1006.3813G Context. NGC 6611 and its parental cloud, the Eagle Nebula (M 16), are well-studied star-forming regions, thanks to their large content of both OB stars and stars with disks and the observed ongoing star formation. In our previous studies of the Eagle Nebula, we identified 834 disk-bearing stars associated with the cloud, after detecting their excesses in NIR bands from J band to 8.0 μ m.
Aims: In this paper, we study in detail the nature of a subsample of disk-bearing stars that show peculiar characteristics. They appear older than the other members in the V vs. V-I diagram, and/or they have one or more IRAC colors at pure photospheric values, despite showing NIR excesses, when optical and infrared colors are compared.
Methods: We confirm the membership of these stars to M 16 by a spectroscopic analysis. The physical properties of these stars with disks are studied by comparing their spectral energy distributions (SEDs) with the SEDs predicted by models of T Tauri stars with disks and envelopes.
Results: We show that the age of these stars estimated from the V vs. V-I diagram is unreliable since their V-I colors are altered by the light scattered by the disk into the line of sight. Only in a few cases their SEDs are compatible with models with excesses in V band caused by optical veiling. Candidate members with disks and photospheric IRAC colors are selected by the used NIR disk diagnostic, which is sensitive to moderate excesses, such as those produced by disks with low masses. In 1/3 of these cases, scattering of stellar flux by the disks can also be invoked.
Conclusions: The photospheric light scattered by the disk grains into the line of sight can affect the derivation of physical parameters of Class II stars from photometric optical and NIR data. Besides, the disks diagnostic we defined are useful for selecting stars with disks, even those with moderate excesses or whose optical colors are altered by veiling or photospheric scattered light.

Table with the data of the stars is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/521/A18 Title: Coronal Fuzziness Modeled with Pulse-heated Multi-stranded Loop Systems Authors: Guarrasi, Massimiliano; Reale, Fabio; Peres, Giovanni Bibcode: 2010ApJ...719..576G Altcode: 2010arXiv1006.3495G Coronal active regions are observed to get increasingly fuzzy (i.e., increasingly confused and uniform) in increasingly hard energy bands or lines. We explain this as evidence of fine multi-temperature structure of coronal loops. To this end, we model bundles of loops made of thin strands, each heated by short and intense heat pulses. For simplicity, we assume that the heat pulses are all equal and triggered only once in each strand at a random time. The pulse intensity and cadence are selected so as to have steady active region loops (~3 MK) on average. We compute the evolution of the confined heated plasma with a hydrodynamic loop model. We then compute the emission along each strand in several spectral lines, from cool (<=1 MK), to warm (2-3 MK) lines, detectable with Hinode/Extreme-ultraviolet Imaging Spectrometer, to hot X-ray lines. The strands are then put side by side to construct an active region loop bundle. We find that in the warm lines (2-3 MK) the loop emission fills all the available image surface. Therefore, the emission appears quite uniform and it is difficult to resolve the single loops, while in the cool lines the loops are considerably more contrasted and the region is less fuzzy. The main reasons for this effect are that, during their evolution, i.e., pulse heating and slow cooling, each strand spends a relatively long time at temperatures around 2-3 MK and it has a high emission measure during that phase, so the whole region appears more uniform or smudged. We predict that fuzziness should be reduced in the hot UV and X-ray lines. Title: VizieR Online Data Catalog: Stars associated to Eagle Nebula (M16=NGC6611) (Guarcello+ 2010) Authors: Guarcello, M. G.; Micela, G.; Peres, G.; Prisinzano, L.; Sciortino, S. Bibcode: 2010yCat..35210061G Altcode: 2010yCat..35219061G This catalog contains coordinates and both optical and infrared photometry, plus usefull tags, of the candidate stars associated to the Eagle Nebula (M16), bost disk-less and disk-bearing, selected in Guarcello et al. 2010: "Chronology of star formation and disks evolution in the Eagle Nebula". The optical photometry in BVI bands comes from observations with WFI@ESO (Guarcello et al. 2007, Cat. J/A+A/462/245); JHK photometry have been obtained from 2MASS/PSC (Bonatto et al. 2006A&A...445..567B, Guarcello et al. 2007, Cat. J/A+A/462/245) and UKIDSS/GPS catalogs (Guarcello et al., 2010, in prep.) ; IRAC data are from GLIMPSE public survey (Indebetouw 2007ApJ...666..321I, Guarcello et al., 2009, Cat. J/A+A/496/453); X-ray data from three observations with Chandra/ACIS-I (Linsky et al., 2007, Cat. J/ApJ/654/347, Guarcello et al., 2007, J/A+A/462/245, Guarcello et al. 2010, in prep.).

(1 data file). Title: Generation of radiative knots in a randomly pulsed protostellar jet. II. X-ray emission Authors: Bonito, R.; Orlando, S.; Miceli, M.; Eislöffel, J.; Peres, G.; Favata, F. Bibcode: 2010A&A...517A..68B Altcode: 2010arXiv1005.2125B Context. Protostellar jets are known to emit in a wide range of bands, from radio to IR to optical bands, and to date about ten jets that also emit X-rays have been detected, with a rate of discovery of about one per year.
Aims: We aim at investigating the mechanism leading to the X-ray emission detected in protostellar jets and in particular at constraining the physical parameters that describe the jet/ambient interaction by comparing our model predictions with observations available from the literature.
Methods: We perform 2D axisymmetric hydrodynamic simulations of the interaction between a supersonic jet and the ambient medium. The jet is described as a train of plasma blobs randomly ejected by the stellar source along the jet axis. We explore the parameter space by varying the ejection rate, the initial Mach number of the jet, and the initial density contrast between the ambient medium and the jet. We synthesize the X-ray emission from the model as it would be observed with the current X-ray telescopes.
Results: The mutual interactions among the ejected blobs themselves and of the blobs with the ambient medium lead to complex X-ray-emitting structures within the jet. The X-ray sources consist of several components: irregular chains of knots, isolated knots with measurable proper motion, apparently stationary knots, and reverse shocks. The predicted X-ray luminosity strongly depends on the ejection rate and on the initial density contrast between the ambient medium and the jet, with a lesser dependence on the jet Mach number.
Conclusions: Our model represents the first attempt to describe the X-ray properties of all X-ray-emitting protostellar jets discovered so far. The comparison between our model predictions and the observations can provide a useful diagnostic tool, which is necessary for a proper interpretation of the observations. We specifically suggest that the observable quantities derived from the spectral analysis of X-ray observations can be used to constrain the ejection rate, a parameter explored in our model that is not measurable by current observations in all wavelength bands. Title: Large Scale Properties of Coronal Heating along the Solar Cycle Authors: Peres, G.; Argiroffi, C.; Orlando, S.; Reale, F. Bibcode: 2010ASPC..428..139P Altcode: We discuss various studies of the global properties of coronal heating. Some of them find power laws tying the X-ray luminosity with the magnetic flux of individual structures, of the whole Sun, and of active solar-type stars. Others are based on methods to model the Sun as an X-ray star. We also briefly discuss solar-like active stars and how the Sun fits in the whole scenario. We use a new model, including all flares, of the Sun as an X-ray star to describe the evolution of the corona along the solar cycle and the implications on the heating of closed coronal structures. We point out that, as activity increases, more heating is released into the confined coronal plasma and such a heating has to be, on average, more intense in order to explain the widespread evidence of a temperature increase with activity. By the same token, nanoflare heating (if existent) has to increase and decrease along the cycle differently from flares. Title: VizieR Online Data Catalog: Blue stars with disk photometry in NGC 6611 (Guarcello+, 2010) Authors: Guarcello, M. G.; Damiani, F.; Micela, G.; Peres, G.; Prisinzano, L.; Sciortino, S. Bibcode: 2010yCat..35210018G Altcode: 2010yCat..35219018G This catalog contains coordinates and both optical and infrared photometry, plus usefull tags, of the Blue Stars With Disk (BWE stars) discussed in detail in Guarcello et al. (2010, in prep): "Pre-main sequence stars with disks in the Eagle nebula observed in scattered light". The optical photometry in BVI bands comes from observations with WFI@ESO (Guarcello et al. 2007, Cat. J/A+A/462/245); JHK photometry have been obtained from 2MASS/PSC and UKIDSS/GPS catalogs (Bonatto et al., 2006A&A...445..567B, Guarcello et al., 2007, Cat. J/A+A/462/245 and 2010, in prep); IRAC data are from GLIMPSE public survey (Indebetouw 2007ApJ...666..321I, Guarcello et al., 2009, Cat. J/A+A/496/453); X-ray data from observations with Chandra/ACIS-I (Linsky et al. 2007, Cat. J/ApJ/654/347, Guarcello et al., 2007, Cat. J/A+A/462/245, Guarcello et al., 2010, in prep). BWE stars have been studied by analizing their Spectral Energy Distribution (SED) using the grid of YSO models developed by Robitaille et al. 2006ApJS..167..256R.

(1 data file). Title: Observability and diagnostics in the X-ray band of shock-cloud interactions in supernova remnants Authors: Orlando, S.; Bocchino, F.; Miceli, M.; Zhou, X.; Reale, F.; Peres, G. Bibcode: 2010A&A...514A..29O Altcode: 2010arXiv1001.2240O Context. X-ray emitting features originating from the interaction of supernova shock waves with small interstellar gas clouds are revealed in many X-ray observations of evolved supernova remnants (e.g., Cygnus Loop and Vela), but their interpretation is not straightforward.
Aims: We develop a self-consistent method for the analysis and interpretation of shock-cloud interactions in middle-aged supernova remnants, which can provide the key parameters of the system and the role of relevant physical effects such as thermal conduction, without the need to perform ad-hoc numerical simulations and bother about morphology details.
Methods: We explore all the possible values of the shock speed and cloud density contrast relevant to middle-aged SNRs with a set of hydrodynamic simulations of shock-cloud interaction including the effects of thermal conduction and radiative cooling. From the simulations, we synthesize spatially and spectrally resolved focal-plane data as they would be collected with XMM-Newton/EPIC, an X-ray instrument commonly used in these studies.
Results: We develop and calibrate two diagnostic tools, the first based on the mean photon energy versus count-rate scatter plot and the second on the spectral analysis of the interaction region, that can be used to highlight the effects of thermal conduction and to derive the shock speed in case of efficient conduction at work. These tools can be used to ascertain information from X-ray observations, without the need to develop detailed and ad-hoc numerical models for the interpretation of the data. Title: Generation of radiative knots in a randomly pulsed protostellar jet. I. Dynamics and energetics Authors: Bonito, R.; Orlando, S.; Peres, G.; Eislöffel, J.; Miceli, M.; Favata, F. Bibcode: 2010A&A...511A..42B Altcode: 2009arXiv0912.2757B Context. Herbig-Haro objects are characterized by a complex knotty morphology detected mainly along the axis of protostellar jets in a wide range of bands: from radio to IR to optical bands, with X-rays knots also detected in the past few years. Evidence of interactions between knots formed in different epochs have been found, suggesting that jets may result from the ejection of plasma blobs from the stellar source.
Aims: We aim at investigating the physical mechanism leading to the irregular knotty structure observed in protostellar jets in different wavelength bands and the complex interactions occurring among blobs of plasma ejected from the stellar source.
Methods: We performed 2D axisymmetric hydrodynamic numerical simulations of a randomly ejected pulsed jet. The jet consists of a train of blobs that ram with supersonic speed into the ambient medium. The initial random velocity of each blob follows an exponential distribution. We explored the ejection rate parameter to derive constraints on the physical properties of protostellar jets by comparing model results with observations. Our model takes the effects of radiative losses and thermal conduction into account.
Results: We find that the mutual interactions of blobs ejected at different epochs and with different speeds lead to a variety of plasma components not described by current models of jets. The main features characterizing the randomly pulsed jet scenario are: single high-speed knots, showing a measurable proper motion in nice agreement with optical and X-rays observations; irregular chains of knots aligned along the jet axis and possibly interacting with each other; reverse shocks interacting with outgoing knots; oblique shock patterns produced by the reflection of shocks at the cocoon surrounding the jet. All these structures work together to help determining the morphology of the jet in different wavelength bands. We also find that the thermal conduction plays a crucial role in damping out hydrodynamic instabilities that would develop within the cocoon and that contribute to the jet breaking. Title: X-ray emitting MHD accretion shocks in classical T Tauri stars. Case for moderate to high plasma-β values Authors: Orlando, S.; Sacco, G. G.; Argiroffi, C.; Reale, F.; Peres, G.; Maggio, A. Bibcode: 2010A&A...510A..71O Altcode: 2009arXiv0912.1799O Context. Plasma accreting onto classical T Tauri stars (CTTS) is believed to impact the stellar surface at free-fall velocities, generating a shock. Current time-dependent models describing accretion shocks in CTTSs are one-dimensional, assuming that the plasma moves and transports energy only along magnetic field lines (β ≪ 1).
Aims: We investigate the stability and dynamics of accretion shocks in CTTSs, considering the case of β ⪆ 1 in the post-shock region. In these cases the 1D approximation is not valid and a multi-dimensional MHD approach is necessary.
Methods: We model an accretion stream propagating through the atmosphere of a CTTS and impacting onto its chromosphere by performing 2D axisymmetric MHD simulations. The model takes into account the stellar magnetic field, the gravity, the radiative cooling, and the thermal conduction (including the effects of heat flux saturation).
Results: The dynamics and stability of the accretion shock strongly depend on the plasma β. In the case of shocks with β > 10, violent outflows of shock-heated material (and possibly MHD waves) are generated at the base of the accretion column and intensely perturb the surrounding stellar atmosphere and the accretion column itself (therefore modifying the dynamics of the shock). In shocks with β ≈ 1, the post-shock region is efficiently confined by the magnetic field. The shock oscillations induced by cooling instability are strongly influenced by β: for β > 10, the oscillations may be rapidly dumped by the magnetic field, approaching a quasi-stationary state, or may be chaotic with no obvious periodicity due to perturbation of the stream induced by the post-shock plasma itself; for β≈ 1 the oscillations are quasi-periodic, although their amplitude is smaller and the frequency higher than those predicted by 1D models.

Three movies are only available in electronic form at http://www.aanda.org Title: The EM(T) of stellar coronae Authors: Argiroffi, Costanza; Peres, Giovanni; Orlando, Salvatore; Reale, Fabio Bibcode: 2010cosp...38.2900A Altcode: 2010cosp.meet.2900A Studying the solar corona, due to its vicinity, is the starting point to understand stellar activity. The emission measure distribution vs temperature, EM(T), is a useful tool to study coronal plasmas, in fact it allows: to investigate the energy balance of coronal plasmas, to easily compare different stars, and also to compare the solar corona to that of other active stars irrespective of the very different observing techniques. The EM(T) of the solar corona differs significantly, in terms of average plasma temperatures, peak temperatures, and total emission measure, with respect to that of active stars. In this work it is discussed how the evaluation of the EM(T) of the solar corona, and of its components (quiescent plasma, active regions, flares, etc.), parallel to the reconstruction of the EM(T) of stars at different activity levels, can be used to investigate coronal physics. Title: Diagnostics of the coronal fine structuring Authors: Guarrasi, Massimiliano; Reale, Fabio; Peres, Giovanni Bibcode: 2010cosp...38.2832G Altcode: 2010cosp.meet.2832G Coronal structures are believed to consist of bundles of fibrils below the resolution of the present-day X-ray and UV telescopes. We model coronal structures as collections of individual thin and independent loop atmospheres, each ignited by a heat pulse, and investigate possible indirect diagnostics of the fine structuring. In particular, we propose that the substructuring is the explanation of the different appearance of the structures in different spectral lines and bands. Title: X-ray optical depth diagnostics of T Tauri accretion shocks Authors: Argiroffi, C.; Maggio, A.; Peres, G.; Drake, J. J.; López-Santiago, J.; Sciortino, S.; Stelzer, B. Bibcode: 2009A&A...507..939A Altcode: 2009arXiv0909.0218A Context: In classical T Tauri stars, X-rays are produced by two plasma components: a hot low-density plasma, with frequent flaring activity, and a high-density lower temperature plasma. The former is coronal plasma related to the stellar magnetic activity. The latter component, never observed in non-accreting stars, could be plasma heated by the shock formed by the accretion process. However its nature is still being debated.
Aims: Our aim is to probe the soft X-ray emission from the high-density plasma component in classical T Tauri stars to check whether this plasma is heated in the accretion shock or whether it is coronal plasma.
Methods: High-resolution X-ray spectroscopy allows us to measure individual line fluxes. We analyze X-ray spectra of the classical T Tauri stars MP Muscae and TW Hydrae. Our aim is to evaluate line ratios to search for optical depth effects, which are expected in the accretion-driven scenario. We also derive the plasma emission measure distributions EMD, to investigate whether and how the EMD of accreting and non accreting young stars differ. The results are compared to those obtained for the non-accreting weak-line T Tauri star TWA 5.
Results: We find evidence of resonance scattering in the strongest lines of MP Mus, supporting the idea that soft X-rays are produced by plasma heated in the accretion shock. We also find that the EMD of MP Mus has two peaks: a cool peak at temperatures expected for plasma heated in the accretion shock, and a hot peak typical of coronal plasma. The shape of the EMD of MP Mus appears to be the superposition of the EMD of a pure coronal source, like TWA 5, and an EMD alike that of TW Hya, which is instead dominated by shock-heated plasma. Title: Modeling accretion shocks on CTTSs and their X-ray emission Authors: Sacco, Giuseppe; Orlando, Salvatore; Argiroffi, Costanza; Maggio, Antonio; Peres, Giovanni; Reale, Fabio Bibcode: 2009cfdd.confE.189S Altcode: Recent high spectral resolution X-ray observations of some CTTSs show the presence of high density plasma (ne=10^{11}-10^{13} cm^{-3}) at temperature T=2-3 MK. This plasma is likely heated up by an accretion shock on the star surface. We investigate this issue by an accurate modelling of the impact of an accretion stream onto the stellar chromosphere. Specifically, we present a large set of 1D hydrodynamical simulations aimed at investigating the physical properties of the system as a function of the density, and the velocity of the accretion stream and of the abundances of the heavy elements. We also synthesize the plasma X-ray emission from the simulations results, in order to link the observed properties of the CTTSs with the accretion shock physics. Furthermore, we present results of 2D magneto-hydrodynamical simulations aimed at investigating the role of the magnetic field in confining the shocked plasma at the base of the accretion column for different magnetic field strength. Title: Multi-wavelength diagnostics of accretion in an X-ray selected sample of CTTSs Authors: Curran, Rachel; Argiroffi, Costanza; Sacco, Giuseppe Germano; Orlando, Salvatore; Reale, Fabio; Peres, Giovanni; Maggio, Antonio Bibcode: 2009cfdd.confE..81C Altcode: The majority of CTTSs observed to date with high spectral resolution X-ray spectroscopy reveal soft X-ray emission (E<0.7 KeV) which originates from cool (1-5 MK), high density (n ∼ 10^{11}-10^{13} cm^{-3}) plasma. This is currently interpreted to be due to mass accretion. Supporting this interpretation is the fact that this plasma component is too dense to have a coronal origin, and it has never been observed in non-accreting stars. Synthesized X-ray spectra from detailed hydrodynamical modelling of the interaction between the accretion flow and the stellar chromosphere also confirm this interpretation. However, the mass accretion rates derived from X-ray data are consistently underestimated when compared to mass accretion rates derived from UV/optical data. We test the hypothesis that this soft X-ray emission originates from accretion by analysing optical, NIR and X-ray data for an X-ray selected sample of CTTSs. We derive mass accretion rates for the sample based on Hα, He I, O I and Ca II emission lines, along with the X-ray data. We draw comparisons between these mass accretion rates to understand the underestimation of the X-ray derived mass accretion rates. We discuss the possibilities of a) the X-ray emission being partially absorbed, b) the optical/NIR emission arising from different parts of the accretion stream and c) the uncertainties involved in the estimation of the mass accretion rates from different spectroscopic diagnostics. Title: Modeling the X-ray emission from jets observed with Chandra Authors: Bonito, Rosaria; Orlando, S.; Peres, G.; Favata, F.; Miceli, M.; Eisloffel, J. Bibcode: 2009cfdd.confE...2F Altcode: In the last decade, X-rays from jets have been discovered thanks to the unprecedented resolving power of Chandra. The first convincing evidence of X-ray jets came from HH2 (2000), HH154 following shortly thereafter. While more sources were detected later, HH154 remains a unique source being the nearest and the most luminous among the nearest jets: the details of the relevant X-ray morphology can be studied to a level impossible with more distant objects.Our group has investigated the mechanisms of X-ray emission from jets both analyzing multi-epochs Chandra data of HH154 and developing numerical models of jets where X-rays are generated by jets shocking onto the circumstellar medium. The analysis of Chandra data of HH 154 in different epochs allowed us to study for the first time the morphology and the evolution of the X-ray source on a time scale of 4years. We found that the source consists of an unresolved, point-like component with no detectable proper motion and an elongated component with a proper motion consistent with a shock moving away from the parent star.We present here the comparison between the Chandra observations of HH jets and our hydrodynamic model of a randomly ejected pulsed jet which reproduces the knotty morphology observed. Title: Modeling SNR shock waves expanding through the magnetized inhomogeneous interstellar medium Authors: Orlando, S.; Bocchino, F.; Miceli, M.; Reale, F.; Peres, G. Bibcode: 2009NCimC..32b..45O Altcode: No abstract at ADS Title: VizieR Online Data Catalog: Photometry of candidate members of NGC6611 (Guarcello+, 2009) Authors: Guarcello, M. G.; Micela, G.; Damiani, F.; Peres, G.; Prisinzano, L.; Sciortino, S. Bibcode: 2009yCat..34960453G Altcode: Optical BVI photometry have been obtained from 2.2m WFI@ESO observations; JHK data are from 2MASS Point Sources Catalog; IRAC data are from GLIMPSE catalog. X-ray data have been obtained from a CHANDRA/ACIS-I observation of a field o f 17'x17' centered on the cluster.

(1 data file). Title: Correlation between the spatial distribution of circumstellar disks and massive stars in the young open cluster NGC 6611. II. Cluster members selected with Spitzer/IRAC Authors: Guarcello, M. G.; Micela, G.; Damiani, F.; Peres, G.; Prisinzano, L.; Sciortino, S. Bibcode: 2009A&A...496..453G Altcode: 2008arXiv0812.0945G Context: The observations of the proplyds in the Orion Nebula Cluster, exhibiting clear evidence of ongoing photoevaporation, have provided clear proof of the role of externally induced photoevaporation in the evolution of circumstellar disks. NGC 6611 is an open cluster suitable for study of disk photoevaporation, due to its significant population of massive members and stars with disk. In a previous paper, we obtained evidence of the influence of the strong UV field generated by the massive cluster members on the evolution of disks around low-mass Pre-Main Sequence members. Our study was based on a multiband BVIJHK and X-ray catalog compiled for the purpose of selecting cluster members with and without disk.
Aims: We attempt to complete the list of candidate cluster members, using data at longer wavelengths obtained with Spitzer/IRAC, and we reinvestigate the effects of UV radiation on the evolution of disks in NGC 6611.
Methods: In a field of view of 33'×34' centered on the cluster, we select the candidate members with disks of NGC 6611 using IRAC color-color diagrams and suitable reddening-free color indices. Using the X-ray data to select Class III cluster members, we also estimate disks frequency relative to the intensity of the incident radiation emitted by massive members.
Results: We identify 458 candidate members with circumstellar disks, among which 146 had not been discovered previously. By comparing all color indices used to select cluster members with disk, we claim that these indices measure the excess of radiation due to the emission of the same physical region of the disk (i.e. the inner rim at the dust sublimation radius). Our new results confirm that UV radiation from massive stars affects the evolution of nearby circumstellar disks.

Optical-infrared catalog of the candidate members of the open cluster NGC 6611 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/496/453 Title: The Sun as a benchmark of flaring activity in stellar coronae Authors: Argiroffi, C.; Peres, G.; Orlando, S.; Reale, F. Bibcode: 2009AIPC.1094..200A Altcode: 2009csss...15..200A The solar corona is a template to study and understand stellar activity. However the solar corona differs from that of active stars: the Sun has lower X-ray luminosity, and on average cooler plasma temperatures. Active stellar coronae have a hot peak in their emission measure distribution, EM (T), at 8-20 MK, while the non-flaring solar corona has a peak at 1-2 MK. In the solar corona significant amounts of plasma at temperature ~10 MK are observed only during flares.

To investigate what is the time-averaged effect of solar flares we measure the disk-integrated time-averaged emission measure, EMF (T), of an unbiased sample of solar flares. To this aim we analyze uninterrupted GOES/XRS light curves over time intervals of one month. We also obtain the EMQ (T) of the quiescent corona for the same time intervals from Yohkoh/SXT data. To investigate variations due to the solar cycle we evaluate EMF (T) and EMQ (T) at different phases of the cycle between December 1991 and April 1998.

Irrespective of the solar cycle phase, EMF (T) appears as a peak in the distribution, and it is significantly larger than the values of EMQ (T) for T~5-10 MK. Adding EMF (T) and EMQ (T) we obtain for the first time a time average EM (T) of the entire solar corona: it is double-peaked, with the hot peak, due to time-averaged flares, being located at temperatures similar to those of active stars, but less enhanced.

In the assumption that the heating of the corona is entirely due to flares, from nano- to macro-flares, a two peak EM (T) distribution suggests that then either the flare distribution or the confined plasma response to flares, or both, are bimodal. Moreover the EMF (T) shape supports the hypothesis that the hot EM (T) peak of active coronae is due to unresolved solar-like flares. If this is the case, quiescent and flare components should follow different scaling laws for increasing stellar activity. Title: Accretion shock on CTTSs and its X-ray emission Authors: Sacco, G. G.; Argiroffi, C.; Orlando, S.; Maggio, A.; Peres, G.; Reale, F. Bibcode: 2009AIPC.1094..329S Altcode: 2009csss...15..329S High spectral resolution X-ray observations of classical T Tauri stars (CTTSs) demonstrate the presence of plasma at T~2-3×106 K and ne~1011-1013 cm-3. Stationary models suggest that this emission is due to shock-heated accreting material. We address this issue by a 1-D hydrodynamic model of the impact of the accretion flow onto a chromosphere of a CTTS with the aim of investigating the stability of accretion shock and the role of the chromosphere. Our simulations include the effects of gravity, radiative losses from optically thin plasma, the thermal conduction and a detailed modeling of the stellar chromosphere. Here we present the results of a simulation based on the parameters of the CTTS MP Mus. Title: The nearest X-ray emitting protostellar jet observed with HST Authors: Bonito, R.; Fridlund, C. V. M.; Favata, F.; Micela, G.; Peres, G.; Djupvik, A. A.; Liseau, R. Bibcode: 2009AIPC.1094..349B Altcode: 2009csss...15..349B The HH 154 jet coming from the YSO binary L1551 IRS5 is one of the closest (about 150 pc) astrophysical jet known. It is therefore a unique laboratory for studies of outflow mechanisms and of the shocks forming at the interaction front between the expanding material and the ambient medium. The substructures (knots) observed within the HH 154 jet were imaged in several spectral bands using the Hubble Space Telescope. This allows us to derive a simple characterization of the physical conditions in different structures as well as to measure the proper motion of the knots in the jet, their flux variability and shock emission over a time base of about ten years. These knots in the jet undergo signicant morphological variations: some of them disappear in a few years and collision between different knots may occur. Our results suggest the presence of a shock front at the base of the jet identified with an internal working surface; more important, the knot at the base of the jet is coincident with the X-ray source discovered in HH 154. In fact the optical data are compared to X-rays data collected with Chandra with the goal of understanding the energetics of this jet, in particular, and of low-mass star jets, in general. Title: The Complex Morphology of the X-ray and Optical Emission from HH 154: The Pulsed Jet Scenario Authors: Bonito, Rosaria; Orlando, Salvatore; Peres, Giovanni; Favata, Fabio; Eislöffel, Jochen Bibcode: 2009ASSP...13..353B Altcode: 2009pjc..book..353B We study the optical and X-ray emission from protostellar jets, focusing, in particular, on the case of HH 154. This project consists of two different and complementary approaches: the development of hydrodynamical models of the jet/ambient interaction, and the analysis of multi-wavelength observations. Comparing the results derived from the simulations with the observations we can infer the physical mechanisms leading to the complex morphology of the X-rays source observed at the base of HH 154. Title: MHD modeling of supernova remnants expanding through inhomogeneous interstellar medium . Authors: Orlando, S.; Bocchino, F.; Miceli, M.; Reale, F.; Peres, G. Bibcode: 2009MSAIS..13...97O Altcode: We model the expansion of a blast wave of a supernova explosion through a magnetized and inhomogeneous interstellar medium (ISM). The aim is to explore the role of an ambient magnetic field and/or a non-uniform ISM in determining the morphology of supernova remnants (SNRs) as observed in different bands. In particular, we investigate: 1) whether the morphology of bilateral SNRs (BSNRs) observed in the radio band is mainly determined by a non-uniform ISM or by a non-uniform ambient magnetic field and 2) the role of thermal conduction and of non-uniform ISM in determining the morphology and physical characteristics of mixed-morphology (MM) SNRs. We present 3-D hydrodynamic and MHD simulations of a spherical SNR shock propagating through an inhomogeneous ISM. We discuss the origin of the asymmetries observed in BSNRs and of the unexpected morphology observed in MM-SNRs. Title: Effects of Flaring Activity on Dynamics of Accretion Disks in YSOs Authors: Yelenina, Tatiana G.; Orlando, Salvatore; Reale, Fabio; Peres, Giovanni; Mignone, Andrea; Matsakos, Titos Bibcode: 2009ASSP...13..631Y Altcode: 2009pjc..book..631Y We investigate the effects of strong flares on the accretion phenomena in YSOs. Among all classical assumptions, the model accounts magnetic-field oriented thermal conduction. We study the global dynamics of the system for two positions of the heating release triggering the flare. Title: Hydrodynamic Modeling of Accretion Shock on CTTSs Authors: Sacco, Germano G.; Argiroffi, Constanza; Orlando, Salvatore; Maggio, Antonio; Peres, Giovanni; Reale, Fabio Bibcode: 2009ASSP...13..607S Altcode: 2009pjc..book..607S High resolution (R ~ 600) X-ray observations of some classical T Tauri stars (CTTSs) (TW Hya, BP Tau, V4046 Sgr, MP Mus and RU Lupi) have shown the presence of X-ray plasma at T ~ 2-3 × 106 K and denser than n e ~ 1011 cm-3 [1, 2, 3, 4, 5], which suggests an origin different from the coronal one (n e ~ 1010 cm_3). Stationary models demonstrated that X-ray emission from CTTSs could also be produced by the accreting material [6]. We address this issue with the aid of a time-dependent hydrodynamic numerical model describing the impact of an accretion stream onto the chromosphere of a CTTS (see [7] for more details). Our simulations include the effects of gravity, radiative losses from optically thin plasma, the thermal conduction and a detailed modeling of the stellar chromosphere. The gravity and the parameters describing the accretion stream (density n e D 1011 cm-3 and velocity v D 450 km s-1) were chosen in order to match the X-ray properties of the CTTSs MP Mus [4]. Title: Round-table discussion Authors: Pasian, F.; Peres, G.; Longo, G. Bibcode: 2009MSAIS..13..125P Altcode: These notes summarise the main topics touched upon during the Round-Table discussion held during the HPC-Grid 08 Workshop, INAF headquarters, March 12th 2008. Title: X-ray emission from dense plasma in classical T Tauri stars: hydrodynamic modeling of the accretion shock Authors: Sacco, G. G.; Argiroffi, C.; Orlando, S.; Maggio, A.; Peres, G.; Reale, F. Bibcode: 2008A&A...491L..17S Altcode: 2008arXiv0810.0192S Context: High spectral resolution X-ray observations of classical T Tauri stars (CTTSs) demonstrate the presence of plasma at temperature T∼ 2-3× 106 K and density n_e∼ 1011-1013 cm-3, which are unobserved in non-accreting stars. Stationary models suggest that this emission is due to shock-heated accreting material, but do not allow us to analyze the stability of the material and its position in the stellar atmosphere.
Aims: We investigate the dynamics and stability of shock-heated accreting material in classical T Tauri stars and the role of the stellar chromosphere in determining the position and thickness of the shocked region.
Methods: We perform one-dimensional hydrodynamic simulations of the impact of an accretion flow on the chromosphere of a CTTS, including the effects of gravity, radiative losses from optically thin plasma, thermal conduction and a well tested detailed model of the stellar chromosphere. We present the results of a simulation based on the parameters of the CTTS MP Mus.
Results: We find that the accretion shock generates an hot slab of material above the chromosphere with a maximum thickness of 1.8 × 109 cm, density n_e∼ 1011-1012 cm-3, temperature T∼ 3× 106 K, and uniform pressure equal to the ram pressure of the accretion flow (~450 dyn cm-2). The base of the shocked region penetrates the chromosphere and remains at a position at which the ram pressure is equal to the thermal pressure. The system evolves with quasi-periodic instabilities of the material in the slab leading to cyclic disappearance and re-formation of the slab. For an accretion rate of ~10-10~M⊙ yr-1, the shocked region emits a time-averaged X-ray luminosity of L_X≈ 7× 1029 erg s-1, which is comparable with the X-ray luminosity observed in CTTSs of identical mass. Furthermore, the X-ray spectrum synthesized from the simulation reproduces in detail all the main features of the O VIII and O VII lines of the star MP Mus. Title: The flaring and quiescent components of the solar corona Authors: Argiroffi, C.; Peres, G.; Orlando, S.; Reale, F. Bibcode: 2008A&A...488.1069A Altcode: 2008arXiv0805.2685A Context: The solar corona is a template to understand stellar activity. The Sun is a moderately active star, and its corona differs from that of active stars: for instance, active stellar coronae have a double-peaked emission measure distribution EM(T) with a hot peak at 8-20 MK, while the non-flaring solar corona has one peak at 1-2 MK and, typically, much cooler plasma.
Aims: We study the average contribution of flares to the solar emission measure distribution to investigate indirectly the hypothesis that the hot peak in the EM(T) of active stellar coronae is due to a large number of unresolved solar-like flares, and to infer properties about the flare distribution from nano- to macro-flares.
Methods: We measure the disk-integrated time-averaged emission measure, EM_F(T), of an unbiased sample of solar flares, analyzing uninterrupted GOES/XRS light curves over time intervals of one month. We obtain the EM_Q(T) of quiescent corona for the same time intervals from Yohkoh/SXT data. To investigate how EM_F(T) and EM_Q(T) vary during the solar cycle, we evaluate them at different phases of the cycle between December 1991 and April 1998.
Results: Irrespective of the solar cycle phase, EM_F(T) appears as a peak in the distribution, and it is significantly larger than the values of EM_Q(T) for T∼5-10 MK. As a result, the time-averaged EM(T) of the entire solar corona is double-peaked, with the hot peak, due to time-averaged flares, being located at temperatures similar to those of active stars, but less enhanced. The EM_F(T) shape supports the hypothesis that the hot EM(T) peak of active coronae is due to unresolved solar-like flares. If this is the case, quiescent and flare components should follow different scaling laws for increasing stellar activity. In the assumption that the heating of the corona is entirely due to flares, from nano- to macro-flares, then either the flare distribution or the confined plasma response to flares, or both, are bimodal. Title: Hinode/XRT Diagnostics of Loop Thermal Structure Authors: Reale, F.; Parenti, S.; Reeves, K. K.; Weber, M.; Bobra, M. G.; Barbera, M.; Kano, R.; Narukage, N.; Shimojo, M.; Sakao, T.; Peres, G.; Golub, L. Bibcode: 2008ASPC..397...50R Altcode: We investigate possible diagnostics of the thermal structure of coronal loops from Hinode/XRT observations made with several filters. We consider the observation of an active region with five filters. We study various possible combinations of filter data to optimize for sensitivity to thermal structure and for signal enhancement. Title: The nearest X-ray emitting protostellar jet (HH 154) observed with Hubble Authors: Bonito, R.; Fridlund, C. V. M.; Favata, F.; Micela, G.; Peres, G.; Djupvik, A. A.; Liseau, R. Bibcode: 2008A&A...484..389B Altcode: Context: The jet coming from the YSO binary L1551 IRS5 is the closest astrophysical jet known. It is therefore a unique laboratory for studies of outflow mechanisms and of the shocks occurring when expanding material hits the ambient medium as well as of how the related processes influence the star- (and planet-) forming process.
Aims: The optical data are related to other data covering the spectrum from the optical band to X-rays with goal of understanding the energetics of low-mass star jets, in general, and of this jet in particular. We study the time evolution of the jet, by measuring the proper motions of knots as they progress outwards from the originating source.
Methods: The nebulosities associated with the jet(s) from the protostellar binary L1551 IRS5 were imaged in a number of spectral bands using the Hubble Space Telescope. This allows the proper motion to be measured and permits a simple characterization of the physical conditions in different structures. To this end we developed a reproducible method of data analysis, which allows us to define the position and shape of each substructure observed within the protostellar jet. Using this approach, we derive the proper motion of the knots in the jet, as well as their flux variability and shock emission.
Results: The time base over which HST observations were carried out is now about ten years. The sub-structures within the jet undergo significant morphological variations: some knots seem to disappear in a few years and collision between different knots, ejected at different epochs and maybe with different speed, may occur. The velocities along the jet vary between ~100 km s-1 and over 400 km s-1, with the highest speed corresponding to the knots at the base of the jet.
Conclusions: There are indications that the HH 154 jet has been active relatively recently. Our results suggest the presence of a new shock front at the base of the jet identified with an internal working surface. From the analysis of the terminal and internal working surfaces within the jet, we find that the more likely scenario for the HH 154 jet is that of a jet traveling through a denser ambient medium (a “light jet”). These results are consistent with the Bonito et al. (2007) model predictions. Furthermore, there is strong evidence that the knots at the base of the northern jet correspond to the location where the highest velocity and the highest excitation component are measured along the jet. More important, this is the location where the X-ray source has been discovered.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programs #6127, #6411 & #10351. Title: The Importance of Magnetic-Field-Oriented Thermal Conduction in the Interaction of SNR Shocks with Interstellar Clouds Authors: Orlando, S.; Bocchino, F.; Reale, F.; Peres, G.; Pagano, P. Bibcode: 2008ApJ...678..274O Altcode: 2008arXiv0801.1403O We explore the importance of magnetic-field-oriented thermal conduction in the interaction of supernova remnant (SNR) shocks with radiative gas clouds and in determining the mass and energy exchange between the clouds and the hot surrounding medium. We perform 2.5-dimensional MHD simulations of a shock impacting on an isolated gas cloud, including anisotropic thermal conduction and radiative cooling; we consider the representative case of a Mach 50 shock impacting on a cloud 10 times denser than the ambient medium. We consider different configurations of the ambient magnetic field and compare MHD models with or without thermal conduction. The efficiency of thermal conduction in the presence of a magnetic field is, in general, reduced with respect to the unmagnetized case. The reduction factor strongly depends on the initial magnetic field orientation, and it is at a minimum when the magnetic field is initially aligned with the direction of the shock propagation. Thermal conduction contributes to the suppression of hydrodynamic instabilities, reducing the mass mixing of the cloud and preserving the cloud from complete fragmentation. Depending on the magnetic field orientation, the heat conduction may determine a significant energy exchange between the cloud and the hot surrounding medium which, while remaining always at levels less than those in the unmagnetized case, leads to a progressive heating and evaporation of the cloud. This additional heating may offset the radiative cooling of some parts of the cloud, preventing the onset of thermal instabilities. Title: A coronal explosion on the flare star CN Leonis Authors: Schmitt, J. H. M. M.; Reale, F.; Liefke, C.; Wolter, U.; Fuhrmeister, B.; Reiners, A.; Peres, G. Bibcode: 2008A&A...481..799S Altcode: 2008arXiv0801.3752S We present simultaneous high-temporal and high-spectral resolution observations of the nearby flare star CN Leo at optical and soft X-ray wavelengths. During our observing campaign a major flare occurred, raising the star's instantaneous energy output by almost three orders of magnitude. The flare shows the often observed impulsive behavior, with a rapid rise and slow decay in the optical and a broad soft X-ray maximum about 200 seconds after the optical flare peak. In addition to this usually encountered flare phenomenology we find, however, an extremely short (τ _dec ≈ 2 s) soft X-ray peak, which is very likely of thermal, rather than nonthermal nature and coincides temporally with the optical flare peak. While at hard X-ray energies nonthermal bursts are routinely observed on the Sun at flare onset, thermal soft X-ray bursts on time scales of seconds have never been observed in a solar, nor stellar context. Time-dependent, one-dimensional hydrodynamic modeling of this event requires an extremely short energy deposition time scale τ _dep of a few seconds to reconcile theory with observations, thus suggesting that we are witnessing the results of a coronal explosion on CN Leo. Thus the flare on CN Leo provides the opportunity to observationally study the physics of the long-sought “micro-flares” thought to be responsible for coronal heating. Title: Numerical Simulations and Diagnostics in Astrophysics: A few Magnetohydrodynamics Examples Authors: Peres, G.; Bonito, R.; Orlando, S.; Reale, F. Bibcode: 2008mss..conf...66P Altcode: No abstract at ADS Title: Fine Thermal Structure of a Coronal Active Region Authors: Reale, Fabio; Parenti, Susanna; Reeves, Kathy K.; Weber, Mark; Bobra, Monica G.; Barbera, Marco; Kano, Ryouhei; Narukage, Noriyuki; Shimojo, Masumi; Sakao, Taro; Peres, Giovanni; Golub, Leon Bibcode: 2007Sci...318.1582R Altcode: The determination of the fine thermal structure of the solar corona is fundamental to constraining the coronal heating mechanisms. The Hinode X-ray Telescope collected images of the solar corona in different passbands, thus providing temperature diagnostics through energy ratios. By combining different filters to optimize the signal-to-noise ratio, we observed a coronal active region in five filters, revealing a highly thermally structured corona: very fine structures in the core of the region and on a larger scale further away. We observed continuous thermal distribution along the coronal loops, as well as entangled structures, and variations of thermal structuring along the line of sight. Title: VizieR Online Data Catalog: BVI photometry in NGC 6611 (Guarcello+, 2007) Authors: Guarcello, M. G.; Prisinzano, L.; Micela, G.; Damiani, F.; Peres, G.; Sciortino, S. Bibcode: 2007yCat..34620245G Altcode: The optical data have been taken in the BVI bands with the Wield Field Camera (WFI), mounted on the 2.2m telescope of the European Southern Observatory (ESO) in La Silla (Chile). The optical images used in this work were taken on 29 July 2000.

(1 data file). Title: On X-Ray Optical Depth in the Coronae of Active Stars Authors: Testa, Paola; Drake, Jeremy J.; Peres, Giovanni; Huenemoerder, David P. Bibcode: 2007ApJ...665.1349T Altcode: 2007arXiv0706.4080T We have investigated the optical thickness of the coronal plasma through the analysis of high-resolution X-ray spectra of a large sample of active stars observed with the High Energy Transmission Grating Spectrometer on Chandra. In particular, we probed for the presence of significant resonant scattering in the strong Lyman series lines arising from hydrogen-like oxygen and neon ions. The active RS CVn-type binaries II Peg and IM Peg and the single M dwarf EV Lac show significant optical depth. For these active coronae, the Lyα/Lyβ ratios are significantly depleted as compared with theoretical predictions and with the same ratios observed in similar active stars. Interpreting these decrements in terms of resonance scattering of line photons out of the line of sight, we are able to derive an estimate for the typical size of coronal structures, and from these we also derive estimates of coronal filling factors. For all three sources we find that both the photon path length as a fraction of the stellar radius and the implied surface filling factors are very small and amount to a few percent at most. The measured Lyα/Lyβ ratios are in good agreement with APED theoretical predictions, thus indicating negligible optical depth, for the other sources in our sample. We discuss the implications for coronal structuring and heating flux requirements. For the stellar sample as a whole, the data suggest increasing quenching of Lyα relative to Lyβ as a function of both LX/Lbol and the density-sensitive Mg XI forbidden-to-intercombination line ratio, as might generally be expected. Title: On the origin of asymmetries in bilateral supernova remnants Authors: Orlando, S.; Bocchino, F.; Reale, F.; Peres, G.; Petruk, O. Bibcode: 2007A&A...470..927O Altcode: 2007arXiv0704.0890O Aims:We investigate whether the morphology of bilateral supernova remnants (BSNRs) observed in the radio band is determined mainly either by a non-uniform interstellar medium (ISM) or by a non-uniform ambient magnetic field.
Methods: We perform 3D MHD simulations of a spherical SNR shock propagating through a magnetized ISM. Two cases of shock propagation are considered: 1) through a gradient of ambient density with a uniform ambient magnetic field; 2) through a homogeneous medium with a gradient of ambient magnetic field strength. From the simulations, we synthesize the synchrotron radio emission, making different assumptions about the details of acceleration and injection of relativistic electrons.
Results: We find that asymmetric BSNRs are produced if the line-of-sight is not aligned with the gradient of ambient plasma density or with the gradient of ambient magnetic field strength. We derive useful parameters to quantify the degree of asymmetry of the remnants that may provide a powerful diagnostic of the microphysics of strong shock waves through the comparison between models and observations.
Conclusions: BSNRs with two radio limbs of different brightness can be explained if a gradient of ambient density or, most likely, of ambient magnetic field strength is perpendicular to the radio limbs. BSNRs with converging similar radio arcs can be explained if the gradient runs between the two arcs. Title: Recent X-ray studies of stellar cycles and long-term variability Authors: Peres, Giovanni; Micela, Giuseppina; Favata, Fabio Bibcode: 2007HiA....14..287P Altcode: We discuss recent X-ray studies of stellar cycles and long-term variability. Title: The X-Ray Telescope (XRT) for the Hinode Mission Authors: Golub, L.; DeLuca, E.; Austin, G.; Bookbinder, J.; Caldwell, D.; Cheimets, P.; Cirtain, J.; Cosmo, M.; Reid, P.; Sette, A.; Weber, M.; Sakao, T.; Kano, R.; Shibasaki, K.; Hara, H.; Tsuneta, S.; Kumagai, K.; Tamura, T.; Shimojo, M.; McCracken, J.; Carpenter, J.; Haight, H.; Siler, R.; Wright, E.; Tucker, J.; Rutledge, H.; Barbera, M.; Peres, G.; Varisco, S. Bibcode: 2007SoPh..243...63G Altcode: The X-ray Telescope (XRT) of the Hinode mission provides an unprecedented combination of spatial and temporal resolution in solar coronal studies. The high sensitivity and broad dynamic range of XRT, coupled with the spacecraft's onboard memory capacity and the planned downlink capability will permit a broad range of coronal studies over an extended period of time, for targets ranging from quiet Sun to X-flares. This paper discusses in detail the design, calibration, and measured performance of the XRT instrument up to the focal plane. The CCD camera and data handling are discussed separately in a companion paper. Title: X-ray emission from MP Muscae: an old classical T Tauri star Authors: Argiroffi, C.; Maggio, A.; Peres, G. Bibcode: 2007A&A...465L...5A Altcode: 2007astro.ph..1765A Aims:We study the properties of X-ray emitting plasma of MP Mus, an old classical T Tauri star. We check whether an accretion process could produce the observed X-ray emission and we derive the accretion parameters and the characteristics of the shock-heated plasma. We compare the properties of MP Mus with those of younger classical T Tauri stars to test whether age is related to the properties of the X-ray emitting plasma.
Methods: XMM-Newton X-ray spectra allow us to measure plasma temperatures, abundances, and electron density. The density of cool plasma probes whether X-ray emission is produced by plasma heated in the accretion process.
Results: X-ray emission from MP Mus originates from high density cool plasma but a hot flaring component is also present, suggesting that both coronal magnetic activity and accretion contribute to the observed X-ray emission. We find a Ne/O ratio similar to that observed in the much younger classical T Tauri star BP Tau. From the soft part of the X-ray emission, mostly produced by plasma heated in the accretion shock, we derive a mass accretion rate of 5×10-11 M_⊙ yr-1. Title: MHD evolution of a fragment of a CME core in the outer solar corona Authors: Pagano, P.; Reale, F.; Orlando, S.; Peres, G. Bibcode: 2007A&A...464..753P Altcode: Context: Detailed hydrodynamic modeling explained several features of a fragment of the core of a Coronal Mass Ejection observed with SoHO/UVCS at 1.7 R⊙ on 12 December 1997, but some questions remained unsolved.
Aims: We investigate the role of the magnetic fields in the thermal insulation and the expansion of an ejected fragment (cloud) traveling upwards in the outer corona.
Methods: We perform MHD simulations including the effects of thermal conduction and radiative losses of a dense spherical or cylindrical cloud launched upwards in the outer corona, with various assumptions on the strength and topology of the ambient magnetic field; we also consider the case of a cylindrical cloud with an internal magnetic field component along its axis.
Results: We find that a weak ambient magnetic field (β ∼ 20) with open topology provides both significant thermal insulation and large expansion. The cylindrical cloud expands more than the spherical one. Title: X-rays from protostellar jets: emission from continuous flows Authors: Bonito, R.; Orlando, S.; Peres, G.; Favata, F.; Rosner, R. Bibcode: 2007A&A...462..645B Altcode: 2006astro.ph.10507B Context: Recently X-ray emission from protostellar jets has been detected with both XMM-Newton and Chandra satellites, but the physical mechanism which can give rise to this emission is still unclear.
Aims: We performed an extensive exploration of the parameter space for the main parameters influencing the jet/ambient medium interaction. Aims include: 1) to constrain the jet/ambient medium interaction regimes leading to the X-ray emission observed in Herbig-Haro objects in terms of the emission by a shock forming at the interaction front between a continuous supersonic jet and the surrounding medium; 2) to derive detailed predictions to be compared with optical and X-ray observations of protostellar jets; 3) to get insight into the protostellar jet's physical conditions.
Methods: We performed a set of two-dimensional hydrodynamic numerical simulations, in cylindrical coordinates, modeling supersonic jets ramming into a uniform ambient medium. The model takes into account the most relevant physical effects, namely thermal conduction and radiative losses.
Results: Our model explains the observed X-ray emission from protostellar jets in a natural way. In particular, we find that a protostellar jet that is less dense than the ambient medium well reproduces the observations of the nearest Herbig-Haro object, HH 154, and allows us to make detailed predictions of a possible X-ray source proper motion (v_sh ≈500 km s-1) detectable with Chandra. Furthermore, our results suggest that the simulated protostellar jets which best reproduce the X-rays observations cannot drive molecular outflows. Title: MHD evolution of a fragment of a CME core in the outer solar corona Authors: Pagano, P.; Reale, F.; Orlando, S.; Peres, G. Bibcode: 2007astro.ph..1145P Altcode: Detailed hydrodynamic modeling explained several features of a fragment of the core of a Coronal Mass Ejection observed with SoHO/UVCS at 1.7 Ro on 12 December 1997, but some questions remained unsolved. We investigate the role of the magnetic fields in the thermal insulation and the expansion of an ejected fragment (cloud) traveling upwards in the outer corona. We perform MHD simulations including the effects of thermal conduction and radiative losses of a dense spherical or cylindrical cloud launched upwards in the outer corona, with various assumptions on the strength and topology of the ambient magnetic field; we also consider the case of a cylindrical cloud with an internal magnetic field component along its axis. We find that a weak ambient magnetic field (beta~20) with open topology provides both significant thermal insulation and large expansion. The cylindrical cloud expands more than the spherical one. Title: Magnetic activity and the solar corona: first results from the Hinode satellite . Authors: Reale, Fabio; Parenti, Susanna; Reeves, Kathy K.; Weber, Mark; Bobra, Monica G.; Barbera, Marco; Kano, Ryohei; Narukage, Noriyuki; Shimojo, Masumi; Sakao, Taro; Peres, Giovanni; Golub, Leon Bibcode: 2007MmSAI..78..591R Altcode: The structure, dynamics and evolution of the solar corona are governed by the magnetic field. In spite of significant progresses in our insight of the physics of the solar corona, several problems are still under debate, e.g. the role of impulsive events and waves in coronal heating, and the origin of eruptions, flares and CMEs. The Hinode mission has started on 22 september 2006 and aims at giving new answers to these questions. The satellite contains three main instruments, two high resolution telescopes, one in the optical and one in the X-ray band, and an EUV imaging spectrometer. On the Italian side, INAF/Osservatorio Astronomico di Palermo has contributed with the ground-calibration of the filters of the X-ray telescope. We present some preliminary mission results, with particular attention to the X-ray telescope data. Title: VizieR Online Data Catalog: X-ray variability of NGC 2516 stars in the XMM-Newton observations. Authors: Marino, A.; Micela, G.; Pillitteri, I.; Peres, G. Bibcode: 2007yCat..34560977M Altcode: We present the characteristics of the X-ray variability of stars in the cluster NGC 2516 as derived from XMM-Newton/EPIC/pn data. The X-ray variations on short (hours), medium (months), and long (years) time scales have been explored. We detected 303 distinct X-ray sources by analysing six EPIC/pn observations; 194 of them are members of the cluster. Stars of all spectral types, from the early-types to the late-M dwarfs, were detected.

(2 data files). Title: Correlation between the spatial distribution of circumstellar disks and massive stars in the open cluster NGC 6611. Compiled catalog and cluster parameters Authors: Guarcello, M. G.; Prisinzano, L.; Micela, G.; Damiani, F.; Peres, G.; Sciortino, S. Bibcode: 2007A&A...462..245G Altcode: 2006astro.ph.10401G Context: The observation of young stars with circumstellar disks suggests that the disks are dissipated, starting from the inner region, by the radiation of the central star and eventually by the formation of rocky planetesimals, over a time scale of several million years. It was also shown that strong UV radiation emitted by nearby massive stars can heat a circumstellar disk up to some thousand degrees, inducing the photoevaporation of the gas. This process strongly reduces the dissipation time scale.
Aims: We study whether there exists a correlation between the spatial distribution of stars with circumstellar disks and the position of massive stars with spectral class earlier than B5, in the open cluster NGC 6611.
Methods: We created a multiband catalog of the cluster, down to V∼ 23^m, using optical data from a WFI observation at 2.2 m of ESO in the BVI bands, the 2MASS public point source catalog and an archival X-ray observation made with CHANDRA/ACIS. We selected the stars with infrared excess (due to the emission of a circumstellar disk) using suitable color indices independent of extinction, and studied their spatial distribution.
Results: The spatial distribution of the stars with K band excess (due to the presence of a circumstellar disk) is anti correlated with that of the massive stars: the disks are more frequent at large distances from these stars. We argue that this is in agreement with the hypothesis that the circumstellar disks are heated by the UV radiation from the massive stars and photoevaporated.

Based on observations made with the European Observatory telescopes obtained from the ESO/ST-ECF Science Archive Facility. Table 3 and Appendix are only available in electronic form at http://www.aanda.org Title: XMM-Newton survey of two upper Scorpius regions Authors: Argiroffi, C.; Favata, F.; Flaccomio, E.; Maggio, A.; Micela, G.; Peres, G.; Sciortino, S. Bibcode: 2006A&A...459..199A Altcode: 2006astro.ph..7298A Aims.We studied X-ray emission from young stars by analyzing the deep XMM-Newton observations of two regions of the Upper Scorpius association with an age of 5 Myr.
Methods: .Based on near infrared and optical photometry we identified 22 Upper Scorpius photometric members among the 224 detected X-ray sources. We derived coronal properties of Upper Scorpius stars by performing X-ray spectral and timing analyses. The study of four strong and isolated stellar flares allowed us to derive the length of the flaring loops.
Results: .Among the 22 Upper Scorpius stars, 13 were identified as Upper Scorpius photometric members for the first time. The sample includes 7 weak-line T Tauri stars and 1 classical T Tauri star, while the nature of the remaining sources is unknown. Except for the intermediate mass star HD 142578, all the detected USco sources are low-mass stars of spectral types ranging from G to late M. The X-ray emission spectrum of the most intense Upper Scorpius sources indicates metal depleted plasma with temperature of ~10 MK, resembling the typical coronal emission of active main sequence stars. At least 59% of the detected members of the association have variable X-ray emission, and the flaring coronal structures appear shorter than or comparable to the stellar radii already at the Upper Scorpius age. We also found indications of increasing plasma metallicity (up to a factor 20) during strong flares. We identified a new galaxy cluster among the 224 X-ray source detected: the X-ray spectrum of its intra cluster medium indicates a redshift of ~0.41 ± 0.02.
Title: Crushing of interstellar gas clouds in supernova remnants. II. X-ray emission Authors: Orlando, S.; Bocchino, F.; Peres, G.; Reale, F.; Plewa, T.; Rosner, R. Bibcode: 2006A&A...457..545O Altcode: 2006astro.ph..7252O Context: .X-ray observations of evolved supernova remnants (e.g. the Cygnus loop and the Vela SNRs) reveal emission originating from the interaction of shock waves with small interstellar gas clouds.
Aims: .We study and discuss the time-dependent X-ray emission predicted by hydrodynamic modeling of the interaction of a SNR shock wave with an interstellar gas cloud. The scope includes: 1) to study the correspondence between modeled and X-ray emitting structures, 2) to explore two different physical regimes in which either thermal conduction or radiative cooling plays a dominant role, and 3) to investigate the effects of the physical processes at work on the emission of the shocked cloud in the two different regimes.
Methods: .We use a detailed hydrodynamic model, including thermal conduction and radiation, and explore two cases characterized by different Mach numbers of the primary shock: M= 30 (post-shock temperature T_psh ≈ 1.7 MK) in which the cloud dynamics is dominated by radiative cooling and M= 50 (T_psh ≈ 4.7 MK) dominated by thermal conduction. From the simulations, we synthesize the expected X-ray emission, using available spectral codes.
Results: .The morphology of the X-ray emitting structures is significantly different from that of the flow structures originating from the shock-cloud interaction. The hydrodynamic instabilities are never clearly visible in the X-ray band. Shocked clouds are preferentially visible during the early phases of their evolution. Thermal conduction and radiative cooling lead to two different phases of the shocked cloud: a cold cooling dominated core emitting at low energies and a hot thermally conducting corona emitting in the X-ray band. The thermal conduction makes the X-ray image of the cloud smaller, more diffuse, and shorter-lived than that observed when thermal conduction is neglected.
Title: VizieR Online Data Catalog: XMM-Newton survey of two Upper Scorpius regions (Argiroffi+ 2006) Authors: Argiroffi, C.; Favata, F.; Flaccomio, E.; Maggio, A.; Micela, G.; Peres, G.; Sciortino, S. Bibcode: 2006yCat..34590199A Altcode: Portions of the Upper Scorpius association were observed with the Curtis Schmidt (CTIO) telescope, with the Danish 1.54m (ESO) telescope, and with the XMM-Newton satellite. The tables contain all the sources detected with the different instruments, and their characteristics.

(4 data files). Title: X-ray variability of NGC 2516 stars in the XMM-Newton observations Authors: Marino, A.; Micela, G.; Pillitteri, I.; Peres, G. Bibcode: 2006A&A...456..977M Altcode: 2008arXiv0801.2300M We present the characteristics of the X-ray variability of stars in the cluster NGC 2516 as derived from XMM-Newton/EPIC/pn data. The X-ray variations on short (hours), medium (months), and long (years) time scales have been explored. We detected 303 distinct X-ray sources by analysing six EPIC/pn observations; 194 of them are members of the cluster. Stars of all spectral types, from the early-types to the late-M dwarfs, were detected. The Kolmogorov-Smirnov test applied to the X-ray photon time series shows that, on short time scales, only a relatively small fraction (ranging from 6% to 31% for dG and dF, respectively) of the members of NGC 2516 are variable with a confidence level ≥99%; however, it is possible that the fraction is small only because of the poor statistics. The time X-ray amplitude distribution functions (XAD) of a set of dF7-dK2 stars, derived on short (hours) and medium (months) time scales, seem to suggest that medium-term variations, if present, have a much smaller amplitude than those on short time scales; a similar result is also obtained for dK3-dM stars. The amplitude variations of late-type stars in NGC 2516 are consistent with those of the coeval Pleiades stars. Comparing these data with those of ROSAT/PSPC, collected 7-8 years earlier, and of ROSAT/HRI, just 4-5 years earlier, we find no evidence of significant variability on the related time scales, suggesting that long-term variations due to activity cycles similar to the solar cycle are not common among young stars. Indications of spectral variability was found in one star whose spectra at three epochs were available. Title: Recent X-ray Observations of Stellar Cycles and Long Term Variability Authors: Peres, G.; Micela, G.; et al. Bibcode: 2006IAUJD...8E..63P Altcode: We will discuss some aspects relevant to the detection of coronal activity cycles in late-type stars. The effects of the activity solar cycle manifest themselves in many bands, and most notably in the X-ray band. Since strong X-ray emission and other forms of activity are very evident in active late-type stars, one would expect detection of analogous X-ray cycles on these stars. However this is not the case. So to which extent can we apply the Solar-stellar connection in the coronal context? Certainly the Sun fits in the general late-type-stars "trend" of activity vs. rotation, age, temperature, flux etc.; on the other side extreme stellar activity shows "saturation" in rapidly rotating stars and the above "trend" may change significantly or break for very active stars, suggesting the action of a mechanism different from the solar-type dynamo. In this context, proving the presence of coronal solar-like cycles and determining their characteristics, hopefully for a large stellar sample, would provide fundamental tests. Detecting stellar coronal cycles is difficult, given the limited availability of present day X-ray telescopes for this purpose. Also, since X-ray observations aimed at cycle determinations cover time intervals much shorter than cycles themselves, and separated by years, it is hard to disentangle the mix of short term variability and cycles. Nonetheless there is some evidence of long term variability in some samples and in relatively old solar-mass stars (but not in young solar-mass stars neither in M stars). Some projects dedicated to find X-ray cycles using present day telescopes (e.g. those driven by Schmitt and by Favata) have started yielding results. A possibly more fruitful approach to detecting X-ray cycles is devising a relatively small X-ray satellite entirely dedicated to a long observing program to monitor active stars, like SADE (recently proposed by P. Martens and collaborators). One of the important goals of this research would be tracing the evolutionary path of the cyclic activity and of dynamo physics by measuring the activity of solar analogs in different stages of their life-cycles and for different stellar masses. Title: The Corona of the Sun as a Star Authors: Peres, Giovanni; Orlando, Salvatore; Reale, Fabio Bibcode: 2006AIPC..848...41P Altcode: We study the physics of the solar corona as a whole, i.e. of the Sun as a Star, in order to understand its global features and to provide a template for stellar coronae. In this process we strive to understand the features of various structures which compose the solar corona. This process in not straightforward given the problems of observing the Sun as a whole: e.g., no recent X-ray wide-band, medium-resolution, spectrum of the Sun is avaible, unlike stars and no X-ray spectral monitoring of the Sun at various activity phases is available. The presentation will discuss our work in this field; we present the method we have devised, based on Yohkoh/SXT data, to derive the Differential Emission Measure vs. Temperature, and the X-ray spectrum, of the solar corona, and the related main results. Finally we present some new results on flares and microflares and the implications on solar-stellar coronal heating. Title: VizieR Online Data Catalog: XMM spectroscopy of TWA 5 (Argiroffi+, 2005) Authors: Argiroffi, C.; Maggio, A.; Peres, G.; Stelzer, B.; Neuhaeuser, R. Bibcode: 2006yCat..34391149A Altcode: TWA 5 was observed for ~30ks with XMM-Newton on January 9, 2003.

(1 data file). Title: The discovery of an expanding X-ray source in the HH 154 protostellar jet Authors: Favata, F.; Bonito, R.; Micela, G.; Fridlund, M.; Orlando, S.; Sciortino, S.; Peres, G. Bibcode: 2006A&A...450L..17F Altcode: 2006astro.ph..3186F Context: .Protostellar jets are a new class of X-ray sources which has been discovered with both XMM-Newton and Chandra. The mechanism responsible for the X-ray emission is still not clear. Self-shocking in jets, shocks where the jet hits the surrounding medium, reflected or scattered stellar X-ray emission have all been invoked as possible explanations.
Aims: .One key diagnostic discriminating among physical emission mechanisms is the motion of the X-ray source: hydrodynamical numerical models of continuous protostellar jets plowing through a uniform medium show an X-ray emitting shock front moving at several hundreds km s-1. In the nearest X-ray emitting protostellar jet, HH 154, this is detectable, with the spatial resolution of the Chandra X-ray observatory, over a few years baseline, allowing a robust discrimination among different mechanisms.
Methods: .We have performed, in October 2005, a deep Chandra X-ray observation of HH 154. Comparison with the previous (2001) Chandra observation allows to detect proper motion down to the level predicted by models of X-ray emitting shocks in the jet.
Results: .The 2005 Chandra observation of HH 154 shows unexpected morphological changes of the X-ray emission in comparison with the 2001 data. Two components are present: a stronger, point-like component with no detectable motion and a weaker component which has expanded in size by approximately 300 AU over the 4 years time base of the two observations. This expansion corresponds to approximately 500 km s-1, very close to the velocity of the X-ray emitting shock in the simple theoretical models.
Conclusions: .The 2005 data show a more complex system than initially thought (and modeled), with multiple components with different properties. The observed morphology is possibly indicating a pulsed jet propagating through a non-homogeneous medium, likely with medium density decreasing with distance from the driving source. Detailed theoretical modeling and deeper X-ray observations will be needed to understand the physics of this fascinating class of sources. Title: VizieR Online Data Catalog: XMM observations of IC 2391 stars (Marino+, 2005) Authors: Marino, A.; Micela, G.; Peres, G.; Pillitteri, I.; Sciortino, S. Bibcode: 2006yCat..34300287M Altcode: We present the analysis of the Guaranteed Time XMM-Newton/EPIC observation pointed on the young open cluster IC 2391. The observation (Obs. No. 0112420101), centered on (RA=8:42:00, DE=-53:00:36), was performed on Nov. 20, 2001 during orbit 357 of XMM-Newton.

(2 data files). Title: Interaction of SNR shocks with thermally conducting, radiative clouds Authors: Orlando, S.; Bocchino, F.; Peres, G.; Reale, F. Bibcode: 2006cosp...36.3172O Altcode: 2006cosp.meet.3172O The shock wave of shell supernova remnants SNRs offers the unique opportunity to detect directly the structures of the interstellar medium both at large and small scales On the other hand the several physical effects at work during the interaction between the shock and the medium along with the superposition effects along a given light of sight require detailed modeling and high resolution observations in order to derive useful diagnostics Here we study the interaction of an evolved SNR shock front impacting on a small interstellar gas cloud through a 3D hydrodynamic model which takes into account the effects of radiative losses and thermal conduction We explore the interplay between the physical processes at work during the cloud evolution and their effect on the mass and energy exchange with the surrounding medium We also investigate the time-dependent X-ray emission originating from the shock-cloud interactions for two prototypical cases in which either thermal conduction or radiative cooling plays a dominant role in the cloud dynamics and energetic We study the correspondence between ideal and observable structures identifing a set of diagnostic tools which may be useful in the comparison with XMM-Newton and Chandra data of SNR shells Title: X-ray properties of NGC 2516 open cluster . Authors: Marino, A.; Pillitteri, I.; Micela, G.; Peres, G. Bibcode: 2006MSAIS...9..256M Altcode: We present the results of a study of X-ray properties of the NGC 2516, obtained with a series of XMM-Newton observations. Deep X-ray photometry has been obtained by summing up six EPIC observations, for a total of 105 ks, thus increasing the sensitivity in the X-ray band in this region by a factor 5 with respect to all previous surveys. Coronal spectra of solar mass stars are generally well described by one or two plasma thermal model, with typical temperatures ranging in 0.5-2.0 keV interval. We have studied the X-ray luminosity functions for different spectral types, confirming that G-M type stars are statistically less luminous than stars in the coeval Pleiades. Analysis of X-ray light-curves shows that only a small fraction of cluster members are variable on short time scales. X-ray variations are also studied on longer time scales. Title: X-ray Emission Mechanisms in Herbig - Haro objects . Authors: Bonito, R.; Orlando, S.; Peres, G.; Favata, F.; Rosner, R. Bibcode: 2006MSAIS...9..226B Altcode: X-ray emission in Herbig - Haro objects is a quite recent and uncommon finding still waiting full explanation. With the scope of explaining this X-ray emission, our project is devoted to model the interaction between a supersonic jet originating from a young stellar object and the ambient medium. We have performed a wide exploration of the parameter space to infer the configuration(s) which can give rise to X-ray emission very similar to what recently observed. Title: Stationary and Flaring Heating Effect on the Coronal Emission Measure . Authors: Peres, G.; Orlando, S.; Di Matteo, V.; Reale, F. Bibcode: 2006MSAIS...9...97P Altcode: Our scope is finding the relative role of "steady" vs. flare coronal heating through the analysis of steady and flaring emission measures, finding whether or not the latter appear as the high-temperature tail of the former and if there is a bi-modality suggesting different heating mechanisms. To this end, we have derived the emission measure vs. temperature for the steady corona, and for flares, from Yohkoh/SXT dataset plus GOES flare data cross-calibrated with the Yohkoh/SXT data. The comparison between the two datasets and the ensuing emission measure distributions vs. temperature is discussed. Title: X-raying the interstellar medium: the study of SNR shells at the OAPa . Authors: Bocchino, F.; Miceli, M.; Reale, F.; Maggio, A.; Orlando, S.; Peres, G. Bibcode: 2006MSAIS...9..223B Altcode: The shock wave of shell supernova remnant (SNR) offers the unique opportunity to detect directly the structures of the interstellar medium, both at large and small scale. Unfortunately, the several physical effects at work in the interaction between the shock and the medium, along with the superposition effects along a given light of sight, require detailed modeling and high resolution observations of the systems in order to understand its physical behavior At the Osservatorio Astronomico di Palermo, we have started a long-term project which will lead, for the first time, to a self-consistent methodological approach for comparison of new accurate numerical models and high resolution multi-wavelength observations. In this poster, we introduce the observation we have already analyzed,while in the talk of S. Orlando the new numerical models we have developed are presented Title: Hydrodynamic interaction of SNR shocks with thermally conducting, radiative clouds . Authors: Orlando, S.; Peres, G.; Reale, F.; Bocchino, F.; Plewa, T.; Rosner, R. Bibcode: 2006MSAIS...9..208O Altcode: Supernova remnants (SNRs) are privileged laboratories to investigate the physical and chemical evolution of the galactic interstellar medium (ISM) and the mass distribution of the plasma in the Galaxy.

Here, we study the interaction of an evolved SNR shock front with on a small interstellar gas cloud. Our model takes into account the hydrodynamics and the effects of the radiative losses and of the thermal conduction. We study the interplay between the radiative cooling and the thermal conduction during the cloud evolution and their effect on the mass and energy exchange between the cloud and the surrounding medium. We find that in cases dominated by the radiative losses the cloud fragments into cold, dense, and compact filaments surrounded by a hot corona which is ablated by the thermal conduction; instead, in cases dominated by the thermal conduction, the shocked cloud evaporates into the ISM in a few dynamical time-scales. In all the cases analyzed we find that the thermal conduction suppresses the hydrodynamic instabilities at the cloud boundaries. Title: Mhd-Modeling of the Propagation of a Coronal Mass Ejection Authors: Pagano, P.; Reale, F.; Orlando, S.; Peres, G. Bibcode: 2005ESASP.600E.161P Altcode: 2005dysu.confE.161P; 2005ESPM...11..161P No abstract at ADS Title: Crushing of interstellar gas clouds in supernova remnants. I. The role of thermal conduction and radiative losses Authors: Orlando, S.; Peres, G.; Reale, F.; Bocchino, F.; Rosner, R.; Plewa, T.; Siegel, A. Bibcode: 2005A&A...444..505O Altcode: 2005astro.ph..8638O We model the hydrodynamic interaction of a shock wave of an evolved supernova remnant with a small interstellar gas cloud like the ones observed in the Cygnus loop and in the Vela SNR. We investigate the interplay between radiative cooling and thermal conduction during cloud evolution and their effect on the mass and energy exchange between the cloud and the surrounding medium. Through the study of two cases characterized by different Mach numbers of the primary shock (M= 30 and 50, corresponding to a post-shock temperature T≈ 1.7× 106 K and ≈ 4.7× 106 K, respectively), we explore two very different physical regimes: for M= 30, the radiative losses dominate the evolution of the shocked cloud which fragments into cold, dense, and compact filaments surrounded by a hot corona which is ablated by the thermal conduction; instead, for M= 50, the thermal conduction dominates the evolution of the shocked cloud, which evaporates in a few dynamical time-scales. In both cases we find that the thermal conduction is very effective in suppressing the hydrodynamic instabilities that would develop at the cloud boundaries. Title: Modeling a Coronal Loop Heated by Magnetohydrodynamic Turbulence Nanoflares Authors: Reale, F.; Nigro, G.; Malara, F.; Peres, G.; Veltri, P. Bibcode: 2005ApJ...633..489R Altcode: 2005astro.ph..6694R We model the hydrodynamic evolution of the plasma confined in a coronal loop, 30,000 km long, subject to the heating of nanoflares due to intermittent magnetic dissipative events in the MHD turbulence produced by loop footpoint motions. We use the time-dependent distribution of energy dissipation along the loop obtained from a hybrid shell model, occurring for a magnetic field of about 10 G in the corona; the relevant heating per unit volume along the loop is used in the Palermo-Harvard loop plasma hydrodynamic model. We describe the results, focusing on the effects produced by the most intense heat pulses, which lead to loop temperatures between 1 and 1.5 MK. Title: XMM-Newton spectroscopy of the metal depleted T Tauri star TWA 5 Authors: Argiroffi, C.; Maggio, A.; Peres, G.; Stelzer, B.; Neuhäuser, R. Bibcode: 2005A&A...439.1149A Altcode: 2005astro.ph..5075A We present results of X-ray spectroscopy for TWA 5, a member of the young TW Hydrae association, observed with XMM-Newton. TWA 5 is a multiple system which shows Hα emission, a signature typical of classical T Tauri stars, but no infrared excess. From this analysis of the RGS and EPIC spectra, we have derived the emission measure distribution vs. temperature of the X-ray emitting plasma, its abundances, and the electron density. The characteristic temperature and density of the plasma suggest a corona similar to that of weak-line T Tauri stars and active late-type main sequence stars. TWA 5 also shows low iron abundance (~0.1 times the solar photospheric one) and a pattern of increasing abundances for elements with increasing first ionization potential reminiscent of the inverse FIP effect observed in highly active stars. The especially high ratio Ne/Fe∼10 is similar to that of the classical T Tauri star TW Hya, where the accreting material has been held responsible for the X-ray emission. We discuss the possible role of an accretion process in this scenario. Since all T Tauri stars in the TW Hydrae association studied so far have very high Ne/Fe ratios, we also propose that environmental conditions may cause this effect. Title: XMM-Newton observations of the Upper Scorpius association Authors: Argiroffi, Costanza; Favata, F.; Maggio, A.; Micela, G.; Peres, G.; Sciortino, S. Bibcode: 2005sfet.confE..47A Altcode: The 5 Myr old Upper Scorpius Association, due to its proximity (145 pc) and low interstellar absorption, allows a detailed study of the X-ray emission from PMS stars. We present the results of the analysis of XMM observations of two Upper Scorpius regions. We detected 224 X-ray sources among which we identified 21 Upper Scorpius probable members on the basis of the 2MASS and DENIS photometry. The selected Upper Scorpius sample includes 7 WTTS, while the nature of the remaining 14 sources it is not known. Except the high mass star HD 142578 (spectral type A2), all the other detected Upper Scorpius sources are low mass stars of spectral type ranging from K to late M. The spectral analysis of the most intense sources indicates plasma temperature of ∼10 MK, resembling the typical coronal emission of active main sequence stars. We study the abundance vs. temperature pattern of Upper Scorpius sources and compare it with that observed for active stars. The Kolmogorov-Smirnov test proves that 67% of the detected X-ray Upper Scorpius sources were variable during the observation, and some of them displayed large flare-like events. Title: X-ray emission mechanisms in protostellar jets Authors: Bonito, R.; Orlando, S.; Peres, G.; Favata, F.; Rosner, R. Bibcode: 2005ESASP.560..185B Altcode: 2005csss...13..185B No abstract at ADS Title: The structure of coronal plasma in active stellar coronae from density measurements Authors: Testa, P.; Drake, J. J.; Peres, G. Bibcode: 2005ESASP.560..997T Altcode: 2005csss...13..997T No abstract at ADS Title: High resolution X-ray spectroscopy of pre-main-sequence stars: TWA 5 and PZ Tel Authors: Argiroffi, C.; Drake, J. J.; Harnden, F. R.; Maggio, A.; Peres, G.; Sciortino, S.; Stelzer, B.; Neuhäuser, R. Bibcode: 2005ESASP.560..399A Altcode: 2004astro.ph.10264A; 2005csss...13..399A We report on the analysis of high resolution X-ray spectra of two pre-main-sequence stars: TWA 5 (observed with XMM-Newton) and PZ Telescopii (observed with Chandra/HETGS). TWA 5 is a classical T Tauri star in the TW Hydrae association while PZ Tel is a rapidly rotating weak-lined T Tauri star in the beta-Pictoris moving group. For both stars we have reconstructed the emission measure distribution and derived the coronal abundances to check for possible patterns of the abundances related to the first ionization potential of the various elements. We have also derived estimates of the plasma density from the analysis of the He-like triplets. We compare the characteristics of our targets with those of other pre-main sequence stars previously analyzed by other authors: TW Hya, HD 98800 and HD 283572. Our findings suggest that X-ray emission from classical T Tauri and weak-lined T Tauri stars is produced in all cases by magnetically-heated coronae, except for TW Hya which has unique plasma temperatures and densities. Moreover we derive that TWA 5 has the same peculiar Ne/Fe abundance ratio as TW Hya. Title: Emission Measure Distribution in Loops Impulsively Heated at the Footpoints Authors: Testa, Paola; Peres, Giovanni; Reale, Fabio Bibcode: 2005ApJ...622..695T Altcode: 2004astro.ph.12482T This work is prompted by evidence of sharply peaked emission measure distributions in active stars and by the claims of isothermal loops in solar coronal observations, at variance with the predictions of hydrostatic loop models with constant cross section and uniform heating. We address the problem with loops heated at the footpoints. Since steady heating does not allow static loop model solutions, we explore whether pulse-heated loops can exist and appear as steady loops on a time average. We simulate pulse-heated loops, using the Palermo-Harvard 1-D hydrodynamic code, for different initial conditions corresponding to typical coronal temperatures of stars ranging from intermediate to active [T~(3-10)×106 K]. We find long-lived quasi-steady solutions even for heating concentrated at the footpoints over a spatial region of the order of ~1/5 of the loop half-length and broader. These solutions yield an emission measure distribution with a peak at high temperature, and the cool side of the peak is as steep as ~T5, in contrast to the usual ~T3/2 of hydrostatic models with constant cross section and uniform heating. Such peaks are similar to those found in the emission measure distribution of active stars around 107 K. Title: X-ray spectral and timing properties of the IC 2391 stars Authors: Marino, A.; Micela, G.; Peres, G.; Pillitteri, I.; Sciortino, S. Bibcode: 2005ESASP.560..787M Altcode: 2005csss...13..787M No abstract at ADS Title: Coronal properties of active G-type stars in different evolutionary phases Authors: Scelsi, L.; Maggio, A.; Peres, G.; Pallavicini, R. Bibcode: 2005ESASP.560..939S Altcode: 2005csss...13..939S No abstract at ADS Title: Coronal properties of G-type stars in different evolutionary phases Authors: Scelsi, L.; Maggio, A.; Peres, G.; Pallavicini, R. Bibcode: 2005A&A...432..671S Altcode: 2005astro.ph..1631S We report on the analysis of XMM-Newton observations of three G-type stars in very different evolutionary phases: the weak-lined T Tauri star HD 283572, the Zero Age Main Sequence star EK Dra and the Hertzsprung-gap giant star 31 Com. They all have high X-ray luminosity (~1031 erg s-1 for HD 283572 and 31 Com and ~1030 erg s-1 for EK Dra). We compare the Emission Measure Distributions (EMDs) of these active coronal sources, derived from high-resolution XMM-Newton grating spectra, as well as the pattern of elemental abundances vs. First Ionzation Potential (FIP). We also perform time-resolved spectroscopy of a flare detected by XMM from EK Dra. We interpret the observed EMDs as the result of the emission of ensembles of magnetically confined loop-like structures with different apex temperatures. Our analysis indicates that the coronae of HD 283572 and 31 Com are very similar in terms of dominant coronal magnetic structures, in spite of differences in the evolutionary phase, surface gravity and metallicity. In the case of EK Dra the distribution appears to be slightly flatter than in the previous two cases, although the peak temperature is similar. Title: Size of coronal structures in active stellar coronae from the detection of X-ray resonant scattering Authors: Testa, P.; Drake, J. J.; Peres, G.; Deluca, E. E. Bibcode: 2005ESASP.560...43T Altcode: 2005csss...13...43T No abstract at ADS Title: X-ray spectral and timing characteristics of the stars in the young open cluster IC 2391 Authors: Marino, A.; Micela, G.; Peres, G.; Pillitteri, I.; Sciortino, S. Bibcode: 2005A&A...430..287M Altcode: 2004astro.ph.10099M We present X-ray spectral and timing analysis of members of the young open cluster IC 2391 observed with the XMM-Newton observatory. We detected 99 X-ray sources by analysing the summed data obtained from MOS1, MOS2 and pn detectors of the EPIC camera; 24 of them are members, or probable members, of the cluster. Stars of all spectral types have been detected, from the early-types to the late-M dwarfs. Despite the capability of the instrument to recognize up to 3 thermal components, the X-ray spectra of the G, K and M members of the cluster are well described with two thermal components (at kT1 ∼ 0.3-0.5 keV and kT2 ∼ 1.0-1.2 keV respectively) while the X-ray spectra of F members require only a softer 1-T model. The Kolmogorov-Smirnov test applied to the X-ray photon time series shows that approximately 46% of the members of IC 2391 are variable with a confidence level >99%. The comparison of our data with those obtained with ROSAT/PSPC, nine years earlier, and ROSAT/HRI, seven years earlier, shows that there is no evidence of significant variability on these time scales, suggesting that long-term variations due to activity cycles similar to that on the Sun are not common, if present at all, among these young stars. Title: The Density of Coronal Plasma in Active Stellar Coronae Authors: Testa, Paola; Drake, Jeremy J.; Peres, Giovanni Bibcode: 2004ApJ...617..508T Altcode: 2004astro.ph..5019T We have analyzed high-resolution X-ray spectra of a sample of 22 active stars observed with the High Energy Transmission Grating Spectrometer on Chandra in order to investigate their coronal plasma density. Densities were investigated using the lines of the He-like ions O VII, Mg XI, and Si XIII. Si XIII lines in all stars of the sample are compatible with the low-density limit (i.e., ne<~1013 cm-3), casting some doubt on results based on lower resolution Extreme Ultraviolet Explorer (EUVE) spectra finding densities ne>1013 cm-3. Mg XI lines betray the presence of high plasma densities up to a few times 1012 cm-3 for most of the sources with higher X-ray luminosity (>~1030 ergs s-1) stars with higher LX and LX/Lbol tend to have higher densities at high temperatures. Ratios of O VII lines yield much lower densities of a few times 1010 cm-3, indicating that the ``hot'' and ``cool'' plasma resides in physically different structures. In the cases of EV Lac, HD 223460, Canopus, μ Vel, TY Pyx, and IM Peg, our results represent the first spectroscopic estimates of coronal density. No trends in density-sensitive line ratios with stellar parameters effective temperature and surface gravity were found, indicating that plasma densities are remarkably similar for stars with pressure scale heights differing by up to 3 orders of magnitude. Our findings imply remarkably compact coronal structures, especially for the hotter (~7 MK) plasma emitting the Mg XI lines characterized by the coronal surface filling factor, fMgXI, ranging from 10-4 to 10-1, while we find fOVII values from a few times 10-3 up to ~1 for the cooler (~2 MK) plasma emitting the O VII lines. We find that fOVII approaches unity at the same stellar surface X-ray flux level as characterizes solar active regions, suggesting that these stars become completely covered by active regions. At the same surface flux level, fMgXI is seen to increase more sharply with increasing surface flux. These results appear to support earlier suggestions that hot 107 K plasma in active coronae arises from flaring activity and that this flaring activity increases markedly once the stellar surface becomes covered with active regions. Comparison of our measured line fluxes with theoretical models suggests that significant residual model inaccuracies might be present and, in particular, that cascade contributions to forbidden and intercombination lines resulting from dielectronic recombination might be to blame. Title: Calibration of the XRT-SOLARB flight filters at the XACT facility of INAF-OAPA Authors: Barbera, Marco; Artale, Maria Antonella; Candia, Roberto; Collura, Alfonso; Lullo, Giuseppe; Peres, Giovanni; Perinati, Emanuele; Varisco, Salvatore; Bookbinder, J. A.; Cheimets, Peter N.; Cosmo, Mario L.; DeLuca, Edward E.; Golub, Leon; Weber, Mark A. Bibcode: 2004SPIE.5488..423B Altcode: The X-Ray Telescope (XRT) experiment on-board the Japanese satellite SOLAR-B (launch in 2006) aimed at providing full Sun field of view at ~ 1.5" angular resolution, will be equipped with two wheels of focal-plane filters to select spectral features of X-ray emission from the Solar corona, and a front-end filter to significantly reduce the visible light contamination. We present the results of the X-ray calibrations of the XRT flight filters performed at the X-ray Astronomy Calibration and Testing (XACT) facility of INAF-OAPA. We describe the instrumental set-up, the adopted measurement technique, and present the transmission vs. energy and position measurements. Title: Calibration of the XRT-SOLARB flat mirror samples at the XACT Facility of INAF-OAPA Authors: Artale, Maria Antonella; Barbera, Marco; Collura, Alfonso; Di Cicca, Gaspare; Peres, Giovanni; Varisco, Salvatore; Bookbinder, J. A.; Cheimets, Peter N.; Cosmo, Mario L.; DeLuca, Edward E.; Golub, Leon; Weber, Mark A. Bibcode: 2004SPIE.5488..440A Altcode: The X-Ray Telescope (XRT) experiment on-board the Japanese satellite SOLAR-B (launch in 2006) is equipped with a modified Wolter I grazing incidence X-ray telescope (focal length 2700 mm) to image the full Sun at ~ 1.5" angular resolution onto a 2048 x 2048 back illuminated CCD focal plane detector. The X-ray telescope consisting of one single reflecting shell is coated with ion beam sputtered Iridium over a binding layer of Chromium to provide nearly 5 square centimetres effective area at 60 Å. We present preliminary results of X-ray calibrations of the XRT flat mirror samples performed at the X-ray Astronomy Calibration and Testing (XACT) facility of INAF-OAPA. We describe the instrumental set-up, the adopted measurement technique, and present the measured reflectivity vs. angle of incidence at few energies. Title: The Sun as an X-ray star: Active region evolution, rotational modulation, and implications for stellar X-ray variability Authors: Orlando, S.; Peres, G.; Reale, F. Bibcode: 2004A&A...424..677O Altcode: We study the contribution of an active region and its core to the luminosity and the spectrum of the Sun in the X-ray band and to the relevant solar emission measure vs. temperature distribution, EM(T). We also study the relevant changes in the course of four solar rotations, and the solar rotational modulation due to this active region, the only one present at that time.

To this end, we have used a large sample of full-disk Yohkoh/SXT observations taken between July and October 1996, covering most of the active region evolution. From the Yohkoh/SXT data we have synthesized the X-ray spectra of the whole solar corona, and the focal plane data as they would be collected with Rosat/PSPC, XMM-Newton/EPIC and Chandra/ACIS. This work is part of a project to study the Sun as an X-ray star, using the solar data as a guide and a template for stellar observations.

We found that the active region contributes significantly to the X-ray spectrum of the Sun mainly during the first month of its evolution. The rotational modulation due to the active region causes a significant variability of the average X-ray flux, with only moderate spectral variation, in the pass-bands of Rosat/PSPC, XMM-Newton/EPIC and Chandra/ACIS.

We investigated the characteristics of the X-ray variability due to the rotational modulation, to the solar cycle, and to flares together with the possible implications on stellar X-ray variability. We derived the diagram of X-ray surface flux vs. spectral hardness ratio in the Rosat/PSPC band; we studied the variability due to the solar cycle and to the rotational modulation and we found that both lead to the same correlation between F_pspc and HR_pspc with a very steep slope. The variability due to the evolution of flares again produces a correlation between F_pspc and HR_pspc but with a much flatter slope than in the other two cases. Analogous results have been found when analyzing data in the format of XMM-Newton/EPIC and Chandra/ACIS. Title: X-Ray Spectroscopy of the Unsteady Quiescent Corona of AD Leonis with Chandra Authors: Maggio, A.; Drake, J. J.; Kashyap, V.; Harnden, F. R., Jr.; Micela, G.; Peres, G.; Sciortino, S. Bibcode: 2004ApJ...613..548M Altcode: 2004astro.ph..5580M We present the analysis and interpretation of an observation of the flare star AD Leo (dM3e) with the Low Energy Transmission Grating of Chandra. The high-resolution X-ray spectrum-dominated by emission lines from O VII-VIII, Ne IX-X, and Fe XVII-Fe XIX-allowed us to infer the plasma emission measure distribution (EMD) versus temperature, as well as the abundances of individual elements in the corona of this magnetically active star, during a typical state characterized by significant variability but no evident flaring event. We have also measured plasma densities at various temperatures using spectroscopic diagnostics provided by He-like triplets and Fe XXI lines. We show that the present EMD is similar, in terms of overall shape and temperature of the peak, to those previously obtained from Extreme Ultraviolet Explorer (EUVE) spectra during quiescent and flaring states confirming the long-term stability of the corona of AD Leo. At variance with the case of other active stars, the EMD of AD Leo is characterized by a significantly shallower slope, compatible with that predicted by static models of isobaric loops with constant cross section and uniform heating. We discuss such coronal modeling to infer the average properties of the corona in terms of loop populations, including estimates of the surface filling factor derived by comparison between the model and the observed EMD. We also show that the EMD is compatible with the model of a corona continuously heated by flares, which predicts an EMD slope slightly steeper than observed, but that can be accommodated by observational uncertainties. The coronal composition is such that the element abundances, relative to solar values, tend to increase with the first ionization potential, with few exceptions. The line-to-continuum ratios suggest a nearly solar metallicity, a result difficult to reconcile with previous determinations based on global fitting of X-ray spectra. Title: The X-ray emission mechanism in the protostellar jet HH 154 Authors: Bonito, R.; Orlando, S.; Peres, G.; Favata, F.; Rosner, R. Bibcode: 2004A&A...424L...1B Altcode: We study the mechanism causing the X-ray emission recently detected in protostellar jets, by performing a detailed modeling of the interaction between a supersonic jet originating from a young stellar object and the ambient medium, for various values of density contrast, ν, between the ambient density and the jet, and of Mach number, M; radiative losses and thermal conduction have been taken into account. Here we report a representative case which reproduces, without any ad hoc assumption, the characteristics of the X-ray emission recently observed in the protostellar jet HH 154. We find that the X-ray emission originates from a localized blob, consistent with observations, which moves with velocity v ∼ 500 km s-1; we therefore predict the X-ray source to have a detectable proper motion. Title: Are Coronae of Late-Type Stars Made of Solar-like Structures? The X-Ray Surface Flux versus Hardness Ratio Diagram and the Pressure-Temperature Correlation Authors: Peres, G.; Orlando, S.; Reale, F. Bibcode: 2004ApJ...612..472P Altcode: 2004astro.ph..5281P This work is dedicated to the solar-stellar connection, i.e., the close similarity of the Sun and late-type stars; in particular, this work shows that stellar coronae can be composed of X-ray-emitting structures similar to those present in the solar corona. To this end we use a large set of ROSAT PSPC observations of late-type stars of all spectral types and activity levels and a large set of solar X-ray data collected with Yohkoh SXT. Solar data have been analyzed and formatted to study the Sun as an X-ray star; they include observations of the solar corona at various phases of the solar cycle and data on various kinds of X-ray coronal structures, from flares to the background corona, i.e., the most quiet regions. We use the X-ray surface flux (FX) versus spectral hardness ratio (HR) diagram as a fundamental tool for our study. We find that FX is strongly correlated to HR in stellar coronae, in the solar corona at all phases of the solar cycle, and in the individual solar coronal structures; all the above follow the same law. Schmitt found the same correlation in stellar coronae. We therefore claim that coronae of late-type stars are formed with X-ray structures very similar to those in the Sun, since their behavior is identical to that of the solar coronal structures and of the whole solar corona. The spatial location of the X-ray structures on the star, however, could be very different from those on the Sun. In this scenario, the fraction of the stellar surface covered with active regions and their bright cores increases with activity; the most active stars are brighter and hotter than if they were entirely covered with active regions, so they can be explained only with the additional presence of several flares (or flarelike structures) at any time. On the basis of the FX versus HR correlation, corresponding to FX~T6, we then derive a set of new laws relating the temperature, pressure, volumetric heating, and characteristic loop length of the coronal plasma on all the late-type stars. In addition, individual solar coronal structures and the whole solar corona follow the same laws. These laws also agree with recent findings of higher plasma density at higher temperatures in stellar coronae. We claim that the strong correlation between surface flux and temperature and the laws mentioned above are just the effect of more fundamental physical mechanisms driving the coronal structures of all the late-type stars from the emergence of new magnetic structures to their dispersal and dissipation. Title: High-Resolution X-Ray Spectroscopy of the Post-T Tauri Star PZ Telescopii Authors: Argiroffi, C.; Drake, J. J.; Maggio, A.; Peres, G.; Sciortino, S.; Harnden, F. R. Bibcode: 2004ApJ...609..925A Altcode: 2004astro.ph..3017A We present an analysis of the Chandra High Energy Transmission Grating Spectrometer observation of the rapidly rotating (Prot=0.94days) post-T Tauri (~20 Myr old) star PZ Telescopii, in the Tucana association. Using two different methods, we have derived the coronal emission measure distribution EM(T) and chemical abundances. The EM(T) peaks at logT=6.9 and exhibits a significant emission measure at temperatures logT>7. The coronal abundances are generally ~0.5 times the solar photospheric values, which are presumed fairly representative of the composition of the underlying star. A minimum in abundance is seen at a first ionization potential (FIP) of 7-8 eV, with evidence for higher abundances at both lower and higher FIP, similar to patterns seen in other active stars. From an analysis of the He-like triplet of Mg XI, we have estimated electron densities of ~1012-1013 cm-3. All the coronal properties found for PZ Tel are much more similar to those of AB Dor, which is slightly older than PZ Tel, than to those of the younger T Tauri star TW Hya. These results support earlier conclusions that the soft X-ray emission of TW Hya is likely dominated by accretion activity rather than by a magnetically heated corona. Our results also suggest that the coronae of pre-main-sequence stars rapidly become similar to those of older active main-sequence stars soon after the accretion stage has ended. Title: Detection of X-Ray Resonance Scattering in Active Stellar Coronae Authors: Testa, Paola; Drake, Jeremy J.; Peres, Giovanni; DeLuca, Edward E. Bibcode: 2004ApJ...609L..79T Altcode: 2004astro.ph..5520T An analysis of Lyman series lines arising from hydrogen-like oxygen and neon ions in the coronae of the active RS CVn-type binaries II Peg and IM Peg, observed using the Chandra High Resolution Transmission Grating Spectrograph, shows significant decrements in the Lyα/Lyβ ratios as compared with theoretical predictions and with the same ratios observed in similar active binaries. We interpret these decrements in terms of resonance scattering of line photons out of the line of sight; these observations present the first strong evidence of this effect in active stellar coronae. The net line photon loss implies a nonuniform and asymmetric surface distribution of emitting structures on these stars. Escape probability arguments, together with the observed line ratios and estimates of the emitting plasma density, imply typical line-of-sight sizes of the coronal structures that dominate the X-ray emission of 1010 cm at temperatures of 3×106 K and of 108 cm at 107 K. These sizes are an order of magnitude larger than predicted by simple quasi-static coronal loops models but are still very small compared to the several 1011 cm radii of the underlying stars. Title: Crushing of Interstellar Gas Clouds in Supernova Remnants: the Role of Thermal Conduction and Radiative Losses Authors: Peres, G.; Orlando, S.; Reale, F.; Rosner, R.; Plewa, T.; Siegel, A. Bibcode: 2004AIPC..703..326P Altcode: We model hydrodynamic interactions of an old supernova remnant shock wave with a small interstellar gas cloud, taking into account the effects of thermal conduction and radiative losses. In particular, we consider a representative case of a Mach 30 shock impacting on an isolated cloud with density contrast χ = 10 with respect to the ambient medium. Thermal conduction appears to be effective in suppressing the Kelvin-Helmholtz and Rayleigh-Taylor instabilities which would develop at the cloud boundaries. We demonstrate that the radiative losses play a crucial role in the dynamics of the shock-cloud interaction, dominating evolution of the shocked cloud medium. Title: Modeling an X-ray flare on Proxima Centauri: Evidence of two flaring loop components and of two heating mechanisms at work Authors: Reale, F.; Güdel, M.; Peres, G.; Audard, M. Bibcode: 2004A&A...416..733R Altcode: 2003astro.ph.12267R We model in detail a flare observed on Proxima Centauri with the EPIC-PN on board XMM-Newton at high statistics and high time resolution and coverage. Time-dependent hydrodynamic loop modeling is used to describe the rise and peak of the light curve, and a large fraction of the decay, including its change of slope and a secondary maximum, over more than 2 h. The light curve, the emission measure and the temperature derived from the data allow us to constrain the loop morphology and the heating function and to show that this flare can be described with two components: a major one triggered by an intense heat pulse injected in a single flaring loop with half-length ≈1.0 × 1010 cm, the other one by less intense heat pulses released about 1/2 h after the first one in related loop systems, probably arcades, with the same half-length. The heat functions of the two loop systems appear very similar: an intense pulse located at the loop footpoints followed by a low gradual decay distributed in the coronal part of the loop. The latter result and the similarity to at least one solar event (the Bastille Day flare in 2000) indicate that this pattern may be common to solar and stellar flares.

Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member states and the USA (NASA). Title: Long-Term X-Ray Spectral Variability of the Nucleus of M81 Authors: La Parola, V.; Fabbiano, G.; Elvis, M.; Nicastro, F.; Kim, D. W.; Peres, G. Bibcode: 2004ApJ...601..831L Altcode: 2003astro.ph.10461L We have analyzed the soft X-ray emission from the nuclear source of the nearby spiral galaxy M81, using the available data collected with ROSAT, ASCA, BeppoSAX, and Chandra. The source flux is highly variable (sometimes dramatic: a factor of 4 in 20 days), showing variability at different timescales, from 2 days to 4 yr, and in particular a steady increase of the flux by a factor of >~2 over 4 yr, broken by rapid flares. After accounting for the extended component resolved by Chandra, the nuclear soft X-ray spectrum (from ROSAT/PSPC, BeppoSAX/LECS, and Chandra data) cannot be fitted well with a single absorbed power-law model. Acceptable fits are obtained by adding an extra component, either a multicolor blackbody (MCBB) or an absorption feature. In the MCBB case, the inner accretion disk would be far smaller than the Schwartzchild radius for the 3-60×106 Msolar nucleus, requiring a strictly edge-on inclination of the disk, even if the nucleus is a rotating Kerr black hole. The temperature is 0.27 keV, larger than expected from the accretion disk of a Schwartzchild black hole but consistent with that expected from a Kerr black hole. In the power law+absorption feature model, we have either high-velocity (0.3c) infalling C V clouds or neutral C I absorption at rest. In both cases the C:O overabundance is a factor of 10. Title: Using AMR to Simulate the 3-D Hydrodynamic Interaction of Supernova Shocks with Interstellar Gas Clouds Authors: Orlando, S.; Peres, G.; Reale, F.; Plewa, T.; Rosner, R.; Siegel, A. Bibcode: 2004MSAIS...4...82O Altcode: We study the 3-D hydrodynamic interaction of supernova shock fronts with interstellar clouds to investigate the evolution, the morphology and the deviations from equilibrium of ionization. To this end, we use the FLASH code including PARAMESH, an advanced and versatile parallel adaptive mesh refinement package. We present here the preliminary results obtained modeling a representative case of a Mach 50 shock impacting on an isolated cloud with density contrast chi = 10 with respect to the ambient medium. The preliminary analysis of the non-equilibrium ionization (NEI) effects on the ionization stages of oxygen, and iron is discussed. Title: X-ray spectroscopy of the Hertzsprung-gap giant star 31 Com, observed with XMM-Newton Authors: Scelsi, L.; Maggio, A.; Peres, G.; Gondoin, Ph. Bibcode: 2004A&A...413..643S Altcode: We have analysed the XMM-Newton X-ray spectra of the yellow giant 31 Com with the aim of deriving information on the coronal structures of this archetypical Hertzsprung-gap star. To determine the emission measure distribution vs. temperature, EM(T), and the elemental abundances of the coronal plasma, with an accurate line-based approach, we have developed a new method for simple and accurate line measurements, based on rebinning and co-adding the two RGS spectra. We have reconstructed the EM(T) independently with both APED and CHIANTI atomic databases in order to investigate possible differences in the final outcome of the analysis, and we have obtained consistent results. The derived emission measure distribution has a well defined peak at T ∼ 107 K and a significant amount of plasma at higher temperatures; there is also evidence for plasma at temperatures below ∼106.5 K, with a mean electron density of ∼3 × 1010 cm-3, as inferred from the line ratio of the O Vii triplet. We have made a global fitting of the EPIC spectra, using multi-component isothermal (3-T) model, and then compared the results with the EM(T), looking for a consistent multi-temperature description of both the RGS and EPIC spectra, over the whole spectral range. While the EM(T) and the 3-T models individually provide a good description of the data set on which they are based, none of them describes adequately the data of all the other instruments; the disagreements may be related, at least in part, to cross-calibration problems. Finally, we have used the EM(T) to derive information about the properties of the coronal structures. Our results indicate that the corona of 31 Com is dominated by a class of magnetic loops with peak temperature ∼107 K and apparently more isothermal than the solar ones.

Table 3 and Appendix A are only available in electronic form at http://www.edpsciences.org Title: Study of the young open cluster IC2391: discovery of X-ray rotational modulation in a supersaturated star Authors: Micela, G.; Marino, A.; Peres, G.; Pillitteri, I.; Sciortino, S. Bibcode: 2004MmSAI..75..442M Altcode: We present the analysis of the Guarantee Time XMM-Newton/EPIC observation pointed on the young open cluster IC 2391. We find evidence, for the first time, of X-ray rotational modulation on a supersaturated star, member of the cluster, implying the presence of structural inhomogeneities. We also present preliminary results of the spectral analysis of the X-ray brightest cluster stars. Title: SADE: The starspot and dynamo explorer Authors: Martens, P. C. H.; Acton, L. W. A.; Klumpar, D.; Kankelborg, C.; Stern, R. A.; Peres, G.; Culhane, J. L. Bibcode: 2003AdSpR..32.1123M Altcode: We propose a mission called SADE, the Starspot And Dynamo Explorer, to study dynamo activity in nearby late-type stars. The onboard instruments will be a Ca-K telescope for magnetically dominated chromospheric emission, and an X-ray grazing incidence telescope to study coronal emission. We design the mission for a life-time of 15 years or longer to capture a full activity cycle for most solar-type stars. We aim to firmly establish the spectrum of the relation between chromospheric and corona' emission in late-type stars, and capture one or more stars going into or coming out of a Maunder type minimum. Operation costs will be kept to a minimum by automating mission operations to a maximum, and have the science operations be carried out by students at Montana State University. Title: Viewing the sun as an X-ray star Authors: Orlando, S.; Peres, G.; Reale, F. Bibcode: 2003AdSpR..32..955O Altcode: The Sun is the late-type star we can study with the highest level of detail. In the interpretation of stellar data, therefore, it is often assumed that the physical processes of the coronae of late-type stars are similar to those of the solar corona, i.e. the "solar-stellar analogy". In order to investigate the validity of this assumption, we have started a program to study systematically the Sun as an X-ray star. Our program aims to explore how far the solar model can be applied to other stars. In this paper we review the results obtained from these studies and, in particular, we discuss the variability of a star identical to the Sun during its cycle, the contribution of different coronal regions to the whole corona X-ray spectrum, and the role of flares. We find that the non-flaring Sun during its cycle fits well in the stellar scenario as a low-activity star. Our results suggest that the coronae of stars from low to intermediate activity may be explained as the effect of structures similar to those present on the Sun, changing the relative weight of the different kinds of non-flaring coronal regions; the coronae of very active stars may be explained if a continuous flaring activity characterizes these stars. Possible implications in stellar data interpretation are discussed. Title: X-ray variability of Pleiades late-type stars as observed with the ROSAT-PSPC Authors: Marino, A.; Micela, G.; Peres, G.; Sciortino, S. Bibcode: 2003A&A...406..629M Altcode: We present a comprehensive analysis of X-ray variability of the late-type (dF7-dM) Pleiades stars, detected in all ROSAT-PSPC observations; X-ray variations on short (hours) and medium (months) time scales have been explored. We have grouped the stars in two samples: 89 observations of 42 distinct dF7-dK2 stars and 108 observations of 61 dK3-dM stars. The Kolmogorov-Smirnov test applied on all X-ray photon time series show that the percentage of cases of significant variability is quite similar on both samples, suggesting that the presence of variability does not depend on mass for the time scales and mass range explored. The comparison between the Time X-ray Amplitude Distribution functions (XAD) of the set of dF7-dK2 and of the dK3-dM show that, on short time scales, dK3-dM stars show larger variations than dF7-dK2. A subsample of eleven dF7-dK2 and eleven dK3-dM Pleiades stars allows the study of variability on longer time scales: we found that variability on medium - long time scales is relatively more common among dF7-dK2 stars than among dK3-dM ones. For both dF7-dK2 Pleiades stars and dF7-dK2 field stars, the variability on short time scales depends on Lx while this dependence has not been observed among dK3-dM stars. It may be that the variability among dK3-dM stars is dominated by flares that have a similar luminosity distribution for stars of different Lx, while flaring distribution in dF7-dK2 stars may depend on X-ray luminosity. The lowest mass stars show significant rapid variability (flares?) and no evidence of rotation modulation or cycles. On the contrary, dF7-dK2 Pleiades stars show both rapid variability and variations on longer time scales, likely associated with rotational modulation or cycles. Title: X-ray rotational modulation of a supersaturated star in IC 2391 Authors: Marino, A.; Micela, G.; Peres, G.; Sciortino, S. Bibcode: 2003A&A...407L..63M Altcode: 2003astro.ph..7170M We present evidence of X-ray rotational modulation on VXR45, a young fast rotator star, member of IC 2391. It is a dG9 spectral type star whose rotational period and X-ray luminosity make it a supersaturated star. Our X-ray observation, made with EPIC/PN on XMM-Newton, covers about two photometric rotational periods. The detection of X-ray rotational modulation implies the presence of structural inhomogeneities. Possible interpretations are presented and discussed. Title: CDS/SoHO multi-line observation of a solar active region: Detection of a hot stable loop and of a cool dynamic loop Authors: Di Giorgio, S.; Reale, F.; Peres, G. Bibcode: 2003A&A...406..323D Altcode: We analyze a space-, time- and spectral-resolved SoHO/CDS observation of the evolution of an active region over a time lapse of approximately three hours in various spectral lines emitted in the interval of temperature 1.3 x 104 < T < 2.5 x 106 K. We identify and characterize two structures of interest: a longer coronal loop (~ 5.5*E9 cm), relatively steady and well visible in lines forming at coronal temperatures (e.g. Fe XIV 334.17 Å, Fe XVI 360.76 Å) and a smaller one (~ 1.8 x 109 cm), transient and visible only in cooler lines (O IV 554.51 Å, O V 629.73 Å). In the hot lines, the longer loop has a bright apex and an emission distribution of constant shape, but of moderately variable absolute intensity; the region around the loop apex shows a distinct brightening practically in all lines. In the hot lines, the brightening appears as a minor perturbation over a steadily high emission level. In the same region the emission measure vs temperature of the hottest lines indicates a temperature of ~ 2 MK, lower than the temperature obtained from Yohkoh data taken just before the CDS observation. Comparison with steady-state loop scaling laws and with plasma time scales, and connection to cooling or heating episodes are discussed. As for the cool loop, its whole evolution, from ignition to disappearance, is directly observed, confirming the highly transient nature of such structures. The O V line is blue-shifted at one footpoint, indicating an upflow associated with the loop ignition. Title: X-Ray Rotational Modulation in VXR45 Authors: Marino, A.; Micela, G.; Peres, G.; Sciortino, S. Bibcode: 2003IBVS.5427....1M Altcode: We present evidence of X-ray rotational modulation on VXR45, a young very fast rotator star member of IC2391 open cluster, observed with XMM-Newton observatory. Title: On coronal structures and their variability in active stars: The case of Capella observed with Chandra/LETGS Authors: Argiroffi, C.; Maggio, A.; Peres, G. Bibcode: 2003A&A...404.1033A Altcode: In this paper we present a detailed analysis of two X-ray spectra of Capella, taken eleven months apart with the Low Energy Transmission Grating Spectrometer (LETGS) of the Chandra Observatory. We have studied variability of the coronal emission over different time scales, both in the whole X-ray band and in narrow temperature ranges identified by lines. The comparison of the two observations shows that the whole coronal emission of Capella in March 2000 was 3% higher than in February 2001; there also appears to be a tendency, albeit a marginal one, for the hottest lines to show the largest changes between the two observations. A detailed search for short-term variability (on time scales ranging from 102 to 104 s) in the emission of individual lines shows that in all cases the emission is compatible with a constant source; the firm upper limits of 5%-10% to the source variability on short time scales suggests that the intense X-ray emission is due to stable coronal structures and not to flaring activity. We have also determined the coronal thermal structure, as described with the emission measure distribution vs. temperature and with the help of plasma density, derived from the analysis of the O Vii, Ne Ix, Mg Xi and Si Xiii He-like ion triplets. The emission measure distribution, em(T), and the element abundances, have been reconstructed with the Markov-Chain Monte Carlo method by \citeauthor{KashyapDrake1998}; the em(T) presents a previously known sharp peak around log T=6.8-6.9, but we have also found evidence of a small amount of plasma at T>107 K. With the help of the em(T) and the density values we have estimated the pressure and volume of the emitting plasma at different temperatures, and we have derived information about the structure of individual loops and about the population of loops having different maximum temperatures. Our results indicate that loops with higher maximum temperature have higher pressure and smaller volume than lower temperature loops.

Appendices A and B are only available in electronic form at http://www.edpsciences.org Title: EBIT diagnostics using X-ray spectra of highly ionized Ne Authors: Matranga, Marco; Barbera, Marco; Maggio, Antonio; Peres, Giovanni; Serio, Salvatore; Takács, Endre; Silver, Eric; Gillaspy, John; Schnopper, Herbert; Laming, Martin; Beeman, Jeff; Haller, Eugen; Madden, Norman Bibcode: 2003NIMPB.205..244M Altcode: We have carried out a detailed analysis of highly ionized neon spectra collected at the NIST EBIT using an NTD germanium X-ray microcalorimeter developed at the Harvard-Smithsonian Center for Astrophysics [Nucl. Instr. and Meth. A 444 (2000) 156]. Our attention was focused especially on the Ne IX He-like triplet to check electron density diagnostics through the intercombination/forbidden line ratio. We have investigated possible effects of the ion dynamics on the plasma emission line intensities, looking at the dependence of the count-rate and the charge state distribution on the electron beam energy and current. The temperature and spatial distribution of the neon ions, and hence the overlap between the electron beam and the ion cloud, depend on the electron beam operating parameters. The overlap affects the average electron density seen by the ions, and in turn the measured line ratio. These results underscore the value of future improved studies of the trapped ion dynamics, both for understanding the EBIT performance and for allowing experimenters to take full advantage of its potential for astrophysical plasma diagnostics. Title: Hydrodynamic Model of Loops Heated by Microflares at the Footpoints Authors: Testa, P.; Peres, G. Bibcode: 2003SPD....34.1702T Altcode: 2003BAAS...35..837T We simulate loops maintained in coronal conditions by random heat pulses concentrated close to the footpoints, by using the Palermo-Harvard 1-D hydrodynamic code. We have explored the existence of dynamic but quasi-static solutions when the heating is very concentrated at footpoints whereas the heating concentrated at footpoints does not allow static loop models solutions. We studied the characteristics of the solutions (in terms of stability, density and temperature structure, emission measure distribution) as a function of the parameters that define the heating function, for three different loop lengths.

We found stable solutions even for heating concentrated over spatial regions of the order of L/5 and higher values. For these stable solutions, the average temperature profiles as a function of the loop coordinate show a flatter or even inverted profile (for the case with more concentrated heating) with respect to the standard static models; the emission measure distribution as a function of temperature is much steeper (up to ∼ T5) than the usual behavior as T3/2 of the hydrostatic standard models. Title: ROSAT HRI Investigation of the NGC 507 X-Ray Halo Authors: Paolillo, M.; Fabbiano, G.; Peres, G.; Kim, D. -W. Bibcode: 2003ApJ...586..850P Altcode: 2002astro.ph.11438P We present an X-ray investigation of the elliptical galaxy NGC 507 in the Pisces cluster. We make use of archival ROSAT HRI and Chandra data, and of previously published PSPC data, to connect the large-scale structure of the halo to the core morphology. Our analysis, based on a bidimensional double beta model of the halo surface brightness, shows that the halo core (r<2-3re) and the external halo (r>3re) are characterized by different dynamical properties and suggests a different origin of the two components. The halo core has a complex morphology with a main X-ray emission peak, coincident with the center of the optical galaxy, and several secondary peaks. The spatial and spectral analysis of the central peak shows that this feature is produced by denser hot gas in the galaxy core. While both homogeneous and inhomogeneous cooling flow models predict a deposited mass exceeding the observed amount, our data support the scenario in which the gas is kinetically heated by stellar mass losses. Comparison with previously published studies suggests that the core of an X-ray extended galaxy is associated with the stellar distribution and has properties similar to the X-ray halos of compact galaxies. The secondary peaks are due instead to interactions between the radio-emitting plasma and the surrounding ISM, producing density fluctuations in the hot gas. We find that the energy input by the central radio source in the ISM is large enough to prevent gas cooling and may explain the failure of the cooling flow models. The total mass profile derived from the bidimensional model shows that a significant amount of dark matter is present at large radii. The dark halo extends on cluster scales and is likely associated with the whole cluster rather than with NGC 507. This structure is typical of many X-ray-bright early-type galaxies and may explain the spatial and spectral differences with X-ray compact galaxies largely debated in the literature. The large-scale surface brightness distribution is irregular and more extended in the northeast direction. The displacement of the cluster halo from the optical galaxy and the filamentary structures observed in the halo core further suggest that the galaxy may be slowly moving within the group potential. Finally, we found that ~20% of the sources detected by Kim & Fabbiano in the NGC 507 halo are due to point sources, while the nature of the remaining population is not clear. If associated with NGC 507, they could be either accreting binaries hosting a massive black hole or density clumps of the X-ray halo. Title: Monitoring the stability of thin and medium back-up filters of the Newton-XMM EPIC camera Authors: Barbera, Marco; Collura, Alfonso; Artale, Marinella A.; Varisco, Salvatore; Peres, Giovanni; Sciortino, Salvatore; Serio, Salvatore; Villa, Gabriele E. Bibcode: 2003SPIE.4851..264B Altcode: We are conducting a measurement program on back-up filters of the XMM-Newton EPIC camera aimed at monitoring possible aging effects during the mission lifetime. One thin and one medium EPIC back-up filters have been stored since 1997 in an environment similar to that one of the flight filters (dry nitrogen box before launch, high vacuum after launch). The transmission of the two filters has been measured periodically in the 1900-10000 angstrom wavelength range where effects of aging would be clearly evident. The preliminary results, after 5 years of monitoring, show that a slight aging effect has occurred on both filters which, however, has no significant impact onto the EPIC calibration for the correct analysis of the X-ray astrophysical observations. Title: Spectral Variability of the Nucleus of M33 in a Chandra/ACIS Observation Authors: La Parola, V.; Damiani, F.; Fabbiano, G.; Peres, G. Bibcode: 2003ApJ...583..758L Altcode: 2002astro.ph.10174L We have analyzed a 90 ks long observation of the bright nucleus of M33 made with Chandra/ACIS. We detected low-amplitude (~10%) highly significant variability on timescales of ~5000 s. We also found associated spectral variability. The two main spectral components [a power law with Γ~2 and a multicolor disk (MCD) with kT~0.9-1.2 keV] vary in relative flux. The MCD temperature also increases with increasing MCD flux. The pattern of variability is reminiscent of (but not identical to) Galactic black hole binaries. An >~5 Msolar accreting black hole may explain this source. Title: Physics of the outer atmosphere of the Sun and of stars along their evolutionary track Authors: Peres, G.; Randich, S. Bibcode: 2003MmSAI..74..403P Altcode: We discuss the activity of a Co-financed project dedicated to study stellar activity and, more in general, stellar properties, through observations in X-ray, UV, optical etc. bands and through modeling. A key aspect of the project is the study of activity of stars in various phases of their evolution so as to ascertain the role of evolution in determining activity and the role of activity in influencing evolution, e.g. through the significant loss of angular momentum. An even more detailed study, thanks to its proximity, of the Sun is a fundamental part of this project; the relevant information is indeed both important for solar physics `per se' and to help defining with a high level of detail, and therefore of insight, the physical conditions in the solar atmosphere which thus is an important reference for the studies of stellar activity. Title: Development and Application of Numerical Modules for FLASH in Palermo: Two Astrophysical Examples Authors: Orlando, S.; Peres, G.; Reale, F.; Rosner, R.; Plewa, T.; Siegel, A. Bibcode: 2003MSAIS...1...45O Altcode: We collaborate with the Flash Center at the University of Chicago to help upgrading and to apply extensively the FLASH code to astrophysical problems. In particular, we have developed new modules for FLASH which extend the field of applicability of the code to some areas in astrophysics, like solar and stellar coronae, and supernova remnants. The new modules so far developed and tested describe: the non-equilibrium ionization effects of the most abundant elements in astrophysical plasmas, the thermal conduction and the viscosity according to the formulation of Spitzer (1962), and the radiative losses from an optically thin plasma according to the Raymond spectral code, and to Peres et al. (1982) for the chromosphere. We show some selected results for a coronal flare and for a supernova remnant, obtained with the version of FLASH 2.0 code including the new modules. Title: Non-Equilibrium Ionization Effects Induced During Coronal Flares Authors: Orlando, S.; Peres, G.; Reale, F.; Rosner, R.; Siegel, A. Bibcode: 2003MmSAI..74..643O Altcode: We present preliminary results of hydrodynamic modeling of flares occurring in plasma confined in coronal loops. Our analysis focuses on the deviations from ionization equilibrium on the population fractions of the most abundant elements in astrophysical plasmas, and on the possible implications for plasma diagnostics. Title: HPC in Astronomy: overview and perspectives Authors: Peres, G. Bibcode: 2003MSAIS...1..223P Altcode: I present a personal overview of italian high performance computing in astronomy, as given with the presentation of this meeting, then suggest ways for a more fruitful interaction with the rest of Astronomy; finally and more important, I suggest a qualitative improvement of high performance computing in astronomy through the support of challenging and competitive key projects. Title: Experimental activity in Palermo related to Solar-B and CALOS satellites Authors: Peres, G.; Barbera, M.; Orlando, S.; Ciaravella, A.; Reale, F.; Serio, S. Bibcode: 2003MmSAI..74..831P Altcode: We report on the experimental activities related to the calibration of the XRT telescope on board the Solar-B satellite and on the design and studies related to the X-ray-calorimeters-based satellite CALOS (Calorimetri per Osservazioni Solari). Title: X-ray variability in ROSAT-PSPC observations of dF7-dK2 stars Authors: Marino, A.; Peres, G.; Micela, G.; Sciortino, S. Bibcode: 2002ASPC..277..539M Altcode: 2002sccx.conf..539M No abstract at ADS Title: AD Leo observed with Chandra LETG: emission measure distribution density, and element abundances of the coronal plasma Authors: Maggio, A.; Drake, J. J.; Kashyap, V.; Micela, G.; Sciortino, S.; Peres, G.; Harnden, F. R., Jr.; Murray, S. S. Bibcode: 2002ASPC..277...57M Altcode: 2002sccx.conf...57M No abstract at ADS Title: XMM-Newton Observations of X-ray Luminous G-type Stellar Coronae Authors: Scelsi, L.; Maggio, A.; Peres, G.; Sanz-Forcada, J.; Gondoin, Ph.; Pallavicini, R. Bibcode: 2002hrxs.confE..22S Altcode: We present results obtained from the analysis of XMM-Newton spectra of the Hertzsprung-gap giant 31 Com and of the pre-main-sequence star HD 283572, two G-type stars in very different evolutionary phases, but having similar high X-ray luminosities. We discuss methodological issues related to the data analysis, as well as the results on the inferred properties of the stellar coronae. Title: Temperature and Density Structure of Hot and Cool Loops Derived from the Analysis of TRACE Data Authors: Testa, P.; Peres, G.; Reale, F.; Orlando, S. Bibcode: 2002ApJ...580.1159T Altcode: We study the transversal structure (in particular the filamentation) and the longitudinal plasma stratification in two sets of solar coronal loops observed with TRACE in the 171 and 195 Å passbands. The density stratification and the thermal structuring of the plasma along the fibrils that form the loops are derived using two techniques simultaneously: (1) a filter ratio diagnostic (195/171 Å) and (2) modeling intensity profiles along the fibrils with hydrostatic models; in both techniques we remove the background flux. We find consistent values of temperature and density with both methods in the coronal structures selected. We find evidence of a very warm fibril (T~5×106 K) and of rather cold ones (T~2×105 K). The hot fibril appears to have a thermal structure with a maximum at the apex compatible with temperature and density stratification, dictated by energy balance, typical of a nonisothermal hydrostatic loop. The cold fibrils appear to be isothermal and are probably in dynamical conditions. Title: Observing the Sun as an X-ray Star: Active Region Evolution and Rotational Modulation Authors: Orlando, S.; Peres, G.; Reale, F. Bibcode: 2002ASPC..277..341O Altcode: 2002sccx.conf..341O No abstract at ADS Title: Temperature and density structure of hot and cool loops derived from the analysis of TRACE data Authors: Testa, P.; Peres, G.; Reale, F.; Orlando, S. Bibcode: 2002ESASP.505..203T Altcode: 2002solm.conf..203T; 2002IAUCo.188..203T We address the plasma structuring both across and along the magnetic field in two sets of solar coronal loops, observed with TRACE in the 171 Å and 195 Å passbands. We derive, after proper background removal, the density stratification and the thermal structure of the plasma in the fibrils forming the loops with two techniques: a) filter ratio diagnostic (195 Å/171 Å) and b) modeling intensity profiles along the fibrils with hydrostatic models. We find evidence of a hot fibril (T ~ 5×106K), with temperature and density stratification well-described with a typical non-isothermal hydrostatic loop model, and evidence of rather cold fibrils (T ~ 2×105K), isothermal and probably in dynamic conditions. Title: Non-equilibrium ionization effects during flares in coronal loops Authors: Orlando, Salvatore; Peres, Giovanni; Reale, Fabio; Rosner, Robert Bibcode: 2002ESASP.505..517O Altcode: 2002solm.conf..517O; 2002IAUCo.188..517O We present preliminary results of hydrodynamic modeling of flares occurring in plasma magnetically confined in coronal loops, using the adaptive-mesh astrophysical hydrodynamic code FLASH. We also investigate the deviations from ionization equilibrium on the population fractions of the most abundant elements in astrophysical plasmas and the relevant consequences on observed spectra. Title: Modeling non-confined coronal flares: Dynamics and X-ray diagnostics Authors: Reale, F.; Bocchino, F.; Peres, G. Bibcode: 2002A&A...383..952R Altcode: 2001astro.ph.12333R Long-lasting, intense, stellar X-ray flares may approach conditions of breaking magnetic confinement and evolving in open space. In the perspective of searching for possible tracers of non-confinement, we explore this hypothesis with hydrodynamic simulations of flares occurring in a non-confined corona: model flares are triggered by a transient impulsive heating injected in a plane-parallel stratified corona. The plasma evolution is described by means of a numerical 2-D model in cylindrical geometry R,Z. We explore the space of fundamental parameters. As a reference model, we consider a flare triggered by a heating pulse of 10 erg cm-3 s-1 lasting 150 s and released in a region ~ 109 cm wide and at a height ~ 2 x 109 cm from the base of the stellar surface. The pressure at the base of the corona of the unperturbed atmosphere is 0.1 dyne cm-2. The heating would cause a 20 MK flare if delivered in a 40 000 km long closed loop. The modeled plasma evolution in the heating phase involves the propagation of a 10 MK conduction front and the evaporation of a shocked bow density front upwards from the chromosphere. As the heating is switched off, the temperature drops in few seconds while the density front still propagates, expanding, and gradually weakening. This kind of evolution is shared by other simulations with different coronal initial pressure, and location, duration and intensity of the heating. The X-ray emission, spectra and light curves at the ASCA/SIS focal plan, and in two intense X-ray lines (Mg XI at 9.169 Åand Fe XXI at 128.752 Å), have been synthesized from the models. The results are discussed and compared to features of confined events, and scaling laws are derived. The light curves invariably show a very rapid rise, a constant phase as long as the constant heating is on, and then a very fast decay, on time scales of few seconds, followed by a more gradual one (few minutes). We show that this evolution of the emission, and especially the fast decay, together with other potentially observable effects, are intrinsic to the assumption of non-confinement. Their lack indicates that observed long-lasting stellar X-ray flares should involve plasma strongly confined by magnetic fields. Title: CALOS: an experiment to study the solar corona with an array of NTD Ge microcalorimeters Authors: Barbera, M.; Peres, G.; Orlando, S.; Reale, F.; Collura, A.; Serio, S.; Silver, E.; Bandler, S.; Schnopper, H. W.; Costa, E.; Bellazzini, R. Bibcode: 2002AIPC..605..547B Altcode: In response to the Italian Space Agency announcement ``New Ideas for Space Missions,'' we have proposed an observatory ``CALorimetri per Osservazioni Solari'' (CALOS) that will perform spatially resolved (Δθ~2') X-ray spectroscopy of the solar corona over the 0.1-10 keV band using an array of NTD germanium microcalorimeters. The observatory will also include an X-ray polarimeter of radically new design that will study the hard X-ray solar emission and its polarization and will serve as a flare alarm. . Title: Deep ROSAT HRI Observations of the NGC 1399/NGC 1404 Region: Morphology and Structure of the X-Ray Halo Authors: Paolillo, M.; Fabbiano, G.; Peres, G.; Kim, D. -W. Bibcode: 2002ApJ...565..883P Altcode: 2001astro.ph..9342P We present the analysis of a deep (167 ks) ROSAT HRI observation of the cD galaxy NGC 1399 in the Fornax Cluster, comparing it with previous work on this galaxy and with recent Chandra data. We find, in agreement with previous observations, an extended and asymmetric gaseous halo with a luminosity (in the 0.1-2.4 keV energy band) of LX=(5.50+/-0.04)×1041 ergs s-1 within 46 kpc (assuming a distance of D=19 Mpc). Using both HRI and, at larger radii, archival PSPC data, we find that the radial behavior of the X-ray surface brightness profile is not consistent with a simple β-model and suggests instead three distinct components. We use a multicomponent bidimensional model to study in detail these three components, which we identify respectively with the cooling flow region, the galactic halo, and the cluster halo. From these data we derive a binding mass distribution in agreement with that suggested by optical dynamical indicators, with an inner core dominated by luminous matter and an extended dark halo differently distributed on galactic and cluster scales. The HRI data and a preliminary analysis of Chandra public data allow us to detect significant density fluctuations in the halo. We discuss possible nonequilibrium scenarios to explain the hot halo structure, including tidal interactions with neighboring galaxies, ram stripping from the intracluster medium, and merging events. In the innermost region of NGC 1399, the comparison between the X-ray and radio emission suggests that the radio-emitting plasma is displacing and producing shocks in the hot X-ray-emitting gas. We do not detect the nuclear source in X-rays, and we pose an upper limit of ~4×1039 ergs s-1 (0.1-2.4 keV) to its X-ray luminosity. We found that the NGC 1404 halo is well represented by a single symmetric β-model and follows the stellar light profile within the inner 8 kpc. The mass distribution is similar to the ``central'' component of the NGC 1399 halo. At larger radii, ram pressure stripping from the intracluster gas produces strong asymmetries in the galactic halo. Finally, we discuss the properties of the point-source population, finding evidence of correlation between the source excess and NGC 1399. Title: Deep ROSAT-HRI Observation of the cD Galaxy NGC 1399 in the Fornax Cluster: Morphology and Dynamical Status of the X-Ray Halo Authors: Paolillo, M.; Peres, G.; Fabbiano, G.; Kim, D. -W. Bibcode: 2002ASPC..268..425P Altcode: 2002tceg.conf..425P No abstract at ADS Title: SADE, the Student Astrophysical Dynamo Explorer Authors: Martens, P.; Acton, L.; Klumpar, D.; Stern, R.; Peres, G.; Culhane, L. Bibcode: 2002cosp...34E1298M Altcode: 2002cosp.meetE1298M In soft x-rays the solar coronal radiance varies by a factor of 10-30 over the solar activity cycle. A similar variation in most stars in the existing x-ray database has not been found (Stern 2001); even stars which exhibit chromospheric activity cycles show only marginal evidence for X-ray cycles. This is rather puzzling as the time span and multiple coverage of the x-ray sky should reveal at least a hint of such a pronounced cyclical variation. We propose a mission called the Student Astrophysical Dynamo Explorer to measure the x-ray brightness of about 75 stars once every 5 days for up to 15 years. Selection of prime stars takes into account location (avoid eclipse), rotation rate, Ca-K observations, and magnetic field strength, to focus on the best candidates for dynamo studies. We baseline a nested 4-5 mirror system with 200 cm^2 geometric area, with a 1.5 to 2 meter focal length, 15 arcsec on-axis resolution, and Au or Ni coatings. The strawman detector is a back-illuminated CCD of 512x512 pixels, with pixels that can be large as a 15 arcseconds. Available exposure time per star per visit is about an hour and a half. We are exploring the option of adding a visible light detector for astroseismology. To minimize operations cost for this long duration mission we envisage tracking and commanding from a simple ground station at Montana State University, operated by students under the auspices of MSU's Space Science and Engineering Lab (SSEL). Title: Spectral and time analysis of the coronal X-ray emission from Capella observed with Chandra/LETG Authors: Argiroffi, C.; Maggio, A.; Peres, G. Bibcode: 2002cosp...34E3034A Altcode: 2002cosp.meetE3034A We present a detailed analysis of two X-ray spectra of Capella, taken eleven months apart, obtained with the LETGS of the handra Observatory. We have identified the strongest emission lines, including several Fe lines with ionization level from FeXVI to FeXXIII. We have determined the electron density Ne and the relevant temperature T using the line ratios of the OVII, NeIX, MgXI and SiXIII He-like triplets, finding Ne values in the range 10^10-10^12 cm-3; from the Ne and T values we have derived volumes and pressures of the emitting plasma vs temperatures. Emission measure distribution em(T), with element abundances, has been reconstructed for each observation with the Markov-Chain Monte Carlo method proposed by Kashyap &Drake (1998). The em(T) distribution obtained has a sharp peak around log T=6.8-6.9 and no significant amount of plasma has been found for log T>7.2. From the em(T) shape we have derived information about the population of loops at different temperatures. We have compared the results obtained from each observation finding that during the first one the whole coronal emission was 3% stronger; there also appears to be a tendency, albeit a marginal one, for hotter lines to show the largest changes between the two observations. We have also performed a detailed search for short-term variability with the Collura et al. (1987) method testing photon arrival times of individual lines (the investigated time scale range is 10^2-10^4 s), thus testing the variability of coronal plasma in a narrow temperature range. In all the cases analysed we have found that emission was compatible with a constant source, therefore we can only pose upper limits of 5%-10% to the source variability, suggesting that the intense X- ray emission is due to stable structures like loops and not to flare activity. Title: Viewing the Sun as an X-ray star Authors: Orlando, S.; Peres, G.; Reale, F. Bibcode: 2002cosp...34E.906O Altcode: 2002cosp.meetE.906O Thanks to its distance from the Earth, the Sun is the late-type star we can study with the highest level of detail. It is not surprising, therefore, that many stellar data are interpreted in the light of the so-called ``solar-stellar analogy", i.e. the assumption that the coronae of late-type stars are the result of physical processes similar to those leading to the solar corona. In this paper we review the results obtained from studying the Sun as an X-ray star in the attempt to explore how far the solar model can be applied to other stars, and how the underlying processes may differ. In particular, we discuss how a star identical to the Sun appears to change during its cycle, how different coronal regions resolved on the Sun contribute to the whole corona X-ray spectrum, and how flares may affect the stellar-like spectrum of the Sun. We discuss also the implications on the X-ray variability we should observe on a star identical to the Sun due to different phenomena, e.g. stellar cycle, flares and rotational modulation. Title: On X-ray variability in ROSAT-PSPC observations of F7-K2 stars Authors: Marino, A.; Micela, G.; Peres, G.; Sciortino, S. Bibcode: 2002A&A...383..210M Altcode: We have analyzed the X-ray variability of dF7-dK2 stars in the solar neighborhood detected with the pointed ROSAT-PSPC observations. Our data base is the sample of all stars listed in the CNS3 catalog (Gliese & Jahreibeta 1991) having a B-V color between 0.5 and 0.9; it includes 70 pointed observations of 40 distinct stars or multiple systems. We have applied the unbinned Kolmogorov-Smirnov test on all X-ray photon time series of our sample: only 10 observations relative to 8 distinct stars are variable at a confidence level greater than 99% and 4 of them belong to multiple systems. For the subsample of 9 stars observed both at the beginning and at the end of the mission, we can study the variability on time scale of years and compare amplitude variations at short and long time scales. Our analysis suggests that, for these stars, the X-ray variability is more likely on longer time scale. All the stars variable on long time scale, and not on short time scale, are relatively quiet and similar to the Sun, suggesting that the variations may be due to cycles. The comparison of our results with those previously obtained for dM stars shows that the amplitude of variability of X-ray emission from dF7-dK2 stars is smaller than that observed in dM stars. Title: The Distribution of the Emission Measure, and of the Heating Budget, among the Loops in the Corona Authors: Peres, G.; Orlando, S.; Reale, F.; Rosner, R. Bibcode: 2001ApJ...563.1045P Altcode: 2001astro.ph.11192P The aim of this paper is to validate a methodology for connecting the emission measure of individual solar coronal loops to the integrated emission measure of the entire solar corona and using this connection to deduce the energetic properties of the corona; we then show how this methodology can be applied to observations of solar-like stellar coronae. The solar validation is carried out by using spatially resolved X-ray observations of the Sun obtained from the Yohkoh satellite. This work is a further step in our effort to place the ``solar-stellar connection'' on a quantitative footing. In particular, we show how this analysis procedure can be used in the context of archival Einstein, ROSAT, and EUVE data, as well as for Chandra and XMM-Newton data, as a complementary analysis tool to existing multithermal component models. Title: Coronal loop hydrodynamics. The solar flare observed on November 12, 1980 revisited: The UV line emission Authors: Betta, R. M.; Peres, G.; Reale, F.; Serio, S. Bibcode: 2001A&A...380..341B Altcode: 2001astro.ph.10514B We revisit a well-studied solar flare whose X-ray emission originating from a simple loop structure was observed by most of the instruments on board SMM on November 12, 1980. The X-ray emission of this flare, as observed with the XRP, was successfully modeled previously. Here we include a detailed modeling of the transition region and we compare the hydrodynamic results with the UVSP observations in two EUV lines, measured in areas smaller than the XRP rasters, covering only some portions of the flaring loop (the top and the foot-points). The single loop hydrodynamic model, which fits well the evolution of coronal lines (those observed with the XRP and the Fe XXI 1354.1 Å line observed with the UVSP) fails to model the flux level and evolution of the O V 1371.3 Åline. Title: The Sun as an X-Ray Star. IV. The Contribution of Different Regions of the Corona to Its X-Ray Spectrum Authors: Orlando, S.; Peres, G.; Reale, F. Bibcode: 2001ApJ...560..499O Altcode: We study X-ray-synthesized spectra of solar regions as templates to interpret analogous stellar spectra. We define three classes of coronal structures of different brightness, low (background quiet corona), medium (active regions), and high (active region cores), and determine their contribution to the solar X-ray emission measure versus temperature, EM(T), luminosity, and spectrum. This study defines the extent of the solar analogy quantitatively and accurately. To this end, we have selected a large sample of full-disk Yohkoh soft X-ray telescope observations taken between the maximum and the minimum of solar cycle 22, obtaining the contribution of each class to the whole Sun's EM(T). From the EM(T) distributions, we synthesize the X-ray spectra of the Sun and of the single classes of solar coronal regions as they would be collected with the ROSAT Position Sensitive Proportional Counter (PSPC) and ASCA Solid-State Imaging Spectrometer. We find that the Sun during the cycle fits well in the stellar scenario as a low-activity star. The ROSAT PSPC hardness ratio (HR) and surface X-ray flux, FPSPC, both increase going from the background corona to the active regions and the cores of the active regions, and range between the values of low and intermediate activity stars. We suggest that the coronae of these stars may be explained as the effect of structures similar to those present on the Sun and that the various levels of X-ray luminosity, HR, and FPSPC are achieved by changing the surface coverage of the different classes of coronal regions. Title: Contribution of various coronal structures to the emission measure vs. temperature distribution and X-ray spectrum of the corona Authors: Orlando, Salvatore; Peres, Giovanni; Reale, Fabio Bibcode: 2001ESASP.493..301O Altcode: 2001sefs.work..301O No abstract at ADS Title: Temperature and density structure of hot and cool loops derived from the analysis of TRACE data Authors: Testa, P.; Peres, G.; Reale, F.; Orlando, S. Bibcode: 2001ESASP.493..389T Altcode: 2001sefs.work..389T No abstract at ADS Title: The OAPA/DPSFA: solar physics, instrumental expertise, and the XACT facility Authors: Barbera, M.; Collura, A.; Peres, G.; Reale, F.; Orlando, S.; Serio, S. Bibcode: 2001ESASP.493..167B Altcode: 2001sefs.work..167B No abstract at ADS Title: The Sun as an X-Ray Star. III. Flares Authors: Reale, F.; Peres, G.; Orlando, S. Bibcode: 2001ApJ...557..906R Altcode: 2001astro.ph..4021R In previous works we have developed a method to convert solar X-ray data, collected with the Yohkoh/SXT, into templates of stellar coronal observations. Here we apply the method to several solar flares, for comparison with stellar X-ray flares. Eight flares, from weak (GOES class C5.8) to very intense ones (X9) are selected as representative of the flaring Sun. The emission measure distribution versus temperature, EM(T), of the flaring regions is derived from Yohkoh/SXT observations in the rise, peak, and decay of the flares. The EM(T) is rather peaked and centered around T~107 K for most of the time. Typically, it grows during the rise phase of the flare, and then it decreases and shifts toward lower temperatures during the decay, more slowly if there is sustained heating. The most intense flare we studied shows emission measure even at very high temperatures (T~108 K). Time-resolved X-ray spectra both unfiltered and filtered through the instrumental responses of the nonsolar instruments ASCA/SIS and ROSAT/PSPC are then derived. Synthesized ASCA/SIS and ROSAT/PSPC spectra are generally well fitted with single thermal components at temperatures close to that of the EM(T) maximum, albeit two thermal components are needed to fit some flare decays. ROSAT/PSPC spectra show that solar flares are in a 2 orders of magnitude flux range (106-108 ergs cm-2 s-1) and a narrow PSPC hardness ratio range, however, higher than that of typical nonflaring solar-like stars. Title: 1997 December 12 Helical Coronal Mass Ejection. II. Density, Energy Estimates, and Hydrodynamics Authors: Ciaravella, A.; Raymond, J. C.; Reale, F.; Strachan, L.; Peres, G. Bibcode: 2001ApJ...557..351C Altcode: We use Ultraviolet Coronagraph and Spectrometer (UVCS) spectra to investigate the density range of the plasma ejected during the coronal mass ejection (CME) on 1997 December 12. Time-dependent ionization states for several phenomenological models, with the boundary conditions derived from the EUV Imaging Telescope (EIT) and UVCS observations, were computed and constraints on the density and temperature of the plasma at the early stage of the ejection are obtained. The role of physical mechanisms such as thermal conduction, radiation, and heating is also studied with a two-dimensional hydrodynamics simulation. The kinetic, thermal, and gravitational energies are estimated as well as the plasma heating. Whenever the ejected plasma has a density >=109cm-3, a continuous supply of heat is required to meet the conditions observed at 1.7 Rsolar. Moreover heating mechanisms that release energy gradually during the outward motion of the plasma seem to be more appropriate than those that dump most of the energy when the plasma is lower in the corona. Our simulations also indicate that a three-dimensional self-similar expansion does not fit the UVCS observations. Comparisons with some CME models from the dynamical and energetics points of view are discussed. Title: The Ultraluminous M81 X-9 Source: 20 Years' Variability and Spectral States Authors: La Parola, V.; Peres, G.; Fabbiano, G.; Kim, D. W.; Bocchino, F. Bibcode: 2001ApJ...556...47L Altcode: 2001astro.ph..3250L The source X-9 was discovered with the Einstein Observatory in the field of M81 and is located in the dwarf galaxy Holmberg IX. X-9 has a 0.2-4.0 keV luminosity in excess of the Eddington limit for a 1 Msolar compact accreting object, if it is at the same distance as Holmberg IX (3.4 Mpc). Past hypotheses on the nature of this super-Eddington source included a supernova remnant or supershell, an accreting compact object, and a background QSO. To shed light on the nature of this source, we have analyzed archival data, including the Einstein data, 23 ROSAT observations, and BeppoSAX and ASCA pointings. Our analysis reveals that most of the emission of X-9 arises from a pointlike highly variable source (0.5-2.4 keV LX~2-8×1039 ergs s-1) and that lower luminosity extended emission may be associated with it. The spectrum of this source changes between low- and high-intensity states, in a way reminiscent of the spectra of galactic black hole candidates. Our result strongly suggest that X-9 is not a background QSO, but a bona fide ``super-Eddington'' source in Ho IX, a dwarf companion of M81. Title: Deep ROSAT-HRI observation of the elliptical galaxy NGC 1399 Authors: Paolillo, M.; Fabbiano, G.; Peres, G.; Kim, D. -W. Bibcode: 2001astro.ph..6309P Altcode: We present the preliminary results of a deep (167 ks) ROSAT HRI observation of the cD galaxy NGC1399 in the Fornax cluster. We find, in agreement with previous observations, an extended (41 Kpc adopting a distance of 19 Mpc) gaseous halo with a luminosity of L_X=(4.41\pm 0.04)x10^{41} erg/s. The 5 arcsec resolution of the data allows us to detect a very complex and asymmetric structure of the halo with respect to the optical galaxy. Moreover the analysis of the radial structure reveals the presence of a multi-component profile not consistent with a simple King model over the whole 40 Kpc. We do not detect the presence of a central source and pose an upper limit to the luminosity of a possible active nucleus. Due to the length of the observation, comparable to that of a deep survey, we detect a large number of sources within the HRI FOV, in slight excess with respect to the estimates based on previous surveys. We study the flux distribution of the sources, their temporal behaviour and their spatial distribution with respect to the central galaxy. Title: Spectral and temporal properties of X-ray emission from the ultra-luminous source X-9 in M81 Authors: La Parola, V.; Fabbiano, G.; Kim, D. W.; Peres, G. Bibcode: 2001astro.ph..2111L Altcode: We have analysed the spectra and the variability of individual X-ray sources in the M-81 field using data from the available ROSAT-PSPC and ROSAT-HRI observations of this nearby spiral galaxy. Here we present the results on the second brightest source in the field (X-9 - Fabbiano, 1988 ApJ 325 544), whose identification and interpretation is still unclear. Our work includes the study of the shape of X-9 from HRI data, the light curve and hardness ratio evolution, and the spectral analysis. Title: Spectra and Evolution of Two X-Ray Sources in M81 Authors: La Parola, V.; Peres, G.; Fabbiano, G.; Kim, D. W. Bibcode: 2001ASPC..230..397L Altcode: 2001gddg.conf..397L; 2001astro.ph..2088L We analyzed the spectral and temporal features of X-ray sources in M81 using data from all the relevant ROSAT-PSPC/HRI observations. We discuss the main features of the point-like nucleus and of the second brightest source in the field (X-9), whose identification is still unclear. Title: The Solar Corona vs. Stellar Coronae Authors: Peres, G. Bibcode: 2001ASPC..234...41P Altcode: 2001xras.conf...41P No abstract at ADS Title: Recent results on the study of the Sun as an X-ray star Authors: Peres, Giovanni; Orlando, Salvatore; Reale, Fabio Bibcode: 2001MmSAI..72..583P Altcode: The Sun is an important test-bed for the physics of stellar coronae. However the context, the methods and the scope of solar studies are different from those of stellar ones and a direct comparison among the two is hard. In order to bridge the gap between solar and stellar coronal physics, we have developed and tested a method to compare directly X-ray data relevant to the solar and the stellar coronae; it is based on the wide band X ray data collected with Yohkoh/SXT. The intermediate results of this analysis (emission measure distributions vs. temperature, solar X-ray spectra, their evolution etc.) are important also in the context of solar physics. We discuss the solar X-ray characteristic in the stellar context, its variability during the cycle, solar X-ray flares, and how these results compare with those of stellar coronae. Title: On Stellar Coronae and Solar Active Regions Authors: Drake, Jeremy J.; Peres, Giovanni; Orlando, Salvatore; Laming, J. Martin; Maggio, Antonio Bibcode: 2000ApJ...545.1074D Altcode: Based on Yohkoh Soft X-Ray Telescope (SXT) observations of the Sun near peak activity level obtained on 1992 January 6, we search for coronal structures that have emission measure distributions EM(T) that match the observed stellar coronal emission measure distributions derived for the intermediate-activity stars ɛ Eri (K2 V) and ξ Boo A (G8 V) from Extreme Ultraviolet Explorer spectroscopic observations. We find that the temperatures of the peaks of the observed stellar distributions EM(T), as well as their slopes in the temperature range 6.0<~logT<~6.5, are very similar to those obtained for the brightest of the solar active regions in the 1992 January 6 SXT images. The observed slopes correspond approximately to EM~Tβ with β~4, which is much steeper than predicted by static, uniformly heated loop models. Plasma densities in the coronae of ɛ Eri and ξ Boo A are also observed to be essentially the same as the plasma densities typical of solar active regions. These data provide the best observational support yet obtained for the hypothesis that solar-like stars up to the activity levels of ɛ Eri (K2 V) and ξ Boo A are dominated by active regions similar to, though possibly considerably larger than, those observed on the Sun. The surface filling factor of bright active regions needed to explain the observed stellar emission measures is approximately unity. We speculate on the scenario in which small-scale ``nanoflares'' dominate the heating of active regions up to activity levels similar to those of ɛ Eri (K2 V) and ξ Boo A. At higher activity levels still, the interactions of the active regions themselves may lead to increasing flaring on larger scales that is responsible for heating plasma to the observed coronal temperatures of T>~107 K on very active stars. Observations of X-ray and EUV light curves using more sensitive instruments than are currently available, together with determinations of plasma densities over the full range of coronal temperatures (106-107 K and higher), will be important to confirm flare heating hypotheses and to elicit further details concerning coronal structures at solar-like active region temperatures (T<~5×106 K) and the temperatures that characterize the most active stars (T>~107 K). Title: Multi-Thread Modeling of Coronal Loops with TRACE Data Authors: Nightingale, R. W.; Aschwanden, M. J.; Alexander, D.; Reale, F.; Peres, G. Bibcode: 2000SPD....31.0211N Altcode: 2000BAAS...32..812N The temperature Te(s) and density structure ne(s) of active region loops in EUV observed with TRACE is modeled with a multi-thread model. The model loops are synthesized from the summed emission of many loop threads that have a distribution of maximum temperatures and that satisfy the steady-state Rosner-Tucker-Vaiana (RTV) scaling law, modified by Serio et al. for gravitational stratification (RTVSp). From model-fitting of the 171 and 195 Angstroms fluxes of 41 loops, which have loop half lengths in the range of L=4-320 Mm, we find: (1) The EUV loops can be explained by near-isothermal loop threads in the temperature range of Te ~ 0.8-1.6 MK with substantially smaller temperature gradients than predicted by the RTVSp model, (2) the loop base pressure, p0 ~ 0.3+/- 0.1 dyne cm-2, is independent of the loop length L, it agrees with the RTVSp model for the shortest loops, but exceeds the RTVSp model up to a factor of 35 for the largest loops, and (3) the pressure scale height is consistent with hydrostatic equilibrium for the shortest loops, but exceeds the temperature scale height up to a factor of ~ 3 for the largest loops. This work was supported by the TRACE project at LMSAL (contract NAS5-38099). Ref.: Aschwanden,M.J., Nightingale,R.W., Alexander,D., Reale,F., and Peres,G. 2000, ApJ, subm., ``Evidence for Nonuniform Heating of Coronal Loops Inferred from Multi-Thread Modeling of TRACE Data'', URL="ftp://sag.lmsal.com/pub/aschwand/2000_reale.ps.gz" Title: A Brightening Coronal Loop Observed by TRACE. I. Morphology and Evolution Authors: Reale, F.; Peres, G.; Serio, S.; DeLuca, E. E.; Golub, L. Bibcode: 2000ApJ...535..412R Altcode: We analyze the transient brightening of a solar coronal loop observed, at high time cadence (30 s) and spatial resolution (0.5" pixel size), with the Transition Region and Coronal Explorer (TRACE) in the 171 Å band on 1998 June 26. The loop, located in AR 8253, is ~1010 cm long and inclined with respect to the vertical to the solar surface. Its geometry and shape do not change significantly during the brightening, which lasts for ~2 hr and is preceded by highly dynamic events in nearby and perhaps interacting loops. The loop footpoints brighten first; after ~10 minutes, moving brightness fronts rise initially from the northern footpoint, and after another ~7 minutes from the southern one, at an apparent speed ~100 km s-1, the whole loop becoming clearly visible afterward. During the rising phase the loop evolves coherently as a single magnetic tube. The brightness profile is asymmetric with respect to the loop apex at all times; the brightness contrast between the footpoints and the apex decreases with time from a ratio of ~10 to ~3. After the loop has become all visible, the several parallel filaments which form it follow an independent evolution. Assuming a plasma temperature of ~1 MK, we infer a plasma density of ~6×108 cm -3 and a pressure of ~0.2 dyn cm-2 close to the loop apex at the luminosity maximum. A companion paper is devoted to modeling the rising phase of this event. Title: A Brightening Coronal Loop Observed by TRACE. II. Loop Modeling and Constraints on Heating Authors: Reale, F.; Peres, G.; Serio, S.; Betta, R. M.; DeLuca, E. E.; Golub, L. Bibcode: 2000ApJ...535..423R Altcode: This is the second of two papers dedicated to the brightening of a coronal loop observed by the Transition Region and Coronal Explorer (TRACE) on 1998 June 26; it aims at hydrodynamic modeling of the brightening. Since the loop geometry is practically unchanged during the brightening, the evolution of the plasma confined in the loop is described with a one-dimensional hydrodynamic time-dependent numerical model, and from the results the emission along the loop in the TRACE 171 Å band is synthesized. The information from Paper I is used to derive the geometry and the initial configuration of the loop as well as for comparison with the results of the model. The modeling is focused to determine the amount, spatial distribution, and evolution of the heating deposited in the loop to make the modeled evolution close to that observed with TRACE. We find that, in order to match the observed evolution and distribution of the brightness along the loop, the heating has to be nonsymmetrical in the loop, in particular, deposited between the apex and one footpoint (3×109 cm from the southern footpoint). A reasonable match with observations is obtained by assuming that the heating is switched on abruptly and then kept constant for the whole rising phase. An even better match is obtained with the heating high and constant for 100 s and then decaying exponentially with an e-folding time of 300 s. We discuss the resulting physical scenario; a bright irregular structure close to the loop in the TRACE images may be a tracer of the heating release. Title: What TRACE Observations tell us about Heating of Coronal Loops Authors: Aschwanden, M. J.; Nightingale, R. W.; Alexander, D.; Reale, F.; Peres, G. Bibcode: 2000SPD....31.0210A Altcode: 2000BAAS...32..812A We analyzed the temperature Te(s) and density structure ne(s) of active region loops in EUV observed with TRACE (see SPD abstract by Nightingale et al.). The observational data indicate that cool EUV loops with maximum temperatures of Tmax ~ 0.8-1.6 MK cannot be explained with the static steady-state scaling law of Rosner, Tucker, & Vaiana (1978) or Serio et al. (1981), in terms of uniform heating. However, they are fully consistent with Serio's model (which includes gravitation and a heating scale height) in the case of nonuniform heating, with heating scale heights in the range of sH=17 +/- 6 Mm. This heating function provides almost uniform heating for small loops (L < 20 Mm), but restricts heating to the footpoints of large loops (L ~ 50-300 Mm). Another observational result of cool EUV loops is that the pressure scale height exceeds the hydrostatic scale height by a factor of ql ~ 1-3. This suggests that the pressure balance of these EUV loops may not be governed by hydrostatic equilibrium, but rather indicates a non-steady state, e.g. caused by dynamic mass flows and/or intermittent heating. Chromospheric upflows may explain the extended scale heights as well as the quasi-isothermal temperature structure of EUV loops. We review and discuss the differences between cool (T ~ 1-2 MK) EUV loops and hot (T ~ 2-8 MK) soft X-ray loops concerning loop scaling laws, radiative equilibrium, hydrostatic equilibrium, and heating function. - This work is supported by the TRACE project at LMSAL (contract NAS5-38099) Ref.: Aschwanden,M.J., Nightingale,R.W., Alexander,D., Reale,F., and Peres,G. 2000, ApJ, subm., ``Evidence for Nonuniform Heating of Coronal Loops Inferred from Multi-Thread Modeling of TRACE Data'' Title: Models of dynamic coronal loops Authors: Peres, Giovanni Bibcode: 2000SoPh..193...33P Altcode: This paper reviews the basic ideas underlying one-dimensional fluid dynamic models of coronal loops and presents some of their most recent applications. These models are an important theoretical support to explore the new scenario provided by the data of Yohkoh, SOHO, and TRACE, and are useful to interpret observations, when supplemented by appropriate spectral synthesis codes. Possible developments are also discussed. Title: The XACT facility at OAPA : ongoing projects and future developments Authors: Barbera, M.; Collura, A.; Artale, M.; Candia, R.; Cavadi, A.; Mirabello, F.; Peres, G.; Perinati, E.; Sciortino, S.; Serio, S.; Varisco, S.; Bandler, S.; Murray, S. S.; Schnopper, H. W.; Silver, E.; Zombeck, M. V. Bibcode: 2000MmSAI..71.1127B Altcode: No abstract at ADS Title: A systematic analysis of X-ray variability of dM stars Authors: Marino, A.; Micela, G.; Peres, G. Bibcode: 2000A&A...353..177M Altcode: We have systematically analyzed X-ray variability of dM stars. Our data base is the sample of all dM stars listed in the CNS3 (Gliese & Jahreiss 1991) catalog which have been observed with the ROSAT PSPC. Our data sample includes 86 pointed observations of 55 distinct stars or multiple systems. A large fraction of stars shows significant variations, regardless of their quiescent flux. Variability is detected on all observable time scales. The amplitudes of these variations are independent of both stellar X-ray and visual luminosity. Compared to solar X-ray variability properties our results suggest that the amplitude distribution of X-ray variability in dM stars is consistent with the analogous distribution for solar flares. We discuss the effect of variability on the spread observed in the X-ray luminosity function of M stars. The comparison of our data with those obtained with Einstein IPC shows that variations on time scales shorter than a few month are more common than long term variations comparable to, e.g., the 11 years solar cycle. Title: The Sun as an X-Ray Star. I. Deriving the Emission Measure Distribution versus Temperature of the Whole Solar Corona from theYohkoh/Soft X-Ray Telescope Data Authors: Orlando, S.; Peres, G.; Reale, F. Bibcode: 2000ApJ...528..524O Altcode: The scope of this work is to obtain the emission measure distributions versus temperature, EM(T), of the whole solar corona from Yohkoh Soft X-ray Telescope images. As discussed in Paper II, the EM(T) is our starting point for studying the Sun as an X-ray star. To this purpose, we need to extract as much information as possible from the Yohkoh/SXT data covering the whole range of the Yohkoh/SXT temperature sensitivity, i.e., 5.5<logT(K)<8. In particular at low photon counts and temperatures below 106 K, errors on the temperature and emission measure determination are expected to be large. To this end, we have made an extensive set of simulations to explore the nominal performance of the entire system (instrument and data analysis system) in the determination of the plasma temperature and emission measure at low, intermediate, and high photon counts per pixel. We have shown that low-count data with a number of photons per pixel nphot<10 are affected by large errors and lead to the derivation of an unrealistic EM(T) characterized by a steep negative slope. As a result, we have devised an analysis method that minimizes the instrumental and statistical effects on the determination of EM(T) and allows us to determine the global coronal EM(T). As a first application to real SXT data, we have derived the EM(T) of the Sun close to the maximum of the solar cycle, a challenging case. The low-temperature part is in agreement with analogous studies made in the UV band, and it shows a well-defined maximum at T~2 MK. Title: The Sun as an X-Ray Star. II. Using theYohkoh/Soft X-Ray Telescope-derived Solar Emission Measure versus Temperature to Interpret Stellar X-Ray Observations Authors: Peres, G.; Orlando, S.; Reale, F.; Rosner, R.; Hudson, H. Bibcode: 2000ApJ...528..537P Altcode: This paper is the second of a project dedicated to using solar Yohkoh/SXT data as a guide and a template to interpret data on stellar coronae. In the light of the large differences in scope and approach between solar and stellar studies, we have developed a method to translate Yohkoh/SXT data of the whole solar corona into stellar-like data, i.e., to put them in the same format and context as the stellar ones. First from the Yohkoh/SXT images we derive the whole-Sun X-ray emission measure versus temperature [EM(T)], in the range 105.5-108 K, during the specific observation. Then, we synthesize the solar X-ray spectrum; finally, we fold the spectrum through the instrumental response of nonsolar X-ray observatories, for instance, ROSAT/PSPC and ASCA/SIS. Finally, we analyze such solar coronal data in the same band and with the same methods used for stellar observations, allowing a direct and homogeneous comparison with them. In this paper we present in detail our method and, as an example of results, we show and discuss EM(T) and stellar-like spectra for three phases of the solar cycle: maximum, intermediate phase, and minimum. The total amount and the distribution of the emission measure change dramatically during the cycle, in particular at temperatures above 106 K. We also show the EM(T) of the whole solar corona during a large flare. The ROSAT/PSPC- and ASCA/SIS-like X-ray spectra of the Sun as a star that we obtain are discussed in the context of stellar coronal physics. The Sun's coronal total luminosity in the ROSAT/PSPC band ranges from ~2.7×1026 ergs s-1 (at minimum) to ~4.7×1027 ergs s-1 (at maximum). We discuss future developments and possible applications of our method. Title: TRACE-derived Temperature and Emission Measure Profiles along Long-lived Coronal Loops: The Role of Filamentation Authors: Reale, F.; Peres, G. Bibcode: 2000ApJ...528L..45R Altcode: 1999astro.ph.11096R In a recent Letter, Lenz et al. have shown evidence of uniform temperature along steady long coronal loops observed by TRACE in two different passbands (171 and 195 Å filters). We propose that such a piece of evidence can be explained by the subarcsecond structuring of the loops across the magnetic field lines. In this perspective, we present a model of a bundle of six thin parallel hydrostatic filaments with temperature stratification dictated by detailed energy balance and with temperatures at their apex ranging between 0.8 and 5 MK. If analyzed as a single loop, the bundle would appear isothermal along most of its length. Title: X-ray spectra of the Sun as a star: how different coronal regions contribute to the observed X-ray spectrum Authors: Orlando, S.; Peres, G.; Reale, F.; Rosner, R.; Hudson, H. Bibcode: 2000ASPC..198..479O Altcode: 2000scac.conf..479O No abstract at ADS Title: On the Dynamics of Siphon Flows in Coronal Loops Authors: Betta, R.; Peres, G.; Serio, S.; Orlando, S. Bibcode: 1999ESASP.448..475B Altcode: 1999ESPM....9..475B; 1999mfsp.conf..475B No abstract at ADS Title: Fast solar wind acceleration by Alfvén waves: observable effects on the EUV lines detected by SOHO/UVCS Authors: Ventura, R.; Orlando, S.; Peres, G.; Spadaro, D. Bibcode: 1999A&A...352..670V Altcode: SOHO/UVCS observations of the most intense EUV spectral lines emitted by the solar corona have been providing us a good opportunity to study in detail the acceleration regions of the solar wind. In this work we aim at deriving useful diagnostics and identifying possible signatures of Alfvén waves momentum deposition. More specifically we investigate, with the help of a detailed wind model (Orlando et al. 1996), the insight and the constraints that these observations give on the presence of Alfvén waves, as deduced from the influence of the waves on the solar wind structure and dynamics. The model developed by Orlando et al. (1996) accounts for the momentum deposition by a spectrum of non-WKB Alfvén waves, generated in the Sun's lower atmosphere and undergoing significant reflection across the transition region. We compute a set of wind solutions characterized by different physical conditions, synthesize, from them, the emission in the Lyalpha , Lybeta and O VI doublet (1032 Ä, 1038 Ä) lines and derive possible diagnostics. We finally compare our results with the most recent SOHO/UVCS data. Title: Long-lived Coronal Loop Profiles from TRACE Authors: Lenz, Dawn D.; DeLuca, Edward E.; Golub, Leon; Rosner, Robert; Bookbinder, Jay A.; Litwin, Christof; Reale, Fabio; Peres, Giovanni Bibcode: 1999SoPh..190..131L Altcode: An initial study of long-lived loops observed with TRACE (Lenz et al., 1999) shows that they have no significant temperature stratification and that they are denser than the classic loop model predicts. Models that agree better with the observations include a loop consisting of a bundle of filaments at different temperatures and a loop with momentum input by MHD waves. Some implications for coronal heating models and mechanisms are discussed. Title: The Dynamics Of The Plasma Confined In Coronal Loops Subject To A Random Heating Authors: Betta, R.; Reale, F.; Peres, G. Bibcode: 1999ESASP.446..179B Altcode: 1999soho....8..179B We model the dynamics of the plasma in the corona and transition region between the chromosphere and the corona using the Palermo-Harvard 1-D hydrodynamic code and the ASAP spectral synthesis code. Assuming that the coronal heating of the loop is entirely due to microflares, a random energy function with a gaussian spatial distribution is applied at the top of the coronal loop. We investigate the hydrodynamic plasma response to such a stocastic heating for a set of loop models; we explore the solution parameter space, varying the loop length, its initial coronal pressure, the typical repetition time of the energy event distribution, while maintaining the same energy rate averaged in time. With the ASAP code we synthesize the emission in some optically thin lines formed in the transition region for all the loop models of the set considered, in order to allow a direct comparison of the model results with line intensities and velocities observed by SUMER. Title: Coronal Heating and Structuring Authors: Peres, G. Bibcode: 1999ESASP.446...43P Altcode: 1999soho....8...43P No abstract at ADS Title: Non-equilibrium ionization and the interpretation of Yohkoh/SXT data during solar dynamic events Authors: Orlando, S.; Bocchino, F.; Peres, G. Bibcode: 1999A&A...346.1003O Altcode: Yohkoh has been dedicated to study the structure and evolution of the solar corona, especially during dynamic events. During such events, however, the collisional equilibrium ionization - which is invariably assumed when deriving the plasma parameters from the observations - may not apply. We explore the influence of the non-equilibrium ionization effects (NEI) on the values of temperature and emission measure derived from the data collected with the Solar X-ray Telescope (SXT) on board the Yohkoh satellite during solar dynamic events. To this end, we have simulated such dynamic events, assuming that the plasma temperature rises instantaneously from the value T_0 to the value T_1, and then it remains constant. We have considered two different temperature jumps: the first one from 10(6.3) K to 10(7.3) K to simulate a flare and the other, from 10(5.7) K to 10(6.5) K, to simulate a microflare. We show that the temperature determination with the SXT wide band filter ratio is, to some extent, affected by non-equilibrium ionization: the maximum fractional error on T_1 amounts to ~ 0.7 for the flare and ~ 0.5 for the microflare we have simulated. On the other hand, the emission measure determination can be considerably affected by such departures: the maximum fractional error on EM amounts to ~ 2 for the flare and ~ 7 for the microflare we have simulated. Therefore NEI effects can be important on the temperature and emission measure determination during fast evolving phenomena. Title: Effects on UV lines observations of stationary plasma flows confined in coronal loops Authors: Orlando, S.; Peres, G. Bibcode: 1999PCEC...24..401O Altcode: We describe a method which uses a rather detailed model of coronal loop hosting a siphon flow as a diagnostic tool to interpret solar UV and X-ray observations in selected bands and lines. We apply the method to investigate the deviations from ionization equilibrium induced by stationary plasma flows confined in coronal loops and their effects on the UV and EUV emission lines observed by the instruments on board SOHO. We present results on the detailed synthesis of loop emission in a set of selected emission lines observed by CDS and SUMER, taking into account the non-equilibrium of ionization effects. Title: Analysis and comparison of loop structures imaged with NIXT and Yohkoh/SXT Authors: Di Matteo, V.; Reale, F.; Peres, G.; Golub, L. Bibcode: 1999A&A...342..563D Altcode: We analyze and compare five coronal regions simultaneously observed by NIXT and Yohkoh/SXT on April 12, 1993. The compact loop structures (length ~ 10(9) cm) imaged in three regions with NIXT and with SXT have a good general morphological correspondence. A large scale (~ 1.7 x 10(10) cm) and an intermediate scale (~ 5 x 10(9) cm) structure observed in the NIXT image have no obvious counterpart in the SXT image. The pressure of the loop plasma detected by NIXT is derived from the brightness profile along the loops by applying a method based on loop models. The pressure of the loop plasma in the SXT band has also been derived from loop models on the basis of the temperature obtained from the standard Yohkoh data analysis. NIXT pressures are systematically lower than those found with SXT. By comparing the measured total loop luminosity to that expected on the basis of loop models, we constrain quantitatively the volume filling factor of the plasma emitting respectively in the NIXT and SXT bands. The filling factors obtained in the NIXT band for the compact and intermediate loops are very low (10(-3) - 10(-2) ), but they are of the order of unity for the large structure. The low filling factors suggest a strong loop filamentation. The filling factors for the compact structures are instead of the order of unity in the SXT band. We discuss our results and propose a scenario for their interpretation. Title: On the Stability of Siphon Flows Confined in Coronal Loops Authors: Betta, Rita; Orlando, Salvatore; Peres, Giovanni; Serio, Salvatore Bibcode: 1999SSRv...87..133B Altcode: We use a time-dependent hydrodynamic model to study the dynamics of siphon flows triggered by differences of pressure or heat deposition asymmetries between the two footpoints of a coronal loop. We show that steady pressure driven flows in a uniformly heated loop are unstable, whereas those driven by asymmetries in the heating function may be stable. We also show that, in these cases, relatively cool loops might be filled far above their static pressure scale height. Title: The Solar-Stellar Connection in X-rays: How to Take Advantage of the YOHKOH data Authors: Peres, G.; Orlando, S.; Reale, F.; Rosner, R.; Hudson, H. Bibcode: 1999ASPC..158..391P Altcode: 1999ssa..conf..391P No abstract at ADS Title: Loop modeling of coronal X-ray spectra. V. One- and two-loop model fitting of G-type star ROSAT/PSPC spectra Authors: Ventura, R.; Maggio, A.; Peres, G. Bibcode: 1998A&A...334..188V Altcode: As part of a systematic study devoted to the diagnostic of solar-like coronal structures in late-type stars, we have analyzed ROSAT/PSPC X-ray spectra of eight main sequence G-type stars in the solar neighborhood. We have fitted the X-ray spectra adopting two different classes of models: the usual two-component isothermal models and the more physically meaningful hydrostatic loop models. The two-component isothermal models yield fairly acceptable chi (2) values, however they allow limited physical insight on the stellar coronal structures; on the other hand, the one-loop model fitting provides unreliable long loops, namely orders of magnitude larger than the stellar radius, in the majority of the cases studied. A more realistic physical description of the observed coronae comes from the two-loop modeling approach: two distinct classes of loops with different characteristics seem to dominate the X-ray emission of our sample of G-type stars: relatively cool loops (T_max =~ 1.5 - 5 x 10(6) K) with pressures ranging from relatively low (p_0 =~ 2 - 10 dyn cm(-2) ) to high values (p_0 =~ 100 dyn cm(-2) ) and hot loops (T_max =~ 1 - 3 x 10(7) ) with very high base pressures (p_0 =~ 10(2) - 2.4 x 10(4) dyn cm(-2) ). We compare the results and the diagnostic power coming from the different approaches and discuss their physical implications. Title: X-ray spectroscopy of the active giant beta Ceti: the SAX LECS view Authors: Maggio, A.; Favata, F.; Peres, G.; Sciortino, S. Bibcode: 1998A&A...330..139M Altcode: We present an X-ray observation of the active giant beta Cet, performed with the Low Energy Concentrator Spectrometer (LECS) on-board the SAX satellite. The resulting X-ray spectrum is well fit by an optically-thin plasma model with two discrete temperatures, and the inferred coronal metallicity is compatible both with the abundances derived from a re-analysis of the ASCA SIS spectrum of the same object, adopting the same plasma emission code (mekal), and with accurate photospheric abundances recently reported in the literature. The remarkable similarity between the X-ray spectrum of beta Cet and the one of the active binary Capella, also observed by SAX, rises again the issue of how some putative He-burning clump giants, like beta Cet, manage to retain (or regain) a relatively high X-ray activity level. Some implications of the new Hipparcos parallaxes, relevant for the coronal properties of beta Cet and the Hyades giants, are also discussed. Title: Momentum deposition by a spectrum of Alfvén waves in fast solar wind: effects on the emission lines observed by SOHO/UVCS Authors: Orlando, S.; Ventura, R.; Peres, G.; Spadaro, D. Bibcode: 1998MmSAI..69..777O Altcode: No abstract at ADS Title: Solar-Stellar Connection: Relevance of YOHKOH Data Authors: Orlando, S.; Peres, G.; Reale, F.; Rosner, R.; Hudson, H. Bibcode: 1998ASPC..154.1130O Altcode: 1998csss...10.1130O The similarity of late-type stars to the Sun is often assumed when studying the physical conditions in their coronae. In order to explore better such a ``solar-stellar'' connection we use the Yohkoh/SXT X-ray images to generate the distribution of the emission measure vs. temperature of the Sun and, from that, the expected emission, as it would be observed by non-solar X-ray telescopes such as ROSAT/PSPC and ASCA/SIS. We discuss the role of the various solar structures in determining the total distribution of the emission measure vs. temperature and in determining the stellar-like synthesized X-ray spectra. Title: Simulating the Source Parameter Recovery Capability from Coronal X-ray Spectra: the SAX/LECS and the ASCA/SIS Cases Authors: Favata, F.; Maggio, A.; Peres, G.; Sciortino, S. Bibcode: 1998ASPC..154.1027F Altcode: 1998csss...10.1027F We have studied the performance of global chi^2 fitting applied to low resolution X-ray spectroscopy, focusing on the retrieval of source parameters, with emphasis on the coronal metallicity. The study has been conducted by fitting large numbers of simulated spectra with known characteristics, and studying the distribution of the best-fit parameters, using the responses of the LECS detector onboard the SAX satellite and of the SIS detector onboard the ASCA satellite. The simulations have been done under the hypothesis that the intrinsic source spectrum can be described by two discrete temperatures. The performance of the fitting process has been studied both in the case in which just the global metallicity is left free to vary and in the case in which the abundance of each element is independently left free to vary in the fit process. Title: Plasma Thermal Structure and Metallicity of Stellar Coronae Observed with XMM Authors: Maggio, A.; Orlando, S.; Favata, F.; Micela, G.; Peres, G.; Sciortino, S. Bibcode: 1998sxmm.confE..70M Altcode: No abstract at ADS Title: X-ray Flare Light Curves and Dimensions of the Flaring Regions Authors: Serio, S.; Reale, F.; Betta, R.; Peres, G.; McTiernan, J. Bibcode: 1998ASPC..154.1161S Altcode: 1998csss...10.1161S The decay of flare X-ray light curves provides information on the dimensions of the flaring region. We show that a reliable estimate of such dimensions must take into account the temperature evolution as tracer of the presence of heating during the decay. Many solar flares appear sustained by prolonged heating episodes. We describe a method to estimate their spatial extension based on the analysis of the decay of their X-ray light curve and temperature, and show that the results are in good agreement with their dimensions measured on Yohkoh/SXT images. Title: The Sun as an X-Ray Star: Overview of the Method Authors: Peres, G.; Orlando, S.; Reale, F.; Rosner, R.; Hudson, H. Bibcode: 1998ASSL..229...29P Altcode: 1998opaf.conf...29P No abstract at ADS Title: Hydrodynamics of the plasma confined inside coronal loops: flare and microflare models Authors: Betta, R.; Peres, G.; Reale, F.; Serio, S. Bibcode: 1998cee..workE..23B Altcode: The plasma contained in coronal loops behaves macroscopically like a fluid and its dynamics and evolution may be described by hydrodynamics provided mass, momentum and energy transport occurs only along magnetic field lines. In fact, coronal loops are very often observed not to change their geometry during a flare, and this suggests that the magnetic field structure may basically act to confine the plasma while chromospheric plasma evaporation and temperature increase cause the increase in brightness. In other words, though the source of the energy release in loops may be of magnetic origin, the subsequent loops response may be adequately described by hydrodynamics in those instances in which the global magnetic field does not change. We have developed such a hydrodynamic model (Peres et al. 1982), which takes into account the main physical effects such as gravity, viscosity, ionization, radiative losses and thermal conduction and which is capable of giving a correct description of the steep and dynamic transition region between the chromosphere and the corona (Betta et al. 1997). Here we show how a plasma confined inside coronal loops responds when it is subject to impulsive heating. We simulate flares by creating a sudden energy release in a localized position along the loop (although the plasma dynamics does not depend crucially on the position of energy release). The initial configuration consists of a loop in hydrostatic equilibrium and steady-state energy balance (i.e.,in which there is an average heating which balances radiation losses and thermal conduction). The hydrodynamic calculations show the formation of an evaporation front propagating from the chromosphere to the corona, while the temperature increases in the loop from the top towards the footpoints anchored in the photosphere and the transition region moves progressively downwards. When the heating is switched off the plasma cools slowly during the decay phase of the flare until a thermal instability occurs (due to the large amount of plasma evaporated in the corona which causes an increased emission): as a consequence, the plasma temperature drops to almost chromosperic values. Continuing the simulations for a few hours, since the steady-state energy balance term is included in the equations, the atmosphere returns to the initial configuration. In the microflare heating model we assume that on average, the loop is maintained close to the coronal conditions predicted by steady-state models. Some preliminary results of microflare simulations may be found in Peres, Reale and Serio (1993). A sequence of short heating pulses is released inside the loop. The time interval between two heating events can be either random following a Poisson distribution, or periodic. The pulse has a different random duration but the total power given during each heating event is such that the integral over time and space equals the total amount of energy needed to balance losses in the hydrostatic equations and mantain a steady-state. We have studied the dynamics using heating pulses of different frequencies in order to find possible diagnostics of random heating in coronal loops. Title: The Sun as an X-ray star: comparing Yohkoh/SXT data with stellar X-ray data Authors: Peres, G. Bibcode: 1998MmSAI..69..693P Altcode: No abstract at ADS Title: X-ray Variability in ROSAT Observations of M Stars Authors: Marino, A.; Micela, G.; Peres, G. Bibcode: 1998ASPC..154.1116M Altcode: 1998csss...10.1116M We present preliminary results of a systematic analysis of X-ray light curves of the PSPC pointed observations of M stars contained in the CNS3 catalog. A large fraction of the sample stars show significant variations, regardless of the quiescent activity level. Variability is observed at all time scales we have explored, and the variations in X-ray luminosity are well related to variations in the hardness ratio of the spectra. We discuss the possible effect of variability on the spread observed in the X-ray luminosity function of M stars. Title: Coronal Loop Modeling of ROSAT/PSPC X-ray Spectra from Solar-Type Stars Authors: Ventura, R.; Maggio, A.; Peres, G. Bibcode: 1998ASPC..154.1183V Altcode: 1998csss...10.1183V In the context of a wide project devoted to the diagnostic of structures in stellar coronae, we have analyzed ROSAT/PSPC X-ray spectra of a selected sample of 8 G-type stars in the solar neighborhood (d< 25 pc). We fitted the X-ray spectra with a few models: the usual two-temperature isothermal model and the more physically detailed one- and two-loop models, in order to compare the diagnostic power coming from the different approaches. The results point out that the one-loop fitting provides acceptable chi^2 values, but it yields, in the majority of the cases, unacceptable long loops, namely, orders of magnitude larger than the stellar radius. A better physical insight on the observed coronae comes from the two-loops model fitting. Two classes of loops with different characteristics seem to dominate the X-ray emission of our sample of solar-like stars: relatively cool loops (T ~= 1.5 - 5 x 10^6 K) with pressures ranging from low (p_0 ~= 2 - 10 dyn cm^{-2}) to relatively high values (p_0 ~= 100 dyn cm^{-2}) and hot loops (T ~= 1 - 3 x 10^7 K) with very high base pressures (p_0 ~= 10^2 - 2.4 x 10^4 dyn cm^{-2}). We discuss these results, their physical implications and possible interpretations. Title: Alfvénic fluctuations in fast and slow solar winds Authors: Orlando, S.; Lou, Y. -Q.; Peres, G.; Rosner, R. Bibcode: 1997JGR...10224139O Altcode: We compute properties of Alfvénic fluctuations within a wide frequency range (i.e., 10-6s-1<~f<~10-2s-1) in both high- and low-speed magnetized radial solar winds. In particular, the radial and frequency variations of the normalized cross helicity σc, the Alfvén ratio <script>R</script>A, and other relevant quantities associated with Alfvénic fluctuations are presented and analyzed. Because of the existence of the Alfvén critical radius rA and a characteristic frequency fc for a given magnetized solar wind profile, continuous reflection effects are clearly manifest at frequencies lower than several times fc. Since outward propagating fluctuations which many authors consider Alfvénic fluctuations have been detected in the high-latitude solar wind (which is fast and steady), such continuous reflection effects in the low-frequency end of the fluctuation spectrum may be found in the Ulysses data. In order to strengthen our case, comparisons are carried out between our theoretical results and previous data analyses of observations from the Helios 1 and 2 spacecraft in the radial range of 0.29AU<~r<~1.0AU. Stronger evidence for continuous reflection effects of low-frequency Alfvénic fluctuations is found in the high-speed solar wind close to the Sun than in the low-speed solar wind in general. Title: On coronal abundances derived with the SAX/LECS and ASCA/SIS detectors. Authors: Favata, F.; Maggio, A.; Peres, G.; Sciortino, S. Bibcode: 1997A&A...326.1013F Altcode: 1997astro.ph..7058F We have studied the performance of global χ^2^ fitting of low-resolution X-ray spectra in retrieving intrinsic source parameters, with emphasis on the coronal metallicity. The study has been conducted by fitting large numbers of simulated spectra with known characteristics, and studying the distribution of best-fit parameters. We have studied the behavior of the LECS detector on board the SAX satellite and the SIS detector on board the ASCA satellite. The fitted source spectra have either two discrete temperature components or a power-law temperature distribution, with metallicity variations modeled by a single global abundance parameter. The model used for the fitting has always been a two-temperature one, with global varying abundance, to explore the influence of the a priori ignorance of the actual temperature stratification in the source being observed. The simulations performed explore the influence of varying statistics in the observed spectrum (spanning a realistic range of values) as well as the effect of varying the intrinsic source metallicity, with values in the range 0.15-1.0 times the solar value. We find that the source metallicity can be retrieved to within a few tens of percent from ASCA/SIS spectra of typical signal to noise ratio, and within few percent from SAX/LECS spectra at the same signal to noise ratio. However, relatively small uncertainties in the detector calibrations and in the plasma emission codes are likely to potentially cause large systematic off-sets in the value of the best-fit parameters. Similar systematic off-sets may derive from assuming too simplistic a temperature distribution for the source plasma. In addition we have re-analyzed the ASCA/SIS spectra of the active giants β Cet and Capella with the same set of assumptions used in the simulations, showing how the best-fit metallicity in these two real cases depends on the details of the fitting process, and in particular on the chosen energy range. Title: Determination of the length of coronal loops from the decay of X-ray flares I. Solar flares observed with YOHKOH SXT. Authors: Reale, F.; Betta, R.; Peres, G.; Serio, S.; McTiernan, J. Bibcode: 1997A&A...325..782R Altcode: We show how to estimate the size of spatially unresolved solar and stellar coronal flaring regions from the X-ray light curve and time-resolved temperature and emission measure values during the flare decay. By means of extensive hydrodynamic modeling of decaying flaring loops, we propose and test a relationship between the decay time of the light curve in the band of a specific instrument and the slope of the trajectory in the density-temperature diagram. From this relationship, we obtain an expression of the loop length as a function of the decay time, the slope and the flare maximum temperature. The novelty of this approach is that it takes into proper quantitative account, and allows us to estimate, the effect of a prolonged heating during the decay. In view of its application to non-imaged solar flares and to stellar flares, we have tested our relationship on spatially resolved solar flares observed with Yohkoh SXT. The comparison of the predictions to the morphology of the structures in the SXT images proves the reliability of our approach under a wide range of conditions. Title: Loop modeling of coronal X-ray spectra. IV. One- and two-loop model fitting of ROSAT PSPC spectra: three test cases. Authors: Maggio, A.; Peres, G. Bibcode: 1997A&A...325..237M Altcode: We present a detailed application of coronal hydrostatic loop models to fit ROSAT/PSPC X-ray spectra, using three late-type stars, Procyon (F5 IV - V), ι Vir (F6 III), and HR 3625 (F9 V), selected as test cases showing different activity levels. We present results of the spectral analysis with models of coronae made of one class of loops or two classes of loops, and compare them with the results obtained with two-component isothermal models. The loop model analysis is performed with a χ^2^ fitting method as accurate as in the case of the two-temperature modeling, including evaluation of statistical confidence regions in the model parameter space. We show that the observed spectra of Procyon and ι Vir can be successfully fitted with one-loop models, while two-loop models are required to fit the spectrum of HR 3625. In the case of Procyon, for which EUVE spectra are also available, we find that a coronal density estimate - based on emission line ratios - is consistent with the results of our one-loop model fitting. The case of HR 3625 is adopted to illustrate how the confidence regions in the two-loop model parameter space can be explored and interpreted. Our final results indicate different coronal conditions for the three selected stars: Procyon emission is dominated by relatively short (L~10^9^cm), low pressure (p_0_~2dyn/cm^2^) and low temperature (T_max_=1.7x10^6^K) loops, covering few tens percent of the stellar surface; the corona of ι Vir appears composed of relatively higher pressure (p_0_=2-10dyn/cm^2^), higher temperature (T_max_=1-2x10^7^K) loops with height (L=1-6x10^10^cm) comparable with the pressure scale height; finally, the coronal emission of HR 3625 can be modelled with low-temperature (T_max_=1-2x10^6^K), short (L<5x10^8^cm) loops, with base pressure p_0_>6dyn/cm^2^, possibly covering a large fraction of the stellar surface, plus higher temperature (T_max_ > 7 x 10^6^ K) loops which may range from compact, very active region loops (L < 10^9^ cm, p_0_ > 10^2^ dyn cm^-2^) covering less than 0.5% of the surface, to larger, relatively less active region loops (L~10^10^cm, p_0_=5-10dyn/cm^2^) covering at most 10% of the surface. Title: The Sun as AN X-Ray Star: Overview of the Method Authors: Peres, G.; Orlando, S.; Reale, F.; Rosner, R.; Hudson, H. Bibcode: 1997SoPh..172..239P Altcode: 1997ESPM....8..239P We present a method to study the solar-stellar connection, i.e., the close similarity of the physical phenomena occurring on the Sun and on late-type active stars, by taking advantage of Yohkoh/SXT X-ray images. From such images, we first generate distribution functions of the whole disk differential emission measure, and then synthesize from these spectra analogous to those collected by X-ray telescope instruments aimed at stars other than the Sun. Here we illustrate the application of this method to the ROSAT/PSPC and ASCA/SIS, and discuss test cases as well as future applications. Title: An adaptive grid code for high resolution 1-D hydrodynamics of the solar and stellar transition region and corona Authors: Betta, R.; Peres, G.; Reale, F.; Serio, S. Bibcode: 1997A&AS..122..585B Altcode: We present a new version of the Palermo-Harvard hydrodynamic model of plasma confined in coronal loops, based on a regridding algorithm which ensures limited variations of thermodynamical variables across grid points. The code provides an accurate description of the dynamics of the chromosphere-corona transition region, in spite of the very steep temperature and density gradients, even during flare-like transients. The new model can be used to synthesize, together with X-ray emission, the optically thin emission originating from the transition region, mainly in the EUV band. Here we show sample results. Title: Loop modeling of coronal X-ray spectra. III. Fitting loop spectra with one- and two-component thermal models. Authors: Ciaravella, A.; Maggio, A.; Peres, G. Bibcode: 1997A&A...320..945C Altcode: In this work we study coronal loops vs. one- and two-component thermal models, the first being the physically realistic description of the X-ray emitting, magnetically-confined solar corona, the others the standard analysis tool of X-ray spectra from stellar coronae. The scope of this work is to compare directly these two paradigms of coronal physics, also to find a possible relation between the loop characteristics and the findings of the thermal components fitting. We simulate observations of coronal spectra using a static loop model, including the effects of stellar gravity and of possible non-uniform loop cross-section. We evaluate the one- and two-temperature fitting results through extensive simulations, varying the loop parameters, the photon counting statistics, and considering two instruments: the ROSAT/PSPC and the ASCA/SIS. We find that one-temperature models do not fit adequately loop spectra with 10^3^ total counts or more, for any of the cases explored. Two-temperature models provide a good fit to single loop spectra in many of the cases explored, with the implication that the two temperatures found when fitting real observations may not necessarily indicate the presence of two classes of dominating loops in the observed corona, but rather may be explained with only one class of loops, as long as the ratio of the emission measures of the hot vs. cool component is larger than unity. The goodness of the fit becomes worse with increasing photon statistics and/or resolving power, especially for loops with relatively intense plasma pressure (p_0_>10dyn/cm^2^) and as high as the pressure scale height. In such cases the two-temperature fitting and the loop modeling are therefore not equivalent, implying that, for such kind of observations, detailed loop fitting should be attempted. We comment on our findings and draw possible guidelines to interpret observations. Title: Radiatively driven downdrafts and redshifts in transition region lines. II. Exploring the parameter space. Authors: Reale, F.; Peres, G.; Serio, S. Bibcode: 1997A&A...318..506R Altcode: We address the hypothesis that downdrafts driven by radiatively-cooling condensations in the solar transition region are able to produce significant redshifts in UV lines as frequently observed on the solar disk and, more recently, on other stars. In a first paper, significant redshifted line components at several km/s have been found from modeling the evolution of an isobaric perturbation twice as dense as the unperturbed atmosphere, almost as large as the thickness of the transition region of an active region loop, and with central temperature higher than the formation temperature of the UV lines. In the present work we show the results of an extensive exploration of the space of the important parameters controlling the evolution of isobaric perturbations: density contrast (δ), dimensions, and ambient pressure. The center of most of the perturbations is placed where the temperature of the unperturbed medium is T_0_=4.5x10^5^K, but higher temperatures do not lead to substantially different results. From the hydrodynamic evolution we synthesize the line flux and effective speed along the line of sight, and examine the distribution of the most intense Doppler-shifted components. In a wide region of the parameter space (δ>0.5, dimensions of the order of the thickness of the transition region) we find redshifted components at speeds of several km/s for ambient pressure values ranging from those typical of quiet Sun to active regions. The assumption of isotropic thermal conduction, or, alternatively, of 1-D hydrodynamics, i.e. mimicking the effect of strong magnetic fields, lead to qualitatively similar results. Our calculations suggest also that redshifts may occur more easily in the higher pressure plasma, typical of active regions, in general consistency to observations. Title: A major optical flare on the recently discovered X-ray active dMe star G 102-21. Authors: Pagano, I.; Ventura, R.; Rodono, M.; Peres, G.; Micela, G. Bibcode: 1997A&A...318..467P Altcode: During the course of UBV photoelectric measurements made at the Catania Astrophysical Observatory we have observed an unusually intense optical flare on the nearby M dwarf G 102-21, the optical counterpart of a strong X-ray source recently detected by ROSAT. The amplitude of the magnitude increase ({DELTA}U~7.3mag) and the total energy release in the UBV bands (E_UBV_~(1.4+/-0.3)x10^35^erg) put this event among the strongest flares ever detected on UV Ceti-type stars. The U-B and B-V colours of the flare emission did not significantly change during most of the flare development. While the B-V is quite a typical colour for stellar flares, the U-B is much bluer when compared with typical values given in literature. This unusually strong ultraviolet component of the flare emission, together with the very fast flare decay, suggest a possible flare site close to the limb. We find that the observed amount of flare energy could be explained by the fast release of energy stored in a magnetic filament located in between the two G 102-21 stellar components, with a surface magnetic field >=3600G. The characteristics of the quiescent optical emission are also discussed. Title: SOHO observations of the north polar solar wind Authors: Peres, G.; Ciaravella, A.; Betta, R.; Orlando, S.; Reale, F.; Kohl, J.; Noci, G.; Fineschi, S.; Romoli, M.; Brekke, P.; Fludra, A.; Gurman, J. B.; Lemaire, P.; Schuhle, U. Bibcode: 1997ESASP.404..587P Altcode: 1997cswn.conf..587P No abstract at ADS Title: Flows of Mass, Momentum and Energy in the Solar Atmosphere: A SOHO-Oriented View of Cold Loops Authors: Peres, G. Bibcode: 1997ESASP.404...55P Altcode: 1997cswn.conf...55P; 1997soho....5...55P No abstract at ADS Title: The Sun as an X-ray Star: Overview of the Method Authors: Peres, G.; Orlando, S.; Reale, F.; Rosner, R.; Hudson, H. Bibcode: 1997IAUJD..19E..37P Altcode: We present a method to study the solar-stellar connection, i.e., the close similarity of the physical phenomena occurring on the Sun and on late-type active stars, by taking advantage of Yohkoh/SXT X-ray images. From such images, we first generate distribution functions of the whole disk differential emission measure, and then synthesize from these spectra analogous to those collected by X-ray telescope instruments aimed at stars other than the Sun. Here we illustrate the application of this method to the ROSAT/PSPC and ASCA/SIS, and discuss test cases as well as future applications. For a more detailed discussion, please refer to a paper, by the same authors in "OBSERVATIONAL PLASMA ASTROPHYSICS: FIVE YEARS OF YOHKOH AND BEYOND", T. Watanabe, T. Kosugi, and A. C. Sterling, eds., Kluwer Academic Publishers, the Proceedings of the Yohkoh 5th Anniversary Symposium, held in November 1996, in Yoyogi, Tokyo, Japan. Title: Stellar Flaring Loop Authors: Reale, F.; Micela, G.; Peres, G.; Betta, R.; Serio, S. Bibcode: 1997MmSAI..68.1103R Altcode: We propose and test a procedure tailored to analyse extensively and systematically stellar X-ray flares, such as those observed in open clusters. The method yields a reliable estimate of the dimensions of the coronal flaring loops and of the presence of a significant heating during the flare decay, it is based on the decay of the light curve and of temperature indicators, and it has been developed and tested successfully on spatially-resolved solar flares. We illustrate the application on flares observed with ROSAT/PSPC. Title: Loop modeling of X-ray coronae: solar-type stars in young clusters and associations. Authors: Maggio, A.; Micela, G.; Peres, G. Bibcode: 1997MmSAI..68.1095M Altcode: No abstract at ADS Title: Radiatively-driven downdrafts and redshifts in transition region lines. I. Reference model. Authors: Reale, F.; Peres, G.; Serio, S. Bibcode: 1996A&A...316..215R Altcode: We show that downdrafts driven by radiatively-cooling subarcsecond perturbations in the solar transition region can produce significant UV line redshifts consistent with those frequently observed on the solar disk, and, by extension, with those more recently observed on stars. We model in detail the thermo-hydrodynamic evolution of a non-linear isobaric spherical condensation inside an initially hydrostatic atmosphere, stratified both in density and temperature, and with initial conditions typical of a solar active region. We study the evolution of the atmosphere both neglecting the effect of magnetic fields and taking it into account by forcing conduction and motions to be only along the vertical direction. The synthesis of emitted flux and effective speed in four UV lines from model results shows the presence of intense redshifted components at speeds between 5 and 10km/s. Title: Propagation of three-dimensional Alfvén waves in a stratified, thermally conducting solar wind Authors: Orlando, S.; Lou, Y. -Q.; Rosner, R.; Peres, G. Bibcode: 1996JGR...10124443O Altcode: 1996JGR...10124433O We model the propagation of three-dimensional, adiabatic, linear Alfvén waves in the solar atmosphere and wind, taking into account relevant physical effects, including gravity stratification, thermal conduction, radiative losses, and heating (via a phenomenological term). Our magnetohydrodynamic solar wind model also accounts for the momentum deposition by a spectrum of non-WKB Alfvén waves. The transmission and reflection of such waves has been previously studied by a variety of techniques, including calculations based on the computation of the ratio between the wavelength and the scale length of the Alfvén speed change and based on a globally-computed transmission coefficient. In this paper we discuss both techniques and show how they are related. We also discuss the physics underlying the reflection process and the possible role wave reflection might play in the acceleration of the solar wind and the winds from other stars. Title: EPIC system onboard the ESA XMM Authors: Villa, Gabriele E.; Abbey, Anthony F.; Arnaud, M.; Balasini, M.; Bignami, Giovanni F.; Boer, M.; Brauninger, H.; Butler, I.; Cafagna, G.; Cara, C.; Chabaud, C.; Chiappetti, L.; Cole, Richard E.; Conte, M.; Dowson, J.; Dhez, Pierre; Duc, R.; Di Cocco, Guido; Ferrando, P.; Goodall, C. V.; Hippman, H.; Holland, Andrew D.; Kendziorra, E.; Labeque, A.; Meidinger, Norbert; Moriggio, C.; La Palombara, N.; Musso, C.; Poindron, E.; Peres, G.; Pferrerman, E.; Pigot, Claude; Pye, John P.; Reppin, C.; Schmitt, D.; Sciortino, Salvatore; Serio, Salvatore; Spragg, J.; Stephen, J. B.; Struder, L.; Trifoglio, M.; Tua, P.; Turner, M. J.; Whitehead, S. Bibcode: 1996SPIE.2808..402V Altcode: The European photon imaging camera (EPIC) is one of the two main instruments onboard the ESA X-Ray Cornerstone Mission XMM. It is devoted to performing imaging and spectroscopy of the x-ray sky in the domain 0.1 10 keV with a peak sensitivity in 10(superscript 5) seconds of 2 multiplied by 10(superscript -15) erg/cm(superscript -2). The x-ray instrumentation is complemented by a radiation monitor which will measure the particle background. The spectral resolution is approximately 140 eV at 6.4 keV and 60 eV at 1 keV. The instrumentation consists of three separate focal plane cameras at the focus of the three XMM telescopes, containing CCDs passively cooled to typically minus 100 degrees via radiators pointing toward the anti-Sun direction. The two cameras with the field of view partially occulted by the RGS grating boxes will have MOS technology CCDs while the third camera, with full field of view, will be based on p-n technology. The CCDs in the focal plane of the cameras will cover the entire 30 foot by 30 foot field of view of the telescope while the pixel size (40 by 40 (mu) for the MOS camera and 150 multiplied by 150 (mu) for the p-n) will be adequate to sample the approximately 20' PSF of the mirrors. In order to cope with a wide range of sky background and source luminosity in the visible/UV band, a filter wheel with six positions has been implemented in each camera. The six positions correspond to: open position, closed position, one thin filter (1600 angstrom of plastic support and 400 angstrom of Al), one medium filter (1600 angstrom of plastic support and 800 angstrom of Al) and one thick filter (approximately 3000 angstrom of plastic support, approximately 1000 angstrom of Al and 300 Angstrom of Sn). The final position will be a redundant filter of type still to be decided. A set of radioactive sources in each camera will allow the calibration of the CCDs in any of the operating modes and with any filter wheel position. Vacuum doors and valves operated will allow the operation of other camera heads on the ground, in a vacuum chamber and/or in a controlled atmosphere, and will protect the CCDs from contamination until the spacecraft is safely in orbit. The MOS camera will have 7 CCDs, each of 600 by 600 pixels arranged in a hexagonal pattern with one central and six peripheral. The p-n camera head will have 12 CCDs, each with 200 multiplied by 64 pixels, in a rectangular arrangement, 4 quadrants of 3 CCDs each. The radiation monitor is based on two separate detectors to monitor the low (electrons greater than 30 keV) and the high (electrons greater than 200 keV and protons greater than 10 MeV) energy particles impinging on the telescope along its orbit. Title: Loop modeling of coronal X-ray spectra. II. Parameter constraints and diagnostics. Authors: Maggio, A.; Peres, G. Bibcode: 1996A&A...306..563M Altcode: We have investigated some aspects of the fitting of stellar X-ray spectra by means of hydrostatic models of coronal loops. We have performed an extensive set of simulations in which we have synthesized and fitted loop model spectra, considering the instrument responses of the Einstein Observatory Imaging Proportional Counter, of the ROSAT Position Sensitive Proportional Counter, and of the ASCA Solid-state Imaging Spectrometer, and we have derived theoretical confidence regions for the determination of the input loop parameters. We have found that our ability to constrain the loop model parameters depends on the distribution of the differential emission measure. More specifically, we have found that, for loops significantly below the pressure scale height, the spectral fitting allows to constrain well the plasma peak temperature and hence the product of the base pressure with the loop semi-length, but not each of the two separately, due to scale invariance of the equations describing the hydrostatic loop model of uniform pressure. For input loops with lengths of the order of the pressure scale height or larger, the spectral fitting yields closed confidence regions in the parameter space, allowing to constrain all the loop parameters, with different levels of discrimination depending on photon counting statistics and instrument energy response. Loops much longer than the star's radius and with relatively high base pressure behave like the short loops, because the pressure decrease along the loop is moderated by the local increase of the pressure scale height. Title: Loop modeling of coronal X-ray spectra. I. General properties. Authors: Ciaravella, A.; Peres, G.; Maggio, A.; Serio, S. Bibcode: 1996A&A...306..553C Altcode: This work is motivated by the widely accepted paradigm that the structure and features of stellar coronae are determined by plasma magnetically confined in coronal loops. In particular we address the question whether it is possible to devise diagnostics amenable to reveal some physical and geometric characteristics of static coronal loops from X-ray observations. We show that single-loop coronal models may give spectra differing in several respects from one- and two-temperature models, the differences being probably observable already with ASCA/SIS. We find that the spectra of our loop models are essentially determined by the plasma below one pressure scale height; therefore we do not need to consider deviations from Spitzer's conductivity which occur in the very diluted upper atmosphere. We also find that widening of cross section of the loops may give observable effects depending on loop parameters and that even a small fraction of the star's surface covered by compact coronal loops with dense plasma can easily dominate the emission from a corona and mask the emission from long and tenuous loops. Finally we present an example of fitting a real ROSAT/PSPC observation with loop-emitted spectra. Title: Radiatively cooling downdrafts as the origin of redshifts in transition region lines Authors: Reale, F.; Peres, G.; Serio, S. Bibcode: 1996ASPC..109..149R Altcode: 1996csss....9..149R No abstract at ADS Title: Loop modeling of ROSAT/PSPC spectra of Hyades and field F-type stars Authors: Maggio, A.; Peres, G.; Pye, J. P.; Hodgkin, S. T.; Morley, J. E. Bibcode: 1996ASPC..109..269M Altcode: 1996csss....9..269M No abstract at ADS Title: Structure and stability of EUV loops originating from sunspots Authors: Peres, G.; Orlando, S. Bibcode: 1996ASPC..109...93P Altcode: 1996csss....9...93P No abstract at ADS Title: Diagnostics of Solar Flares from Hydrodynamic Modeling: Using the Decay Phase to Derive the Loop Dimensions Authors: Reale, F.; Peres, G.; Betta, R. Bibcode: 1996ASPC..111..232R Altcode: 1997ASPC..111..232R The authors describe a method to derive the size of solar and stellar flaring loops from the decay time of the light curve and the slope of the decay trajectory in the density-temperature diagram of the flare. They have calibrated the method with extensive numerical hydrodynamic modeling. The method is designed for observations without angular resolution, and, in view of its application to stellar flares, is tested on solar flares observed by the Yohkoh SXT, which provides light curves and temperature diagnostics, together with a direct measure of the loop length. Title: A Regridding Algorithm for High Resolution Hydrodynamics of Flaring Coronal Loops Authors: Betta, R. M.; Peres, G.; Serio, S.; Reale, F. Bibcode: 1996mpsa.conf..473B Altcode: 1996IAUCo.153..473B No abstract at ADS Title: Detailed fitting of coronal X-ray spectra with loop models: model characteristics and simulations Authors: Ciaravella, A.; Maggio, A.; Peres, G.; Serio, S. Bibcode: 1996ASPC..109..257C Altcode: 1996csss....9..257C No abstract at ADS Title: Hydrodynamic Modeling of Flares Well-Observed by Yohkoh/SXT Authors: Reale, F.; Peres, G.; Hudson, H. Bibcode: 1996mpsa.conf..311R Altcode: 1996IAUCo.153..311R No abstract at ADS Title: Observation of small flares and a long-lasting pre-flare dip. on V 1054 Ophiuchi Authors: Ventura, R.; Peres, G.; Pagano, I.; Rodono, M. Bibcode: 1995A&A...303..509V Altcode: We report on the observations and analysis of two small flares and of an exceptionally long pre-flare dip observed on V 1054 Oph by a twin-beam UBV photometer that permit to reach high photometric precision. The monitoring of V 1054 Oph has been carried out as part of a project mainly devoted to the detection of low amplitude optical variability and microflares in very active red dwarf flare stars. We discuss our results in the framework of present theories on stellar flares and pre-flare dips, including the particularly severe constraints that the present results - and previous observations of pre-flare dips on FF And - pose on the most relevant flare models. Title: Effects of flows and non-equilibrium ionization on some C IV line ratios on the Sun. Authors: Spadaro, D.; Orlando, S.; Peres, G.; Leto, P. Bibcode: 1995A&A...302..285S Altcode: We investigate the deviations from ionization equilibrium occurring in coronal loops hosting steady-state siphon flows, and their effects on the EUV emission line ratios of C IV recently considered by Keenan et al. (1992) and found discrepant with observations of highly dynamics events. We use the same electron impact excitation rates adopted by Keenan et al. and two independent siphon flow models. The C IV line ratios predicted by our models do not differ appreciably between equilibrium and non-equilibrium conditions, and therefore indicate that the discrepancy found by Keenan et al. (1992) cannot be solved by steady flows. We discuss alternative solutions, also in view of forthcoming observations by CDS/SOHO. Title: Models of stationary siphon flows in stratified, thermally conducting coronal loops. II. Shocked solutions. Authors: Orlando, S.; Peres, G.; Serio, S. Bibcode: 1995A&A...300..549O Altcode: We use a detailed hydrodynamic model which takes into account the effects of gravity, of radiative losses, of thermal conduction and of a parametrized heating term to explore the conditions for the presence of stationary shocks in critical and supersonic siphon flows in coronal loops. We consider separately isothermal and adiabatic shocks. The exploration of the parameter space shows that in every solution, as characterized by loop length, base dynamical pressure and base velocity, the shock position depends on the volumetric heating rate of the loop. In addition, with the exception of the critical solutions with isothermal shocks, there exists a range of volumetric heating rates for which there are two alternative positions where the shock can form. Scaling laws for supersonic solutions are derived. Title: Solar flare X-ray imaging: coronal loop hydrodynamics and diagnostics of the rising phase. Authors: Reale, F.; Peres, G. Bibcode: 1995A&A...299..225R Altcode: We discuss the flare diagnostics possible with the images of flaring loops obtained with the Soft X-ray Telescope (SXT) on board Yohkoh. We use hydrodynamic models of individual flaring loops to synthesize in detail the distribution of surface brightness, and its evolution, in the pass-band of the SXT during the rising thermal phase. The point is to use our synthesized sequences as a guide to interpret data from Yohkoh SXT. On the basis of synthesized images (and the insight on hydrodynamics), we recognize four phases of the flare: the conduction phase (practically unobservable, whenever present), the chromospheric evaporation, the brightening at the loop apex and the peak phase. For each phase we discuss the possible diagnostics and, when possible, we discuss its connection with the heating mechanisms. A common feature of our results for observations through the Be filter is the initial propagation of brightness fronts upwards from the base to the top of the loop, in fact associated with chromospheric evaporation fronts: only the details of this propagation can depend on the flare heating mechanism and on its location. Bright evaporation fronts are preferentially produced by a localized thermal heating (especially at the loop footpoints); heating by electron beams instead produce a more gradual emission evolution. All our models predict a brightening of the top of the loop after the evaporation phase, independently of the site and mechanism of energy release. We find that the hardest SXT filter (the Be filter) should access a wider variety of emission structures and therefore provide a richer diagnostics, while the other filters should preferentially show bright loop feet, especially during hot (~2x10^7^K) flares in relatively long loops (e.g. 6x10^9^cm). We show that, for a correct analysis of the images, it is crucial to evaluate accurately loop length and the plasma temperature; for instance, some observable features can be properly interpreted only knowing the loop temperature. Title: Models of stationary siphon flows in stratified, thermally conducting coronal loops. I. Regular solutions. Authors: Orlando, S.; Peres, G.; Serio, S. Bibcode: 1995A&A...294..861O Altcode: We present a detailed hydrodynamic model of stationary siphon flows in a semicircular solar coronal loop of constant cross section, which takes into account simultaneously gravity, thermal conduction, radiative losses and a phenomenological heating term. The model has been designed to provide a tool for insight into coronal loop physics and to help interpretation of the observations of new UV and X-ray instruments. In order to achieve high reliability of the calculations even close to critical points, we have used two very accurate and independent algorithms, and compared their results. Extensive analytical derivations have supplemented numerical calculations to study the characteristics of solutions. We distinguish four classes of solutions: subsonic, critical, supersonic and unphysical. The characteristics of the solutions space depend significantly on the boundary conditions, for instance supersonic solutions exist only for negative heat flux at the base of the loop. We have explored thoroughly the space of solutions, and have derived scaling laws for subsonic solutions which relate the dynamic pressure p+ρv^2^, the maximum loop temperature, the loop semilength and the volumetric heating; such scaling laws extend those of Rosner Tucker & Vaiana (1978) and of Serio et al. (1981) to the siphon flows. Additional laws and conditions for various classes of siphon flows are derived. Title: A Long-Duration Pre-flare Dip in the Light Curve of V1054 OPH Authors: Ventura, R.; Pagano, L.; Peres, G.; Rodòn, M. Bibcode: 1995LNP...454..108V Altcode: 1995flfl.conf..108V; 1995IAUCo.151..108V No abstract at ADS Title: A Major Optical Flare on the Recently Discovered X-ray Active dMe Star G102-21 Authors: Pagano, I.; Ventura, R.; Rodonò, M.; Peres, G.; Micela, G. Bibcode: 1995LNP...454...95P Altcode: 1995flfl.conf...95P; 1995IAUCo.151...95P No abstract at ADS Title: Thermally Unstable Perturbations in Stratified Conducting Atmospheres Authors: Reale, Fabio; Serio, Salvatore; Peres, Giovanni Bibcode: 1994ApJ...433..811R Altcode: We investigate the thermal stability of isobaric perturbations in a stratified isothermal background atmosphere with solar abundances, as resulting from the competition of optically thin plasma radiative cooling and of heating conducted from the surrounding atmosphere. We have analyzed the threshold line between stable and unstable perturbations, in the plane of the two important control parameters: the initial size of the perturbation and the temperature of the unperturbed medium; this line changes with the pressure of the unperturbed atmosphere. We have extended the results of linear perturbation analysis by means of numerical calculations of the evolution of spherical isobaric perturbations, using a two-dimensional hydrodynamic code including Spitzer heat conduction. We explore a wide range of the parameters appropriate to the solar and stellar upper atmospheres: the background uniform temperature is between 105 K and 107 K, the initial pressure betweeen 0.1 and 10 dyn/sq cm, and the perturbation size between 105 and 1010 cm. The numerical results are in substantial agreement with the linear analysis. We discuss possible implications of our results also in terms of observable effects, especially concerning plasma downflows, and propose thermal instability as a possible candidate to explain the observed redshifts in solar and stellar transition region lines. Title: ASAP: A systematic approach to plasma spectral synthesis Authors: Reale, F.; Maggio, A.; Ciaravella, A.; Peres, G. Bibcode: 1994SSRv...70..211R Altcode: ASAP (Analysis System for Astrophysical Plasmas), developed at the Osservatorio Astronomico di Palermo, is a package of procedures based on IDL. It is aimed at the detailed presentation of theoretical models of astrophysical plasmas and the accurate comparison with observational data, up to fittings of specific observations. It is also useful for the prediction of specific observations, and for the simulation of the expected performances of forthcoming instruments, for instance those on board SOHO. Title: Stationary flows in coronal loops Authors: Orlando, S.; Serio, S.; Peres, G. Bibcode: 1994SSRv...70..203O Altcode: We present a study of stationary flows in closed solar coronal loops. The hydrodynamic differential equations of plasma flow and energy balance are integrated with algorithms which achieve high reliability. We present here results on the detailed synthesis of loop emission in specific bands and lines, taking into account also non-equilibrium ionization. Title: Energy Transport and Dynamics Authors: Schmieder, B.; Peres, G.; Enome, S.; Falciani, R.; Heinzel, P.; Hénoux, J. C.; Mariska, J.; Reale, F.; Rilee, M. L.; Rompolt, B.; Shibasaki, K.; Stepanov, A. V.; Wülser, J. P.; Zarro, D.; Zharkova, V. Bibcode: 1994SoPh..153...55S Altcode: We report findings concerning energy transport and dynamics in flares during the impulsive and gradual phases based on new ground-based and space observations (notably fromYohkoh). A preheating sometimes occurs during the impulsive phase. CaXIX line shifts are confirmed to be good tracers of bulk plasma motions, although strong blue shifts are not as frequent as previously claimed. They often appear correlated with hard X-rays but, forsome events, the concept that electron beams provide the whole energy input to the thermal component seems not to apply. Theory now yields: new diagnostics of low-energy proton and electron beams; accurate hydrodynamical modeling of pulse beam heating of the atmosphere; possible diagnostics of microflares (based on X-ray line ratio or on loop variability); and simulated images of chromospheric evaporation fronts. For the gradual phase, the continual reorganization of magnetic field lines over active regions determines where and when magnetic reconnection, the mechanism favoured for energy release, will occur. Spatial and temporal fragmentation of the energy release, observed at different wavelengths, is considered to be a factor as well in energy transport and plasma dynamics. Title: The analysis system for astrophysical plasmas (ASAP) of the Osservatorio Astronomico di Palermo Authors: Maggio, A.; Reale, F.; Peres, G.; Ciaravella, A. Bibcode: 1994CoPhC..81..105M Altcode: We present the software package ASAP, mainly devoted to the presentation and analysis of fluid models of astrophysical plasmas. ASAP allows to generate optically thin X-ray and UV spectra emitted by plasma of given density, temperature and velocity, by means of well established spectral synthesis codes; the emission can then be easily folded with the instrumental response of interest and compared directly with measurements, or can be used to predict the performance of planned instruments.

As examples of typical applications of the ASAP package, we show high resolution emission spectra derived from 1D static models of coronal loops, detailed fitting with coronal loop models of observations made with the ROSAT/PSPC instrument, hydrodynamic simulations of a solar flaring loop as seen by the Yohkoh satellite, and the evolution of line emission from a dense blob of interstellar plasma subject to the passage of a shock.

The package, designed to be highly flexible, modular, and easily expandable, is expected to evolve and grow rapidly in response to new needs and required tasks. It operates within the IDL environment, and it is under consideration for being included in the SOHO Archive System. Title: Accurate Period Determination of an Eclipsing Binary X-Ray Source in M33 Authors: Schulman, Eric; Bregman, Joel N.; Collura, Alfonso; Reale, Fabio; Peres, Giovanni Bibcode: 1994ApJ...426L..55S Altcode: In the Letter "Accurate Period Determination of an Eclipsing Binary X- Ray Source in M33" by Eric Schulman, Joel N. Bregman, Alfonso Collura, Fabio Reale, and Giovanni Peres (ApJ, 418, L67 [1993]), the time between the ROSAT and Einstein Observatory observations was incorrectly calculated, leading to an incorrect period determination of P_orb_ = 154286 s (1.78572^d^). The correct period is P_orb_ = 154298 s (1.78586^d^) and the center of the first eclipse observed by Einstein occurred at J.D. 2,444,087.6061. The figures are unchanged, except that the times shown in Figures 1d and 1e are too large by 4.32 x 10^5^s. The correct stop time of the ROSAT observation section RH600020b is 00^h^32^m^52^s^ (Table 1). Title: Loop Models of Low Coronal Structures Observed by the Normal Incidence X-Ray Telescope (NIXT) Authors: Peres, G.; Reale, F.; Golub, L. Bibcode: 1994ApJ...422..412P Altcode: The X-ray pictures obtained with the Normal Incidence X-Ray Telescope (NIXT), apart from the ubiquitous coronal loops well known from previous X-ray observations, show a new and peculiar morphology: in many active regions there are wide and apparently low-lying areas of intense emission which resemble H alpha plages. By means of hydrostatic models of coronal arches, we analyze the distribution of temperature, density, emission measure, and plasma emissivity in the spectral band to which NIXT is sensitive, and we show that the above morphology can be explained by the characteristics of high pressure loops having a thin region of high surface brightness at the base. We therefore propose that this finding might help to identify high-pressure X-ray emitting coronal regions in NIXT images, and it is in principle applicable to any imaging instrument which has high sensitivity to 104 - 106 K plasma within a narrow coronal-temperature passband. As a more general result of this study, we propose that the comparison of NIXT observations with models of stationary loops might provide a new diagnostic: the determination of the loop plasma pressure from measurements of brightness distribution along the loop. Title: 15-30 arcsec resolution replica x-ray optics for AXAF-S Authors: Stella, Luigi; Chincarini, G.; Citterio, Oberto; Conconi, Paolo; Maccacaro, T.; Tagliaferri, G.; Trinchieri, G.; Wolter, A.; Bignami, Giovanni F.; Bocchino, F.; Maggio, A.; Micela, G.; Sciortino, Salvatore; Serio, Salvatore; Collura, Alfonso; Giommi, P.; Maraschi, L.; Pallavicini, R.; Pellegrini, S.; Peres, G. Bibcode: 1994SPIE.2011..149S Altcode: This document describes a proposal for a replica X-ray optics to be developed in Italy for NASA's X-ray spectroscopy mission AXAF-S. The program is based on state of the art technology for the production of replica X-ray optics. On the basis of the experience with the Jet-X mirror shells (to be flown on the Spectrum X-(gamma) satellite), a spatial resolution of 15 - 30 arcsec half power diameter (HPD) can be achieved for the AXAF-S optics. The characteristics of the proposed optical system are described and its performances evaluated by using the current baseline configuration for the array of X-ray calorimeters in the focal plane. The impact of the proposed replica X-ray optics is briefly outlined and a comparison with foil optics (1 - 3 arcmin HPD resolution) is carried out. Title: Siphon Flows in Stratified Coronal Loops Authors: Orlando, S.; Peres, G.; Serio, S. Bibcode: 1994scs..conf..185O Altcode: 1994IAUCo.144..185O The authors have developed a detailed siphon flow model for coronal loops. They find scaling laws relating the characteristic parameters of the loop, explore systematically the space of solutions and show that supersonic flows are impossible for realistic values of heat flux at the base of the upflowing leg. Title: Impulsive Heating of Coronal Loops Authors: Reale, F.; Peres, G.; Serio, S. Bibcode: 1994scs..conf..215R Altcode: 1994IAUCo.144..215R With the aid of hydrodynamic modeling of coronal plasma, the authors have investigated the signatures of a heating composed by a sequence of time-limited episodes (microflares) in possible continuous observations of loops by several instruments, like Yohkoh/SXT and the forthcoming SOHO/EIT. Title: Pressure diagnostics of coronal loops observed by NIXT Authors: Peres, G.; Reale, F.; Golub, L. Bibcode: 1994LNP...432..179P Altcode: 1994LNPM...11..179P The Normal Incidence X-ray Telescope (NIXT sounding rocket payload — a set of multilayer telescopes of novel design — provides images of the corona at sub-arcsec angular resolution in narrow X-ray spectral bands centered at the wavelengths of particular coronal emission lines. The NIXT 63.5 angstrom coronal images show the well-known and ubiquitous coronal loops but also, mostly in active regions, shallow and bright areas of intense emission not resembling loops. We have explained such areas within the traditional physics of coronal loop models as intense emission in the NIXT band coming from a narrow region at the base of high pressure loops; the particular nature of the NIXT temperature sensitivity, with its bimodal temperature response, is the key to detecting such a feature. We discuss the implications of this finding and, in particular, we show the possibility of new diagnostics of plasma pressure independent of the traditional one based on the determination of emission measure. In addition, we show that there is very little dependence of the spatial distribution of the X-ray emissivity in the NIXT passband on the details of the spatial distribution of the heating function. Title: Accurate Period Determination of an Eclipsing Binary X-Ray Source in M33 Authors: Schulman, Eric; Bregman, Joel N.; Collura, Alfonso; Reale, Fabio; Peres, Giovanni Bibcode: 1993ApJ...418L..67S Altcode: We have analyzed the time variability of one of the X-ray sources in M33 observed by both the ROSAT and Einstein Observatory telescopes. The light curve of M33 X-7 exhibits a variability pattern of high and low states, suggesting an eclipsing binary X-ray source. The data suggest a binary period P = 1.78572 days (very close to that of Her X-1) and an eclipse duration of ~0.4 days. The low phase lasts about one-fourth of the period as in Cen X-3. Title: Low amplitude variability and transient periodicity in FF Andromedae and other active stars. Authors: Peres, G.; Ventura, R.; Pagano, I.; Rodono, M. Bibcode: 1993A&A...278..179P Altcode: This work presents the first results of a project mainly dedicated to the detection of low amplitude variability in stars with the URSULA double channel photometer fed by the 91 cm telescope of Catania Astrophysical Observatory. URSULA simultaneously observes the target star and a nearby comparison star with two identical photometers and achieves a relative photometric precision down to a few thousandths of magnitude because rapid fluctuations of sky transparency equally affect both stars and are easily removed. We can, therefore, use short integration times and attain high time resolution. The analysis of our observations is based on the methods of periodograms, according to the formulation for unequally spaced time sequences devised by Scargle (1982). We have observed FF And, BY Dra, V 1054 Oph and V 1396 Cyg and detected, at a high level of confidence, several cases of small scale variability, particularly evident in the data of FF And and of V1054 Oph. From FF And we have detected small, but highly significant, dips in the light curve, approximately 25 minutes before a large flare. We compare this finding with the known phenomenology of pre-flare dips and, among other interpretations, we propose them as possible signatures of pre-flare atmospheric activation. We also discuss the data set pertaining to all observed stars. Title: Simulations of the CA XIX Spectral Emission from a Flaring Solar Coronal Loop. II. Impulsive Heating by Accelerated Electrons Authors: Antonucci, E.; Dodero, M. A.; Martin, R.; Peres, G.; Reale, F.; Serio, S. Bibcode: 1993ApJ...413..786A Altcode: In this paper we analyze the response of the plasma inside a coronal loop to the passage of a beam of accelerated electrons and study it systematically as a function of the parameters and temporal profile of the beam, as well as of the initial conditions of the corona. Our approach consists in computing the spectral emission from an impulsively heated loop in the Ca XIX line complex in the range 3.165-3.231 A and in analyzing the simulated spectra with the same technique used for the observations. This allows a direct comparison of the simulations with the results of the spectral observations of the Bent Crystal Spectrometer on board the SMM. While in some cases the values and evolution of average plasma properties, as derived from the analysis of spectra, are reproduced with good accuracy, the detailed shape of the Ca XIX lines cannot be accurately reproduced by this kind of model of flares heated by nonthermal electron beams. Title: Detectability of chromospheric evaporation fronts in solar flares Authors: Peres, G.; Reale, F. Bibcode: 1993A&A...275L..13P Altcode: We compute the surface brightness distribution of the chromospheric evaporation front during the flare impulsive phase in the pass-band of the imaging Soft X-ray Telescope (SXT) on board Yohkoh. Hydrodynamic simulations of the flaring plasma allow us to derive the evolution of density and temperature distribution and therefore of emission distribution across the evaporation fronts. We show that SXT - as well as imaging X-ray telescopes with equivalent or better spatial and temporal resolution - should be able to detect clearly the front and follow its evolution along the loop. Title: Dynamics of the decay of confined stellar X-ray flares Authors: Reale, F.; Serio, S.; Peres, G. Bibcode: 1993A&A...272..486R Altcode: We extend and generalize to stellar X-ray flares the analysis of the decay phase of flares inside solar coronal loops of constant cross-section, and investigate related diagnostics from X-ray observations. The work is based on hydrodynamic simulations of compact stellar flares for different values of stellar surface gravity, loop length and peak temperature, and characteristics of loop heating during the decay phase. We discuss the validity of the thermodynamic scaling law of Serio et al. (1991) in the wider context of stellar flares, and find that a generalized version of this law holds if the loop half-length L is smaller than the peak pressure scale height h. In the light of the diagnostics which can be derived from the density-temperature (n-T) diagram, already tested for solar flares, we consider how the ratio ζ of the density decay time to the temperature decay time depends on gravity. We find that ζ, as computed at the top of the loop, has a maximum value ζ ∼ 3 for very small L/h and decreases as L/h increases. Alternatively we consider the average flare density and temperature as they would be derived from observations with the ROSAT PSPC or other moderate energy resolution detectors, we obtain a flatter curve, with a maximum value around 2. We discuss also how energy deposition during the flare decay phase influences ζ, and propose a combined diagnostic approach on stellar flares based on the joint use of the n-T diagrams and of the fitting of the light curve with hydrodynamic models. As a first practical result we can safely exclude the presence of sustained heating during the decay of an X-ray flare observed on Prox Cen by the EINSTEIN/IPC instrument. Title: Thermal Conduction and Modeling of Static Stellar Coronal Loops Authors: Ciaravella, A.; Peres, G.; Serio, S. Bibcode: 1993SoPh..145...45C Altcode: We have modeled stellar coronal loops in static conditions for a wide range of loop length, plasma pressure at the base of the loop and stellar surface gravity, so as to describe physical conditions that can occur in coronae of stars ranging from low mass dwarfs to giants as well as on a significant fraction of the Main-Sequence stars. Title: Stellar XUV Spectra from Coronal Loop Models Authors: Maggio, A.; Serio, S.; Peres, G. Bibcode: 1993AAS...182.4126M Altcode: 1993BAAS...25..864M Following the analogy with the solar corona, we describe the observed EUV and X-ray emission from late-type stars as originating from optically thin plasma magnetically confined in closed loops, according to the model by Serio et al. (1981). This model takes into account optically thin radiative losses, thermal conduction, and heating terms in the energy balance equation, as well as gravity in the hydrostatic equilibrium equation, thus providing a description physically more appropriate than that obtained by simple one- or two-temperature thermal models. We generate loop models with a variety of characteristic parameters (such as loop base pressure, loop length, or stellar surface gravity), to synthesize XUV spectra from the emission measure vs. temperature model distribution, and to fold them through the relevant instrumental response, in order to allow a direct comparison with observational data. In this paper, we show some examples of this approach pertaining to ROSAT and EUVE data. Title: Sky simulations for X-ray telescopes Authors: Peres, G.; Micela, G.; Barbera, M.; Sciortino, S.; Serio, S. Bibcode: 1993MmSAI..64..716P Altcode: No abstract at ADS Title: The Palermo analysis system for astrophysical plasmas (ASAP): rationale, development, and applications Authors: Maggio, A.; Reale, F.; Peres, G. Bibcode: 1993MmSAI..64.1023M Altcode: No abstract at ADS Title: The importance of plasma viscosity on X-ray line diagnostics of solar flares Authors: Peres, G.; Reale, F. Bibcode: 1993A&A...267..566P Altcode: We discuss the importance of compressional viscosity for a correct description of flare plasma dynamics and for its diagnostics with high resolution soft X-ray spectroscopy. We have separated the influence of viscosity from that of other physical effects at work during flares by comparing hydrodynamic simulations differing only for the presence or absence of viscosity. We show that the influence is significant on the flaring plasma dynamics as well as on the relevant spectra, when the flare heating is mostly delivered either at the loop apex or by means of low-energy-cutoff nonthermal electron beams. For base-heated loop flares, instead, the effect of viscosity is significant only with very intense heating. We also discuss possible markers of shocks in the observed spectra of Ca XIX (3.18 A) and Fe XXV (1.85 A) and show that viscosity influences time-integrated spectra and, a fortiori, spectra with one second-time resolution, as available with the Bragg Crystal Spectrometer on board YOHKOH. Title: Thermodynamic decay of stellar coronal flares Authors: Reale, F.; Serio, S.; Peres, G. Bibcode: 1993MmSAI..64..555R Altcode: No abstract at ADS Title: Hydrodynamics and Diagnostics of Coronal Loops Subject to Dynamic Heating Authors: Peres, G.; Reale, F.; Serio, S. Bibcode: 1993ASSL..183..151P Altcode: 1993pssc.symp..151P No abstract at ADS Title: Scaling Laws for the Decay Phase of Stellar Loop Flares Authors: Reale, F.; Serio, S.; Peres, G. Bibcode: 1993ASSL..183..599R Altcode: 1993pssc.symp..599R No abstract at ADS Title: Soft X-ray Line Shifts as Signature of the Flare Heating Process Authors: Antonucci, E.; Dodero, M. A.; Peres, G.; Reale, F.; Serio, S.; Somov, B. V. Bibcode: 1993ASSL..183..159A Altcode: 1993pssc.symp..159A No abstract at ADS Title: The G. S. Vaiana X-ray Astronomy Calibration and Testing (XACT) Facility Authors: Collura, A.; Barbera, M.; Inzerillo, G.; Maggio, A.; Micela, G.; Mirabello, F.; Sciortino, S.; Serio, S.; Peres, G. Bibcode: 1993ASSL..183..275C Altcode: 1993pssc.symp..275C No abstract at ADS Title: Thermal Transport in Static Stellar Coronal Loops Authors: Ciaravella, A.; Peres, G.; Serio, S. Bibcode: 1993ASSL..183..595C Altcode: 1993pssc.symp..595C No abstract at ADS Title: Observations of Low Level Optical Variability in Active Stars Authors: Ventura, R.; Pagano, I.; Peres, G.; Rodonò, M. Bibcode: 1993ASSL..183..463V Altcode: 1993pssc.symp..463V No abstract at ADS Title: A tool for X-Ray sky simulations. Authors: Peres, G.; Micela, G.; Sciortino, S.; Serio, S. Bibcode: 1993MmSAI..64..385P Altcode: No abstract at ADS Title: Siphon Flow Models of Coronal Loops Authors: Orlando, S.; Serio, S.; Peres, G. Bibcode: 1993ASSL..183..163O Altcode: 1993pssc.symp..163O No abstract at ADS Title: Steady Siphon Flows in Closed Coronal Structures: Comparison with Extreme-Ultraviolet Observations Authors: Peres, G.; Spadaro, D.; Noci, G. Bibcode: 1992ApJ...389..777P Altcode: Models of steady siphon flow in solar coronal loops are computed and the intensities of some EUV transition region emission lines synthesized from these models are compared with representative obsevations of typical solar regions. It is found that siphon flow models of active region loops are in better agreement with observations of EUV transition region lines, while for large loops interconnecting different active regions, static models work slightly better. The role of siphon flows in determining the structure of the outer solar atmosphere is discussed based on these results. Title: The effect of viscosity on hydrodynamics of coronal flares Authors: Reale, F.; Peres, G. Bibcode: 1992AIPC..267..140R Altcode: 1992ecsa.work..140R We investigate the effect of plasma viscosity in the hydrodynamics of coronal flares. To this end we compute two otherwise identical models of a typical coronal compact flare, one including and the other neglecting viscosity terms from the relevant hydrodynamic equations. We find significant differences which may affect the diagnosis of observed high resolution X-ray spectra. Title: Dynamics of flaring loops. II - Flare evolution in the density-temperature diagram Authors: Jakimiec, J.; Sylwester, B.; Sylwester, J.; Serio, S.; Peres, G.; Reale, F. Bibcode: 1992A&A...253..269J Altcode: The evolution of basic thermodynamic parameters of a single flaring solar loop has been investigated in terms of density-temperature (N-T) diagram. A grid of hydrodynamic models has been calculated for this purpose, using the Palermo-Harvard code. The calculated models differ in their initial conditions and the form of the energy imput (i.e., the heating rate value, the heating duration, the assumed time profile). The consequences of variation of these model parameters on the evolutionary paths in the density-temperature diagrams is considered; over a substantial duration, the decay occurs along a T varies as N-squared trajectory when the impulsive flare heating function is switched-off abruptly. The results obtained can be very useful as diagnostics of the flare heating process, based on soft X-ray observations. Title: Thermodynamic decay scaling laws in solar loop flares Authors: Serio, S.; Reale, F.; Peres, G.; Jakimiec, J.; Sylwester, B.; Sylwester, J. Bibcode: 1992MmSAI..63..763S Altcode: Results of numerical calculations are used, together with analytical considerations, as a guide to set up diagnostic tools for the flare decay phase in terms of the temperature-density (n-T) diagram. The decaying loop was modeled as a hydrodynamic process in a rigid semicircular tube. It is shown that the trajectory on the n-T plane has a slope of about 2 for a wide set of peak model flare conditions. Title: Modelling of open and closed coronal structures - Comparison with detailed EUV observations Authors: Peres, G.; Spadaro, D. Bibcode: 1992sws..coll...87P Altcode: We consider the modelling and EUV diagnostics of plasma in steady state motion both within closed coronal structures (siphon flows) and outflowing from coronal holes toward interplanetary space. We take into account nonequilibrium ionization in the synthesis of emission lines originating from the modelled closed structures and compare the computed line intensities with detailed EUV observations, in order to constrain significantly the model. We evaluate the importance of nonequilibrium ionizations effects for some published coronal hole models. Title: Hydrostatic models of X-ray coronal loops observed by NIXT Authors: Peres, G.; Reale, F.; Golub, Leon Bibcode: 1992AIPC..267..136P Altcode: 1992ecsa.work..136P Observations made with the Normal Incidence X-ray Telescope (NIXT) have shown that some X-ray emitting structures observed with NIXT resemble very closely the corresponding Hα plages. We have used hydrostatic models of coronal loops to explain such observations as strong emission from the lower section of high-presssure coronal arches. Title: Flare Evolution in the Density - Temperature Diagram Authors: Serio, S.; Reale, F.; Peres, G.; Jakimiec, J.; Sylwester, B.; Sylwester, J. Bibcode: 1992LNP...399..135S Altcode: 1992esf..coll..135S; 1992IAUCo.133..135S No abstract at ADS Title: Non-local Thermal Conduction in Solar and Stellar Coronal Loops Authors: Ciaravella, A.; Peres, G.; Serio, S. Bibcode: 1992LNP...397..261C Altcode: 1992sils.conf..261C No abstract at ADS Title: Numerical simulations of thermal instabilities in stratified gases. II - Exploration of the parameter space Authors: Reale, F.; Rosner, R.; Malagoli, A.; Peres, G.; Serio, S. Bibcode: 1991MNRAS.251..379R Altcode: The temporal evolution of density perturbations in an initially hydrostatic isothermal atmosphere consisting of an optically thin radiating compressible plasma is studied. Numerical techniques are used to describe the nonlinear evolution of the perturbations, and the relative equilibrium between dynamic and thermal instabilities as governed by three independent control parameters are examined, namely, the initial density contrast of the perturbation, the ratio of the local buoyancy oscillation period to the local radiative cooling time, and the ratio of the perturbation radius to the local scaleheight. Four orders of magnitude of initial density contrasts and ratios of buoyancy and cooling times, and one order of magnitude of the bubble dimensions are explored. Well-defined oscillations were found to occur in a limited parameter range, and thermal instability to occur even within secondary condensations deriving from the bubble fragmentation. Title: Non-local heat transport in static solar coronal loops Authors: Ciaravella, A.; Peres, G.; Serio, S. Bibcode: 1991SoPh..132..279C Altcode: We investigate the limits of applicability of the Spitzer-Harm thermal conductivity in solar coronal loops and show that the ratio λ0/LTof electron mean-free path to temperature scale height in large-scale structures can approach the limits of the Spitzer-Harm theory. We use a non-local formulation of heat transport to compute a grid of loop models: the effects of non-local transport on the distribution of differential emission measure are particularly important in the coronal part of loops longer than the pressure scale height sp.We derive a scaling law for λ0/LTin the corona, showing that it grows exponentially with L/sp, and discuss effects of non-local heat transport in the transition region. Title: European Photon Imaging Camera (EPIC) for X-ray astronomy. Authors: Bignami, G. F.; Villa, G. E.; Boella, G.; Bonelli, G.; Caraveo, P.; Chiappetti, L.; Quadrini, M. E.; Di Cocco, G.; Trifoglio, M.; Ubertini, P.; Peres, G.; Sciortino, S.; Serio, S.; Vaiana, G.; Rothenflug, R.; Vigroux, L.; Koch, L.; Rio, Y.; Pigot, C.; Cretolle, J.; Gabriel, A.; Foing, B.; Atteia, J. L.; Roques, J. P.; Bräuninger, H.; Pietsch, W.; Predehl, P.; Reppin, C.; Struder, L.; Trümper, J.; Lutz, G.; Kendziorra, E.; Staubert, R.; Holland, A. D.; Cole, R. E.; Wells, A.; Pounds, K.; Lumb, D. A.; Pye, J.; Turner, M. J. L.; Goodall, C. V.; Ponman, T. J.; Skinner, G. K.; Willmore, A. P. Bibcode: 1990SPIE.1344..144B Altcode: 1990exrg.conf..144B ESA has selected the final payload for its "Cornerstone" mission in X-Ray astronomy with multiple mirrors (XMM), to be flown in the late nineties in the context of the "Horizon 2000" long term science plan. EPIC represents the main instrument of the mission, to include three CCD arrays in the focal planes of the three telescopes of the spacecraft. They will be dedicated to source imaging, photometry, spectroscopy and timing. The goals of EPIC are described. Title: A two-dimensional hydrodynamic code for astrophysical flows. Authors: Reale, F.; Peres, G.; Serio, S. Bibcode: 1990NCimB.105.1235R Altcode: The authors present a two-dimensional hydrodynamic code suited to study astrophysical flows in many different environments. The code solves the hydrodynamic equations in conservative form in the most used coordinate systems and is based on an explicit fully two-dimensional flux corrected transport (FCT) technique, which ensures an accurate description of steep gradient regions and shocks, a relatively ample flexibility to include a variety of physical effects, and a good efficiency for speed on vector or array processors. Extensive testing has allowed an accurate "tuning" of the FCT numerical parameters. This code is among the best FCT codes and performs well in a whole set of demanding strongly nonlinear hydrodynamic tests, getting close to performances of more complex codes, while remaining less computationally expensive and more than sufficient for most astrophysical applications. Title: One-dimensional hydrodynamic modeling of coronal plasmas on transputer arrays Authors: Reale, F.; Brugé, F.; Peres, G.; Fornili, S. L.; Martorana, V.; Serio, S. Bibcode: 1990CoPhC..60..201R Altcode: We describe a concurrent implementation of the Palermo-Harvard hydrodynamic code on cost-effective and modularity expandable transputer arrays. We have tested the effectiveness of our approach by simulating an already well-studied compact solar-flare model on different transputer configurations and compared their performances with those of other machines. We have found that the speed of the concurrent program on a 16-T800 transputers array is ~1/9 of that of the equivalent code optimized for a CRAY X-MP/48. This work clearly shows that transputer-based arrays provide locally available high computing-power tools to extend the investigation of compact solar flares and similar astrophysical problems. Title: Two Variable X-Ray Sources in M31 Authors: Collura, A.; Reale, F.; Peres, G. Bibcode: 1990ApJ...356..119C Altcode: We have analyzed the entire set of Einstein observations of M31, consisting of five imaging proportional counter and nine high resolution imager observations, for variability of the X-ray sources. The time scales explored range up to ~ 10^5^ s, the maximum time span of individual observations. We detect variability above the adopted 99.73% significance level in two sources and derive constraints on their possible periodicity. The observed X-ray luminosities, the pattern of variability, and the optical identifications suggest the possibility that one of them is a low- mass X-ray binary and the other one a compact massive X-ray binary. We also discuss the analysis of the nonvariable sources and derive upper limits on the detectable variability of the various sources. The number of variable sources detected is in agreement with the expectations based on the features of intense Galactic X-ray sources and on the characteristics of the observations. Title: X-ray spectral synthesis in hydrodynamic flare models Authors: Serio, S.; Antonucci, E.; Dodero, M. A.; Peres, G.; Reale, F. Bibcode: 1990hrxr.conf..126S Altcode: 1990IAUCo.115..126S Compact solar flares are triggered by sudden energy release in magnetically confined plasma. This class of flares is well suited to be studied with numerical hydrodynamic models. In particular, it is possible to compare the evolution of observed and synthetic X-ray spectra, computed under various assumptions for the mechanism of impulsive energy deposition, to constrain theoretical models and their parameter space. Recent results on solar flares along this line, nonthermal to models of energy depositions by relativistic electron beams are discussed. Possible applications of X-ray spectral synthesis to stellar flares are also discussed. Title: X-ray observations, scaling laws and magnetic fields Authors: Peres, G.; Vaiana, G. S. Bibcode: 1990MmSAI..61..401P Altcode: Sounding-rocket, satellite, and Skylab X-ray observations of the sun, obtained at high spatial resolution using grazing-incidence-telescopes during the period 1963-1975, are reviewed, with an emphasis on their astrophysical implications. The history of the observation programs is discussed; sample images and tables of numerical data are presented; and it is shown how the X-ray data revealed the magnetic confinement of the hot coronal plasma and permitted the formulation of scaling laws, which relate the loop length to the maximum plasma temperature and pressure at the base of the corona and account for gravitational effects and magnetic-field heating. More recent Einstein Observatory, Exosat, and Rosat X-ray observations of stellar coronae are briefly described, and the applicability of the solar coronal-loop models to late-type stars is considered. Title: Variability of the X-Ray Sources in M33 and M31 Authors: Peres, G.; Reale, F.; Collura, A.; Fabbiano, G. Bibcode: 1990ixra.conf..267P Altcode: 1990ixra.symp..267P Contents: Introduction. M33: an eclipsing binary and an "X-ray only" AGN. M31: two binaries? AXAF prospects. Title: Variability of the X-Ray Sources in M31 Authors: Collura, A.; Reale, F.; Peres, G. Bibcode: 1989ESASP.296..359C Altcode: 1989ttxa.symp..359C The Einstein observations of M31, looking for variability of the X-ray sources up to time scales of approximately 100,000 are analyzed. Two sources are significantly variable. The characteristics of their variability are discussed and constraints on their possible periodicity are derived. The observed X-ray luminosities, the pattern of variability, and the optical identifications suggest the possibility that one of them is low mass X-ray binary and the other one a compact massive X-ray binary. Title: Hydrodynamic Models of Solar and Stellar Flares Authors: Peres, Giovanni Bibcode: 1989SoPh..121..289P Altcode: 1989IAUCo.104..289P This paper discusses the hydrodynamic modeling of flaring plasma confined in magnetic loops and its objectives within the broader scope of flare physics. In particular, the Palermo-Harvard model is discussed along with its applications to the detailed fitting of X-ray light curves of solar flares and to the simulation of high-resolution Ca xix spectra in the impulsive phase. These two approaches provide complementary constraints on the relevant features of solar flares. The extension to the stellar case, with the fitting of the light curve of an X-ray flare which occurred on Proxima Centauri, demonstrates the feasibility of using this kind of model for stars too. Although the stellar observations do not provide the wealth of details available for the Sun, and, therefore, constrain the model more loosely, there are strong motivations to pursue this line of research: the wider range of physical parameters in stellar flares and the possibility of studying further the solar-stellar connection. Title: Time Variability of the X-Ray Sources in M33 Authors: Peres, G.; Reale, F.; Collura, A.; Fabbiano, G. Bibcode: 1989ApJ...336..140P Altcode: We have analyzed the variability of the X-ray sources in M33 observed by the Einstein Observatory telescope. Our analysis has been based on statistical techniques sensitive to variability over several time scales and has shown that two of the 15 known sources are variable above the 99.73% confidence level. The light curve of one of these sources, M33 X-7, exhibits a variability pattern of high and low states, suggesting an eclipsing binary X-ray source. The result would be the first identification of a close accreting binary system with an X-ray source in an external galaxy other than the Magellanic Clouds. The data suggest a binary period P = 1.7857 days and an eclipse duration of ~0.4 days. The nuclear source M33 X-8 varies only in the softest part of the spectrum (kT <= 1.2 keV). The observations suggest a rapid variability (t ~ 3000 s) and show a rapid flare with a rise time shorter than 3 days together with longer time scale variability. Title: Variability of the X-ray sources in M 33. Authors: Peres, G.; Reale, F.; Collura, A.; Fabbiano, G. Bibcode: 1989MmSAI..60..221P Altcode: 1989MSAIt..60..221P Variability analysis of the X-ray sources in M33 observed with the Einstein telescope is presented. Two of the fifteen sources of M33 have been found to be variable beyond 99.77 percent significance level: M33 X-7 and M33 X-8. M33 X-7 showed a very strong variability on many time scales ranging from months to less than hours. This result signifies the first observation of a close accreting binary system with an X-ray source in an external galaxy other than the Magellanic Clouds. The nuclear source M33 X-8 is variable on a six-month time scale in the High Resolution Imager (HRI), the Imaging Proportional Counter (IPC), and the nonimaging Monitor Proportional Counter (MPC). The spectral characteristics of the X-ray emission of this source and the similarity with other low luminosity nuclei favor the possibility of galactic nuclear activity. Title: Numerical simulations of thermal instabilities in galactic and cluster halos Authors: Peres, G.; Rosner, R.; Bodo, G. Bibcode: 1989MmSAI..60..139P Altcode: X-ray observations have shown the presence of diffuse hot gas in many clusters of galaxies and around some elliptical galaxies. It is typically expected that radiative cooling in the central region of these halos drives significant subsonic inflow of gas to the center of the system. Steady infall models of such flows typically predict that the gas will 'drop out' of the flow, so that the mass accretion rate is a function of radius. The plausible mechanism that removes some fraction of the gas from the flow at various radii is thermal instability in the cooling flow itself. Here, results are reported of a fully nonlinear analysis of the monotonic radial modes in a cooling flow which uses physical parameters appropriate to the cooling flow of M87. A case of spontaneous formation of the radiative instability at the center of the halo is described and the relevance of thermal conductivity in determining the stability is shown. Title: Flare energetics. Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.; Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres, G.; Serio, S.; Sylwester, J.; Veck, N. J. Bibcode: 1989epos.conf..377W Altcode: The authors have sought to establish a comprehensive and self-consistent picture of the sources and transport of energy within a flare. To achieve this goal, they chose five flares in 1980 that were well observed with instruments on the Solar Maximum Mission, and with other space-borne and ground-based instruments. The events were chosen to represent various types of flares. Details of the observations available for them and the corresponding physical parameters derived from these data are presented. The flares were studied from two perspectives, the impulsive and gradual phases, and then the results were compared to obtain the overall picture of the energetics of these flares. The authors also discuss the role that modeling can play in estimating the total energy of a flare when the observationally determined parameters are used as the input to a numerical model. Finally, a critique of our current understanding of flare energetics and the methods used to determine various energetics terms is outlined, and possible future directions of research in this area are suggested. Title: Intercomparison of numerical models of flaring coronal loops. Authors: Kopp, R. A.; Fisher, G. H.; MacNiece, P.; McWhirter, R. W. P.; Peres, G. Bibcode: 1989epos.conf..597K Altcode: The numerical modelling group was concerned with computations relevant to the problem of the hydrodynamic and radiative response of a single magnetic flux tube to a sudden release of energy in it. The group initially considered a simple "benchmark model" in which the physics of real loops - radiation, thermal conduction, compressible hydrodynamics, gravity and nonthermal heating - could be incorporated with some degree of realism. The primary goal of the benchmark model was to intercompare code calculations on a standardized, although hypothetical, problem, rather than to establish the best possible physical model. Title: Hydrodynamic simulations of stellar flares in various magnetic field geometries Authors: Reale, F.; Peres, G.; Serio, S. Bibcode: 1989MmSAI..60..203R Altcode: Hydrodynamic simulations of coronal flares from the solar to the stellar environment have recently been extended to a flare that occurred on Proxima Centauri and that was observed by the imaging proportional counter (IPC) instrument on the Einstein X-ray satellite. Numerical simulations of this flare have provided information about the dimensions of the flaring region. The moderate spectral resolution of the IPC also made it possible to set constraints on the energetics of the flare. A new flare model was considered for an X-ray flare observed by the Exosat on the RS CVn stellar system Sigma CrB on September 29, 1983 at 4:07 UT, with the medium energy detector in the 2-8 keV energy band, which is considerably harder than the Einstein-IPC energy band of 0.2-4.0 keV. Preliminary results on the simulation of the initial phase of the flare are presented. Title: Two-dimensional hydrodynamics of astrophysical plasmas Authors: Reale, F.; Peres, G.; Serio, S. Bibcode: 1989MmSAI..60..207R Altcode: A preliminary version of a fully two-dimensional flux corrected transport (CFT) code which can solve hydrodynamic equations of mass and momentum conservation is presented. A set of tests is being carried out to verify stability, accuracy, and efficiency of the code. Results of a properly two-dimensional test describing the rigid clockwise revolution of a fluid cylinder around the center of a spatial grid (100 x 100 points) are reported. Title: Variability of sources in the M33 and M31 Galaxies Authors: Peres, G.; Collura, A.; Fabbiano, G. Bibcode: 1988feta.conf...23P Altcode: No abstract at ADS Title: Morphology and Spectral Characteristics of the X-Ray Emission of M33: Erratum Authors: Trinchieri, G.; Fabbiano, G.; Peres, G. Bibcode: 1988ApJ...329.1037T Altcode: In regard to the paper "Morphological and Spectral Characteristics of the X-ray Emission of M33" by G. Trinchieri, G. Fabbiano, and G. Peres (Ap. J., 325, 531 [1988]), several additional remarks should be made. Title: Hydrodynamic Modeling of an X-Ray Flare on Proxima Centauri Observed by the Einstein Telescope Authors: Reale, F.; Peres, G.; Serio, S.; Rosner, R.; Schmitt, J. H. M. M. Bibcode: 1988ApJ...328..256R Altcode: Hydrodynamic numerical calculations of a flare which occurred on Proxima Centauri and was observed by the Einstein satellite on August 20, 1980 at 12:50 UT are presented. The highlights of the hydrodynamic code are reviewed, and the physical and geometrical parameters necessary for the calculations are derived and compared with observations. The results are consistent with the stellar flare being caused by the rapid dissipation of 5.9 x 10 to the 31st ergs, within a magnetic loop structure whose semilength is 7 x 10 to the 9th cm and cross-sectional radius is 7.3 x 10 to the 8th cm. The results provide evidence that flares on late-type stars can be described by a hydrodynamic model with a relatively simple geometry, similar to solar compact flares. Title: Morphology and Spectral Characteristics of the X-Ray Emission of M33 Authors: Trinchieri, G.; Fabbiano, G.; Peres, G. Bibcode: 1988ApJ...325..531T Altcode: We have extended previous analysis of the X-ray data on M33 to include a detailed study of the morphology and spectral characteristics of its X-ray emission. In addition to the bright nuclear source and to several bright pointlike sources in the disk and arms of this galaxy, which had already been reported, we find a lower surface brightness, diffuse emission from the plane of M33. This extended emission is further separable in a hard (>2 keV) and in a soft (<1 keV) component, similarly distributed in the plane of the galaxy. We argue that the hard component is most likely the result of the integrated contribution of several lower luminosity discrete sources, similar to the ones observed in the Milky Way, e.g., compact accreting systems and young SNRs; the soft component is most likely due to the integrated emission of stellar coronae and old SNRs, although diffuse hot gas in the disk of M33 could also contribute. The brightest source in M33 is in the nuclear region, and its average luminosity (L_x_~ 4 x 10^39^ ergs s^-1^) is >=10 times higher than the next brightest source in the plane. The nature of this source remains undetermined. The spectral results indicate a soft and intrinsically absorbed spectrum, consistent with those of low-luminosity active galactic nuclei, although the lack of signs of activity at frequencies other than the X-ray ones would make this source a unique example of " X-ray-only" activity. Alternatively the source could be explained as a new type of X-ray binary system. Title: Two-Dimensional Numerical Hydrodynamics Authors: Reale, F.; Peres, G.; Serio, S. Bibcode: 1988sca..conf...41R Altcode: No abstract at ADS Title: The theoretical and computational activity at the Osservatorio DI Palermo Authors: Peres, Giovanni Bibcode: 1988MmSAI..59..389P Altcode: The construction of theoretical models of solar flares and magnetically confined stellar coronal plasmas and the use of these models in the analysis of observational data are discussed, reviewing the results of recent investigations at the Osservatorio di Palermo. The governing equations for the Palermo-Harvard one-dimensional hydrodynamic/thermodynamic plasma model (Peres et al., 1982; Pallavicini et al., 1983) are presented; the numerical implementation of the model is described; computed and observed X-ray light curves of solar flares are compared; and the treatment of nonlocal thermal conduction is considered. The steps in a typical simulation project are outlined, and the potential advantages of transputers for astrophysical simulations are indicated. Title: Simulations of the CA XIX Spectral Emission from a Flaring Solar Crornal Loop. I. Thermal Case Authors: Antonucci, E.; Dodero, M. A.; Peres, G.; Serio, S.; Rosner, R. Bibcode: 1987ApJ...322..522A Altcode: Spectral profiles synthesized from the results of numerical simulations of solar coronal loop plasma are compared with the observed line profiles, and the sensitivity of this comparison to variations in the parameter values is tested. The comparison allows the procedures commonly adopted for obtaining temperatures and velocities directly from the bent crystal spectrometer observations, as well as the interpretation of parameters obtained from hydrodynamical calculations, to be independently validated. The characteristic spectral signatures of different models for the deposition of impulsive thermal energy in the loop are derived assuming two distinct spatial distributions of thermal heating: in one case, the heating function peaks near the apex of the loop, while in the second, energy is deposited directly in the lower atmosphere. Title: Hydrodynamics of an X-Ray Flare on Proxima Centauri Authors: Reale, F.; Peres, G.; Serio, S.; Rosner, R.; Schmitt, J. H. M. M. Bibcode: 1987LNP...291..179R Altcode: 1987csss....5..179R; 1987LNP87.291..179R We apply the Palermo-Harvard hydrodynamic numerical code to compute the evolution of temperature, density, pressure and velocity in a semicircular symmetric rigid loop to reproduce the Einstein IPC observations of the 20 August 1980 flare on Proxima Centauri. Title: Explosive chromospheric instability in hydrodynamic loopflare models:the problem and its cure. Authors: Peres, G.; Serio, S.; Rosner, R. Bibcode: 1987NCimB..99...15P Altcode: No abstract at ADS Title: Nonlocal thermal conduction in hydrodynamic loop flare models. Authors: Peres, G.; Rosner, R.; Serio, S. Bibcode: 1987NCimB..99...29P Altcode: No abstract at ADS Title: LEBAN diagnostic for basic flaring loop parameters. Authors: Sylwester, B.; Sylwester, J.; Jakimiec, J.; Fludra, A.; Peres, G.; Serio, S. Bibcode: 1987PAICz..66..229S Altcode: 1987eram....1..229S The authors describe the diagnostic procedure called LEBAN which may be helpful in deriving basic geometrical loop parameters. "Palermo Code" hydrodynamic flare model calculations have been used to test reliability of the procedure. Title: High-Temperature Plasma Diagnostics of Solar Flares and Comparison with Model Calculations Authors: Jakimiec, J.; Sylwester, B.; Sylwester, J.; Lemen, J. R.; Mewe, R.; Bentley, R. D.; Peres, G.; Serio, S.; Schrijver, J. Bibcode: 1987sman.work...91J Altcode: The present state of flare diagnostics from X-ray spectra is briefly outlined. The authors discuss how improved diagnostic results can be used in flare heating process investigations. Title: Hydrodynamic Flare Modeling: Comparison of Numerical Calculations with SMM Observations of the 1980 November 12 17:00 UT Flare Authors: Peres, G.; Reale, F.; Serio, S.; Pallavicini, R. Bibcode: 1987ApJ...312..895P Altcode: The results of calculations of the evolution of flaring plasma in a simple coronal loop are compared with SMM observations of a well-studied compact flare. Calculations assuming different models of impulsive heating are performed, all starting from the same initial static loop configuration. A mechanism of local heat deposition and a mechanism of flare heating by energetic electron beams are tested with a variety of parameters. It is found that the evolution of the soft X-ray compact flare is mainly dominated by the thermodynamic and hydrodynamic properties of the confined plasma. In order to obtain a close match between calculated and observed X-ray polychromator light curves, a sizable fraction of impulsive energy must be deposited directly in the corona. Local Fe XXV brightenings in the flaring loop do not provide a good diagnostic of the site of energy deposition in solar flares. The total amount of impulsive energy necessary to match the calculated light curves with the observations depends strongly on the effective deposition depth. Title: Characterization of the Total Flare Energy Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.; Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres, G.; Serio, S.; Sylwester, J.; Veek, N. J. Bibcode: 1986epos.conf.5.41W Altcode: 1986epos.confE..41W No abstract at ADS Title: Energetics of the Impulsive Phase Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.; Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres, G.; Serio, S.; Sylwester, J.; Veek, N. J. Bibcode: 1986epos.conf..5.5W Altcode: 1986epos.confE...5W No abstract at ADS Title: Energetics of the Gradual Phase Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.; Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres, G.; Serio, S.; Sylwester, J.; Veek, N. J. Bibcode: 1986epos.conf.5.20W Altcode: 1986epos.confE..20W No abstract at ADS Title: Review of Impulsive Phase Phenomena Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.; Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres, G.; Serio, S.; Sylwester, J.; Veek, N. J. Bibcode: 1986epos.conf.5.60W Altcode: 1986epos.confE..60W No abstract at ADS Title: Intercomparison of numerical models of flaring coronal loops Authors: Kopp, R. A.; Fisher, G. H.; MacNeice, P.; McWhirter, R. W. P.; Peres, G. Bibcode: 1986epos.conf..7.1K Altcode: 1986epos.confG...1K The proposed Benchmark Problem consists of an infinitesimal magnetic flux tube containing a low-beta plasma. The field strength is assumed to be so large that the plasma can move only along the flux tube, whose shape remains invariant with time (i.e., the fluid motion is essentially one-dimensional). The flux tube cross section is taken to be constant over its entire length. In planar view the flux tube has a semi-circular shape, symmetric about its midpoint s = smax and intersecting the chromosphere-corona interface (CCI) perpendicularly at each foot point. The arc length from the loop apex to the CCI is 10,000 km. The flux tube extends an additional 2000 km below the CCI to include the chromosphere, which initially has a uniform temperature of 8000 K. The temperature at the top of the loop was fixed initially at 2 X 1 million K. The plasma is assumed to be a perfect gas (gamma = 5/3), consisting of pure hydrogen which is considered to be fully ionized at all temperatures. For simplicity, moreover, the electron and ion temperatures are taken to be everywhere equal at all times (corresponding to an artificially enhanced electron-ion collisional coupling). While there was more-or-less unanimous agreement as to certain global properties of the system behavior (peak temperature reached, thermal-wave time scales, etc.), no two groups could claim satisfactory accord when a more detailed comparison of solutions was attempted. Title: Flares Chosen for Energetics Study Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.; Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres, G.; Serio, S.; Sylwester, J.; Veek, N. J. Bibcode: 1986epos.conf.5.47W Altcode: 1986epos.confE..47W No abstract at ADS Title: Relationships among the Phases Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.; Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres, G.; Serio, S.; Sylwester, J.; Veek, N. J. Bibcode: 1986epos.conf.5.39W Altcode: 1986epos.confE..39W No abstract at ADS Title: The X-Ray Morphology and Spectrum of M33 Authors: Trinchieri, G.; Fabbiano, G.; Peres, G. Bibcode: 1986BAAS...18..998T Altcode: No abstract at ADS Title: Investigation of flare heating based on X-ray observations Authors: Jakimiec, J.; Sylwester, B.; Sylwester, J.; Mewe, R.; Peres, G.; Serio, S.; Schrijver, J. Bibcode: 1986CoSka..15..123J Altcode: Using X-ray data recorded by the Solar Maximum Mission Hard X-ray Imaging Spectrometer the authors have investigated flare evolution in a (Tm, N)-diagram, where Tm is the maximum temperature and N is the mean density in the flare volume. This flare diagnostics allows one to show that most large flares achieve a quasi-steady-state during their decay, which means that the cooling is then so slow that a flare evolves along the line of steady-state loops in the (Tm, N)-diagram. The diagnostics allows one to determine the time evolution of the flare heating function, which gives the rate of thermal energy release, per unit volume. Title: Investigation of flare heating based on X-ray observations Authors: Jakiemiec, J.; Sylwester, B.; Sylwester, J.; Mewe, R.; Peres, G.; Serio, S.; Schrijver, J. Bibcode: 1986AdSpR...6f.237J Altcode: 1986AdSpR...6..237J Using X-ray data recorded by the Solar Maximum Mission Hard X-ray Imaging Spectrometer we have investigated flare evolution in a (Tm, N)-diagram, where Tm is the maximum temperature and N is the mean density in the flare volume. It is important that the behaviour of a flare in such a diagram does not depend significantly on details of the flare geometry and therefore can be effectively compared with simplified model calculations of flare loops. This flare diagnostics allows us to show that most large flares achieve a quasi-steady-state during their decay, which means that the cooling is then so slow that a flare evolves along the line of steady-state loops in the (Tm, N)-diagram. The diagnostics allows us to determine the time evolution of the flare heating function, EH(t), which gives the rate of thermal energy release, per unit volume. For the flares which achieve the quasi-steady-state branch it gives a new valuable method of estimation of the electron density in the flare loops. Title: Hydrodynamics of coronal loops: A comparison of observed and simulated spectra emitted from flaring coronal loops Authors: Antonucci, E.; Dodero, M. A.; Peres, G.; Serio, S. Bibcode: 1986AdSpR...6f.151A Altcode: 1986AdSpR...6..151A The data obtained at high temporal and spectral resolution with the Bent Crystal Spectrometer of the Solar Maximum Mission have shown that the soft x-ray spectra emitted by highly ionized heavy ions are characterised during the impulsive phase of flares by blue-shifted components. The blue emission has been interpreted as due to convective plasma flows from the chromosphere, which in flare conditions is heated to coronal temperatures. In the present study, the properties of the Ca XIX solar spectra are compared with the results of numerical calculations simulating the behaviour of coronal plasma subject to impulsive heating. The plasma hydrodynamic response to transient heating is modeled in the assumption of a fluid confined in a semicircular loop of uniform cross-section. Two different forms of heating are considered: direct thermal heating, and heating due to injection in the loop of non-thermal electrons. Title: Flare energetics. Authors: Wu, S. T.; de Jager, C.; Dennis, B. R.; Hudson, H. S.; Simnett, G. M.; Strong, K. T.; Bentley, R. D.; Bornmann, P. L.; Bruner, M. E.; Cargill, P. J.; Crannell, C. J.; Doyle, J. G.; Hyder, C. L.; Kopp, R. A.; Lemen, J. R.; Martin, S. F.; Pallavicini, R.; Peres, G.; Serio, S.; Sylwester, J.; Veck, N. J. Bibcode: 1986NASCP2439....5W Altcode: In this investigation of flare energetics, the authors establish a comprehensive and self-consistent picture of the sources and transport of energy within a flare. They chose five flares in 1980 that were well observed with instruments on the SMM, and with other space-borne and ground-based instruments. Details of the observations available for them and the corresponding physical parameters derived from these data are presented. The flares were studied from two perspectives, the impulsive and gradual phases, and then the results were compared to obtain the overall picture of the energetics of these flares. The authors also discuss the role that modeling can play in estimating the total energy of a flare when the observationally determined parameters are used as the input to a numerical model. Title: Intercomparison of numerical models of flaring coronal loops. Authors: Kopp, R. A.; Fisher, G. H.; MacNiece, P.; McWhirter, R. W. P.; Peres, G. Bibcode: 1986NASCP2439....7K Altcode: The authors were concerned with computations relevant to the problem of the hydrodynamic and radiative response of a single magnetic flux tube to a sudden release of energy in it. They considered a simple "Benchmark Model" in which the physics of real loops - radiation, thermal conduction, compressible hydrodynamics, gravity and nonthermal heating could be incorporated with some degree of realism. The primary goal of the Benchmark Model was to intercompare code calculations on a standardized, although hypothetical, problem, rather than to establish the best possible physical model. Title: Solar X-ray spectrum simulations for flaring loop models with emphasis on transient ionization effects during the impulsive phase Authors: Mewe, R.; Lemen, J. R.; Peres, G.; Schrijver, J.; Serio, S. Bibcode: 1985A&A...152..229M Altcode: X-ray spectra are simulated for observations with the Bent Crystal Spectrometer (BCS) on the Solar Maximum Mission (SMM) spacecraft (around 1.9 A and 3 A) using results of a time-dependent numerical code for a dynamic flaring loop model. The spectra are integrated over the whole loop and fully take into account the effects of deviations from ionization equilibrium and Doppler shifts resulting from plasma movements. The effect of transient ionization on the spectra emitted during the initial impulsive phase is emphasized. In all the cases considered a strong depletion (relative to the continuum) of high-ionization spectral lines takes place during about the first minute of the flare for electron densities below about 10 to the 11th/cu cm. A preliminary comparison to observations made with the BCS show some indications for such effects in a few strong flares, but more sensitive instruments will be needed in the future to exploit such transient ionization effects as a possible valid density diagnostic for hot solar flares. Title: More on momentum deposition by electron beams in solar flares Authors: Reale, F.; Peres, G.; Serio, S. Bibcode: 1985A&A...152L...5R Altcode: Numerical experiments were carried out to determine if the momentum deposited by electron beams is significant in the evolution of a solar flare. The Palermo-Harvard numerical model, with its hydrodynamic differential equations of conservation of mass, momentum and energy for a solar magnetic loop, was modified to include thick target heating by nonthermal electrons. The model was used to simulate flares forming 20,000 km loops, both with and without inclusion of beam momentum transfer. The results of the simulation were compared to Ca XIX spectral data for a solar flare event. Inclusion of the momemtum transfer factor caused significant changes in the simulated impulsive phase and in the associated spectral signature. Downward pressure from the electron beam reduced the upward moving velocity by as much as 50 km/sec. The results underscore the necessity of considering electron beam momentum transfer effects in modeling of solar flare events. Title: Closed coronal structures. VI. Far-ultraviolet and X-ray emission from active late-type stars and the applicability of coronal loop models. Authors: Giampapa, M. S.; Golub, L.; Peres, G.; Serio, S.; Vaiana, G. S. Bibcode: 1985ApJ...289..203G Altcode: We present far-ultraviolet line fluxes of prominent transition region emission lines, as obtained with the International Ultraviolet Explorer satellite, for a sample of solar-type stars. We combine the ultraviolet observations with existing soft X-ray measurements obtained by the Einstein Observatory (HEAO 2). We utilize the resulting data set and a new coronal loop model numerical code developed at the Harvard-Smithsonian Center for Astrophysics to perform a preliminary investigation of the applicability of coronal loop models to solar-type stars. In a few cases, reasonable agreement between the predictions of single-component, coronal loop model atmospheres and the observational data is achieved for a relatively well-defined, plausible range of values in the pressure-filling factor (p, f) plane. In general, however, we find that the addition of non- simultaneous ultraviolet observations to a previously acquired soft X-ray data set does not provide a sufficient constraint on the range of possible loop filling factors and pressures for loop model atmospheres that may be producing the observed X-ray and transition region emissions. We discuss the origins of the discrepancies between the model results and the observations within the context of (1) stellar variability, (2) multiple coronal components, and (3) the presence of relatively low temperature loops that give rise to far-ultraviolet emission but not to coronal X-ray emission. We suggest on the basis of the results presented in this investigation that in order to verify the applicability of coronal loop models to solar-type stars, simultaneous far-ultraviolet and moderate spectral resolution X-ray observations will eventually have to be obtained. Title: The X-ray corona of Procyon. Authors: Schmitt, J. H. M. M.; Harnden, F. R., Jr.; Rosner, R.; Peres, G.; Serio, S. Bibcode: 1985ApJ...288..751S Altcode: X-ray emission from the nearby system Procyon A/B (F5 IV + DF) was detected, using the IPC (Imaging Proportional Counter) on board the Einstein Observatory. Analysis of the X-ray pulse height spectrum suggests that the observed X-ray emission originates in Procyon A rather than in the white dwarf companion Procyon B, since the derived X-ray temperature, log T = 6.2, agrees well with temperatures found for quiescent solar X-ray emission. Modeling Procyon's corona with loops characterized by some apex temperature Tmax and emission length scale L, it is found that Tmax is well constrained, but L, and consequently the filling factor of the X-ray emitting gas, are essentially unconstrained even when EUV emission from the transition region is included in the analysis. Title: Momentum deposition by electron beams in solar flares Authors: Reale, F.; Peres, G.; Serio, S. Bibcode: 1985MmSAI..56..801R Altcode: In a study of coronal plasma dynamics, to include electron beam interactions with the ambient plasma, the effects of electron beam momentum deposition have been computed in detail. The differences in the evolution of a model flare when this effect is not taken into account is studied. Important differences are detectable from velocity profiles at advanced times (greater than 100 sec). Large differences in bulk plasma velocity, which manifest over extended portions of the loop and for a long time, suggest a corresponding discrepancy between the predicted line shapes. Ongoing work aims to delineate the observable differences between the spectra predicted in the two cases. Title: Dynamic heating of stationary coronal loops Authors: Reale, F.; Peres, G.; Serio, S. Bibcode: 1985MmSAI..56..805R Altcode: The influence of dynamic heating of constant average intensity on apparently stationary loop features is studied. The intent is to investigate the possibility that the 'steady state' coronal heating might be due, at least partially, to the superposition of frequent coronal microflares of hard X-rays (events such as were reported by Lin et al., 1984). A few cases of coronal loops heated by periodic pulses of electron beams were simulated by means of the Palermo-Harvard hydrodynamic code. It is shown that if the solar corona is entirely heated by microflares, the X-ray emission would fluctuate, as expected. Detecting such fluctuations is not straightforward, but chances of detecting them if they exist would be higher if observations are made with high angular resolution instruments such as the recently proposed normal incidence X-ray telescope. Title: Active late-type stars and the applicability of coronal loop models. Authors: Giampapa, M. S.; Golub, L.; Peres, G.; Serio, S.; Vaiana, G. S. Bibcode: 1984NASCP2349..454G Altcode: 1984fiue.rept..454G; 1984IUE84......454G Far ultraviolet IUE observations of a sample of solar type stars were combined with existing soft X-ray measurements obtained by HEAO B. The resulting data set was utilized and a new coronal loop model numerical code was developed to perform a preliminary investigation of the applicability of coronal loop models to solar-type stars. Reasonable agreement was found to exist between the predictions of single-component coronal loop model atmospheres. It was demonstrated that semi-empirical, coronal loop models can be applied to account for observed stellar transition region and coronal emission. This result is corroborative evidence for the presence of magnetic field structures analogous to solar coronal loops on the surfaces of solar-type stars. It is suggested that stellar transition region emission arises predominantly from the base of quiescent coronal loop configurations. Title: Solar flare hydrodynamics Authors: Peres, G.; Serio, S. Bibcode: 1984MmSAI..55..749P Altcode: The Palermo-Harvard numerical code for confined solar loop atmospheres is presented, evidencing general problems and advantages of the numerical modelling of flares, and discussing recent results of this code and their comparison with SMM observations. Future improvements and probable evolutions of this kind of models are discussed. Title: Closed coronal structures. V - Gasdynamic models of flaring loops and comparison with SMM observations Authors: Pallavicini, R.; Peres, G.; Serio, S.; Vaiana, G.; Acton, L.; Leibacher, J.; Rosner, R. Bibcode: 1983ApJ...270..270P Altcode: The hydrodynamic response of confined magnetic structures to strong heating perturbations is investigated by means of a time-dependent one-dimensional colde which incorporates the energy, momentum, and mass conservation equations. The entire atmospheric structure from the chromosphere to the corona is taken into account. Models with different energy input, heating time dependence, preflare conditions and heating location have been examined in the numerical simulations.

The result of model calculations are compared with observations of flares obtained with the X-ray Polychromator experiment on the Solar Maximum Mission. These include light curves of spectral lines formed over a wide range of coronal flare temperatures, as well as determinations of Doppler shifts for the high temperature plasma. Several examples are used to illustrate the range of the observational variation.

It is shown that the predictions of the numerical simulations are in good overall agreement with the observed evolution of the flare coronal plasma. The model reproduces correctly the temporal profile of X-ray spectral lines and -- to first order at least -- their relative intensities. The upflow velocities predicted by model calculations are in agreement with the observed blueshifts, supporting the interpretation of the blueshifts as due to evaporation of chromospheric material. The possibility of using the comparison of model predictions with observations to derive information on the processes of energy release and transfer in solar flares is discussed. Title: Hydrodynamics of Flaring Loops - SMM Observations and Numerical Simulations Authors: Pallavicini, R.; Peres, G. Bibcode: 1983SoPh...86..147P Altcode: The hydrodynamic response of confined magnetic structures to strong heating perturbations is investigated by means of a timedependent one-dimensional code which incorporates the energy, momentum and mass conservation equations. The entire atmospheric structure from the chromosphere to the corona is taken into account. The results of model calculations are compared with observations of flares obtained with the X-Ray Polychromator experiment on the Solar Maximum Mission. Title: The Palermo-Harvard numerical code for the dynamics of confined coronal plasma. Authors: Peres, G.; Serio, S.; Vaiana, G. S.; Rosner, R. Bibcode: 1983pig..conf...96P Altcode: No abstract at ADS Title: Temporal evolution of soft X-ray emitting plasma in solar flares. Authors: Pallavicini, R.; Peres, G. Bibcode: 1982BAAS...14..776P Altcode: No abstract at ADS Title: Coronal closed structures. IV - Hydrodynamical stability and response to heating perturbations Authors: Peres, G.; Serio, S.; Vaiana, G. S.; Rosner, R. Bibcode: 1982ApJ...252..791P Altcode: The response of magnetically confined atmospheres to perturbations in the temperature and density distribution, and the local heating rate by means of a one-dimensional time-dependent hydrodynamical code, which incorporates the full energy, momentum and mass conservation equations is studied. These studies extend the linear instability analysis of Habbal and Rosner (1979) into the finite-amplitude regime, and generalize the confined atmosphere models of Serio et al., to the time-dependent domain. The results show that closed coronal atmospheres are stable against finite-amplitude perturbations if the chromospheric response is taken into account; and observed correlated increases in coronal density and temperature can only be achieved under quiescent conditions by increasing the heat deposition rate relatively more in the chromosphere than in the corona. Title: Il sistema di trattamento di immagini presso l'Osservatorio Astronomico di Palermo. Authors: Sciortino, S.; Peres, G.; Serio, S.; Vaiana, G. S. Bibcode: 1982MmSAI..53..115S Altcode: No abstract at ADS Title: X-Ray Flare Spectroscopy: SMM Observations and Loop Modeling. Authors: Acton, L.; Pallavicini, R.; Peres, G.; Vaiana, G. Bibcode: 1982uxsa.coll....1A Altcode: 1982IAUCo..73....1A No abstract at ADS Title: Closed coronal structures. III - Comparison of static models with X-ray, EUV, and radio observations Authors: Pallavicini, R.; Peres, G.; Serio, S.; Vaiana, G. S.; Golub, L.; Rosner, R. Bibcode: 1981ApJ...247..692P Altcode: Numerical models of static coronal loops in energy balance are compared with high spatial resolution observations of extreme ultraviolet lines, broad-band X-ray emission, and interferometric observations at 2.8 cm of a solar active region. Difficulties of using scaling laws to test static models of coronal loops are reviewed. The theoretical model used for the comparison is summarized; the detailed X-ray, EUV, and microwave observations of the selected active region are presented; and the comparison of the model with the observations is performed. It is shown that simple static models with conductive flux vanishing at the loop base reproduce satisfactorily the observed properties in the upper portion of loop structures from compact, high-pressure loops in the core of the region to more extended, fainter loops and to large-scale loops interconnecting different active regions. Effects of changing loop parameters are investigated, and it is argued, that in contrast to the present approach, scaling laws cannot be used to discriminate between different static energy balance models. Some discrepancy is found between model predictions and observations for the lower sections of loop structures. Possible causes of the discrepancy are discussed. Title: Closed coronal structures. II - Generalized hydrostatic model Authors: Serio, S.; Peres, G.; Vaiana, G. S.; Golub, L.; Rosner, R. Bibcode: 1981ApJ...243..288S Altcode: Numerical computations of stationary solar coronal loop atmospheres are used to extend earlier analytical work. Two classes of loops are examined, namely symmetric loops with a temperature maximum at the top but now having a length greater than the pressure scale height and loops which have a local temperature minimum at the top. For the first class, new scaling laws are found which relate the base pressure and loop length to the base heating, the heating deposition scale height, and the pressure scale height. It is found that loops for which the length is greater than about two to three times the pressure scale height do not have stable solutions unless they have a temperature minimum at the top. Computed models with a temperature inversion at the top are permitted in a wider range of heating deposition scale height values than are loops with a temperature maximum at the top. These results are discussed in relation to observations showing a dependence of prominence formation and stability on the state of evolution of magnetic structures, and a general scenario is suggested for the understanding of loop evolution from emergence in active regions through the large-scale structure phase to opening in coronal holes. Title: Transition Region and Corona in Solar Active Regions: Observations and Numerical Modeling Authors: Golub, L.; Pallavicini, R.; Peres, G.; Rosner, R.; Serio, S.; Vaiana, G. S. Bibcode: 1980BAAS...12..908G Altcode: No abstract at ADS Title: Transienti nella corona solare: determinazione dei parametri fisici del plasma e leggi di scala. Authors: Peres, G.; Sciortino, S.; Serio, S.; Vaiana, G. S. Bibcode: 1979RSAI...22..165P Altcode: No abstract at ADS Title: Rivelazione quantica di immagini ad alta risoluzione spazio-temporale: il progetto Photicon. Authors: Peres, G.; Sciortino, S.; Serio, S.; Vaiana, G. S. Bibcode: 1979RSAI...22..237P Altcode: No abstract at ADS Title: Sistema analisi dati per fotometria nel lontano infrarosso della collaborazione Palermo-Harvard. Risultati preliminari per la regione intorno a W42. Authors: Peres, G.; Sciortino, S.; Serio, S.; Vaiana, G. S. Bibcode: 1979RSAI...22..190P Altcode: No abstract at ADS Title: The Palermo four-color far infrared program Authors: Peres, G.; Serio, S.; Vaiana, G. S.; Cali, C.; Daneu, V.; Maxson, C.; Steier, M. Bibcode: 1978MmSAI..49...95P Altcode: A four-channel far-IR photometer designed for use with a balloon-borne 102-cm telescope is described. The data analysis system for this photometer is discussed, and a typical observational program is outlined. Possible objects that may be observed with the four-color far-IR photometer include molecular clouds, the galactic-center region, H II regions, quasars, Seyfert galaxies, and BL Lacertae objects. Title: Four Color Infrared Bolometer System for One-Meter Telescope Authors: Daneu, V.; Maxson, C.; Peres, G.; Serio, S.; Vaiana, G. Bibcode: 1978ASIC...38..335D Altcode: 1978infa.proc..335D A far-infrared photometer has been developed as a focal plane instrument for a balloon-borne 102-cm telescope. The four gallium-doped Ge bolometers adopted for the system operate at 1.8 K in a liquid-helium dewar. Wavelength bands of the four-bolometer system are a narrow spectrum between 18 and 22 microns, and bands centered at 42, 70, and 140 microns. The responsivity and Johnson noise of the detectors limit the performance of the instrument. Title: Solar X-ray transients in magnetically confined plasma: observational data and hydrodynamic model. Authors: Peres, G.; Serio, S.; Vaiana, G. S.; Rosner, R. Bibcode: 1978spre.conf..341P Altcode: 1978spre.proc..341P No abstract at ADS Title: The Palermo four-color far infrared program. Authors: Peres, G.; Serio, S.; Vaiana, G. S.; Cali', C.; Daneu, V.; Maxson, C.; Steier, M. Bibcode: 1978ASIC...38...95P Altcode: No abstract at ADS