Author name code: lanz ADS astronomy entries on 2022-09-14 author:"Lanz, Thierry M." ------------------------------------------------------------------------ Title: Properties of Primitive Galaxies Authors: Heap, Sara R.; Hubeny, I.; Bouret, J. -C.; Lanz, T.; Brinchmann, J. Bibcode: 2022arXiv220704553H Altcode: We report on a study of 9 nearby star-forming, very low-metallicity galaxies observed by Hubble's COS far-UV spectrograph that can serve as templates of high-z galaxies to be observed by JWST. We find that the nebular spectra of these primitive galaxies show evidence of irradiation by X-ray emitters. Following Thuan et al. (2004), we identify the sources of X-ray emission as massive X-ray binaries containing a massive accreting stellar black hole. We further find that the lower the metallicity, the higher the probability of strong X-irradiation. Following Heger et al. (2003), we suggest that these accreting black holes are produced by direct collapse of stars having initial masses greater than $\sim50\, M_\odot$. Our models of young star clusters with an embedded stellar black hole produce effects on the surrounding gaseous medium that are consistent with the observed spectra. We conclude that primitive galaxies are qualitatively different from more metal-rich galaxies in showing evidence of hard radiation that can best be explained by the presence of one or more embedded stellar black holes. Title: High Resolution Spectroscopy of the Unusual Spectral Feature in the Central Compact Object PSR J0821-4300 Authors: Gotthelf, Eric; Bogdanov, Slavko; Paerels, Frits; Groger, John; Helfand, David; Halpern, Jules; Hubeny, Ivan; Lanz, Thierry Bibcode: 2022HEAD...1911006G Altcode: We present Chandra LETG high-resolution X-ray spectroscopy of the unusual line feature seen in the blackbody emission from the central compact object (CCO) in supernova remnant (SNR) Puppis~A. This feature is likely associated with one of the antipodal hot-spots on the NS surface and may provide the first detection of atomic photospheric absorption lines, allowing a model independent, direct measurement of the mass-radius relation, a strong constrain on the equation of state of nuclear matter. If generated by cyclotron resonance scattering or possibly from accretion of supernova debris, the line emission may provide a localized measurement of the magnetic field, important for reconciling the observed bright surface hot-spots unprecedented for a neutron star with such a weak (2.8E-10 G) dipole magnetic field. Title: The Photospheric X-ray Spectrum of the Neutron Star in Puppis A Authors: Groger, John; Paerels, Frits; Gotthelf, Eric; Halpern, Jules; Helfand, David; Bogdanov, Slavko; Hubeny, Ivan; Lanz, Thierry Bibcode: 2022HEAD...1910107G Altcode: We have collected 352.6 ksec (as of September 7, 2021) of a 400 ksec Chandra observation of the central compact object PSR J0821-4300 in the supernova remnant Puppis A using the Low Energy Transmission Grating Spectrometer. We have clearly detected photospheric radiation from the neutron star with a temperature of approximately 2 million degrees. We anticipate that we will have the sensitivity to detect absorption features from highly-ionized metals that may be present at the surface of the star. In this poster, we discuss our preliminary analysis of the spectral data. Title: The Neutron Star in Puppis A: Atomic Photospheric Spectroscopy at Last? Authors: Paerels, Frits; Groger, John John; Helfand, David; Bogdanov, Slavko; Gotthelf, Eric; Halpern, Jules; Hubeny, Ivan; Lanz, Thierry Bibcode: 2022HEAD...1910106P Altcode: We have accumulated 400 ksec of exposure time with the Chandra Low Energy Transmission Grating Spectrometer on the neutron star in the supernova remnant Puppis A. We clearly detect photospheric radiation from a ~ 2 million degree star. We anticipate that the spectrum will have sufficient sensitivity for us to detect atomic absorption features from highly ionized mid-Z atoms, which may be present at the surface of the star. We discuss the data and our preliminary spectroscopic analysis. Title: TLUSTY and SYNSPEC Users's Guide IV: Upgraded Versions 208 and 54 Authors: Hubeny, Ivan; Allende Prieto, Carlos; Osorio, Yeisson; Lanz, Thierry Bibcode: 2021arXiv210402829H Altcode: We present a brief description of the newly upgraded versions of the programs TLUSTY, version 208, and SYNSPEC, version 54. TLUSTY is used to generate model stellar atmospheres or accretion disks, and SYNSPEC produces detailed synthetic spectra and/or opacity tables. This paper complements published guides that provide a detailed description of earlier versions of the codes, TLUSTY205, and SYNSPEC51. The main upgrades include the flexible construction of opacity tables in SYNSPEC, and their use in producing hybrid models with TLUSTY}, with important species treated in NLTE, while the bulk of opacity of atomic and molecular lines and continua are considered in LTE using a pre-calculated opacity table. There is also a number of additional changes and upgrades that increase the versatility and flexibility of these codes. Title: Massive stars in the Small Magellanic Cloud. Evolution, rotation, and surface abundances Authors: Bouret, J. -C.; Martins, F.; Hillier, D. J.; Marcolino, W. L. F.; Rocha-Pinto, H. J.; Georgy, C.; Lanz, T.; Hubeny, I. Bibcode: 2021A&A...647A.134B Altcode: 2021arXiv210109269B Context. The evolution of massive stars depends on several physical processes and parameters. Metallicity and rotation are among the most important, but their quantitative effects are not well understood.
Aims: To complement our earlier study on main-sequence stars, we study the evolutionary and physical properties of evolved O stars in the Small Magellanic Cloud (SMC). We focus in particular on their surface abundances to further investigate the efficiency of rotational mixing as a function of age, rotation, and global metallicity.
Methods: We analysed the UV and optical spectra of 13 SMC O-type giants and supergiants using the stellar atmosphere code CMFGEN to derive photospheric and wind properties. We compared the inferred properties to theoretical predictions from evolution models. For a more comprehensive analysis, we interpret the results together with those we previously obtained for O-type dwarfs.
Results: Most dwarfs of our sample lie in the early phases of the main sequence. For a given initial mass, giants are farther along the evolutionary tracks, which confirms that they are indeed more evolved than dwarfs. Supergiants have higher initial masses and are located past the terminal-age main-sequence in each diagram. We find no clear trend of a mass discrepancy, regardless of the diagram that was used to estimate the evolutionary mass. Surface CNO abundances are consistent with nucleosynthesis from the CNO cycle. Comparisons to theoretical predictions reveal that the initial mixture is important when the observed trends in the N/C versus N/O diagram are to be reproduced. A trend for stronger chemical evolution for more evolved objects is observed. Above about 30 M⊙, more massive stars are on average more chemically enriched at a given evolutionary phase. Below 30 M⊙, the trend vanishes. This is qualitatively consistent with evolutionary models. A principal component analysis of the abundance ratios for the whole (dwarfs and evolved stars) sample supports the theoretical prediction that massive stars at low metallicity are more chemically processed than their Galactic counterparts. Finally, models including rotation generally reproduce the surface abundances and rotation rates when different initial rotational velocities are considered. Nevertheless, for some objects, a stronger braking and/or more efficient mixing is required.

This research is based on observations made with the NASA/ESA Hubble Space Telescope obtained from 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 programmes GO 7437, GO 9434, and GO 11625.

Based on data products from observations made with ESO Telescopes at the La Silla Paranal Observatory under programmes ID 67.D-0238, 70.D-0164, 074.D-0109, 079.D-0073, and 079.D-0562. Title: Supernovae Chemical Yields in Magellanic Cloud Environments Authors: Peters, G. J.; Lanz, T.; Bouret, J.; Proffitt, C. R.; Adelman, S. J.; Hubeny, I. Bibcode: 2020AAS...23511025P Altcode: Recently there has been interest in the abundance of Mn and other Fe group elements as diagnostics for determining the progenitors of SNe Ia and their role in the chemical evolution of a galaxy. We have combined recent spectroscopic observations from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope with archival data from the Far Ultraviolet Spectroscopic Explorer (FUSE) and ESO's VLT/UVES to determine the abundances of the Fe group elements (Ti, V, Cr, Mn, Fe, Co, & Ni) in main sequence B stars in the Magellanic Clouds. Here we report results for NGC 1818-D1 (LMC) and AV 304 (SMC). The analysis was carried through using the Hubeny/Lanz NLTE programs TLUSTY/SYNSPEC. The COS observations were secured with the G130M, G160M, G185M, and G225M gratings. Combined with the FUSE data, we have achieved spectral coverage in the UV from 950 to 2400 A. Measurable lines from the Fe group, except for a very few multiplets of Fe II, III are not found in optical spectra. The following stellar parameters were adopted: Teff = 24700 K, log g = 4.0 cm/s2, Vturb = 0 km/s, and v sin i = 30 km/s for NGC 1818-D1, and Teff = 27500 K, log g = 3.7 cm/s2, Vturb = 1 km/s, and v sin i = 8 km/s for AV 304. Both stars show Fe group abundances about 0.2-0.3 dex smaller than their lighter elements when compared to solar values and the LMC/SMC averages. However Ti, an alpha-process element, appears to have an abundance more in line with LMC/SMC values. [Mn/Fe] ranges from 0.2 dex (NGC 1818-D1) to 0.1 dex (AV 304) with an uncertainty of 0.2 dex, and implies that the progenitors that produced their Fe group material probably had Chandrasekhar masses (cf. Seitenzahl et al. 2013, A&A, 559, L5). Support from STScI grants HST-GO-14081.002 and HST-GO-13346.022, and USC's Women in Science and Engineering (WiSE) program are greatly appreciated. Title: New insight on accretion shocks onto young stellar objects. Chromospheric feedback and radiation transfer Authors: de Sá, L.; Chièze, J. -P.; Stehlé, C.; Hubeny, I.; Lanz, T.; Cayatte, V. Bibcode: 2019A&A...630A..84D Altcode: Context. Material accreted onto classical T Tauri stars is expected to form a hot quasi-periodic plasma structure that radiates in X-rays. Simulations of this phenomenon only partly match observations. They all rely on a static model for the chromosphere and on the assumption that radiation and matter are decoupled.
Aims: We explore the effects of a shock-heated chromosphere and of the coupling between radiation and hydrodynamics on the structure and dynamics of the accretion flow.
Methods: We simulated accretion columns that fall onto a stellar chromosphere using the 1D ALE code AstroLabE. This code solves the hydrodynamics equations along with the first two moment equations for radiation transfer, with the help of a dedicated opacity table for the coupling between matter and radiation. We derive the total electron and ion densities from collisional-radiative model.
Results: The chromospheric acoustic heating affects the duration of the cycle and the structure of the heated slab. In addition, the coupling between radiation and hydrodynamics leads to a heating of the accretion flow and of the chromosphere: the whole column is pushed up by the inflating chromosphere over several times the steady chromosphere thickness. These last two conclusions are in agreement with the computed monochromatic intensity. Acoustic heating and radiation coupling affect the amplitude and temporal variations of the net X-ray luminosity, which varies between 30 and 94% of the incoming mechanical energy flux, depending on which model is considered. Title: Cosmic Evolution Through UV Spectroscopy (CETUS) Probe-Class Mission Concept Authors: Danchi, William; Arenberg, J.; Bartoszyk, A.; Bezanson, R.; Bianchi, L.; Bowen, D.; Burge, J.; Cenko, B.; Choi, M.; Danchi, W.; Dodson, K.; Driver, S.; Durbeck, G.; Dwek, E.; Eckles, W.; Fleming, B.; France, K.; Gatkine, P.; Gezari, S.; Gong, Q.; Greene, J.; Heap, S.; Heckman, T.; Hodges-Kluck, E.; Hull, T.; Kendrick, S.; King, R.; Kutyrev, A.; Lanz, T.; MacKenty, J.; McCandliss, S.; Mehle, G.; Mentzell, E.; Moseley, S.; Nikzad, S.; Purves, L.; Rauscher, B.; Rhee, M.; Riall, S.; Rigby, J.; Roederer, I.; Schur, N.; Seipel, S.; Sheikh, D.; Siegmund, O.; Simmons, R.; Smith, W.; Spergel, D.; Stark, D.; Stoneking, E.; Szalay, A.; Tompkins, S.; Trump, J.; Vandewel, A.; Valente, M.; Valerga, J.; Veilleux, S.; Waczynski, A.; Whitaker, K.; Woodruff, R.; Wyse, R. Bibcode: 2019BAAS...51g..84D Altcode: 2019astro2020U..84D CETUS is a concept for an affordable, Probe-Class mission based on a 1.5-m-aperture space telescope with a wide FOV covering UV wavelengths from 100 to 400 nm. The set of science instruments consists of a wide-field camera, a wide-field Multi-Object Spectrograph, and a Point/slit Source echelle Spectrograph with high spectral resolution. Title: The Probe-class mission concept, Cosmic Evolution Through UV Surveys (CETUS) Authors: Heap, Sara; Hull, Tony; Kendrick, Steve; Woodruff, Bob; Arenberg, Jonathan; Baes, Maarten; Bezanson, Rachel; Bianchi, Luciana; Bowen, David; Cenko, Brad; Chiang, Yi-Kuan; Cochrane, Rachel; Corcoran, Mike; Crowther, Paul; Driver, Simon; Danchi, Bill; Dwek, Eli; Fleming, Brian; France, Kevin; Gatkine, Pradip; Gezari, Suvi; Hayward, Chris; Hayes, Matthew; Heckman, Tim; Hodges-Kluck, Edmund; Kutyrev, Alexander; Lanz, Thierry; MacKenty, John; McCandliss, Steve; Moseley, Harvey; Neiner, Coralie; Pacifici, Camilla; Rafelski, Marc; Rauscher, Bernie; Rigby, Jane; Roederer, Ian; Spergel, David; Stark, Dan; Szalay, Alexander; Terrazas, Bryan; Trump, Jonathan; van der Wel, Arjun; Veilleux, Sylvain; Whitaker, Kate; Wold, Isak; Wyse, Rosemary; Burge, Jim; Dodson, Kelly; Eckles, Chip; Fleming, Brian; MacKenty, John; McCandliss, Steve; Mehle, Greg; Nikzad, Shouleh; Purves, Lloyd; Quijada, Manuel; Siegmund, Ossy; Sheik, Dave; Vallerga, John; Valente, Marty Bibcode: 2019BAAS...51g.159H Altcode: 2019astro2020U.159H CETUS is a 1.5-m, wide-field UV observatory that will be a worthy successor to Hubble. Its distinguishing characteristics include multi-object slit spectroscopy, long-slit spectroscopy, spectroscopy in the Lyman-UV, prompt-response observations, and detection of low-surface brightness objects. These new capabilities ensure future discoveries. Title: Stars and Stellar Black Holes in the Low-metallicity Galaxy I Zw 18 Authors: Heap, S. R.; Hubeny, I.; Lanz, T. M. Bibcode: 2019ASPC..519..267H Altcode: I Zw 18 is a star-forming dwarf galaxy having a very low metal content, O/H ∼ 1/50 solar (Skillman & Kennicutt 1993). While galaxies with such low metallicity are rare in the low-redshift universe, they are likely to be common in galaxies at cosmic dawn. Thus, I Zw 18 is a “living” template for z > 6 galaxies. We have obtained HST/COS far-UV spectra of the northwest star cluster in I Zw 18 and have compared them to SYNSPEC model spectra by Lanz & Hubeny (2003, 2007) in order to determine the properties of the stellar population. We have also compared the observed spectra of I Zw 18-NW to the CLOUDSPEC models (Hubeny et al. 2000) of the stellar cluster with an embedded ultra-luminous X-ray source (ULX). This comparison reveals feedback of the stellar black hole in the form of photoionization and heating. Such models can be used as starting points to explore the physical conditions in which stars and black holes form and evolve in an extremely low-metallicity environment at high redshift. Title: The Radial Oxygen Abundance Gradient from OB Stars in the Outer Galactic Disk Authors: Daflon, Simone; Bragança, Gustavo A.; Lanz, Thierry; Cunha, Katia; Hubeny, Ivan; Bensby, Thomas; McMillan, Paul J.; Garmany, Kate; Glaspey, John W.; Borges Fernandes, Marcelo; Oey, Sally Bibcode: 2019ASPC..519..213D Altcode: Metallicity gradients are important constraints for models of chemical evolution of the Galaxy. We present the current radial gradient of oxygen abundances using a sample of main-sequence OB stars located in the outer Galactic Disk, considering non-NLTE abundances, and distances based on GAIA DR2 results. We compare the obtained gradient with results from other young objects of the Galactic Disk. Title: 3D Spectral Radiative Transfer and Perspectives for Spectroscopic Diagnostics Authors: Ibgui, Laurent; Hubeny, Ivan; Lanz, Thierry; González, Matthias; Stehlé, Chantal; Orlando, Salvatore; Colombo, Salvatore Bibcode: 2019ASPC..519...21I Altcode: We present the features of the three-dimensional (3D) radiative transfer code IRIS, which synthesizes spectra emitted from hydrodynamics structures. We discuss our first application of IRIS to the analysis of a laboratory radiative shock. We demonstrate, with the help of the radiation hydrodynamics (RHD) code HERACLES, the existence of a radiative precursor. We validate the gray approximated model M1 used by HERACLES. We show a couple of synthesized X-UV spectra of such a shock. We finally open up our discussion to the future possibilities of spectroscopic diagnostics of accreting streams onto the surface of Classical T Tauri stars (CTTSs). Title: Hot UV-bright stars of galactic globular clusters Authors: Moehler, S.; Landsman, W. B.; Lanz, T.; Miller Bertolami, M. M. Bibcode: 2019A&A...627A..34M Altcode: 2019arXiv190506718M Context. We have performed a census of the UV-bright population in 78 globular clusters using wide-field UV telescopes. This population includes a variety of phases of post-horizontal branch (HB) evolution, including hot post-asymptotic giant branch (AGB) stars, and post-early AGB stars. There are indications that old stellar systems like globular clusters produce fewer post-(early) AGB stars than currently predicted by evolutionary models, but observations are still scarce.
Aims: We wish to derive effective temperatures, surface gravities, and helium abundances of the luminous hot UV-bright stars in these clusters to determine their evolutionary status and compare the observed numbers to predictions from evolutionary theory.
Methods: We obtained FORS2 spectroscopy of eleven of these UV-selected objects (covering a range of -2.3 < [Fe/H] < -1.0), which we (re-)analysed together with previously observed data. We used model atmospheres of different metallicities, including super-solar ones. Where possible, we verified our atmospheric parameters using UV spectrophotometry and searched for metal lines in the optical spectra. We calculated evolutionary sequences for four metallicity regimes and used them together with information about the HB morphology of the globular clusters to estimate the expected numbers of post-AGB stars.
Results: We find that metal-rich model spectra are required to analyse stars hotter than 40 000 K. Seven of the eleven new luminous UV-bright stars are post-AGB or post-early AGB stars, while two are evolving away from the HB, one is a foreground white dwarf, and another is a white dwarf merger. Taking into account published information on other hot UV-bright stars in globular clusters, we find that the number of observed hot post-AGB stars generally agrees with the predicted values, although the numbers are still low.
Conclusions: Spectroscopy is clearly required to identify the evolutionary status of hot UV-bright stars. For hotter stars, metal-rich model spectra are required to reproduce their optical and UV spectra, which may affect the flux contribution of hot post-AGB stars to the UV spectra of evolved populations. While the observed numbers of post-AGB and post-early AGB stars roughly agree with the predictions, our current comparison is affected by low number statistics.

The extracted spectra, their best-fitting model spectra, and the evolutionary tracks used in this paper are available at 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/627/A34Based on observations with the ESO Very Large Telescope at Paranal Observatory, Chile (proposal ID 089.C-0210). Title: Radiation Feedback in Accretion Shocks on Young Stars Authors: de Sá, Lionel; Stehlé, Chantal; Chièze, Jean-Pierre; Hubeny, Ivan; Lanz, Thierry; Colombo, Salvatore; Cayatte, Véronique; Ibgui, Laurent; Orlando, Salvatore Bibcode: 2019ASPC..519..281D Altcode: The first stages of pre-main sequence stars evolution are governed by exchanges of mass and momentum between the proto-star and its accretion disk. These quantities remain uncertain due to numerous unanswered questions concerning the topology of the accretion flow, its temperature, and its dynamics. In this work, we first present 1D simulations of accretion columns falling onto a a stellar chromosphere; the goal is to inspect the feedback of the dynamics of the chromosphere on the accretion flow. Then, we analyze the effect of radiation absorption and emission on the flow. The simulations take benefit of a newly generated base of opacities computed by SYNSPEC code for the conditions of the present astrophysical process. We finally present our perspectives on this topic. Title: Radiative Signatures from the Cosmos Authors: Werner, K.; Stehle, C.; Rauch, T.; Lanz, T. Bibcode: 2019ASPC..519.....W Altcode: No abstract at ADS Title: The Galactic Abundance Gradient for the Fe Group Elements in Early B Stars Authors: Peters, Geraldine J.; Bouret, Jean-Claude; Lanz, Thierry; Proffitt, Charles R. Bibcode: 2019hst..prop15869P Altcode: We propose HST spectroscopic observations of twelve sharp-lined early B main-sequence stars with galactocentric distances ranging from 3-14 kpc in order to determine the metallicity gradient for the Fe group elements (Ti, V, Cr, Mn, Fe, Co, Ni) in our galaxy. This will be the first attempt to establish abundance gradients for all of the Fe-peak elements found in these young stars, that were formed <100 Myr ago. The result will be an assessment of the chemical evolution of our galaxy. Whereas the light elements are mostly delivered to the ISM by core-collapse supernovae (CCSNe), the Fe group elements are believed to primarily come from low/intermediate mass binaries containing white dwarfs that undergo SNe Ia explosions. According to some references, a single SNe Ia can deliver as much as 0.5 solar masses of pure Fe and Mn to the ISM compared with about 0.07 solar masses from a CCSNe. The older stellar populations were formed from an ISM that was enriched primarily by CCSNe, but the Fe group elements in B stars probably owe their origin mostly to SN Ia. Recently there has been a great deal of interest in using the observed Mn/Fe ratio to determine the progenitors of SNe Ia. Since the SNe models predict that Mn/Fe varies with metallicity, a good determination of the Mn abundance across our galaxy would also be useful for SN science. For each Fe group element we will determine its abundance relative to H and Fe. The unique spectra will have legacy value, as strong lines from the Fe group elements (except for a very few features of Fe itself) are found only in the FUV/NUV. Both COS and STIS will be used. The analysis will be carried through with TLUSTY/SYNSPEC. Title: VizieR Online Data Catalog: GC hot UV-bright stars model spectra (Moehler+, 2019) Authors: Moehler, S.; Landsman, W. B.; Lanz, T.; Miller Bertolami, M. M. Bibcode: 2019yCat..36270034M Altcode: Optical medium- and low-resolution spectra (flux-calibrated and normalized) of 13 UV bright stars in globular clusters are presented together with the atmospheric parameters derived from them. The following globular clusters have been observed: NGC5139, NGC6121, NGC6656, NGC6712, NGC6779, NGC7099. The flux-calibrated spectra were corrected to laboratory wavelengths and their continua were normalized. Effective temperatures, surface gravities, and helium abundances were derived from line profile fits and the corresponding best-fitting model spectra are also presented here. We also present post-helium core burning evolutionary sequences calculated low mass stars for [M/H] = -2.3, -2.0, -1.5, -1.0.

Grid of post-horizontal branch evolutionary sequences for four different metallicities. Models at the Zero Age Horizontal Branch (ZAHB) were constructing by removing mass at the tip of the Red Giant Branch (RGB). Details about the initial model on the main sequence are provided in table B.1 of the article. Each file contains all the sequences with a given initial metallicity and are named as CDSFeH-1.0.dat, CDSFeH-1.5.dat, CDSFeH-2.0.dat, and CDSFeH-2.3.dat (corresponding to metallicities [Fe/H]=-1, -1.5, -2, -2.3, see Table 8 of the Article)

All sequences are presented at similar timesteps

Sequences are presented until their luminosity drops to 10 solar luminosities.

(11 data files). Title: Radial abundance gradients in the outer Galactic disk as traced by main-sequence OB stars Authors: Bragança, G. A.; Daflon, S.; Lanz, T.; Cunha, K.; Bensby, T.; McMillan, P. J.; Garmany, C. D.; Glaspey, J. W.; Borges Fernandes, M.; Oey, M. S.; Hubeny, I. Bibcode: 2019A&A...625A.120B Altcode: 2019arXiv190404340B Context. Elemental abundance gradients in galactic disks are important constraints for models of how spiral galaxies form and evolve. However, the abundance structure of the outer disk region of the Milky Way is poorly known, which hampers our understanding of the spiral galaxy that is closest to us and that can be studied in greatest detail. Young OB stars are good tracers of the present-day chemical abundance distribution of a stellar population and because of their high luminosities they can easily be observed at large distances, making them suitable to explore and map the abundance structure and gradients in the outer regions of the Galactic disk.
Aims: Using a sample of 31 main-sequence OB stars located between galactocentric distances 8.4-15.6 kpc, we aim to probe the present-day radial abundance gradients of the Galactic disk.
Methods: The analysis is based on high-resolution spectra obtained with the MIKE spectrograph on the Magellan Clay 6.5-m telescope on Las Campanas. We used a non-NLTE analysis in a self-consistent semi-automatic routine based on TLUSTY and SYNSPEC to determine atmospheric parameters and chemical abundances.
Results: Stellar parameters (effective temperature, surface gravity, projected rotational velocity, microturbulence, and macroturbulence) and silicon and oxygen abundances are presented for 28 stars located beyond 9 kpc from the Galactic center plus three stars in the solar neighborhood. The stars of our sample are mostly on the main-sequence, with effective temperatures between 20 800-31 300 K, and surface gravities between 3.23-4.45 dex. The radial oxygen and silicon abundance gradients are negative and have slopes of -0.07 dex kpc-1 and -0.09 dex kpc-1, respectively, in the region 8.4 ≤ RG ≤ 15.6 kpc.
Conclusions: The obtained gradients are compatible with the present-day oxygen and silicon abundances measured in the solar neighborhood and are consistent with radial metallicity gradients predicted by chemodynamical models of Galaxy Evolution for a subsample of young stars located close to the Galactic plane.

Based on data obtained with the Magellan Clay telescope at the Las Campanas observatory and the ESO/MPI telescope at La Silla under the ESO-ON agreement. Title: New insight on Young Stellar Objects accretion shocks -- a claim for NLTE opacities Authors: de Sá, Lionel; Chièze, Jean-Pierre; Stehlé, Chantal; Hubeny, Ivan; Lanz, Thierry; Cayatte, Véronique Bibcode: 2019arXiv190409156D Altcode: Context. Accreted material onto CTTSs is expected to form a hot quasi-periodic plasma structure that radiates in X-rays. Simulations of this phenomenon only partly match with observations. They all rely on a static model for the chromosphere model and on the assumption that radiation and matter are decoupled. Aims. We explore the effects on the structure and on the dynamics of the accretion flow of both a shock-heated chromosphere and of the coupling between radiation and hydrodynamics. Methods. We simulate accretion columns falling onto a stellar chromosphere using the 1D ALE code AstroLabE. This code solves the hydrodynamics equations along with the two first momenta equations for radiation transfer, with the help of a dedicated opacity table for the coupling between matter and radiation. We derive the total electron and ions densities from collisional-radiative NLTE ionisation equilibrium. Results. The chromospheric acoustic heating has an impact on the duration of the cycle and on the structure of the heated slab. In addition, the coupling between radiation and hydrodynamics leads to a heating of the accretion flow and the chromosphere, inducing a possible unburial of the whole column. These two last conclusions are in agreement with the computed monochromatic intensity. Both effects (acoustic heating and radiation coupling) have an influence on the amplitude and temporal variations of the net X-ray luminosity, which varies between 30 and 94% of the incoming mechanical energy flux, depending on the model considered. Title: Modelling the Accretion on Young Stars, Recent Results and Perspectives Authors: de Sá, L.; Stehlé, C.; Chièze, J. P.; Hubeny, I.; Lanz, T.; Colombo, S.; Ibgui, L.; Orlando, S. Bibcode: 2019ASSP...55...29D Altcode: No abstract at ADS Title: The Abundances of the Fe Group Elements in AV 304, an Abundance Standard in the Small Magellanic Cloud Authors: Peters, Geraldine J.; Lanz, Thierry; Bouret, Jean-Claude; Proffitt, Charles R.; Adelman, Saul J.; Hubeny, Ivan Bibcode: 2018AAS...23231704P Altcode: AV 304 is a B0.5 IV field star in the Small Magellanic Cloud with ultra-sharp spectral lines that has emerged as an abundance standard. We have combined recent spectroscopic observations from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope with archival data from the Far Ultraviolet Spectroscopic Explorer (FUSE) and ESO’s VLT/UVES to determine the abundances of the Fe group elements (Ti, V, Cr, Mn, Fe, Co, & Ni). The analysis was carried through using the Hubeny/Lanz NLTE programs TLUSTY/SYNSPEC. The COS observations were secured with the G130M, G160M, G185M, and G225M gratings. Combined with the FUSE data, we have achieved spectral coverage in the UV from 950 to 2400 A. Measurable lines from the Fe group, except for a very few multiplets of Fe II, III are not observed in optical spectra. The following stellar parameters were found: Teff = 27500±500 K, log g = 3.7±0.1 cm/s2, Vturb= 1±1 km/s, and v sin i = 8 ±2 km/s. The Fe abundance appears to be only slightly lower than the mean depletion in the SMC, but the other Fe group elements are underabundant by 0.3 dex or more. This study confirmed the low abundance of nitrogen (-1.25 dex relative to the solar value) that was reported by Peters & Adelman (ASP Conf. Series, 348, p. 136, 2006). Whereas the light elements are delivered to the ISM by core-collapse supernovae (CCSNe), the Fe group elements are believed to come mostly from low/intermediate mass binaries containing white dwarfs that undergo SNe Ia explosions. A single SNe Ia can deliver 0.5 solar masses of pure Fe (and maybe Mn) to the ISM compared with about 0.07 solar masses from a CCSNe. It appears that there is very little processed material from its interior in the atmosphere of AV 304 and that the star did not form from an interstellar cloud that was enriched by material from earlier supernova activity. Support from STScI grants HST-GO-14081.002 and HST-GO-13346.022, and USC’s Women in Science and Engineering (WiSE) program is greatly appreciated. Title: Mid-infrared observations of O-type stars: spectral morphology Authors: Marcolino, W. L. F.; Bouret, J. -C.; Lanz, T.; Maia, D. S.; Audard, M. Bibcode: 2017MNRAS.470.2710M Altcode: 2017arXiv170504244M We present mid-infrared (mid-IR) observations for a sample of 16 O-type stars. The data were acquired with the NASA Spitzer Space Telescope, using the IRS instrument at moderate resolution (R ∼ 600), covering the range of ∼10-37 μm. Our sample includes early, mid and late O supergiants and dwarfs. We explore for the first time their mid-IR spectral morphology in a quantitative way. We use NLTE expanding atmosphere models to help with line identifications, analyse profile contributions and line-formation regions. The O supergiants present a rich emission line spectra. The most intense features are from hydrogen - 6 α, 7 α and 8 α - that have non-negligible contributions of He I or He II lines, depending on the spectral type. The spectrum of early O supergiants is a composite of H I and He II lines, He I lines being absent. On the other hand, late O supergiants present features composed mainly by H I and He I lines. All emission lines are formed throughout the stellar wind. We found that O dwarfs exhibit a featureless mid-IR spectrum. Two stars of our sample exhibit very similar mid-IR features, despite having a very different optical spectral classification. The analysis of O-type stars based on mid-IR spectra alone to infer spectral classes or to estimate physical parameters may thus be prone to substantial errors. Our results may therefore inform spectroscopic observations of massive stars located in heavily obscured regions and help establish an initial framework for observations of massive stars using the Mid-Infrared Instrument on the James Webb Space Telescope. Title: Uncrowding R 136 from VLT/SPHERE extreme adaptive optics Authors: Khorrami, Z.; Vakili, F.; Lanz, T.; Langlois, M.; Lagadec, E.; Meyer, M. R.; Robbe-Dubois, S.; Abe, L.; Avenhaus, H.; Beuzit, J. L.; Gratton, R.; Mouillet, D.; Origné, A.; Petit, C.; Ramos, J. Bibcode: 2017A&A...602A..56K Altcode: 2017arXiv170302876K This paper presents the sharpest near-IR images of the massive cluster R 136 to date, based on the extreme adaptive optics of the SPHERE focal instrument implemented on the ESO Very Large Telescope and operated in its IRDIS imaging mode.The crowded stellar population in the core of the R 136 starburst compact cluster remains still to be characterized in terms of individual luminosities, age, mass and multiplicity. SPHERE/VLT and its high contrast imaging possibilities open new windows to make progress on these questions.Stacking-up a few hundreds of short exposures in J and Ks spectral bands over a field of view (FoV) of 10.9″ × 12.3″ centered on the R 136a1 stellar component, enabled us to carry a refined photometric analysis of the core of R 136. We detected 1110 and 1059 sources in J and Ks images respectively with 818 common sources. Thanks to better angular resolution and dynamic range, we found that more than 62.6% (16.5%) of the stars, detected both in J and Ks data, have neighbours closer than 0.2'' (0.1''). The closest stars are resolved down to the full width at half maximum (FWHM) of the point spread function (PSF) measured by Starfinder. Among resolved and/or detected sources R 136a1 and R 136c have optical companions and R 136a3 is resolved as two stars (PSF fitting) separated by 59 ± 2 mas. This new companion of R 136a3 presents a correlation coefficient of 86% in J and 75% in Ks. The new set of detected sources were used to re-assess the age and extinction of R 136 based on 54 spectroscopically stars that have been recently studied with HST slit-spectroscopy (Crowther et al. <xref ref-type="bibr" rid="R11">2016</xref>, MNRAS, 458, 624) of the core of this cluster. Over 90% of these 54 sources identified visual companions (closer than 0.2''). We found the most probable age and extinction for these sources are 1.8+1.2-0.8 Myr, AJ = (0.45 ± 0.5) mag and AK = (0.2 ± 0.5) mag within the photometric and spectroscopic error-bars. Additionally, using PARSEC evolutionary isochrones and tracks, we estimated the stellar mass range for each detected source (common in J and K data) and plotted the generalized histogram of mass (MF with error-bars). Using SPHERE data, we have gone one step further and partially resolved and studied the initial mass function covering mass range of (3-300) M⊙ at the age of 1 and 1.5 Myr. The density in the core of R 136 (0.1-1.4 pc) is estimated and extrapolated in 3D and larger radii (up to 6 pc). We show that the stars in the core are still unresolved due to crowding, and the results we obtained are upper limits. Higher angular resolution is mandatory to overcome these difficulties.

Based on data collected at the European Southern Observatory, Chile, Guaranteed Time Observation 095.D-0309(K).

The catalogue is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/602/A56 Title: TLUSTY User's Guide II: Reference Manual Authors: Hubeny, Ivan; Lanz, Thierry Bibcode: 2017arXiv170601935H Altcode: This is the second part of a three-volume guide to TLUSTY and SYNSPEC. It presents a detailed reference manual for TLUSTY, which contains a detailed description of basic physical assumptions and equations used to model an atmosphere, together with an overview of the numerical methods to solve these equations. Title: A brief introductory guide to TLUSTY and SYNSPEC Authors: Hubeny, Ivan; Lanz, Thierry Bibcode: 2017arXiv170601859H Altcode: This is the first of three papers that present a detailed guide for working with the codes {\sc tlusty} and {\sc synspec} to generate model stellar atmospheres or accretion disks, and to produce detailed synthetic spectra. In this paper, we present a very brief manual intended for casual users who intend to use these codes for simple, well defined tasks. This paper does not present any background theory, or a description of the adopted numerical approaches, but instead uses simple examples to explain how to employ these codes. In particular, it shows how to produce a simple model atmosphere from the scratch, or how to improve an existing model by considering more extended model atoms. This paper also presents a brief guide to the spectrum synthesis program {\sc synspec}. Title: Towards a Sharper Picture of R136 with SPHERE Extreme Adaptive Optics Authors: Khorrami, Z.; Vakili, F.; Lanz, T.; Langlois, M.; Lagadec, E.; Meyer, M. R.; Gratton, R.; Beuzit, J. -L.; Mouillet, D. Bibcode: 2017Msngr.168...32K Altcode: The SPHERE extreme adaptive optics instrument was used to observe the central core of the Large Magellanic Cloud, R136, in the near-infrared. This challenging observation demonstrated the capabilities of SPHERE for imaging distant clusters. More than one thousand sources have been detected in Ks- and J-band images in the small field of view of IRDIS covering almost 2.7 × 3.1 pc of the core of R136. Based on isochrone fitting of the colour-magnitude diagram, ages of 1 and 1.5 Myr for the inner 3-arcsecond core and the outer core of R136 fit our data best. The mass function slope is -0.96 ± 0.22 over the mass range of 3 to 300 M⊙. Using SPHERE data, we have gone one step further in partially resolving the core of R136, but this is certainly not the final step and higher resolution is still required. Title: TLUSTY User's Guide III: Operational Manual Authors: Hubeny, Ivan; Lanz, Thierry Bibcode: 2017arXiv170601937H Altcode: This paper presents a detailed operational manual for TLUSTY. It provides a guide for understanding the essential features and the basic modes of operation of the program. To help the user, it is divided into two parts. The first part describes the most important input parameters and available numerical options. The second part covers additional details and a comprehensive description of all physical and numerical options, and a description of all input parameters, many of which needed only in special cases. Title: VizieR Online Data Catalog: R136 JKs photometry from VLT/SPHERE EAO (Khorrami+, 2017) Authors: Khorrami, Z.; Vakili, F.; Lanz, T.; Langlois, M.; Lagadec, E.; Meyer, M. R.; Robbe-Dubois, S.; Abe, L.; Avenhaus, H.; Beuzit, J. L.; Gratton, R.; Mouillet, D.; Origne, A.; Petit, C.; Ramos, J. Bibcode: 2017yCat..36020056K Altcode: The SPHERE/IRDIS catalog of the common sources between J and Ks-band data on R136. The ID, Xpix and Ypix are the identification and pixel position in the IRDIS K and J image. σK and σJ are the total error (combination of PSF-fitting error, residual errors and the calibration error) in Ks and J images. CK and CJ are the Correlation coefficients between the input PSF and the star, in Ks and J data.

(1 data file). Title: Masses of the Planetary Nebula Central Stars in the Galactic Globular Cluster System from HST Imaging and Spectroscopy Authors: Jacoby, George H.; De Marco, Orsola; Davies, James; Lotarevich, I.; Bond, Howard E.; Harrington, J. Patrick; Lanz, Thierry Bibcode: 2017ApJ...836...93J Altcode: 2017arXiv170103516J The globular cluster (GC) system of our Galaxy contains four planetary nebulae (PNe): K 648 (or Ps 1) in M15, IRAS 18333-2357 in M22, JaFu 1 in Pal 6, and JaFu 2 in NGC 6441. Because single-star evolution at the low stellar mass of present-epoch GCs was considered incapable of producing visible PNe, their origin presented a puzzle. We imaged the PN JaFu 1 with the Hubble Space Telescope (HST) to obtain photometry of its central star (CS) and high-resolution morphological information. We imaged IRAS 18333-2357 with better depth and resolution, and we analyzed its archival HST spectra to constrain its CS temperature and luminosity. All PNe in Galactic GCs now have quality HST data, allowing us to improve CS mass estimates. We find reasonably consistent masses between 0.53 and 0.58 M ⊙ for all four objects, though estimates vary when adopting different stellar evolutionary calculations. The CS mass of IRAS 18333-2357, though, depends strongly on its temperature, which remains elusive due to reddening uncertainties. For all four objects, we consider their CS and nebula masses, their morphologies, and other incongruities to assess the likelihood that these objects formed from binary stars. Although generally limited by uncertainties (∼0.02 M ⊙) in post-AGB tracks and core mass versus luminosity relations, the high-mass CS in K 648 indicates a binary origin. The CS of JaFu 1 exhibits compact, bright [O III] and Hα emission, like EGB 6, suggesting a binary companion or disk. Evidence is weaker for a binary origin of JaFu 2.

Based, in part, on observations made with the NASA/ESA Hubble Space Telescope, obtained [from the Data Archive] 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 program GO-11558. Title: Probing Supernovae Chemical Yields in Low Metallicity Environments with UV Spectroscopy of Magellanic Cloud B-type Stars Authors: Lanz, Thierry; Adelman, Saul J.; Bouret, Jean-Claude; Gulliver, Austin F.; Hill, Graham; Hubeny, Ivan; Peters, Geraldine J.; Pickering, Juliet C.; Proffitt, Charles R. Bibcode: 2016hst..prop14081L Altcode: Spectrum synthesis studies of the UV spectra of sharp-lined main sequence B stars provide astronomers with some of the best determinations of the abundances of the light, Fe group, and neutron capture elements. B stars are therefore best-suited to study the chemical evolution of the Magellanic Clouds. But the HST archive is virtually devoid of high resolution spectra of such objects. We propose FUV and NUV observations with the COS G130M, G160M, G185M, and G225M gratings. The four program stars have been observed with the FUSE spacecraft, hence this project will produce continuous high-resolution spectral coverage from 950 to 2400 A and provide a permanent archive of fundamental spectra from which ground-breaking studies of the Magellanic Clouds can be performed in the decades to come. This limited program aims at producing an extragalactic, low-metallicity counterpart to the bright star library of early B stars that is currently being obtained as part of the HST Cycle 21 Treasury program "Advanced Spectral Library II: Hot Stars" (GO 13346, PI T. Ayres).

Spectral lines from most Fe group and s-process elements are found only in the UV region in B stars and information on their abundances is important for studying the chemical evolution of a galaxy, computing opacities for stellar evolution calculations, and assessing the validity of theoretical calculations of explosive nucleosynthesis. Comparing the derived abundances of iron-peak and heavier elements in galactic and Magellanic Cloud B main sequence stars will provide an empirical probe of chemical yields ejected by evolved stars and supernovae in different environments. Title: Grown-up stars physics with MATISSE Authors: Millour, F.; Hron, J.; Chiavassa, A.; Weigelt, G.; Soulain, A.; Khorrami, Z.; Meilland, A.; Nardetto, N.; Paladini, C.; Domiciano de Souza, A.; Niccolini, G.; Hofmann, K. -H.; Schertl, D.; Stee, P.; Bendjoya, P.; Thévenin, F.; Vakili, F.; Berio, P.; Lanz, T.; Matter, A.; Cruzalèbes, P.; Petrov, R.; Lopez, B. Bibcode: 2016SPIE.9907E..3QM Altcode: 2016arXiv160702393M MATISSE represents a great opportunity to image the environment around massive and evolved stars. This will allow one to put constraints on the circumstellar structure, on the mass ejection of dust and its reorganization, and on the dust-nature and formation processes. MATISSE measurements will often be pivotal for the understanding of large multiwavelength datasets on the same targets collected through many high-angular resolution facilities at ESO like sub-millimeter interferometry (ALMA), near-infrared adaptive optics (NACO, SPHERE), interferometry (PIONIER, GRAVITY), spectroscopy (CRIRES), and mid-infrared imaging (VISIR). Among main sequence and evolved stars, several cases of interest have been identified that we describe in this paper. Title: An overview of the mid-infrared spectro-interferometer MATISSE: science, concept, and current status Authors: Matter, A.; Lopez, B.; Antonelli, P.; Lehmitz, M.; Bettonvil, F.; Beckmann, U.; Lagarde, S.; Jaffe, W.; Petrov, R.; Berio, P.; Millour, F.; Robbe-Dubois, S.; Glindemann, A.; Bristow, P.; Schoeller, M.; Lanz, T.; Henning, T.; Weigelt, G.; Heininger, M.; Morel, S.; Cruzalebes, P.; Meisenheimer, K.; Hofferbert, R.; Wolf, S.; Bresson, Y.; Agocs, T.; Allouche, F.; Augereau, J. -C.; Avila, G.; Bailet, C.; Behrend, J.; van Belle, G.; Berger, J. -P.; van Boekel, R.; Bourget, P.; Brast, R.; Clausse, J. -M.; Connot, C.; Conzelmann, R.; Csepany, G.; Danchi, W. C.; Delbo, M.; Dominik, C.; van Duin, A.; Elswijk, E.; Fantei, Y.; Finger, G.; Gabasch, A.; Gonté, F.; Graser, U.; Guitton, F.; Guniat, S.; De Haan, M.; Haguenauer, P.; Hanenburg, H.; Hofmann, K. -H.; Hogerheijde, M.; ter Horst, R.; Hron, J.; Hummel, C.; Isderda, J.; Ives, D.; Jakob, G.; Jasko, A.; Jolley, P.; Kiraly, S.; Kragt, J.; Kroener, T.; Kroes, G.; Kuindersma, S.; Labadie, L.; Laun, W.; Leinert, C.; Lizon, J. -L.; Lucuix, C.; Marcotto, A.; Martinache, F.; Martinot-Lagarde, G.; Mauclert, N.; Mehrgan, L.; Meilland, A.; Mellein, M.; Menardi, S.; Merand, A.; Neumann, U.; Nussbaum, E.; Ottogalli, S.; Palsa, R.; Panduro, J.; Pantin, E.; Percheron, I.; Phan Duc, T.; Pott, J. -U.; Pozna, E.; Roelfsema, R.; Rupprecht, G.; Schertl, D.; Schmidt, C.; Schuil, M.; Spang, A.; Stegmeier, J.; Tromp, N.; Vakili, F.; Vannier, M.; Wagner, K.; Venema, L.; Woillez, J. Bibcode: 2016SPIE.9907E..0AM Altcode: 2016arXiv160802350M MATISSE is the second-generation mid-infrared spectrograph and imager for the Very Large Telescope Interferometer (VLTI) at Paranal. This new interferometric instrument will allow significant advances by opening new avenues in various fundamental research fields: studying the planet-forming region of disks around young stellar objects, understanding the surface structures and mass loss phenomena affecting evolved stars, and probing the environments of black holes in active galactic nuclei. As a first breakthrough, MATISSE will enlarge the spectral domain of current optical interferometers by offering the L and M bands in addition to the N band. This will open a wide wavelength domain, ranging from 2.8 to 13 μm, exploring angular scales as small as 3 mas (L band) / 10 mas (N band). As a second breakthrough, MATISSE will allow mid-infrared imaging - closure-phase aperture-synthesis imaging - with up to four Unit Telescopes (UT) or Auxiliary Telescopes (AT) of the VLTI. Moreover, MATISSE will offer a spectral resolution range from R ∼ 30 to R ∼ 5000. Here, we present one of the main science objectives, the study of protoplanetary disks, that has driven the instrument design and motivated several VLTI upgrades (GRA4MAT and NAOMI). We introduce the physical concept of MATISSE including a description of the signal on the detectors and an evaluation of the expected performances. We also discuss the current status of the MATISSE instrument, which is entering its testing phase, and the foreseen schedule for the next two years that will lead to the first light at Paranal. Title: Science with MATISSE Authors: Wolf, Sebastian; Lopez, Bruno; Augereau, Jean-Charles; Delbo, Marco; Dominik, Carsten; Henning, Thomas; Hofmann, Karl-Heinz; Hogerheijde, Michiel; Hron, Josef; Jaffe, Walter; Lanz, Thierry; Meisenheimer, Klaus; Millour, Florentin; Pantin, Eric; Petrov, Roman; Schertl, Dieter; van Boekel, Roy; Weigelt, Gerd; Chiavassa, Andrea; Juhasz, Attila; Matter, Alexis; Meilland, Anthony; Nardetto, Nicolas; Paladini, Claudia Bibcode: 2016SPIE.9907E..3SW Altcode: 2016arXiv160906152W We present an overview of the scientific potential of MATISSE, the Multi Aperture mid-Infrared SpectroScopic Experiment for the Very Large Telescope Interferometer. For this purpose we outline selected case studies from various areas, such as star and planet formation, active galactic nuclei, evolved stars, extrasolar planets, and solar system minor bodies and discuss strategies for the planning and analysis of future MATISSE observations. Moreover, the importance of MATISSE observations in the context of complementary high-angular resolution observations at near-infrared and submillimeter/millimeter wavelengths is highlighted. Title: A Definitive Test of Rotational Mixing in Massive Stars Authors: Proffitt, Charles R.; Brott, Ines; Cunha, Katia; Dufton, Philip; Langer, Norbert; Lanz, Thierry; Lennon, Daniel J.; Simon-Diaz, Sergio; de Mink, Selma E. Bibcode: 2016hst..prop14673P Altcode: CNO processed surface material found in OB stars may originate either from internal mixing or from binary interaction, but incomplete boron depletion is an unambiguous sign of internal mixing. Existing boron observations indeed suggest that internal mixing occurs in some stars at a level that is consistent with the low end of the efficiency range predicted by the different models of rotationally driven mixing. However, current results are too sparse to directly confirm the expected relation between boron depletion and rotation, and leave room to interpret boron depletion through other mixing processes. We propose to observe boron in ten rather rapidly rotating early-B stars in the 10 Myr old open cluster NGC 3293. Together with our previous data on stars in this cluster, this increased sample with an expanded range of V sin(i) values will provide a definitive test of rotational mixing, and --- assuming that rotation actually drives the expected mixing --- will allow for a tight calibration of its efficiency, which is of critical importance for modeling the interior of massive stars, with wide implications for their advanced evolutionary stages. Title: Before the Burst: The Properties of Rapidly Rotating, Massive Supergiants Authors: Bouret, Jean-Claude; Hillier, D. John; Depagne, Eric; Fullerton, Alexander W.; Herve, Anthony; Lanz, Thierry; Marcolino, Wagner; Martins, Fabrice; Walborn, Nolan R. Bibcode: 2016hst..prop14683B Altcode: Rapidly rotating, evolved massive stars are thought to be the progenitors of long gamma-ray bursts (LGRBs). Their rapid rotation may be the result of (a) reduced angular-momentum loss by a single star due to reduced mass loss at low metallicity; or (b) spin-up via mass and angular momentum transfer from a companion or a stellar merger. Independent of their formation mechanism, the properties of their stellar winds are strongly affected by their rapid rotation. We have recently obtained optical spectroscopy to study the properties of a sample of ten fast rotating, evolved massive stars in the Large Magellanic Cloud. Here, we propose to use HST FUV spectroscopy to determine the wind properties of these objects. The information needed to characterize the stellar winds of massive stars is encoded in FUV spectra. Establishing the properties of these rotating outflows is a critical issue, since the present uncertainties restrict our understanding of the evolution of the angular momentum, the initial-to-final mass relation, and the ultimate fate of a massive star.

By fitting line profiles from sophisticated model atmospheres to the FUV spectra, we will measure photospheric CNO abundances, probe the rotation law of the stellar wind, and study the influence of rapid rotation on the profiles of wind lines, and hence on the determination of mass-loss rates. We will investigate how rapid rotation affects the onset of clumping, the presence of which reduces estimates of mass-loss rates. This program will enable decisive steps toward determining the nature of rapidly rotating, evolved, massive stars, as required to evaluate their suitability as progenitors of LGRBs. Title: VLT/SPHERE deep insight of NGC 3603's core: Segregation or confusion? Authors: Khorrami, Z.; Lanz, T.; Vakili, F.; Lagadec, E.; Langlois, M.; Brandner, W.; Chesneau, O.; Meyer, M. R.; Carbillet, M.; Abe, L.; Mouillet, D.; Beuzit, J. L.; Boccaletti, A.; Perrot, C.; Thalmann, C.; Schmid, H. -M.; Pavlov, A.; Costille, A.; Dohlen, K.; Le Mignant, D.; Petit, C.; Sauvage, J. F. Bibcode: 2016A&A...588L...7K Altcode: 2016arXiv160300457K We present new near-infrared photometric measurements of the core of the young massive cluster NGC 3603 obtained with extreme adaptive optics. The data were obtained with the SPHERE instrument mounted on ESO's Very Large Telescope, and cover three fields in the core of this cluster. We applied a correction for the effect of extinction to our data obtained in the J and K broadband filters and estimated the mass of detected sources inside the field of view of SPHERE/IRDIS, which is 13.5''×13.5''. We derived the mass function (MF) slope for each spectral band and field. The MF slope in the core is unusual compared to previous results based on HST and VLT observations. The average slope in the core is estimated as -1.06 ± 0.26 for the main sequence stars with 3.5 M⊙ < M < 120M⊙. Thanks to the SPHERE extreme adaptive optics, 814 low-mass stars were detected to estimate the MF slope for the pre-main sequence stars with 0.6 M⊙ < M < 3.5M⊙, Γ = -0.54 ± 0.11 in the K-band images in two fields in the core of the cluster. For the first time, we derive the MF of the very core of the NGC 3603 young cluster for masses in the range 0.6-120 M⊙. Previous studies were either limited by crowding, lack of dynamic range, or a combination of both.

Based on data collected at the European Southern Observatory, Chile (guaranteed time observation 095.D-0309(A) and 095.D-0309(E)) Title: The Massive Stars Nursery R136 Authors: Khorrami, Z.; Vakili, F.; Chesneau, O.; Lanz, T. Bibcode: 2015EAS....71..331K Altcode: As most stars are born in a clustered mode, young massive star clusters are the best places to find and study the formation and evolution of massive stars. R136 is one of the most massive nearby clusters in the LMC. It contains at least 72 known O and Wolf-Rayet stars. These young stars are usually embedded in dust and gas so that correcting the local extinction plays an important role to estimate their mass from their luminosity. The extinction is derived for 26 O stars in different HST filters using TLUSTY model atmospheres of O stars. We derived the stellar masses, and hence the Mass Function (MF), using HST multi-color photometry. In parallel, we simulated series of R136-like clusters using the NBODY6 code to test the segregation scenario for R136. We could check whether massive stars are preferentially formed in the cluster core or formed homogeneously. By comparing the surface brightness profiles (SBP) of simulated clusters mimicking R136 with HST data, we could determine which scenario best represents R136. We present here a method that we used to connect the results of the NBODY6 simulations to R136 HST imaging data. The results of these studies bring a new insight to the understanding of R136 and similar clusters, pending future VLT and E-ELT high-contrast imaging observations at the diffraction limit at visible and IR wavelengths. Title: Foreword Authors: Lagadec, E.; Millour, F.; Lanz, T. Bibcode: 2015EAS....71....1L Altcode: No abstract at ADS Title: The Wind and Photosphere of the Unique DO White Dwarf RE J0503-289 Authors: Barstow, Martin; Holberg, Jay B.; Hubeny, Ivan; Lanz, Thierry; Sion, Edward M. Bibcode: 2015hst..prop.6628B Altcode: We propose to obtain high S/N GHRS spectra of the key He-richwhite dwarf RE J0503-289. These observations have twoprincipal aims. The first is to investigate the phenomenon ofmass loss in this star. Excluding planetary nebulae nuclei, REJ0503-289 is the only white dwarf in which evidence of ongoingmass loss has been observed with IUE. However, the reporteddetection is near the limit of the sensitivity of IUEpreventing study of the wind during its less intense periods.Mass loss may well be the missing ingredient in thetheoretical description of heavy element abundances in whitedwarfs. RE J0503-289 is the only known white dwarf where thiseffect can be studied as an on going phenomenon. The secondaim is to search for Fe and Ni in the photosphere of thisstar. The presence of these elements is required to explainthe large flux deficit observed in the EUV. Detailed models,including those elements so far detected (C, N, O and Si),substantially over predict EUV flux levels. RE J0503-289 isalso the only DO white dwarf for which a joint analysis of EUVand UV data will be possible and for which we can conduct adefinitive study of the composition and structure of thephotosphere. Title: Probing Supernovae Chemical Yields in Low Metallicity Environments with UV Spectroscopy of Magellanic Cloud B-type Stars Authors: Lanz, Thierry Bibcode: 2015hst..prop14081L Altcode: Spectrum synthesis studies of the UV spectra of sharp-lined main sequence B stars provide astronomers withsome of the best determinations of the abundances of the light, Fe group, and neutron capture elements. B stars are therefore best-suited to study the chemical evolution of the Magellanic Clouds. But the HST archive is virtually devoid of high resolution spectra of such objects. We propose FUV and NUV observations with the COS G130M, G160M, G185M, and G225M gratings. The four program stars have been observed with the FUSE spacecraft, hence this project will produce continuous high-resolution spectral coverage from 950 to 2400 A and provide a permanent archive of fundamental spectra from which ground-breaking studies of the Magellanic Clouds can be performed in the decades to come. This limited program aims at producing an extragalactic, low-metallicity counterpart to the bright star library of early B stars that is currently being obtained as part of the HST Cycle 21 Treasury program Advanced Spectral Library II: Hot Stars (GO 13346, PI T. Ayres).Spectral lines from most Fe group and s-process elements are found only in the UV region in B stars and information on their abundances is important for studying the chemical evolution of a galaxy, computing opacities for stellar evolution calculations, and assessing the validity of theoretical calculations of explosive nucleosynthesis. Comparing the derived abundances of iron-peak and heavier elements in galactic and Magellanic Cloud B main sequence stars will provide an empirical probe of chemical yields ejected by evolved stars and supernovae in different environments. Title: No breakdown of the radiatively driven wind theory in low-metallicity environments Authors: Bouret, J. -C.; Lanz, T.; Hillier, D. J.; Martins, F.; Marcolino, W. L. F.; Depagne, E. Bibcode: 2015MNRAS.449.1545B Altcode: 2015arXiv150205641B We present a spectroscopic analysis of Hubble Space Telescope/Cosmic Origins Spectrograph observations of three massive stars in the low metallicity dwarf galaxies IC 1613 and WLM. These stars, were previously observed with Very Large Telescope (VLT)/X-shooter by Tramper et al., who claimed that their mass-loss rates are higher than expected from theoretical predictions for the underlying metallicity. A comparison of the far ultraviolet (FUV) spectra with those of stars of similar spectral types/luminosity classes in the Galaxy, and the Magellanic Clouds provides a direct, model-independent check of the mass-loss-metallicity relation. Then, a quantitative spectroscopic analysis is carried out using the non-LTE (NLTE) stellar atmosphere code CMFGEN. We derive the photospheric and wind characteristics, benefiting from a much better sensitivity of the FUV lines to wind properties than Hα. Iron and CNO abundances are measured, providing an independent check of the stellar metallicity. The spectroscopic analysis indicates that Z/Z⊙ = 1/5, similar to a Small Magellanic Cloud-type environment, and higher than usually quoted for IC 1613 and WLM. The mass-loss rates are smaller than the empirical ones by Tramper et al., and those predicted by the widely used theoretical recipe by Vink et al. On the other hand, we show that the empirical, FUV-based, mass-loss rates are in good agreement with those derived from mass fluxes computed by Lucy. We do not concur with Tramper et al. that there is a breakdown in the mass-loss-metallicity relation. Title: Non-LTE Abundances in OB stars: Preliminary Results for 5 Stars in the Outer Galactic Disk Authors: Bragançca, G. A.; Lanz, T.; Daflon, S.; Cunha, K.; Garmany, C. D.; Glaspey, J. W.; Borges Fernandes, M.; Oey, M. S.; Bensby, T.; Hubeny, I. Bibcode: 2015IAUS..307...90B Altcode: 2014arXiv1408.5807B The aim of this study is to analyse and determine elemental abundances for a large sample of distant B stars in the outer Galactic disk in order to constrain the chemical distribution of the Galactic disk and models of chemical evolution of the Galaxy. Here, we present preliminary results on a few stars along with the adopted methodology based on securing simultaneous O and Si ionization equilibria with consistent NLTE model atmospheres. Title: 3D Gray Radiative Properties of a Radiation Hydrodynamic Model of a YSO Accretion Shock Authors: Ibgui, L.; de Sá, L.; Stehlé, C.; Chièze, J. -P.; Orlando, S.; Hubeny, I.; Lanz, T.; Matsakos, T.; González, M.; Bonito, R. Bibcode: 2014ASPC..488...83I Altcode: We present preliminary results of radiative properties of a 1D gray radiation hydrodynamic (RHD) model of an accretion shock on a young stellar object (YSO). This model takes into account the transition between the collisional equilibrium regime (local thermodynamic equilibrium, LTE), and the coronal equilibrium regime. Based on the 1D planar structure, we built a 3D cylindrical one. Most notably, the post-shock region obtained in our case is far less extended (by a factor of 10 000) than the typical one obtained with models that assume gray optically thin radiative losses. Moreover, we find that the column is optically thin in its longitudinal dimension, and in the transverse dimension, except over an extremely narrow region (≲ 700 m). Consequently, still under the gray assumption, the photons emitted by the hot slab can propagate through the column and escape freely in all directions, including towards the chromosphere. The radiation flux has therefore components that are perpendicular to the accretion column, which demonstrates that a multidimensional (2D or 3D) radiative model is necessary for such a cylindrical structure. This study needs to be taken forward and expanded, by improving the radiative treatment of the RHD model, through relaxation of both the gray and the LTE approximations for the calculation of opacities, in order to clarify the structure of the post-shock region, which is a major source of emission probed by observations. Title: An Overview of the MATISSE Instrument — Science, Concept and Current Status Authors: Lopez, B.; Lagarde, S.; Jaffe, W.; Petrov, R.; Schöller, M.; Antonelli, P.; Beckmann, U.; Berio, P.; Bettonvil, F.; Glindemann, A.; Gonzalez, J. -C.; Graser, U.; Hofmann, K. -H.; Millour, F.; Robbe-Dubois, S.; Venema, L.; Wolf, S.; Henning, T.; Lanz, T.; Weigelt, G.; Agocs, T.; Bailet, C.; Bresson, Y.; Bristow, P.; Dugué, M.; Heininger, M.; Kroes, G.; Laun, W.; Lehmitz, M.; Neumann, U.; Augereau, J. -C.; Avila, G.; Behrend, J.; van Belle, G.; Berger, J. -P.; van Boekel, R.; Bonhomme, S.; Bourget, P.; Brast, R.; Clausse, J. -M.; Connot, C.; Conzelmann, R.; Cruzalèbes, P.; Csepany, G.; Danchi, W.; Delbo, M.; Delplancke, F.; Dominik, C.; van Duin, A.; Elswijk, E.; Fantei, Y.; Finger, G.; Gabasch, A.; Gay, J.; Girard, P.; Girault, V.; Gitton, P.; Glazenborg, A.; Gonté, F.; Guitton, F.; Guniat, S.; De Haan, M.; Haguenauer, P.; Hanenburg, H.; Hogerheijde, M.; ter Horst, R.; Hron, J.; Hugues, Y.; Hummel, C.; Idserda, J.; Ives, D.; Jakob, G.; Jasko, A.; Jolley, P.; Kiraly, S.; Köhler, R.; Kragt, J.; Kroener, T.; Kuindersma, S.; Labadie, L.; Leinert, C.; Le Poole, R.; Lizon, J. -L.; Lucuix, C.; Marcotto, A.; Martinache, F.; Martinot-Lagarde, G.; Mathar, R.; Matter, A.; Mauclert, N.; Mehrgan, L.; Meilland, A.; Meisenheimer, K.; Meisner, J.; Mellein, M.; Menardi, S.; Menut, J. -L.; Merand, A.; Morel, S.; Mosoni, L.; Navarro, R.; Nussbaum, E.; Ottogalli, S.; Palsa, R.; Panduro, J.; Pantin, E.; Parra, T.; Percheron, I.; Duc, T. P.; Pott, J. -U.; Pozna, E.; Przygodda, F.; Rabbia, Y.; Richichi, A.; Rigal, F.; Roelfsema, R.; Rupprecht, G.; Schertl, D.; Schmidt, C.; Schuhler, N.; Schuil, M.; Spang, A.; Stegmeier, J.; Thiam, L.; Tromp, N.; Vakili, F.; Vannier, M.; Wagner, K.; Woillez, J. Bibcode: 2014Msngr.157....5L Altcode: MATISSE, a second generation Very Large Telescope Interferometer (VLTI) instrument, is a combined imager and spectrograph for interferometry in the 3-5 μm region (L- and M-bands) and the 8-13 μm window (N-band). MATISSE builds on the experience gained with the VLTI's first generation instruments. It employs multi-axial beam combination while also providing wavelength differential visibility and phase, and closure-phase aperture-synthesis imaging at a range of spectral resolutions. MATISSE is designed for a broad range of science goals, and its potential for studies of the discs around young stars and active galactic nuclei are highlighted. The instrument concept and operating modes are described; construction is in progress towards installation at the VLTI in 2016. 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: Type II Plateau supernovae as metallicity probes of the Universe Authors: Dessart, L.; Gutierrez, C. P.; Hamuy, M.; Hillier, D. J.; Lanz, T.; Anderson, J. P.; Folatelli, G.; Freedman, W. L.; Ley, F.; Morrell, N.; Persson, S. E.; Phillips, M. M.; Stritzinger, M.; Suntzeff, N. B. Bibcode: 2014MNRAS.440.1856D Altcode: 2014arXiv1403.1167D; 2014MNRAS.tmp..580D We explore a method for metallicity determinations based on quantitative spectroscopy of Type II-Plateau supernovae (SNe II-P). For consistency, we first evolve a set of 15 M⊙ main-sequence stars at 0.1, 0.4, 1, and 2 times the solar metallicity. At the onset of core collapse, we trigger a piston-driven explosion and model the resulting ejecta and radiation. Our theoretical models of such red supergiant star explosions at different metallicity show that synthetic spectra of SNe II-P possess optical signatures during the recombination phase that are sensitive to metallicity variations. This sensitivity can be quantified and the metallicity inferred from the strengths of metal-line absorptions. Furthermore, these signatures are not limited to O, but also include Na, Ca, Sc, Ti, or Fe. When compared to a sample of SNe II-P from the Carnegie SN Project and previous SN followup programmes, we find that most events lie at a metallicity between 0.4 and 2 times solar, with a marked scarcity of SN II-P events at small magellanic cloud metallicity. This most likely reflects the paucity of low-metallicity star-forming regions in the local Universe.

SNe II-P have high-plateau luminosities that make them observable spectroscopically at large distances. Because they exhibit signatures of diverse metal species, in the future they may offer a means to constrain the evolution of the composition (e.g. the O/Fe ratio) in the Universe out to a redshift of 1 and beyond. Title: The Discovery of Pulsating Hot Subdwarfs in NGC 2808 Authors: Brown, T.; Landsman, W.; Randall, S.; Sweigart, A.; Lanz, T.; Hubeny, I. Bibcode: 2014ASPC..481...29B Altcode: We present preliminary results of a Hubble Space Telescope program to search for pulsating hot subdwarfs in the core of NGC 2808. These observations, obtained in March of 2013, were motivated by the recent discovery of such stars in the outskirts of ω Cen. Both ω Cen and NGC 2808 are massive globular clusters exhibiting complex stellar populations and large numbers of extreme horizontal branch stars. Our far-UV photometric monitoring of over 100 UV-bright stars has revealed at least six pulsating subdwarfs with periods ranging from 100 to 150 seconds. In the UV color-magnitude diagram of NGC 2808, all six of these stars lie immediately below the canonical horizontal branch, a region populated by the subluminous “blue hook” stars. Three of these six pulsators also have low-resolution far-UV spectroscopy that is sufficient to broadly constrain their atmospheric abundances and effective temperatures. Curiously, the spectroscopic and photometric data do not exhibit the uniformity one might expect from a well-defined instability strip. 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: 3D Gray Radiative Properties of Accretion Shocks in Young Stellar Objects Authors: Ibgui, L.; Orlando, S.; Stehlé, C.; Chièze, J. -P.; Hubeny, I.; Lanz, T.; de Sá, L.; Matsakos, T.; González, M.; Bonito, R. Bibcode: 2014EPJWC..6404005I Altcode: We address the problem of the contribution of radiation to the structure and dynamics of accretion shocks on Young Stellar Objects. Solving the 3D RTE (radiative transfer equation) under our "gray LTE approach", i.e., using appropriate mean opacities computed in local thermodynamic equilibrium, we post-process the 3D MHD (magnetohydrodynamic) structure of an accretion stream impacting the stellar chromosphere. We find a radiation flux of ten orders of magnitude larger than the accreting energy rate, which is due to a large overestimation of the radiative cooling. A gray LTE radiative transfer approximation is therefore not consistent with the given MHD structure of the shock. Further investigations are required to clarify the role of radiation, by relaxing both the gray and LTE approximations in RHD (radiation hydrodynamics) simulations. Post-processing the obtained structures through the resolution of the non-LTE monochromatic RTE will provide reference radiation quantities against which RHD approximate solutions will be compared. Title: Accretion shock stability on a dynamically heated YSO atmosphere with radiative transfer Authors: de Sá, Lionel; Chièze, Jean-Pierre; Stehlé, Chantal; Matsakos, Titos; Ibgui, Laurent; Lanz, Thierry; Hubeny, Ivan Bibcode: 2014EPJWC..6404002D Altcode: Theory and simulations predict Quasi-Periodic Oscillations of shocks which develop in magnetically driven accretion funnels connecting the stellar disc to the photosphere of Young Stellar Objects (YSO). X-ray observations however do not show evidence of the expected periodicity. We examine here, in a first attempt, the influence of radiative transfer on the evolution of material impinging on a dynamically heated stellar atmosphere, using the 1D ALE-RHD code ASTROLABE. The mechanical shock heating mechanism of the chromosphere only slightly perturbs the flow. We also show that, since the impacting flow, and especially the part which penetrates into the chromosphere, is not treated as a purely radiating transparent medium, a sufficiently efficient coupling between gas and radiation may affect or even suppress the oscillations of the shocked column. This study shows the importance of the description of the radiation effects in the hydrodynamics and of the accuracy of the opacities for an adequate modeling. Title: The Discovery of Pulsating Hot Subdwarfs in NGC 2808 Authors: Brown, Thomas M.; Landsman, Wayne B.; Randall, Suzanna K.; Sweigart, Allen V.; Lanz, Thierry Bibcode: 2013ApJ...777L..22B Altcode: 2013arXiv1310.0835B We present the results of a Hubble Space Telescope program to search for pulsating hot subdwarfs in the core of NGC 2808. These observations were motivated by the recent discovery of such stars in the outskirts of ω Cen. Both NGC 2808 and ω Cen are massive globular clusters exhibiting complex stellar populations and large numbers of extreme horizontal branch stars. Our far-UV photometric monitoring of over 100 hot evolved stars has revealed six pulsating subdwarfs with periods ranging from 85 to 149 s and UV amplitudes of 2.0%-6.8%. In the UV color-magnitude diagram of NGC 2808, all six of these stars lie immediately below the canonical horizontal branch, a region populated by the subluminous "blue-hook" stars. For three of these six pulsators, we also have low-resolution far-UV spectroscopy that is sufficient to broadly constrain their atmospheric abundances and effective temperatures. Curiously, and in contrast to the ω Cen pulsators, the NGC 2808 pulsators do not exhibit the spectroscopic or photometric uniformity one might expect from a well-defined instability strip, although they all fall within a narrow band (0.2 mag) of far-UV luminosity.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at STScI, which is operated by AURA, Inc., under NASA contract NAS 5-26555. 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: Massive stars at low metallicity. Evolution and surface abundances of O dwarfs in the SMC Authors: Bouret, J. -C.; Lanz, T.; Martins, F.; Marcolino, W. L. F.; Hillier, D. J.; Depagne, E.; Hubeny, I. Bibcode: 2013A&A...555A...1B Altcode: 2013arXiv1304.6923B
Aims: We aim to study the properties of massive stars at low metallicity, with an emphasis on their evolution, rotation, and surface abundances. We focus on O-type dwarfs in the Small Magellanic Cloud. These stars are expected to have weak winds that do not remove significant amounts of their initial angular momentum.
Methods: We analyzed the UV and optical spectra of twenty-three objects using the NLTE stellar atmosphere code cmfgen and derived photospheric and wind properties.
Results: The observed binary fraction of the sample is ≈26%, which is consistent with more systematic studies if one considers that the actual binary fraction is potentially larger owing to low-luminosity companions and that the sample was biased because it excluded obvious spectroscopic binaries. The location of the fastest rotators in the Hertzsprung-Russell (H-R) diagram built with fast-rotating evolutionary models and isochrones indicates that these could be several Myr old. The offset in the position of these fast rotators compared with the other stars confirms the predictions of evolutionary models that fast-rotating stars tend to evolve more vertically in the H-R diagram. Only one star of luminosity class Vz, expected to best characterize extreme youth, is located on the zero-age main sequence, the other two stars are more evolved. We found that the distribution of O and B stars in the ɛ(N) - vsin i diagram is the same, which suggests that the mechanisms responsible for the chemical enrichment of slowly rotating massive stars depend only weakly on the star's mass. We furthermore confirm that the group of slowly rotating N-rich stars is not reproduced by the evolutionary tracks. Even for more massive stars and faster rotators, our results call for stronger mixing in the models to explain the range of observed N abundances. All stars have an N/C ratio as a function of stellar luminosity that match the predictions of the stellar evolution models well. More massive stars have a higher N/C ratio than the less massive stars. Faster rotators show on average a higher N/C ratio than slower rotators, again consistent with the expected trend of stronger mixing as rotation increases. When comparing the N/O versus N/C ratios with those of stellar evolution models, the same global qualitative agreement is reached. The only discrepant behavior is observed for the youngest two stars of the sample, which both show very strong signs of mixing, which is unexpected for their evolutionary status.

Based on observations made with the NASA-ESA Hubble Space Telescope (program GO 11625), obtained at STScI, which is operated by AURA, Inc., under NASA contract NAS 5-26555. Based on observations collected at the European Southern Observatory Very Large Telescope, program 079.D-0073.Appendix B is available in electronic form at http://www.aanda.org Title: Wind properties in hot massive stars at low metallicity Authors: Lanz, Thierry; Bouret, J. C.; Hillier, D. J.; Martins, F.; Marcolino, W. L. F. Bibcode: 2013msao.confE..41L Altcode: We present our latest results on the wind properties of 40 O-type stars in the SMC observed with STIS and COS aboard HST. We reexamine the metallicity dependence of the wind parameters and the weak winds in SMC O dwarfs. We discuss the presence and origin of discrete absorption components in the FUV wind lines of some of these stars. Title: 3D Spectral Radiative Transfer with IRIS: Application to the Simulation of Laboratory Models of Accretion Shocks in Young Stellar Objects Authors: Ibgui, L.; Hubeny, I.; Lanz, T.; Stehlé, C.; González, M.; Chièze, J. P. Bibcode: 2013ASPC..474...66I Altcode: IRIS is a new three-dimensional radiative transfer code that provides spectroscopic diagnostics of hydrodynamics structures. We present its major numerical features and capabilities. We investigate the dynamical, radiative and spectral properties of laboratory generated radiative shocks, with the aim of improving our understanding of the mechanisms of accretion in young stellar objects. We employed IRIS to generate X-UV theoretical spectra, while three-dimensional non-stationary radiation hydrodynamics simulations were performed with the code HERACLES. Title: IRIS: a generic three-dimensional radiative transfer code Authors: Ibgui, L.; Hubeny, I.; Lanz, T.; Stehlé, C. Bibcode: 2013A&A...549A.126I Altcode: 2012arXiv1211.4870I Context. For most astronomical objects, radiation is the only probe of their physical properties. Therefore, it is important to have the most elaborate theoretical tool to interpret observed spectra or images, thus providing invaluable information to build theoretical models of the physical nature, the structure, and the evolution of the studied objects.
Aims: We present IRIS, a new generic three-dimensional (3D) spectral radiative transfer code that generates synthetic spectra, or images. It can be used as a diagnostic tool for comparison with astrophysical observations or laboratory astrophysics experiments.
Methods: We have developed a 3D short-characteristic solver that works with a 3D nonuniform Cartesian grid. We have implemented a piecewise cubic, locally monotonic, interpolation technique that dramatically reduces the numerical diffusion effect. The code takes into account the velocity gradient effect resulting in gradual Doppler shifts of photon frequencies and subsequent alterations of spectral line profiles. It can also handle periodic boundary conditions. This first version of the code assumes local thermodynamic equilibrium (LTE) and no scattering. The opacities and source functions are specified by the user. In the near future, the capabilities of IRIS will be extended to allow for non-LTE and scattering modeling.
Results: IRIS has been validated through a number of tests. We provide the results for the most relevant ones, in particular a searchlight beam test, a comparison with a 1D plane-parallel model, and a test of the velocity gradient effect.
Conclusions: IRIS is a generic code to address a wide variety of astrophysical issues applied to different objects or structures, such as accretion shocks, jets in young stellar objects, stellar atmospheres, exoplanet atmospheres, accretion disks, rotating stellar winds, cosmological structures. It can also be applied to model laboratory astrophysics experiments, such as radiative shocks produced with high power lasers. Title: Hydrodynamic modeling of accretion shocks on a star with radiative transport and a chromospheric model Authors: de Sá, L.; Chièze, J. -P.; Stehlé, C.; Hubeny, I.; Delahaye, F.; Lanz, T. Bibcode: 2012sf2a.conf..309D Altcode: The aim of the project (ANR STARSHOCK) is to understand the dynamics and the radiative properties of accretion columns, linking the circumstellar disk to the surface photosphere of Young Stellar Objects. The hydrodynamics is computed first, using a high resolution hydrodynamic 1D ALE code (ASTROLABE) coupled to radiative transfer and line cooling, along with a model for the acoustic heating of the chromospheric plasma. Spectra are then post-processed with a 1D radiative transfer code (SYNSPEC), using DFE solver and an extended atomic database covering a wavelength range from X rays to visible. Title: 3D numerical simulations of laboratory models of accretion shocks in young stellar objects Authors: Ibgui, L.; Hubeny, I.; Lanz, T.; Stehlé, C.; González, M.; Chièze, J. -P. Bibcode: 2012sf2a.conf..347I Altcode: We show preliminary results of our numerical simulations of laboratory experiments of radiative shocks. Such experiments aim at understanding accretion shocks in young stellar objects. Three-dimensional non-stationary radiation hydrodynamics calculations were performed with the code HERACLES. X-UV spectra were then generated with the new three-dimensional radiative transfer code IRIS. Title: Modeling periodic media with the three-dimensional radiative transfer code IRIS Authors: Ibgui, L.; Hubeny, I.; Lanz, T.; Stehlé, C. Bibcode: 2012sf2a.conf..343I Altcode: We describe the implementation in our generic three-dimensional radiative transfer code, IRIS, of an algorithm that allows the modeling of periodic infinite media. We show how this algorithm has been validated by comparison with well-established 1D plane-parallel models. A particularly interesting astrophysical application will be the calculation of synthetic spectra of the fully three-dimensional solar atmosphere. Title: The influence of rotation on optical emission profiles of O stars Authors: Hillier, D. John; Bouret, Jean-Claude; Lanz, Thierry; Busche, Joseph R. Bibcode: 2012MNRAS.426.1043H Altcode: 2012arXiv1207.0822H We study the formation of photospheric emission lines in O stars and show that the rectangular profiles, sometimes double peaked, that are observed for some stars are a direct consequence of rotation, and it is unnecessary to invoke an enhanced density structure in the equatorial regions. Emission lines, such as N IV λ4058 and the N III λλ4634-4640-4642 multiplet, exhibit non-standard 'limb-darkening' laws. The lines can be in absorption for rays striking the centre of the star and in emission for rays near the limb. Weak features in the flux spectrum do not necessarily indicate an intrinsically weak feature - instead the feature can be weak because of cancellation between absorption in 'core' rays and emission from rays near the limb. Rotation also modifies line profiles of wind diagnostics such as He II λ4686 and Hα and should not be neglected when inferring the actual stratification, level and nature of wind structures. Title: The Wind of Massive Stars in Low-Metallicity Galaxies Authors: Lanz, Thierry Bibcode: 2012hst..prop12867L Altcode: The ultimate fate of massive stars rests on the mass and angular momentum that they loose during their evolution, hence depends on the properties of their winds. Because of the radiation-driven nature of these winds, we expect the mass loss rates to scale with stellar metallicity. Such a relation has been established theoretically and empirically with spectroscopic analyses of O stars in the Galaxy {Z/Zsun=1} and in the LMC and SMC {Z/Zsun = 1/2 and 1/5}. Tramper et al. {2011} derived mass loss rates of 6 O stars in galaxies with low metallicities {Z/Zsun 1/7}. Based on the Halpha line, they derived mass loss rates that are similar to those of LMC stars. This result, if upheld, would have far-reaching consequences beyond our understanding of radiatively-driven winds, and would imply for instance a lower number of collapsars and of SN Ib and Ic in low metallicity environments, hence in the high-z Universe.We propose to coroborate or refute Tramper et al.'s result by obtaining FUV spectra of 3 O stars in the IC 1613 and WLM galaxies with HST/COS. Because of numerous iron lines and lines sensitive to wind properties, FUV spectroscopy is the adequate tool to fully address and resolve this outstanding issue of the dependence of hot, massive star mass loss rates with metallicity. A comparison of these spectra with extant STIS or COS spectra of LMC and SMC stars will provide a direct, model-independent check of the mass loss - metallicity relation. A quantitative analysis will be carried out using our state-of-the-art NLTE unified model atmospheres calculated with the CMFGEN code to establish robustly the wind properties of low-metallicity massive stars. Title: Properties of Galactic early-type O-supergiants. A combined FUV-UV and optical analysis Authors: Bouret, J. -C.; Hillier, D. J.; Lanz, T.; Fullerton, A. W. Bibcode: 2012A&A...544A..67B Altcode: 2012arXiv1205.3075B
Aims: We aim to constrain the properties and evolutionary status of early and mid-spectral type supergiants (from O4 to O7.5). These posses the highest mass-loss rates among the O stars, and exhibit conspicuous wind profiles.
Methods: Using the non-LTE wind code cmfgen we simultaneously analyzed the FUV-UV and optical spectral range to determine the photospheric properties and wind parameters. We derived effective temperatures, luminosities, surface gravities, surface abundances, mass-loss rates, wind terminal velocities, and clumping filling factors.
Results: The supergiants define a very clear evolutionary sequence, in terms of ages and masses, from younger and more massive stars to older stars with lower initial masses. O4 supergiants cluster around the 3 Myr isochrone and are more massive than 60 M⊙, while the O5 to O7.5 stars have masses in the range 50-40 M⊙ and are 4 ± 0.3 Myr old. The surface chemical composition is typical of evolved O supergiants (nitrogen-rich, carbon- and oxygen-poor). While the observed ranges of carbon and nitrogen mass-fractions are compatible with those expected from evolutionary models for the measured stellar masses, the N/C ratios as a function of age are inconsistent with the theoretical predictions for the four earliest (O4 spectral type) stars of the sample. We question the efficiency of rotational mixing as a function of age for these stars and suggest that another mechanism may be needed to explain the observed abundance patterns. Mass-loss rates derived with clumped-models range within a factor of three of the theoretical mass-loss rates. The corresponding volume-filling factors associated with small-scale clumping are 0.05 ± 0.02. Clumping is found to start close to the photosphere for all but three stars, two of which are fast rotators.

Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer (FUSE) and by the NASA-ESA-SERC International Ultraviolet Explorer (IUE), and retrieved from the Multimission Archive at the Space Telescope Science Institute (MAST). Based on observations collected with the ELODIE spectrograph on the 1.93-m telescope (Observatoire de Haute-Provence, France). Based on observations collected with the FEROS instrument on the ESO 2.2 m telescope, program 074.D-0300 and 075.D-0061.Appendix A is available in electronic form at http://www.aanda.org Title: Flash Mixing on the White Dwarf Cooling Curve: Spectroscopic Confirmation in NGC 2808 Authors: Brown, Thomas M.; Lanz, Thierry; Sweigart, Allen V.; Cracraft, Misty; Hubeny, Ivan; Landsman, Wayne B. Bibcode: 2012ApJ...748...85B Altcode: 2012arXiv1201.4070B We present new Hubble Space Telescope far-UV spectroscopy of two dozen hot evolved stars in NGC 2808, a massive globular cluster with a large population of "blue-hook" (BHk) stars. The BHk stars are found in ultraviolet color-magnitude diagrams of the most massive globular clusters, where they fall at luminosities immediately below the hot end of the horizontal branch (HB), in a region of the H-R diagram unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that these subluminous HB stars are very likely the progeny of stars that undergo extensive internal mixing during a late He-core flash on the white dwarf cooling curve. This flash mixing leads to hotter temperatures and an enormous enhancement of the surface He and C abundances; these hotter temperatures, together with the decrease in H opacity shortward of the Lyman limit, make the BHk stars brighter in the extreme UV while appearing subluminous in the UV and optical. Our far-UV spectroscopy demonstrates that, relative to normal HB stars at the same color, the BHk stars of NGC 2808 are hotter and greatly enhanced in He and C, thus providing unambiguous evidence of flash mixing in the subluminous population. Although the C abundance in the BHk stars is orders of magnitude larger than that in the normal HB stars, the atmospheric C abundance in both the BHk and normal HB stars appears to be affected by gravitational settling. The abundance variations seen in Si and the Fe-peak elements also indicate that atmospheric diffusion is at play in our sample, with all of our hot subdwarfs at 25,000-50,000 K exhibiting large enhancements of the iron-peak elements. The hottest subdwarfs in our BHk sample may be pulsators, given that they fall in the temperature range of newly discovered pulsating subdwarfs in ω Cen. In addition to the normal hot HB and BHk stars, we also obtain spectra of five blue HB stars, a post-HB star, and three unclassified stars with unusually blue UV colors.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at STScI, which is operated by AURA, Inc., under NASA contract NAS 5-26555. Title: New Observational Evidence of Flash Mixing on the White Dwarf Cooling Curve Authors: Brown, T. M.; Lanz, T.; Sweigart, A. V.; Cracraft, M.; Hubeny, I.; Landsman, W. B. Bibcode: 2012ASPC..452...23B Altcode: 2012arXiv1201.4204B Blue hook stars are a class of subluminous extreme horizontal branch stars that were discovered in UV images of the massive globular clusters ω Cen and NGC 2808. These stars occupy a region of the HR diagram that is unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that the blue hook stars are very likely the progeny of stars that undergo extensive internal mixing during a late helium-core flash on the white dwarf cooling curve. This “flash mixing” produces hotter-than-normal EHB stars with atmospheres significantly enhanced in helium and carbon. The larger bolometric correction, combined with the decrease in hydrogen opacity, makes these stars appear subluminous in the optical and UV. Flash mixing is more likely to occur in stars born with a high helium abundance, due to their lower mass at the main sequence turnoff. For this reason, the phenomenon is more common in those massive globular clusters that show evidence for secondary populations enhanced in helium. However, a high helium abundance does not, by itself, explain the presence of blue hook stars in massive globular clusters. Here, we present new observational evidence for flash mixing, using recent HST observations. These include UV color-magnitude diagrams of six massive globular clusters and far-UV spectroscopy of hot subdwarfs in one of these clusters (NGC 2808). Title: 3D modeling of accretion shocks in young stellar objects : Simulation of laboratory experiments Authors: Ibgui, L.; González, M.; Stehlé, C.; Hubeny, I.; Lanz, T. Bibcode: 2012EAS....58..149I Altcode: In order to improve the understanding of the physics of accretion shocks around young stellar objects, we have performed a three dimensional simulation of a radiative shock generated in a laser installation. We depict the 3D structure of such a shock. Radiation hydrodynamics is modeled with the HERACLES code; then, radiative transfer post-processing is performed with the IRIS code. Title: Chemical homogeneity in the Orion Association: Oxygen abundances of B stars Authors: Cunha, K.; Hubeny, I.; Lanz, T. Bibcode: 2012EPJWC..1908005C Altcode: 2011arXiv1110.5879C We present non-LTE oxygen abundances for a sample of B stars in the Orion association. The abundance calculations included non-LTE line formation and used fully blanketed non-LTE model atmospheres. The stellar parameters were the same as adopted in the previous study by Cunha & Lambert (1994). We find that the young Orion stars in this sample of 10 stars are described by a single oxygen abundance with an average value of A(O) = 8.78 and a small dispersion of ±0.05, dex which is of the order of the uncertainties in the analysis. This average oxygen abundance compares well with the average oxygen abundance obtained previously in Cunha & Lambert (1994): A(O) = 8.72 ± 0.13 although this earlier study, based upon non-blanketed model atmospheres in LTE, displayed larger scatter. Small scatter of chemical abundances in Orion B stars had also been found in our previous studies for neon and argon; all based on the same effective temperature scale. The derived oxygen abundance distribution for the Orion association compares well with other results for the oxygen abundance in the solar neighborhood. Title: SPEC3D: a three-dimensional radiative transfer code for astrophysical and laboratory applications Authors: Ibgui, L.; Hubeny, I.; Lanz, T.; Stehlé, C. Bibcode: 2011sf2a.conf..485I Altcode: We have developed a generic three-dimensional radiative transfer code, SPEC3D, aimed at post-processing 3D radiation magnetohydrodynamics simulations. SPEC3D solves the monochromatic 3D radiative transfer equation. The numerical approach and the major features of the code are presented. The wide range of applications includes the modeling of a number of astrophysical objects and structures, such as accretion shocks around young stellar objects, stellar and exoplanets atmospheres, cosmological structures, but also the modeling of laboratory astrophysics experiments such as magnetohydrodynamics jets and radiative shocks. Title: Synspec: General Spectrum Synthesis Program Authors: Hubeny, Ivan; Lanz, Thierry Bibcode: 2011ascl.soft09022H Altcode: Synspec is a user-oriented package written in FORTRAN for modeling stellar atmospheres and for stellar spectroscopic diagnostics. It assumes an existing model atmosphere, calculated previously with Tlusty or taken from the literature (for instance, from the Kurucz grid of models). The opacity sources (continua, atomic and molecular lines) are fully specified by the user. An arbitrary stellar rotation and instrumental profile can be applied to the synthetic spectrum. Title: TLUSTY: Stellar Atmospheres, Accretion Disks, and Spectroscopic Diagnostics Authors: Hubeny, Ivan; Lanz, Thierry Bibcode: 2011ascl.soft09021H Altcode: TLUSTY is a user-oriented package written in FORTRAN77 for modeling stellar atmospheres and accretion disks and wide range of spectroscopic diagnostics. In the program's maximum configuration, the user may start from scratch and calculate a model atmosphere of a chosen degree of complexity, and end with a synthetic spectrum in a wavelength region of interest for an arbitrary stellar rotation and an arbitrary instrumental profile. The user may also model the vertical structure of annuli of an accretion disk. Title: VizieR Online Data Catalog: Hot HB stars in ω Cen (Moehler+, 2011) Authors: Moehler, S.; Dreizler, S.; Lanz, T.; Bono, G.; Sweigart, A. V.; Calamida, A.; Nonino, M. Bibcode: 2011yCat..35260136M Altcode: 2011yCat..35269136M Moderately high resolution spectra of hot HB stars in the globular cluster omega Cen were analysed for radial velocity variations, atmospheric parameters, and abundances using LTE and non-LTE model atmospheres.

The spectroscopic data were obtained in 2005 (4 observations) and in 2006 (5 observations) in Service Mode using the MEDUSA mode of the multi-object fiber spectrograph FLAMES + GIRAFFE on the UT2 Telescope of the VLT. We used the low spectroscopic resolution mode with the spectral range 3964Å-4567Å (LR2, R=6400) and observed spectra for a total of 109 blue hook and canonical blue HB/EHB star candidates (see Table 1) and for 17 sky background positions. Each observation had an exposure time of 2550s to keep the total execution time of the observing block shorter than one hour.

(3 data files). Title: The hot horizontal-branch stars in ω Centauri Authors: Moehler, S.; Dreizler, S.; Lanz, T.; Bono, G.; Sweigart, A. V.; Calamida, A.; Nonino, M. Bibcode: 2011A&A...526A.136M Altcode: 2010arXiv1009.3191M Context. UV observations of some massive globular clusters have revealed a significant population of stars hotter and fainter than the hot end of the horizontal branch (HB), the so-called blue hook stars. This feature might be explained either by the late hot flasher scenario where stars experience the helium flash while on the white dwarf cooling curve or by the progeny of the helium-enriched sub-population postulated to exist in some clusters. Previous spectroscopic analyses of blue hook stars in ω Cen and NGC 2808 support the late hot flasher scenario, but the stars contain much less helium than expected and the predicted C and N enrichment cannot be verified.
Aims: We compare the observed effective temperatures, surface gravities, helium abundances, and carbon line strengths (where detectable) of our targets stars with the predictions of the two aforementioned scenarios.
Methods: Moderately high resolution spectra of hot HB stars in the globular cluster ω Cen were analysed for radial velocity variations, atmospheric parameters, and abundances using LTE and non-LTE model atmospheres.
Results: We find no evidence of close binaries among our target stars. All stars below 30 000 K are helium-poor and very similar to HB stars observed in that temperature range in other globular clusters. In the temperature range 30 000 K to 50 000 K, we find that 28% of our stars are helium-poor ({log{n_He}/{n_H}} < -1.6), while 72% have roughly solar or super-solar helium abundance ({log{n_He}/{n_H}} ≥ -1.5). We also find that carbon enrichment is strongly correlated with helium enrichment, with a maximum carbon enrichment of 3% by mass.
Conclusions: A strong carbon enrichment in tandem with helium enrichment is predicted by the late hot flasher scenario, but not by the helium-enrichment scenario. We conclude that the helium-rich HB stars in ω Cen cannot be explained solely by the helium-enrichment scenario invoked to explain the blue main sequence.

Based on observations with the ESO Very Large Telescope at Paranal Observatory, Chile (proposal IDs 075.D-0280(A) and 077.D-0021(A)). Title: Hot, Massive Stars in I Zw 18 Authors: Heap, Sara R.; Hubeny, Ivan; Lanz, Thierry; Landsman, Wayne; Lindler, Don; Malumuth, Eliot Bibcode: 2011BSRSL..80..149H Altcode: We present the far-ultraviolet spectrum of the northwest component of I Zw 18, a blue compact galaxy having a very low metallicity. The spectrum is compatible with continuous star-formation over the past ∼ 15 Myr (CSF age), and a very low metallicity, log Z/Z_⊙ ∼ -1.7, although the stellar surface may be enhanced in carbon. Stellar winds are very weak, and the edge velocity of wind lines is very low (∼ 250 km/s). Title: HST/WFC3 Photometry of NGC 2808 and Its Multiple Main Sequences Authors: Jeffery, Elizabeth; Brown, T.; Dotter, A.; Hubeny, I.; Landsman, W. B.; Lanz, T.; Sweigart, A. V. Bibcode: 2011AAS...21715212J Altcode: 2011BAAS...4315212J Recent high precision photometric observations of several globular clusters have revealed that these simple stellar populations may not be as simple as we once believed. Some globular clusters have shown interesting features in their color-magnitude diagrams indicating the presence of multiple populations. Recent HST observations of the massive globular cluster NGC 2808 have revealed the presence of three main sequences. We present new observations of NGC 2808 using the new WFC3 camera on HST. Observations are in five photometric bands covering a large wavelength range from the UVIS to the IR channels of WFC3. Moreover, these observations extend below the "turn down" in the lower main sequence (as seen in the IR CMD) and therefore reach deeper than past data sets on this cluster. We discuss various model fits and compare with other observed CMDs of other globular clusters to explore the origin of these multiple sequences. Title: Multiwavelength Observations of the Hot DB Star PG 0112+104 Authors: Dufour, P.; Desharnais, S.; Wesemael, F.; Chayer, P.; Lanz, T.; Bergeron, P.; Fontaine, G.; Beauchamp, A.; Saffer, R. A.; Kruk, J. W.; Limoges, M. -M. Bibcode: 2010ApJ...718..647D Altcode: 2010arXiv1006.0365D We present a comprehensive multiwavelength analysis of the hot DB white dwarf PG 0112+104. Our analysis relies on newly acquired FUSE observations, on medium-resolution FOS and GHRS data, on archival high-resolution GHRS observations, on optical spectrophotometry both in the blue and around Hα, as well as on time-resolved photometry. From the optical data, we derive a self-consistent effective temperature of 31,300 ± 500 K, a surface gravity of log g = 7.8 ± 0.1 (M = 0.52 M sun), and a hydrogen abundance of log N(H)/N(He)< -4.0. The FUSE spectra reveal the presence of C II and C III lines that complement the previous detection of C II transitions with the GHRS. The improved carbon abundance in this hot object is log N(C)/N(He) = -6.15 ± 0.23. No photospheric features associated with other heavy elements are detected. We reconsider the role of PG 0112+104 in the definition of the blue edge of the V777 Her instability strip in light of our high-speed photometry and contrast our results with those of previous observations carried out at the McDonald Observatory.

Based on observations with the FUSE satellite, which is operated by the Johns Hopkins University under NASA contract NAS 5-32985; 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 the NASA contract NAS 5-26555; and with the Kitt Peak National Observatory, National Optical Astronomy Observatories, operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation. Title: The Blue Hook Populations of Massive Globular Clusters Authors: Brown, Thomas M.; Sweigart, Allen V.; Lanz, Thierry; Smith, Ed; Landsman, Wayne B.; Hubeny, Ivan Bibcode: 2010ApJ...718.1332B Altcode: 2010arXiv1006.1591B We present new Hubble Space Telescope ultraviolet color-magnitude diagrams of five massive Galactic globular clusters: NGC 2419, NGC 6273, NGC 6715, NGC 6388, and NGC 6441. These observations were obtained to investigate the "blue hook" (BH) phenomenon previously observed in UV images of the globular clusters ω Cen and NGC 2808. Blue hook stars are a class of hot (approximately 35,000 K) subluminous horizontal branch stars that occupy a region of the HR diagram that is unexplained by canonical stellar evolution theory. By coupling new stellar evolution models to appropriate non-LTE synthetic spectra, we investigate various theoretical explanations for these stars. Specifically, we compare our photometry to canonical models at standard cluster abundances, canonical models with enhanced helium (consistent with cluster self-enrichment at early times), and flash-mixed models formed via a late helium-core flash on the white dwarf cooling curve. We find that flash-mixed models are required to explain the faint luminosity of the BH stars, although neither the canonical models nor the flash-mixed models can explain the range of color observed in such stars, especially those in the most metal-rich clusters. Aside from the variation in the color range, no clear trends emerge in the morphology of the BH population with respect to metallicity.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at STScI, and associated with proposal 10815. Title: Pulsed Thermal Emission from the Accreting Pulsar XMMU J054134.7-682550 Authors: Manousakis, A.; Walter, R.; Audar, M.; Lanz, T. Bibcode: 2010ASPC..424..138M Altcode: Soft X-ray excesses have been detected in several Be/X-ray binaries and interpreted as the signature of hard X-ray reprocessing in the inner accretion disk. The system XMMU J054134.7- 682550, located in the LMC, featured a giant Type II outburst in August 2007. The geometry of this system can be understood by studying the response of the soft excess emission to the hard X-ray pulses. We have analyzed series of simultaneous observations obtained with XMM-Newton/EPIC-MOS and RXTE/PCA in order to derive spectral and temporal characteristics of the system, before, during and after the giant outburst. Spectral fits were performed and a timing analysis has been carried out. Spectral variability, spin period evolution and energy dependent pulse shapes are analysed. The outburst (LX ≍ 3×1038 erg/s ∼ LEDD) spectrum could be modeled successfully using a cutoff powerlaw, a cold disk emission, a hot blackbody, and a cyclotron absorption line. The magnetic field and magnetospheric radius could be constrained. The thickness of the inner accretion disk is broadened to a width of 75 km. The hot blackbody component features sinusoidal modulations indicating that the bulk of the hard X-ray emission is emitted preferentially along the magnetic equator. The spin period of the pulsar decreased very significantly during the outburst. This is consistent with a variety of neutron star equations of state and indicates a very high accretion rate. Title: Experimental study of radiative shocks at PALS facility Authors: Stehlé, Chantal; González, Matthias; Kozlova, Michaela; Rus, Bedrich; Mocek, Tomas; Acef, Ouali; Colombier, Jean Philippe; Lanz, Thierry; Champion, Norbert; Jakubczak, Krzysztof; Polan, Jiri; Barroso, Patrice; Bauduin, Daniel; Audit, Edouard; Dostal, Jan; Stupka, Michal Bibcode: 2010LPB....28..253S Altcode: 2010arXiv1003.2739S We report on the investigation of strong radiative shocks generated with the high energy, sub-nanosecond iodine laser at PALS. These shock waves are characterized by a developed radiative precursor and their dynamics is analyzed over long time scales (~50 ns), approaching a quasi-stationary limit. We present the first preliminary results on the rear side XUV spectroscopy. These studies are relevant to the understanding of the spectroscopic signatures of accretion shocks in Classical T Tauri Stars. Title: The Determination of the Abundances of the Fe Group Elements in Early B Stars from High Resolution FUV Spectra Authors: Peters, Geraldine J.; Adelman, Saul J.; Hubeny, Ivan; Lanz, Thierry Bibcode: 2010IAUS..265..213P Altcode: We present selected results from an investigation that is currently underway to determine the abundances of the Fe group elements in early B stars and assess the extent to which contemporary NLTE and LTE models represent their atmospheres. High resolution UV and optical spectra of B stars that display ultrasharp lines are compared with computations from TLUSTY/SYNSPEC and SYNTHE. Some results from our analysis of the abundance standard ι Her (B3V) are presented here. Title: The Blue Hook Populations of Massive Globular Clusters Authors: Brown, Thomas M.; Sweigart, A. V.; Lanz, T.; Smith, E.; Landsman, W. B.; Hubeny, I. Bibcode: 2010AAS...21542516B Altcode: 2010BAAS...42..338B We present new HST ultraviolet color-magnitude diagrams of 5 massive Galactic globular clusters: NGC 2419, NGC 6273, NGC 6715, NGC 6388, and NGC 6441. &nbspThese observations were obtained in order to investigate the "blue hook" phenomenon previously observed in HST ultraviolet images of the globular clusters omega Cen and NGC 2808. &nbspBlue hook stars are a class of hot (approximately 35,000 K) subluminous horizontal branch stars that occupy a region of the HR diagram that is unexplained by canonical stellar evolution theory. &nbspBy coupling new stellar evolution models to appropriate non-LTE synthetic spectra, we investigate various theoretical explanations for these stars. &nbspSpecifically, we compare the color-magnitude diagrams to stellar models that assume normal evolution at cluster abundances, models with enhanced helium resulting from cluster self-enrichment, and flash-mixed models formed via late helium core flash on the white dwarf cooling curve. Title: Quantitative Analysis of the Spectra of Early B Stars with Ultrasharp Lines Authors: Peters, Geraldine J.; Adelman, S.; Hubeny, I.; Lanz, T. Bibcode: 2009AAS...21460512P Altcode: The extent to which contemporary NLTE and LTE models represent the atmospheres of the early B stars with the sharpest lines is discussed. Due to the low rotational broadening in these stars, line blending presents minimal problems and the intrinsic line profiles are revealed. Spectral data from the HST and FUSE spacecraft and the KPNO Coude Feed Telescope are compared with computations from the Hubeny/Lanz codes TLUSTY/SYNSPEC and the Kurucz code SYNTHE. The B stars include ι Her (B3V), HR 1886 (B1V), HR 1887 (B0.5V), and AV 304 (B0.5V) in the SMC, all of which are considered to be abundance standards. Contemporary NLTE model atmospheres represent the photospheres of early B stars remarkably well. Some lines are fit much better in NLTE even for the B3 stars, but widespread discrepancies seem to emerge for stars hotter than B0.5, which also show dynamic atmospheres. Future work need to be done on the following: 1) The determination and meaning of microturbulence and its effect on the atmosphere, 2) Further attention to the strong Si II lines, especially Multiplet 4, 3) Treatment of Ti, V, Cr, Mn, Co, and Ni in NLTE, 4) Wind models for B0-B0.5 stars.

The authors appreciate support from NASA grants NAG5-11802, NAG5-12239, NAG5-13212, and STScI grants GO-09848 & GO-06709. Title: On The Importance Of The Interclump Medium For Superionization: O VI Formation In The Wind Of ζ Pup. Authors: Zsargo, Janos; Hillier, J. D.; Bouret, J. C.; Lanz, T.; Leutenegger, M. A.; Cohen, D. H. Bibcode: 2009AAS...21340815Z Altcode: We have studied superionization and X-ray line formation in the spectra of ζ Pup using our new stellar atmosphere code (XCMFGEN) that can be used to simultaneously analyze optical, UV, and X-ray observations. Here, we present results on the formation of the O VI λλ1032, 1038 doublet. Our simulations, supported by simple theoretical calculations, show that clumped wind models that assume void in the interclump space cannot reproduce the observed O VI profiles. However, enough O VI can be produced if the voids are filled by a low density gas. The recombination of O VI is very efficient in the dense material but in the tenuous interclump region an observable amount of O VI can be maintained. We also find that different UV resonance lines are sensitive to different density regimes in ζ Pup : C IV is almost exclusively formed within the densest regions, while the majority of O VI resides between clumps. N V is an intermediate case, with contributions from both the tenuous gas and clumps.

Acknowledgments: This research was supported by STScI grant HST-AR-10693.02 and by SAO grant TM6-7003X. M. A. L. acknowledges support from a fellowship administered by Oak Ridge Associated Universities under the NASA Postdoctoral Program. J.-C. B. acknowledges financial support from the French National Research Agency (ANR) through program number ANR-06-BLAN-0105. Title: Scaling stellar jets to the laboratory: The power of simulations Authors: Stehlé, C.; Ciardi, A.; Colombier, J. -P.; González, M.; Lanz, T.; Marocchino, A.; Kozlova, M.; Rus, B. Bibcode: 2009LPB....27..709S Altcode: 2009arXiv0912.4279S Advances in laser and Z-pinch technology, coupled with the development of plasma diagnostics and the availability of high-performance computers, have recently stimulated the growth of high-energy density laboratory astrophysics. In particular a number of experiments have been designed to study radiative shocks and jets with the aim of shedding new light on physical processes linked to the ejection and accretion of mass by newly born stars. Although general scaling laws are a powerful tools to link laboratory experiments with astrophysical plasmas, the phenomena modelled are often too complicated for simple scaling to remain relevant. Nevertheless, the experiments can still give important insights into the physics of astrophysical systems and can be used to provide the basic experimental validation of numerical simulations in regimes of interest to astrophysics. We will illustrate the possible links between laboratory experiments, numerical simulations and astrophysics in the context of stellar jets. First we will discuss the propagation of stellar jets in a cross-moving interstellar medium and the scaling to Z-pinch produced jets. Our second example focuses on slab-jets produced at the PALS (Prague Asterix Laser System) laser installation and their practical applications to astrophysics. Finally, we illustrate the limitations of scaling for radiative shocks, which are found at the head of the most rapid stellar jets. Title: Quantitative Analysis of the Spectra of Early B Stars with Ultrasharp Lines Authors: Peters, Geraldine J.; Adelman, Saul J.; Hubeny, Ivan; Lanz, Thierry Bibcode: 2009AIPC.1171..345P Altcode: We present selected results from an investigation that is currently underway to determine the abundances of light and Fe group elements in early B stars and assess the extent to which contemporary NLTE and LTE models represent their atmospheres. Spectral data of B stars that display ultrasharp lines, obtained with HST, FUSE, and the KPNO Coudé Feed Telescope, are compared with computations from TLUSTY/SYNSPEC and SYNTHE. The B stars include the abundance standards ι Her (B3V), HR 1886 (B1V), and HR 1887 (B0.5V). Title: Pulsed Thermal Emission from the Accreting Pulsar XMMU J054134.7-682550 Authors: Manousakis, Antonis; Walter, Roland; Audard, Marc; Lanz, Thierry Bibcode: 2009AIPC.1126..325M Altcode: 2009arXiv0903.0580M XMMU J054134.7-682550, located in the LMC, featured a type II outburst in August 2007. We analyzed XMM-Newton (EPIC-MOS) and RXTE (PCA) data in order to derive the spectral and temporal characteristics of the system throughout the outburst. Spectral variability, spin period evolution, energy dependent pulse shape are discussed. The outburst (LX~3×1038 erg/s~LEDD) spectrum can be modeled using, cutoff power law, soft X-ray blackbody, disk emission, and cyclotron absorption line. The blackbody component shows a sinusoidal behavior, expected from hard X-ray reprocessing on the inner edge of the accretion disk. The thickness of the inner accretion disk (width of ~75 km) can be constrained. The spin-up of the pulsar during the outburst is the signature of a (huge) accretion rate. Simbol-X will provide similar capabilities as XMM-Newton and RXTE together, for such bright events. Title: Analysis of Galactic late-type O dwarfs: more constraints on the weak wind problem Authors: Marcolino, W. L. F.; Bouret, J. -C.; Martins, F.; Hillier, D. J.; Lanz, T.; Escolano, C. Bibcode: 2009A&A...498..837M Altcode: 2009arXiv0902.1833M Aims: We investigate the stellar and wind properties of a sample of late-type O dwarfs. Previous analyses of such stars have found very low mass-loss rates; rates much lower than predicted by theory (the weak wind problem).
Methods: Far-UV to optical spectra of five Galactic O stars were analyzed: HD 216898 (O9IV/O8.5V), HD 326329 (O9V), HD 66788 (O8V/O9V), ζ Oph (O9.5Vnn), and HD 216532 (O8.5V((n))). We used a grid of TLUSTY models to obtain effective temperatures, gravities, rotational velocities, and to identify wind lines. Wind parameters for each object were obtained using expanding atmosphere models calculated with the CMFGEN code.
Results: The spectra of our sample have primarily a photospheric origin. A weak wind signature is seen in C iv λλ1548, 1551, from which mass-loss rates consistent with previous CMFGEN results for O8-O9V stars were derived (∼10-10-10-9 M_⊙ yr-1). A discrepancy of roughly two orders of magnitude is found between these mass-loss rates and the values predicted by theory (dot{M}_Vink), confirming a breakdown or a steepening of the modified wind momentum-luminosity relation at log L_star/L_⊙ ⪉ 5.2. We have estimated the carbon abundance for the stars of our sample and concluded that its value cannot be reduced to sufficiently small values to solve the weak wind problem. Upper limits on dot{M} were established for all objects using lines of different ions: P v λλ1118, 1128, C iii λ 1176, N v λλ1239, 1243, Si iv λλ1394, 1403, and N iv λ1718. All the values obtained are in disagreement with theoretical predictions, bringing support to the reality of weak winds. Together with C iv λλ1548, 1551, the use of N v λλ1239, 1243 results in the lowest mass-loss rates: the upper limits indicate that dot{M} must be less than about -1.0 dex dot{M}_Vink. Upper mass-loss rate limits obtained for other transitions are also low: they indicate that dot{M} must be less than about (-0.5 ± 0.2) dex dot{M}_Vink. We studied the behavior of the Hα line with different mass-loss rates. For two stars, only models with very low dot{M}'s provide the best fit to the UV and optical spectra. We also explored ways to fit the observed spectra with the theoretical mass-loss rates. By using large amounts of X-rays, we could reduce the predicted wind emission to the observed levels. However, unrealistic X-ray luminosities had to be used (log L_X/L_Bol ⪆ -3.5). The validity of the models used in our analyses is discussed.

Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer and by the NASA-ESA-SERC International Ultraviolet Explorer , and retrieved from the Multimission Archive at the Space Telescope Science Institute (MAST). Based on observations collected with the ELODIE spectrograph on the 1.93-m telescope (Observatoire de Haute-Provence, France). Based on observations collected with the FEROS instrument on the ESO 2.2 m telescope, program 074.D-0300 and 075.D-0061. Appendices A and B are only available in electronic form at http://www.aanda.org Title: Effective area calibration of the reflection grating spectrometers of XMM-Newton. II. X-ray spectroscopy of DA white dwarfs Authors: Kaastra, J. S.; Lanz, T.; Hubeny, I.; Paerels, F. B. S. Bibcode: 2009A&A...497..311K Altcode: 2009arXiv0902.2282K Context: White dwarf spectra have been widely used as a calibration source for X-ray and EUV instruments. The in-flight effective area calibration of the reflection grating spectrometers (RGS) of XMM-Newton depend upon the availability of reliable calibration sources.
Aims: We investigate how well these white dwarf spectra can be used as standard candles at the lowest X-ray energies in order to gauge the absolute effective area scale of X-ray instruments.
Methods: We calculate a grid of model atmospheres for Sirius B and HZ 43A, and adjust the parameters using several constraints until the ratio of the spectra of both stars agrees with the ratio as observed by the low energy transmission grating spectrometer (LETGS) of Chandra. This ratio is independent of any errors in the effective area of the LETGS.
Results: We find that we can constrain the absolute X-ray spectrum of both stars with better than 5% accuracy. The best-fit model for both stars is close to a pure hydrogen atmosphere, and we put tight limits to the amount of helium or the thickness of a hydrogen layer in both stars. Our upper limit to the helium abundance in Sirius B is 4 times below the previous detection based on EUVE data. We also find that our results are sensitive to the adopted cut-off in the Lyman pseudo-continuum opacity in Sirius B. We get best agreement with a long wavelength cut-off.
Conclusions: White dwarf model atmospheres can be used to derive the effective area of X-ray spectrometers in the lowest energy band. An accuracy of 3-4% in the absolute effective area can be achieved. Title: Pulsed thermal emission from the accreting pulsar XMMU J054134.7-682550 Authors: Manousakis, A.; Walter, R.; Audard, M.; Lanz, T. Bibcode: 2009A&A...498..217M Altcode: Aims: Soft X-ray excesses have been detected in several Be/X-ray binaries and interpreted as the signature of hard X-ray reprocessing in the inner accretion disk. The system XMMU J054134.7-682550, located in the LMC, featured a giant Type II outburst in August 2007. The geometry of this system can be understood by studying the response of the soft excess emission to the hard X-ray pulses.
Methods: We have analyzed series of simultaneous observations obtained with XMM-Newton/EPIC-MOS and RXTE/PCA in order to derive spectral and temporal characteristics of the system, before, during and after the giant outburst. Spectral fits were performed and a timing analysis has been carried out. Spectral variability, spin period evolution and energy dependent pulse shapes are analysed.
Results: The outburst ({L}_X= 3× 1038 erg/s≈ {L}_EDD) spectrum could be modeled successfully using a cutoff powerlaw, a cold disk emission, a hot blackbody, and a cyclotron absorption line. The magnetic field and magnetospheric radius could be constrained. The thickness of the inner accretion disk is broadened to a width of 75 km. The hot blackbody component features sinusoidal modulations indicating that the bulk of the hard X-ray emission is emitted preferentially along the magnetic equator. The spin period of the pulsar decreased very significantly during the outburst. This is consistent with a variety of neutron star equations of state and indicates a very high accretion rate.

Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. Title: Recent Progress in Modeling Stellar Atmospheres Authors: Hubeny, I.; Lanz, T. Bibcode: 2009ASSP....7..211H Altcode: 2009nqsa.conf..211H We briefly review recent progress in modeling stellar atmospheres. We first summarize various levels of approximations being used in constructing the models, and present a brief overview of existing model atmosphere codes. We then concentrate on a description of our universal computer program Tlusty, and our recent grids of NLTE metal line blanketed model photospheres of O-stars - OSTAR2002 - and early B-stars - BSTAR2006. Title: Radiative Shocks in the Context of Young Stellar Objects: A Combined Analysis from Experiments and Simulations Authors: Stehlé, Chantal; González, Matthias; Audit, Edouard; Lanz, Thierry Bibcode: 2009ASSP...13..359S Altcode: 2009pjc..book..359S Hypersonic flows occurring during stellar formation are structured by radiation. When radiation is reabsorbed in different locations of the accretion flows or in the bow shocks of highly supersonic jets, the coupling between hydrodynamics and radiation becomes an important feature that significantly affects the hydrodynamical structure as well as spectroscopic signatures of these shocks. Radiative shocks can now be studied in the laboratory using high-energy lasers. Using recent laboratory experiment and state-of-the-art multi-dimensional radiative hydrodynamics simulations, we present an up to date description of the physical and hydrodynamical properties of radiative shocks, with an emphasis on the aspects that are important for stellar hypersonic flows. Title: The magnetic field of the supergiant star ζ Ori A Authors: Escolano, C.; Bouret, J. -C.; Donati, J. -F.; Martins, F.; Lanz, T.; Marcolino, W.; Howarth, I. Bibcode: 2008sf2a.conf..465E Altcode: We present the results obtained on the O9.7 supergiant ζ Ori with the spectropolarimeter NARVAL at the 2M Telescope Bernard Lyot atop Pic du Midi (France). We detected the presence of a weak magnetic field of about 50-100G, making ζ Ori the third O star known to host a magnetic field and the first magnetic O star with a 'normal' rotationnal velocity. The magnetic field of Zeta Ori is the weakest magnetic field ever detected on a massive star and is lower than the thermal equipartition limit (about 100 G). By fitting synthetic spectra (obtained from NLTE stellar atmosphere models), we derived the physical properties of ζ Ori. This lattest is a 40 M_{⊙} star, with a radius of 25 R_{⊙} and appears quite evolved with an age of 5-6Myr. Despite its evolutionnary status, ζ Ori does not show signs of nitrogen surface enrichment. Concerning the wind of ζ Ori, we estimated a mass loss rate of about 2×10^{-6} M_{⊙}.yr^{-1}. The magnetic topology of ζ Ori is apparently more complex than a simple dipole and involves two main magnetic polarities located on both sides of the northern hemisphere. Our data also suggest that ζ Ori rotates in about 7.0 days and is about 40 degrees away from pole-on to an Earth-based observer. Despite its weakness, the detected field appears sufficient to affect significantly the wind structure: the corresponding Alfvén radius is however very close to the surface of the star, thus generating a rotational modulation in wind lines different than that reported on the two other known magnetic O stars.

Finally, the rapid rotation of ζ Ori with respect to θ^{1} Ori C is surprising since both stars have similar unsigned magnetic fluxes (once rescaled to the same radius). This may indicate that the field of ζ Ori is not a fossil remnant (as opposed to that of θ^{1} Ori C and HD191612) but rather the result of exotic dynamo processes produced through MHD instabilities. Title: The Gaia satellite: a tool for Emission Line Stars and Hot Stars Authors: Martayan, C.; Frémat, Y.; Blomme, R.; Jonckheere, A.; Borges, M.; de Batz, B.; Leroy, B.; Sordo, R.; Bouret, J. -C.; Martins, F.; Zorec, J.; Neiner, C.; Nazé, Y.; Alecian, E.; Floquet, M.; Hubert, A. -M.; Briot, D.; Miroshnichenko, A.; Kolka, I.; Stee, P.; Lanz, T.; Meynet, G. Bibcode: 2008sf2a.conf..499M Altcode: 2008arXiv0809.4417M The Gaia satellite will be launched at the end of 2011. It will observe at least 1 billion stars, and among them several million emission line stars and hot stars. Gaia will provide parallaxes for each star and spectra for stars till V magnitude equal to 17. After a general description of Gaia, we present the codes and methods, which are currently developed by our team. They will provide automatically the astrophysical parameters and spectral classification for the hot and emission line stars in the Milky Way and other close local group galaxies such as the Magellanic Clouds. Title: On the Importance of the Interclump Medium for Superionization: O VI Formation in the Wind of ζ Puppis Authors: Zsargó, J.; Hillier, D. J.; Bouret, J. -C.; Lanz, T.; Leutenegger, M. A.; Cohen, D. H. Bibcode: 2008ApJ...685L.149Z Altcode: 2008arXiv0808.2233Z We have studied superionization and X-ray line formation in the spectra of ζ Pup using our new stellar atmosphere code (XCMFGEN) that can be used to simultaneously analyze optical, UV, and X-ray observations. Here, we present results on the formation of the O VI λλ1032, 1038 doublet. Our simulations, supported by simple theoretical calculations, show that clumped wind models that assume void in the interclump space cannot reproduce the observed O VI profiles. However, enough O VI can be produced if the voids are filled by a low-density gas. The recombination of O VI is very efficient in the dense material, but in the tenuous interclump region an observable amount of O VI can be maintained. We also find that different UV resonance lines are sensitive to different density regimes in ζ Pup: C IV is almost exclusively formed within the densest regions, while the majority of O VI resides between clumps. N V is an intermediate case, with contributions from both the tenuous gas and clumps. Title: The weak magnetic field of the O9.7 supergiant ζOrionisA Authors: Bouret, J. -C.; Donati, J. -F.; Martins, F.; Escolano, C.; Marcolino, W.; Lanz, T.; Howarth, I. D. Bibcode: 2008MNRAS.389...75B Altcode: 2008arXiv0806.2162B; 2008MNRAS.tmp..842B We report here the detection of a weak magnetic field of 50-100G on the O9.7 supergiant ζOrionisA (ζOriA), using spectropolarimetric observations obtained with NARVAL at the 2-m Télescope Bernard Lyot atop Pic du Midi (France). ζOriA is the third O star known to host a magnetic field (along with θ1OriC and HD191612), and the first detection on a `normal' rapidly rotating O star. The magnetic field of ζOriA is the weakest magnetic field ever detected on a massive star. The measured field is lower than the thermal equipartition limit (about 100G). By fitting non-local thermodynamic equilibrium (NLTE) model atmospheres to our spectra, we determined that ζOriA is a 40Msolar star with a radius of 25Rsolar and an age of about 5-6Myr, showing no surface nitrogen enhancement and losing mass at a rate of about 2 × 10-6Msolaryr-1.

The magnetic topology of ζOriA is apparently more complex than a dipole and involves two main magnetic polarities located on both sides of the same hemisphere; our data also suggest that ζOriA rotates in about 7.0d and is about 40° away from pole-on to an Earth-based observer. Despite its weakness, the detected magnetic field significantly affects the wind structure; the corresponding Alfvén radius is however very close to the surface, thus generating a different rotational modulation in wind lines than that reported on the two other known magnetic O stars.

The rapid rotation of ζOriA with respect to θ1OriC appears as a surprise, both stars having similar unsigned magnetic fluxes (once rescaled to the same radius); it may suggest that the subequipartition field detected on ζOriA is not a fossil remnant (as opposed to that of θ1 OriC and HD191612), but the result of an exotic dynamo action produced through magnetohydrodynamics (MHD) instabilities.

Based on observations obtained at the Télescope Bernard Lyot (TBL), operated by the Institut National des Science de l'Univers of the Centre National de la Recherche Scientifique of France.

E-mail: jean-claude.bouret@oamp.fr Title: CNO abundance determination in massive fast rotating stars Authors: Frémat, Y.; Zorec, J.; Martayan, C.; Lanz, T. Bibcode: 2008RMxAC..33...53F Altcode: Be stars are early-type Main Sequence objects which have shown at least once some emission in the hydrogen line series. Their most important characteristic is fast rotation, which on average reach ∼ 90% of the critical rate, i.e. Ω/Ω_{c} %∼ 0.90. Since a number of them have masses M > 15 M_⊙, they are ideal targets to study the effects of rotational mixing on the atmospheric CNO abundance and to perform comparisons with the predictions obtained from recent evolutionary models with rotation. Title: The spectra of massive stars with Gaia Authors: Bouret, J. -C.; Lanz, T.; Frémat, Y.; Martins, F.; Lefever, K.; Blomme, R.; Martayan, C.; Neiner, C.; Quinet, P.; Zorec, J. Bibcode: 2008RMxAC..33...50B Altcode: ESA's cornerstone mission Gaia is scheduled to be launched in late 2011. It will determine the positions and distances of more than 24 million stars within 3 kpc of the Sun with a 1% relative accuracy. About 150,000 O and B-type stars are expected in the whole sample, as well as additional peculiar stars such as Be, B[e], and WR stars. Gaia will also provide spectrophotometry over the spectral range 330-1150 nm (RP/BP bands) and medium-resolution spectroscopy in the domain ranging from 847 to 874 nm (RVS spectral range). A large effort is now under way to develop the necessary software aiming at automatically derive the astrophysical parameters and radial velocities. One component of this development is the computation of grids of synthetic spectra for different types of stars and, in particular, for massive stars. Title: Pulsed thermal emission in the accreting pulsar HMXB XMMU J054134.7-682550 reveals accretion geometry Authors: Manousakis, Antonia; Walter, Roland; Audard, Marc; Lanz, Thierry Bibcode: 2008xru..confE..51M Altcode: No abstract at ADS Title: Argon Abundances in the Solar Neighborhood: Non-LTE Analysis of Orion Association B-Type Stars Authors: Lanz, Thierry; Cunha, Katia; Holtzman, Jon; Hubeny, Ivan Bibcode: 2008ApJ...678.1342L Altcode: 2007arXiv0709.2147L Argon abundances have been derived for a sample of B main-sequence stars in the Orion association. The abundance calculations are based on non-LTE (NLTE) metal line-blanketed model atmospheres calculated with the NLTE code TLUSTY and an updated and complete argon model atom. We derive an average argon abundance for this young population of A(Ar) = 6.66 +/- 0.06. While our result is in excellent agreement with a recent analysis of the Orion Nebula, it is significantly higher than the currently recommended solar value, which is based on abundance measurements in the solar corona. Moreover, the derived argon abundances in the Orion B stars agree very well with a measurement from a solar impulsive flare during which unmodified solar photospheric material was brought to flare conditions. We therefore argue that the argon abundances obtained independently for both the Orion B stars and the Orion Nebula are representative of the disk abundance value in the solar neighborhood. The lower coronal abundance may reflect a depletion related to the first ionization potential (FIP) effect. We propose a new reference value for the abundance of argon in the solar neighborhood, A(Ar) = 6.63 +/- 0.10, corresponding to Ar/O = 0.009 +/- 0.002.

Based on observations obtained with the Apache Point Observatory 3.5 m telescope, which is owned and operated by the Astrophysical Research Consortium. Title: Clumping in O-type Supergiants Authors: Bouret, J. -C.; Lanz, T.; Hillier, D. J.; Foellmi, C. Bibcode: 2008cihw.conf...31B Altcode: We have analyzed the spectra of seven Galactic O4 supergiants, with the NLTE wind code CMFGEN. For all stars, we have found that clumped wind models match well lines from different species spanning a wavelength range from FUV to optical, and remain consistent with Hα data. We have achieved an excellent match of the P V λλ1118, 1128 resonance doublet and N IV λ1718, as well as He II λ4686 suggesting that our physical description of clumping is adequate. We find very small volume filling factors and that clumping starts deep in the wind, near the sonic point. The most crucial consequence of our analysis is that the mass loss rates of O stars need to be revised downward significantly, by a factor of 3 and more compared to those obtained from smooth-wind models. Title: Orbital Parameters and Chemical Composition of Four White Dwarfs in Post-Common-Envelope Binaries Authors: Kawka, Adela; Vennes, Stéphane; Dupuis, Jean; Chayer, Pierre; Lanz, Thierry Bibcode: 2008ApJ...675.1518K Altcode: 2007arXiv0711.1526K We present FUSE observations of the hot white dwarfs in the post-common-envelope binaries Feige 24, EUVE J0720-317, BPM 6502, and EUVE J2013+400. The spectra show numerous photospheric absorption lines, which trace the white dwarf orbital motion. We report the detection of C III, O VI, P V, and Si IV in the spectra of Feige 24, EUVE J0720-317, and EUVE J2013+400 and the detection of C III, N II, Si III, Si IV, and Fe III in the spectra of BPM 6502. Abundance measurements support the possibility that white dwarfs in post-common-envelope binaries accrete material from the secondary star wind. The FUSE observations of BPM 6502 and EUVE J2013+400 cover a complete binary orbit. We used the FUSE spectra to measure the radial velocities traced by the white dwarf in the four binaries, where the zero-point velocities were fixed using the ISM velocities in the line of sight of the stellar systems. For BPM 6502 we determined a white dwarf velocity semiamplitude of KWD = 18.6 +/- 0.5 km s-1, and with the velocity semiamplitude of the red dwarf companion (KRD = 75.2 +/- 3.1 km s-1), we estimate the mass ratio to be q = 0.25 +/- 0.01. Adopting a spectroscopic mass determination for the white dwarf, we infer a low secondary mass of MRD = 0.14 +/- 0.01 M⊙. For EUVE J2013+400 we determine a white dwarf velocity semiamplitude of KWD = 36.7 +/- 0.7 km s-1. The FUSE observations of EUVE J0720-317 cover approximately 30% of the binary period and, combined with the HST GHRS measurements, we update the binary properties. FUSE observations of Feige 24 cover approximately 60% of the orbit, and we combine this data set with HST STIS data to update the binary properties. Title: White Dwarf Spectra and Calibration of X-ray Grating Spectrometers Authors: Kaastra, Jelle S.; Lanz, T.; Hubeny, I.; Paerels, F. Bibcode: 2008HEAD...10.0406K Altcode: White dwarf spectra have been widely used as calibration sources for X-ray and EUV instruments. The in-flight effective area calibration of the RGS of XMM-Newton and of the LETGS of Chandra depend upon the availability of reliable calibration sources. We calculate a grid of model atmospheres for Sirius B and HZ 43A, and adjust the parameters using several constraints until the ratio of the spectra of both stars agrees with the ratio observed with the LETGS of Chandra. This ratio is independent of any errors in the effective area of the LETGS. We determine how accurately the effective area of the LETGS is determined using our method, and find interesting constraints on the parameters for both stars. We discuss the role of the Lyman pseudo-continuum in the calculation of the spectrum of Sirius B. The treatment of that pseudo-continuum appears to play a fundamental role in the ultimate accuracy that can be reached. With the proper treatment of the pseudo-continuum, the soft X-ray flux of both stars and thereby the absolute effective area of the LETGS can be determined with an uncertainty of less than 5 %. Title: CAL 83 detected again in X-rays, dimming in optical Authors: Lanz, T.; Audard, M.; Walter, F. Bibcode: 2008ATel.1438....1L Altcode: We report a detection of CAL 83 with the Swift X-ray telescope during a short exposure (490 sec) started on 2008 March 20 at 10:27 UT, with a count rate of about 0.024 ct/s, corresponding to about 15% of the normal X-ray flux of CAL 83. The latest X-ray off-state of CAL 83 was discovered with Swift on 2008 Jan 2 (Greiner et al. 2008, ATel #1357), following an increase of brightness in the optical a week earlier. Title: Testing the Evolution of Young Stars with FU Orionis Objects Authors: Audard, Marc; Baldovin Saavedra, Carla; Carmona, Andres; Lanz, Thierry Bibcode: 2008sptz.prop50654A Altcode: FU Orionis objects (FUors) play a central role in the study of the evolution of young stars. They undergo accretion outbursts that can bring up to about 0.01 solar masses per outburst, therefore providing a significant amount of mass in the pre-main sequence life of a star. Spitzer IRS observations of FUors have shown either silicate features in absorption and ice bands or silicates in emission with no ice bands. This led some authors to propose an evolutionary paradigm in which the former FUors are younger than the latter FUors. The FUor phase might be a common but rarely observed phase (about 20 FUors known) of most young low-mass stars and might be the link between Class I and Class II sources. So far, Spitzer has obtained IRS spectra of about half of the FUor catalog, focusing on its most famous class members. With this short Spitzer proposal, we aim to observe FUors that have not yet been observed with the IRS or had no high-resolution module spectra. We will study the composition and evolution of dust grains and of ices in FUors surrounded by envelopes, and we aim at detecting faint molecular hydrogen and atomic lines with the high-resolution module spectra. The IRS spectra will probe the stellar environment, and thus test the above evolutionary paradigm and the different theories for the origin of FUor outburst. The Spitzer IRS observations of our sample of FUors will therefore provide additional insights about a significant phase in the life of young stars with the aim to better describe the evolutionary sequence from Class I star to Class II sources. Title: New Views of the Horizontal Branch in ω Centauri Authors: Moehler, S.; Dreizler, S.; Lanz, T.; Bono, G.; Sweigart, A. V.; Calamida, A.; Monelli, M.; Nonino, M. Bibcode: 2008ASPC..392...47M Altcode: UV observations of some massive globular clusters uncovered a significant population of very hot stars below the hot end of the horizontal branch, the so-called blue hook stars. This feature might be explained either by the late hot flasher scenario where stars experience the helium flash while on the white dwarf cooling curve or by the helium-rich sub-population recently postulated to exist in some clusters. Previous spectroscopic analyses of blue hook stars in ω Cen and NGC 2808 support the late hot flasher scenario, but the stars were found to contain much less helium than expected and the predicted carbon and nitrogen enrichment could not be verified. New moderately high resolution spectra of stars at the hot end of the blue horizontal branch in ω Cen were analysed for atmospheric parameters ( Teff, log g, and log{nHe}/{nH}) and abundances using LTE and non-LTE model atmospheres. In the temperature range 30,000 K to 50,000 K we find that 35% of our stars are helium-poor (log{nHe/{nH} < -2), 51% have solar helium abundance within a factor of 3 (-1.5 ≤ log{nHe}/{nH} ≤ -0.5) and 14% are helium-rich (log{nHe/{nH} > -0.4). We also find carbon enrichment along with helium enrichment, with a maximum carbon abundance of 3% by mass. At least 14% of the hottest horizontal branch stars in ωCen show helium abundances well above the highest predictions from the helium enrichment scenario (Y≈ 0.42, corresponding to log{nHe}/{nH} ≈ -0.74). In addition, the most helium-rich stars show high carbon abundances as predicted by the late hot flasher scenario. We conclude that the helium-rich horizontal branch stars in ω Cen cannot be explained solely by the helium-enrichment scenario invoked to explain the blue main sequence. Title: The hottest horizontal-branch stars in ω Centauri. Late hot flasher vs. helium enrichment Authors: Moehler, S.; Dreizler, S.; Lanz, T.; Bono, G.; Sweigart, A. V.; Calamida, A.; Monelli, M.; Nonino, M. Bibcode: 2007A&A...475L...5M Altcode: 2007arXiv0709.2839M Context: UV observations of some massive globular clusters have revealed a significant population of very hot stars below the hot end of the horizontal branch (HB), the so-called blue hook stars. This feature might be explained either as a result of the late hot flasher scenario where stars experience the helium flash while on the white dwarf cooling curve or by the progeny of the helium-enriched sub-population recently postulated to exist in some clusters. Previous spectroscopic analyses of blue hook stars in ω Cen and NGC 2808 support the late hot flasher scenario, but the stars contain much less helium than expected and the predicted C, N enrichment could not be verified.
Aims: We compare observed effective temperatures, surface gravities, and abundances of He, C, and N of blue hook and canonical extreme horizontal branch (EHB) star candidates to the predictions of the two scenarios.
Methods: Moderately high resolution spectra of stars at the hot end of the blue HB in the globular cluster ω Cen were analysed for atmospheric parameters and abundances using LTE and non-LTE model atmospheres.
Results: In the temperature range 30 000 K to 50 000 K we find that 35% of our stars are helium-poor (log{{n_He}/{n_H}} < -2), 51% have solar helium abundance within a factor of 3 (-1.5 ⪉ log{{n_He}/{n_H}} ⪉ -0.5) and 14% are helium-rich (log{{n_He}/{n_H}}} > -0.4). We also find carbon enrichment strongly correlated with helium enrichment, with a maximum carbon enrichment of 3% by mass.
Conclusions: At least 14% of the hottest HB stars in ω Cen show helium abundances well above the highest predictions from the helium enrichment scenario (Y = 0.42 corresponding to log{{n_He}/{n_H}} ≈ -0.74). In addition, the most helium-rich stars show strong carbon enrichment, as predicted by the late hot flasher scenario. We conclude that the helium-rich HB stars in ω Cen cannot be explained solely by the helium-enrichment scenario invoked to explain the blue main sequence.

Based on observations

with the ESO Very Large Telescope at Paranal Observatory, Chile

(proposal IDs 075.D-0280(A) and 077.D-0021(A)). Title: VizieR Online Data Catalog: VLT-FLAMES survey of massive stars (Hunter+, 2007) Authors: Hunter, I.; Dufton, P. L.; Smartt, S. J.; Ryans, R. S. I.; Evans, C. J.; Lennon, D. J.; Trundle, C.; Hubeny, I.; Lanz, T. Bibcode: 2007yCat..34660277H Altcode: Files table3.dat, table4.dat and table5.dat contain the equivalent widths and abundances estimates of the observed metal lines for B-type stars in NGC 6611, N11 and NGC 346 respectively. The equivalent widths of the spectral lines have been measured using the line fitting program ELF in the spectral analysis package DIPSO (Howarth et al., 1994, Starlink User Note, No. 50.15). The nitrogen equivalent widths and abundances are upper limits for objects N11-037, N11-047, N11-101, NGC 346-029, NGC 346-039, NGC 346-040, NGC 346-043, NGC 346-044 and NGC 346-075.

table6.dat contains details of the adopted atomic data for the absorption lines considered in this analysis. Model atoms are discussed in Lanz & Hubeny (2003ApJS..146..417L) and Allende Prieto et al. (2003ApJS..147..363A), see Dufton et al. (2005, Cat. ) for further details. Note, the loggf values of the Si II lines at 4128 and 4131 angstroms have been updated to include improved atomic data (A. Korn, private communication).

(5 data files). Title: The VLT-FLAMES survey of massive stars: surface chemical compositions of B-type stars in the Magellanic Clouds Authors: Hunter, I.; Dufton, P. L.; Smartt, S. J.; Ryans, R. S. I.; Evans, C. J.; Lennon, D. J.; Trundle, C.; Hubeny, I.; Lanz, T. Bibcode: 2007A&A...466..277H Altcode: 2006astro.ph..9710H We present an analysis of high-resolution FLAMES spectra of approximately 50 early B-type stars in three young clusters at different metallicities, NGC 6611 in the Galaxy, N 11 in the Large Magellanic Cloud (LMC) and NGC 346 in the Small Magellanic Cloud (SMC). Using the tlusty non-LTE model atmospheres code, atmospheric parameters and photospheric abundances (C, N, O, Mg and Si) of each star have been determined. These results represent a significant improvement on the number of Magellanic Cloud B-type stars with detailed and homogeneous estimates of their atmospheric parameters and chemical compositions. The relationships between effective temperature and spectral type are discussed for all three metallicity regimes, with the effective temperature for a given spectral type increasing as one moves to a lower metallicity regime. Additionally the difficulties in estimating the microturbulent velocity and the anomalous values obtained, particularly in the lowest metallicity regime, are discussed. Our chemical composition estimates are compared with previous studies, both stellar and interstellar with, in general, encouraging agreement being found. Abundances in the Magellanic Clouds relative to the Galaxy are discussed and we also present our best estimates of the base-line chemical composition of the LMC and SMC as derived from B-type stars. Additionally we discuss the use of nitrogen as a probe of the evolutionary history of stars, investigating the roles of rotational mixing, mass-loss, blue loops and binarity on the observed nitrogen abundances and making comparisons with stellar evolutionary models where possible.

Based on observations at the European Southern Observatory in programmes 171.0237 and 073.0234. Tables 3-6 and Figs. 7-31 are only available in electronic form at http://www.aanda.org Title: A Grid of NLTE Line-blanketed Model Atmospheres of Early B-Type Stars Authors: Lanz, Thierry; Hubeny, Ivan Bibcode: 2007ApJS..169...83L Altcode: 2006astro.ph.11891L We have constructed a comprehensive grid of 1540 metal line-blanketed, NLTE, plane-parallel, hydrostatic model atmospheres for the basic parameters appropriate to early B-type stars. The BSTAR2006 grid considers 16 values of effective temperatures, 15,000 K<=Teff<=30,000 K with 1000 K steps, 13 surface gravities, 1.75<=logg<=4.75 with 0.25 dex steps, six chemical compositions, and a microturbulent velocity of 2 km s-1. The lower limit of logg for a given effective temperature is set by an approximate location of the Eddington limit. The selected chemical compositions range from twice to one-tenth of the solar metallicity and metal-free. Additional model atmospheres for B supergiants (logg<=3.0) have been calculated with a higher microturbulent velocity (10 km s-1) and a surface composition that is enriched in helium and nitrogen and depleted in carbon. This new grid complements our earlier OSTAR2002 grid of O-type stars (our Paper I). The paper contains a description of the BSTAR2006 grid and some illustrative examples and comparisons. NLTE ionization fractions, bolometric corrections, radiative accelerations, and effective gravities are obtained over the parameter range covered by the grid. By extrapolating radiative accelerations, we have determined an improved estimate of the Eddington limit in absence of rotation between 55,000 and 15,000 K. The complete BSTAR2006 grid is available at the TLUSTY Web site. Title: A Mid-Infrared Spitzer Study of the Herbig Be Star R Mon and the Associated HH 39 Herbig-Haro Object Authors: Audard, M.; Skinner, S.; Guedel, M.; Lanz, T.; Paerels, F.; Arce, H. Bibcode: 2007astro.ph..1906A Altcode: We report on initial results of our Spitzer Cycle 2 program to observe the young massive star R Mon and its associated HH 39 Herbig-Haro object in the mid-infrared. Our program used all instruments on-board Spitzer to obtain deep images with IRAC of the HH 39 complex and of R Mon and its surroundings, a deep image of HH 39 at 24 and 70 $\mu$m with MIPS, and mid-infrared spectra with the SH, LH, and LL modules of IRS. The aim of this program is to study the physical links in a young massive star between accretion disk, outflows and jets, and sh ocks in the associated HH object. Our preliminary analysis reveals that several knots of HH 39 are clearly detected in most IRAC bands. In IRAC4 (8 $\mu$m), diffuse emission, probably from PAHs, appears as foreground emission covering the HH 39 emission. The HH 39 knots are detected at 24 microns, despite the fact that dust continuum emission covers the knots and shows the same structure as observed with IRAC4. The IRS spectra of HH 39 show weak evidence of [Ne II] 12.8 $\mu$m and 0--0 S(1) H$_2$ 17.0 $\mu$m lines. A more detailed analysis is, however, required due to the faintness of the Herbig-Haro knots. Finally, we obtained the SH and MIPS SED spectra of R Mon. A PAH emission feature at 11.3 $\mu$m is detected on top of the strong continuum; although no strong emission or absorption lines are observed, we will seek to detect faint lines. The combined IRAC, IRS, and MIPS data of the R Mon/HH 39 system will help us to understand circumstellar disk processing, and the connection between jets, outflows, and HH objects. Title: Iron in Hot DA White Dwarfs Authors: Vennes, Stéphane; Chayer, Pierre; Dupuis, Jean; Lanz, Thierry Bibcode: 2006ApJ...652.1554V Altcode: 2006astro.ph..8416V We present a study of the iron abundance pattern in hot, hydrogen-rich (DA) white dwarfs. The study is based on new and archival far-ultraviolet spectroscopy of a sample of white dwarfs in the temperature range 30,000 K<~Teff<~64,000 K. The spectra obtained with the Far Ultraviolet Spectroscopic Explorer, along with spectra obtained with the Hubble Space Telescope Imaging Spectrograph and the International Ultraviolet Explorer, sample Fe III-Fe VI absorption lines, enabling a detailed iron abundance analysis over a wider range of effective temperatures than previously afforded. The measurements reveal abundance variations in excess of 2 orders of magnitude between the highest and the lowest temperatures probed, but also show considerable variations (over 1 order of magnitude) between objects with similar temperatures and surface gravities. Such variations in cooler objects may be imputed to accretion from unseen companions or so-called circumstellar debris, although the effect of residual mass loss and selective radiation pressure in the hottest objects in the sample remain dominant. Title: Modeling multidimensional effects in the propagation of radiative shocks Authors: Leygnac, Sébastien; Boireau, Laurent; Michaut, Claire; Lanz, Thierry; Stehlé, Chantal; Clique, Christine; Bouquet, Serge Bibcode: 2006PhPl...13k3301L Altcode: 2006astro.ph.10876L Radiative shocks (also called supercritical shocks) are high Mach number shock waves that photoionize the medium ahead of the shock front and give rise to a radiative precursor. They are generated in the laboratory using high-energy or high-power lasers and are frequently present in a wide range of astronomical objects. Their modelization in one dimension has been the subject of numerous studies, but generalization to three dimensions is not straightforward. We calculate analytically the absorption of radiation in a gray uniform cylinder and show how it decreases with χR, the product of the opacity χ and of the cylinder radius R. Simple formulas, whose validity range increases when χR diminishes, are derived for the radiation field on the axis of symmetry. Numerical calculations in three dimensions of the radiative energy density, flux, and pressure created by a stationary shock wave show how the radiation decreases with R. Finally, the bidimensional structures of both the precursor and the radiation field are calculated with time-dependent radiation hydrodynamics numerical simulations and the influence of two-dimensional effects on the electron density, the temperature, the shock velocity, and the shock geometry are exhibited. These simulations show how the radiative precursor shortens, cools, and slows down when R is decreased. Title: The Origin of CAL 83 X-Ray Off-States Authors: Lanz, Thierry Bibcode: 2006xmm..prop...26L Altcode: The close binary supersoft X-ray source CAL 83 is a most promising candidate of SN Ia progenitors. Our XMM-Newton and Chandra observations show a massive white dwarf, exhibit large amplitude short-term variations of the X-ray flux, and establish definitively its recurrent nature. We propose now to conduct a campaign with EPIC and OM on XMM-Newton to substantiate the correlation between X-ray and optical variations on a short, medium, and long-term basis. This campaign will trace the physical relation between the accretion disk and the white dwarf, and will discriminate between several models explaining the origin of CAL 83 off-states. XMM-Newton data will offer unique and timely insights into accretion and unstable nuclear burning during the last evolution stages of SN Ia progenitors. Title: On the sensitivity of He I singlet lines to the Fe IV model atom in O stars Authors: Najarro, F.; Hillier, D. J.; Puls, J.; Lanz, T.; Martins, F. Bibcode: 2006A&A...456..659N Altcode: 2006astro.ph..5211N Aims.Recent calculations and analyses of O star spectra have revealed discrepancies between theory and observations, and between different theoretical calculations, for the strength of optical He I singlet transitions. We investigate the source of these discrepancies.
Methods: .Using a non-LTE radiative transfer code we have undertaken detailed test calculations for a range of O star properties. Our principal test model has parameters similar to those of the O9V star, 10 Lac.
Results: .We show that the discrepancies arise from uncertainties in the radiation field in the He I 1s2 ^1S-1s 2p ^1P° transition near 584 Å. The radiation field at 584 Å is influenced by model assumptions, such as the treatment of line-blanketing and the adopted turbulent velocity, and by the Fe IV atomic data. It is shown that two Fe IV transitions near 584 Å can have a substantial influence on the strength of the He I singlet transitions.
Conclusions: .Because of the difficulty of modeling the He I singlet lines, particularly in stars with solar metalicity, the He I triplet lines should be preferred in spectral analyses. These lines are much less sensitive to model assumptions. Title: Supersoft Sources at Low Metallicity: SMC 13 Authors: Lanz, Thierry Bibcode: 2006cxo..prop.2120L Altcode: The identification of SN Ia progenitors remains elusive. A promising channel leading to a SN Ia event involves accreting white dwarfs (WD) that sustain steady nuclear burning. They are observed as close binary X-ray sources with a supersoft spectrum (CBSS). We propose to obtain the first Chandra LETGS spectrum of SMC 13, a CBSS in the SMC. From absorption line spectroscopy, we will achieve significant advances in determing the properties of SMC 13 and provide unique data to constrain new evolutionary calculations of accreting hot white dwarfs. At low metallicities, the population of SN Ia progenitors shifts toward systems with higher initial WD masses and lower mass donor stars. This difference might turn out to be crucial to understanding the physics of local vs. distant SN Ia. Title: High Resolution Fe Lyman and Balmer Band Spectroscopy of the Bursts in EXO 0748-676 Authors: Cottam, Jean; Paerels, F.; Telis, G.; Audard, M.; Mendez, M.; Boirin, L.; Lanz, T.; Lewin, W.; Marshall, H.; Bildsten, L.; Chang, P. Bibcode: 2006HEAD....9.0205C Altcode: 2006BAAS...38..345C We present the results of high resolution spectroscopy of the x-ray bursts in the LMXB EXO 0748-676. Early observations with XMM/RGS showed evidence for gravitationally redshifted absorption lines, which can be identified with the n=2-3 transitions of H- and He-like Fe arising in the photosphere of the neutron star at z=0.35. We will discuss the results from a 600 ks follow-up observation with the XMM/RGS, and from a 300 ks observation with the Chandra/HETGS. Title: Neon Abundances in B Stars of the Orion Association: Solving the Solar Model Problem? Authors: Cunha, Katia; Hubeny, Ivan; Lanz, Thierry Bibcode: 2006ApJ...647L.143C Altcode: 2006astro.ph..6738C We report on non-LTE Ne abundances for a sample of B-type stellar members of the Orion association. The abundances were derived by means of non-LTE fully metal-blanketed model atmospheres and extensive model atoms with updated atomic data. We find that these young stars have a very homogeneous abundance of A(Ne)=8.11+/-0.04. This abundance is higher by ~0.3 dex than the currently adopted solar value, A(Ne)=7.84, which is derived from lines produced in the corona and active regions. The general agreement between the abundances of C, N, and O derived for B stars with the solar abundances of these elements derived from three-dimensional hydrodynamical models atmospheres strongly suggests that the abundance patterns of the light elements in the Sun and B stars are broadly similar. If this hypothesis is true, then the Ne abundance derived here will help to reconcile solar models with helioseismological observations. Title: VizieR Online Data Catalog: SMC NGC 346-11 and AV 304 spectra (Hunter+, 2005) Authors: Hunter, I.; Dufton, P. L.; Ryans, R. S. I.; Lennon, D. J.; Rolleston, W. R. J.; Hubeny, I.; Lanz, T. Bibcode: 2006yCat..34360687H Altcode: High-resolution spectra have been obtained for both NGC 346-11 and AV 304 using the Ultraviolet and Visual Echelle Spectrograph, UVES, on the UT2 (Kueyen) telescope at the European Southern Observatory. NGC 346-11 was observed during a three night run in November 2001, whilst the observations of AV 304, taken during a two night run in January 2001, have been previously discussed by Rolleston et al. (2003A&A...400...21R).

(1 data file). Title: Hydrogen Column Density Measurements toward White Dwarfs for D/H Studies Authors: Vennes, S.; Chayer, P.; Dupuis, J.; Moos, H. W.; Lanz, T. Bibcode: 2006ASPC..348..460V Altcode: Hot white dwarfs provide sources of extreme ultraviolet (EUV) radiation against which the photoionization continua of interstellar H I, He I, and He II are measured. The column density measurements depend on a detailed knowledge of the intrinsic EUV luminosity of the sources. We discuss recent progress in the study of low- and high-metallicity white dwarfs enabling a study of interstellar hydrogen and helium continuum absorptions toward many lines of sight. We compare the EUV-based measurements to recent hydrogen column density measurements based on Lyα line profiles and discuss the implications for D/H and O/H ratios in the local interstellar medium. Title: FUSE Spectroscopy of Hybrid White Dwarfs Authors: Lanz, T.; Holberg, J. B.; Hubeny, I. Bibcode: 2006ASPC..348..197L Altcode: We have obtained FUSE spectra of three white dwarfs showing hybrid helium and hydrogen spectra. We present initial results of our NLTE model atmosphere analysis, and discuss their implication for understanding the origin of DAB white dwarfs and the physical processes in white dwarf atmospheres. Title: High Resolution Fe Lyman and Balmer Band Spectroscopy of the Bursts in EXO 0748-676 Authors: Cottam, J.; Paerels, F. B. S.; Telis, G. A.; Mendez, M.; Boirin, L.; Audard, M.; Lanz, T.; Lewin, W. H. G.; Marshall, H. L.; Bildsten, L.; Chang, P. Bibcode: 2006AAS...207.3208C Altcode: We present the results of high resolution spectroscopy of the X-ray bursts in the LMXB EXO 0749-676. Early observations with XMM/RGS showed evidence for gravitationally redshifted absorption lines, which can be identified with the n=2-3 transitions in H- and He-like Fe arising in the stellar photosphere at z=0.35. We will discuss the results of spectroscopy of the Fe Lyman band with Chandra/HETGS, and a reobservation in the Fe Balmer band with XMM/RGS. Title: The Density Structure of O Star Winds Authors: Lanz, Thierry; Audard, Marc; Bouret, Jean-Claude; Hillier, John Bibcode: 2006sptz.prop30030L Altcode: Because of their dense hypersonic winds, O stars have a great influence on their environment by the deposition of mechanical energy and momentum, as well as by their copious emission of ionizing photons. There is a mounting evidence that O stars have highly-structured winds, ranging from X-ray wind emission, line profile variability, and from detailed NLTE analyses of UV wind line profiles. Substantially reduced mass loss rates are derived from simple clumped wind models compared to the classical analyses performed with smooth, homogeneous wind models. Mid-infrared recombination lines and free-free continuum are particularly sensitive to the density at the base of flow and thus allow us to investigate the region where clumping is believed to be initiated. Moreover, the IR lines are not as sensitive to the wind ionization as UV lines. We propose therefore to obtain IRS high resolution spectroscopy and MIPS photometry of a sample of O dwarfs and supergiants to empirically characterize the density structure of O star winds, and derive robust estimates of the mass loss rates. Because the ionizing spectrum of O stars is very sensitive to the wind properties, the Spitzer data will be an essential step towards a reliable prediction of ionizing fluxes of massive stars. Title: Fundamental Properties of O-Type Stars Authors: Heap, Sara R.; Lanz, Thierry; Hubeny, Ivan Bibcode: 2006ApJ...638..409H Altcode: 2004astro.ph.12345H We present a comprehensive analysis of high-resolution, far-ultraviolet HST STIS, FUSE, and optical spectra of 18 O stars in the Small Magellanic Cloud. Our analysis is based on the OSTAR2002 grid of NLTE metal-line-blanketed model atmospheres calculated with our code TLUSTY. We systematically explore and present the sensitivity of various UV and optical lines to different stellar parameters. We have obtained consistent fits of the UV and the optical spectrum to derive the effective temperature, surface gravity, surface composition, and microturbulent velocity of each star. Stellar radii, masses, and luminosities follow directly. For stars of the same spectral subtype, we find a general good agreement between effective temperature determinations obtained with TLUSTY, CMFGEN, and FASTWIND models, which are all lower than the standard Teff calibration of O stars. We propose a new calibration between the spectral type and effective temperature based on our results from UV metal lines, as well as optical hydrogen and helium lines. The lower effective temperatures translate into ionizing luminosities that are smaller by a factor of 3 compared to luminosities inferred from previous standard calibrations. The chemical composition analysis reveals that the surface of about 80% of the program stars is moderately to strongly enriched in nitrogen, while showing the original helium, carbon, and oxygen abundances. Our results support the new stellar evolution models that predict that the surface of fast rotating stars becomes nitrogen-rich during the main-sequence phase because of rotationally induced mixing. Enrichment factors are, however, larger than predicted by stellar evolution models. Most stars exhibit the ``mass discrepancy'' problem, which we interpret as a result of fast rotation that lowers the measured effective gravity. Nitrogen enrichment and low spectroscopic masses are therefore two manifestations of fast rotation. Our study thus emphasizes the importance of rotation in our understanding of the properties of massive stars and provides a framework for investigating populations of low-metallicity massive stars at low and high redshifts.

Based on observations 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 NAS5-2655. Also based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer, which is operated for NASA by Johns Hopkins University under NASA contract NAS5-32985. Title: Lower Mass Loss Rates in O Stars Authors: Bouret, J. -C.; Lanz, T.; Hillier, D. J. Bibcode: 2005AAS...20718223B Altcode: 2005BAAS...37.1467B We have analyzed the far-UV and optical spectra of 5 Galactic O4 supergiants with the NLTE wind code CMFGEN. Stellar and wind parameters, and the surface composition of the stars have been derived. The surface composition for all 5 stars is typical of an evolved O supergiant (nitrogen-rich, carbon and oxygen-poor), and reflects the appearance of CNO processed material at the stellar surface. For all stars only highly-clumped wind models match key lines of different species in the far-UV and optical spectrum, in particular the \ion{P}{5} resonance doublet, \ion{N}{4}1718, and Hα . Compared to homogeneous wind models, models with clumping exhibit increased recombination in the clumps and provide a better description of the wind ionization structure. We also achieve a good fit to \ion{He}{2}4686 (especially the electron scattering wings) with clumping. The sharp transition from absorption to emission in the profile of UV wind lines indicate that clumping starts deep in the wind, just above the sonic point. The most crucial consequence of wind clumping is that the mass loss rates of O stars need to be revised downward significantly, by a factor of 3 and more. Similarly, we also recently derived lower mass loss rates for a few O-type dwarfs. Our study therefore calls for a fundamental revision in our understanding of mass loss and of O-type star stellar winds.

Support from the NASA Astrophysics Data Program (grant NNG04GC81G) is gratefully acknowledged. Title: Non-LTE Line-Blanketed Model Atmospheres of B-type Stars Authors: Lanz, T.; Hubeny, I. Bibcode: 2005AAS...20718221L Altcode: 2005BAAS...37.1467L We present an extension of our OSTAR2002 grid of NLTE model atmospheres to B-type stars. We have calculated over 1,300 metal line-blanketed, NLTE, plane-parallel, hydrostatic model atmospheres for the basic parameters appropriate to B stars. The grid covers 16 effective temperatures from 15,000 to 30,000 K, with 1000 K steps, 13 surface gravities, log g≤ 4.75 down to the Eddington limit, and 5 compositions (2, 1, 0.5, 0.2, and 0.1 times solar). We have adopted a microturbulent velocity of 2 km/s for all models. In the lower surface gravity range (log g≤ 3.0), we supplemented the main grid with additional model atmospheres accounting for higher microtutbulent velocity (10 km/s) and for alterated surface composition (He and N-rich, C-deficient), as observed in B supergiants. The models incorporate basically all known atomic levels of 46 ions of H, He, C, N, O, Ne, Mg, Al, Si, S, and Fe, which are grouped into 1127 superlevels. Models and spectra will be available at our Web site, http://nova.astro.umd.edu. Title: A Spectroscopic Analysis of Blue Stragglers, Horizontal Branch Stars, and Turnoff Stars in Four Globular Clusters Authors: De Marco, Orsola; Shara, Michael M.; Zurek, D.; Ouellette, John A.; Lanz, Thierry; Saffer, Rex A.; Sepinsky, Jeremy F. Bibcode: 2005ApJ...632..894D Altcode: 2005astro.ph.11497D We present a spectroscopic analysis of HST STIS and FOS low- and intermediate-resolution spectroscopy of 55 stars in four globular clusters (47 Tucanae, M3, NGC 6752, and NGC 6397). Stars hotter than Teff=5750 K and with a signal-to-noise ratio larger than 15 were analyzed with non-local thermodynamic equilibrium model atmospheres, and values for their effective temperatures and gravities were obtained. Using photometric fluxes, we also obtained radii, luminosities, and spectroscopic masses.

Twenty-four stars in our sample are blue stragglers (BSs). Their photometric colors and magnitudes place these BSs above and redward of the clusters' zero-age main sequence: this is consistent with the gravities we find for these stars, which are lower than zero-age main-sequence gravities. A comparison with stellar evolutionary tracks shows that almost all of our BSs are in the Hertzsprung gap. This is contrary to theory, because of the short timescale expected for stars in this evolutionary phase.

The mean BS mass is 1.04 Msolar for 14 nonvariable stars, or 1.07 Msolar counting all 24 BSs in our sample. For the nonvariable stars the mean BS masses for individual clusters are 1.73, 1.01, 0.95, and 0.72 Msolar for NGC 6397, NGC 6752, 47 Tuc, and M3, respectively. Adding the variable stars (which improves the statistics but increases the uncertainty), the mean masses become 1.27, 1.05, 0.99, and 0.99 Msolar, respectively. Although there is considerable scatter, the BS spectroscopic masses correlate with both effective temperature and brightness of the stars, as expected.

The mean nonvariable turnoff star mass (0.58 Msolar) is significantly below the values determined for the BSs and below the main-sequence turnoff mass. The mean nonvariable horizontal-branch (HB) star mass is higher than expected (0.79 Msolar). In particular, several HB stars have masses well above the main-sequence turnoff mass. Some of these HB stars are suspected of actually being BSs, since most of them reside at ambiguous locations on the CMD, making them prone to misclassification.

Values and limits to the stellar rotation rates (vsini) are imposed by fitting weak metal lines, the Ca II K line wings, or the helium lines for the hotter stars. Five BSs with reasonably constrained rotations show average and median vsini values of 109 and 100 km s-1, respectively, suggesting v~160 km s-1. At least some GC BSs are very rapid rotators, but this information cannot yet constrain their origin as stellar collision or binary mergers because of the lack of clear theoretical predictions. Six extreme HB stars have rotation rates vsini between 50 and 200 km s-1, which are high for these stars and might indicate a binary origin.

De Marco et al. found that four BSs and two HB stars in our sample have Balmer jumps that are too large for the effective temperatures implied by the slopes of their Paschen continua. Two additional HB stars are now identified in the current study as having the same feature. For these stars, the presence of a disk of partly ionized material is suspected, although high stellar rotation rates could also partly explain the data. Title: The Supersoft X-Ray Source CAL 83: A Massive White Dwarf Authors: Lanz, Thierry; Audard, Marc; Paerels, Frits; Telis, Gisela A. Bibcode: 2005AIPC..797..313L Altcode: We have obtained Chandra HRC-S/LETG spectroscopy of the prototypical supersoft source CAL 83 in the Large Magellanic Cloud. The data reveal a very rich absorption line spectrum from the hot white dwarf photosphere. We have recently completed the analysis of the Chandra spectrum and of an earlier XMM-Newton RGS spectrum of CAL 83 with new non-LTE line-blanketed model atmospheres that explicitly include 74 ions of the 11 most abundant species. We have successfully matched the Chandra and XMM-Newton spectra, and have thus derived the basic stellar parameters of the hot white dwarf. In particular, we have obtained the first direct spectroscopic evidence that the white dwarf is massive (M > 1Msolar). We also found no spectral signatures of a wind from the white dwarf. These results provide direct support for supersoft sources as likely progenitors of SN Ia. Title: Characterizing Type Ia Supernova Progenitor Candidates Authors: Lanz, Thierry Bibcode: 2005cxo..prop.4038L Altcode: The identification of SN Ia progenitors still remains elusive. The most promising channel to form SN Ia involves accreting white dwarfs that sustain steady nuclear burning. They are observed as ultrasoft X-ray sources. Our NLTE analysis of the LETGS spectrum of the supersoft source CAL 83 gives the first direct spectroscopic evidence of a massive WD. We need now to show that massive WDs are common in supersoft sources. We propose to perform a similar NLTE model atmosphere analysis of the 5 other supersoft sources that have been observed with Chandra. We will derive the properties of the WDs and of the binary systems, thus constraining the evolution of Close Binary Supersoft X-ray Sources. We will explore the role of nova-like winds in CBSS for understanding the WD evolution. Title: Lyman Line Spectroscopy of Hot Hydrogen-Rich White Dwarfs Revisited Authors: Vennes, S.; Chayer, P.; Dupuis, J.; Lanz, T. Bibcode: 2005ASPC..334..185V Altcode: We present a study of 40 hot DA white dwarfs observed with the Far Ultraviolet Spectroscopic Explorer (FUSE). We measured the effective temperatures and surface gravities using a grid of pure-hydrogen model atmospheres in local thermodynamic equilibrium (LTE). Systematic differences with similar measurements based on Balmer line spectroscopy are in part due to non-LTE departure in the hydrogen atom, but for the most part they are due to the presence of heavy-elements in many white dwarf atmospheres throughout the range of effective temperatures covered in our sample. Title: Lower mass loss rates in O-type stars: Spectral signatures of dense clumps in the wind of two Galactic O4 stars Authors: Bouret, J. -C.; Lanz, T.; Hillier, D. J. Bibcode: 2005A&A...438..301B Altcode: 2004astro.ph.12346B We have analyzed the far-ultraviolet spectrum of two Galactic O4 stars, the O4If+ supergiant HD 190429A and the O4V((f)) dwarf HD 96715, using archival FUSE and IUE data. We have conducted a quantitative analysis using the two NLTE model atmosphere and wind codes, tlusty and cmfgen, which incorporate a detailed treatment of NLTE metal line blanketing. From the far-UV spectrum, we have derived the stellar and wind parameters and the surface composition of the two stars. The surface of HD 190429A has a composition typical of an evolved O supergiant (nitrogen-rich, carbon and oxygen-poor), while HD 96715 exhibits surface nitrogen enhancement similar to the enrichment found in SMC O dwarfs which has been attributed to rotationally-induced mixing. Following studies of Magellanic Cloud O stars, we find that homogeneous wind models could not match the observed profile of O vλ1371 and require very low phosphorus abundance to fit the P vλλ1118-1128 resonance lines. We show, on the other hand, that we are able to match the O v and P v lines using clumped wind models. In addition to these lines, we find that N ivλ1718 is also sensitive to wind clumping. For both stars, we have calculated clumped wind models that match well all these lines from different species and that remain consistent with Hα data. In particular, we have achieved an excellent match of the P v resonance doublet, indicating that our physical description of clumping is adequate. These fits therefore provide a coherent and thus much stronger evidence of wind clumping in O stars than earlier claims. We show that the success of the clumped wind models in matching these lines results from increased recombination in the clumps, hence from a better description of the wind ionization structure. We find that the wind of these two stars is highly clumped, as expressed by very small volume filling factors, namely f_∞ = 0.04 for HD 190429A and f_∞ = 0.02 for HD 96715. In agreement with our analysis of SMC stars, clumping starts deep in the wind, just above the sonic point. The most crucial consequence of our analysis is that the mass loss rates of O stars need to be revised downward significantly, by a factor of 3 and more. These lower mass loss rates will affect substantially the evolution of massive stars. Accounting for wind clumping is essential when determining the wind properties of O stars. Our study therefore calls for a fundamental revision in our understanding of mass loss and of O-type star stellar winds. Title: Searching for the Donor Star in Close Binary Supersoft Sources Authors: Lanz, Thierry; Audard, Marc; Hubeny, Ivan; Paerels, Frits Bibcode: 2005sptz.prop20130L Altcode: The identification of Type Ia Supernova progenitors still remains elusive. The most promising channel to form SNe Ia involves accreting white dwarfs (WDs) in close binary systems. The hot WDs sustain steady nuclear burning and are observed as ultrasoft X-ray sources. The companions have remained until now undetected because the UV and optical spectrum is dominated by the emission from the accretion disk around the WDs. At present, these close binary supersoft sources (CBSS) have not been observed in the infrared, with the single exception of near-IR photometry of the Galactic CBSS QR And that provides the first hint of the secondary. Because of the general lack of a direct detection, the nature of the donor stars in these systems is still debated. By combining ultraviolet and optical SEDs with Spitzer observations of 4 CBSS in the mid-infrared, we will be able to definitively identify the nature of the donor stars in CBSS systems, as well as determining the size of the accretion disks. We propose to observe eclipsing systems at different phases to disentangle the contribution of the companions from the emission of the disks. Spitzer observations of CBSS will therefore represent a significant step forward in characterizing the binary systems that are best candidates for becoming SNe Ia. Title: Shocks and Outflows in Young Massive Stars: The R Mon / HH 39 System Authors: Audard, Marc; Guedel, Manuel; Lanz, Thierry; Paerels, Frederik; Skinner, Stephen Bibcode: 2005sptz.prop20034A Altcode: We propose to use Spitzer's sensitive imaging and spectroscopic capabilities to undertake a comprehensive infrared study of the massive young star R Mon and its associated Herbig-Haro object HH 39. We will reach unprecendeted sensitivities with IRAC to search for mid-IR emission from R Mon's circumstellar disk, and from its powerful jets and outflow. We will obtain the first deep mid-IR image of the HH 39 complex located 7' North of R Mon with IRAC and MIPS. We will search for mid and far-IR emission from shocked material, similar to that recently detected by IRAC in HH 46/47. With IRS, we will obtain the first mid-IR spectra of R Mon's bipolar jet and of several emission knots in HH 39. We will also derive the low-resolution SED of R Mon with MIPS. Despite its spectacular nature, R Mon has received little attention in the mid-IR and was never observed by ISO. This short Spitzer proposal aims at studying in young massive stars the physical links between accretion disks, outflows and jets, and shocks in the associated HH objects. The latter can help us understand the importance of the mechanical energy input of young stars for the origin of turbulence in molecular clouds. The R Mon/HH 39 system is ideal to fulfill our goals. Title: A non-LTE analysis of the spectra of two narrow lined main sequence stars in the SMC Authors: Hunter, I.; Dufton, P. L.; Ryans, R. S. I.; Lennon, D. J.; Rolleston, W. R. J.; Hubeny, I.; Lanz, T. Bibcode: 2005A&A...436..687H Altcode: 2005astro.ph..3662H An analysis of high-resolution VLT/UVES spectra of two B-type main sequence stars, NGC 346-11 and AV 304, in the Small Magellanic Cloud (SMC), has been undertaken, using the non-LTE tlusty model atmospheres to derive the stellar parameters and chemical compositions of each star. The chemical compositions of the two stars are in reasonable agreement. Moreover, our stellar analysis agrees well with earlier analyses of H II regions. The results derived here should be representative of the current base-line chemical composition of the SMC interstellar medium as derived from B-type stars. Title: B-type supergiants in the SMC: Chemical compositions and comparison of static and unified models Authors: Dufton, P. L.; Ryans, R. S. I.; Trundle, C.; Lennon, D. J.; Hubeny, I.; Lanz, T.; Allende Prieto, C. Bibcode: 2005A&A...434.1125D Altcode: 2004astro.ph.12367D High-resolution UCLES/AAT spectra are presented for nine B-type supergiants in the SMC, chosen on the basis that they may show varying amounts of nuclear-synthetically processed material mixed to their surface. These spectra have been analysed using a new grid of approximately 12 000 non-LTE line blanketed tlusty model atmospheres to estimate atmospheric parameters and chemical composition. The abundance estimates for O, Mg and Si are in excellent agreement with those deduced from other studies, whilst the low estimate for C may reflect the use of the C II doublet at 4267 Å. The N estimates are approximately an order of magnitude greater than those found in unevolved B-type stars or H II regions but are consistent with the other estimates in AB-type supergiants. These results have been combined with results from a unified model atmosphere analysis of UVES/VLT spectra of B-type supergiants (Trundle et al. 2004, A&A, 417, 217) to discuss the evolutionary status of these objects. For two stars that are in common with those discussed by Trundle et al., we have undertaken a careful comparison in order to try to understand the relative importance of the different uncertainties present in such analyses, including observational errors and the use of static or unified models. We find that even for these relatively luminous supergiants, tlusty models yield atmospheric parameters and chemical compositions similar to those deduced from the unified code fastwind. Title: VizieR Online Data Catalog: B-type Supergiants in the SMC (Dufton+, 2005) Authors: Dufton, P. L.; Ryans, R. S. I.; Trundle, C.; Lennon, D. J.; Hubeny, I.; Lanz, T.; Allende Prieto, C. Bibcode: 2005yCat..34341125D Altcode: Table 4 contains the adopted atomic data, equivalent widths and abundance estimates for all the metal lines observed in the SMC supergiants. It also contains data for the SMC near main sequence star AzV 304, which has been used in a differential analysis.

(2 data files). Title: Constraining Galaxy Formation Epoch Authors: Yi, S.; Brown, T.; Heap, S.; Hubeny, I.; Landsman, W.; Lanz, T.; Sweigart, A. Bibcode: 2005IAUS..201..536Y Altcode: 2000IAUS..201E.108Y The galaxy formation epoch (zf) is an important and unique product of cosmological models, and thus much effort has been made to constrain it. The most direct way of constraining zf is to estimate the ages of bright elliptical galaxies at high redshifts arrucately, but it has not been an easy task. We have finally reached the stage where the stellar ages can be determined within 15% uncertainty. Using this advantage, the recent spectral analysis of LBDS 53W091 by Spinrad and his collaborators (1997) has suggested that this red galaxy at z=1.552 is at least 3.5 Gyr old. This provides a significant constraint to cosmology, i.e., zf > 6.5, as LBDS 53W091 is the most distant passiviely evolving galaxy whose UV spectrum is available. The results, based on our population synthesis models, however, suggest a much smaller age for this galaxy, i.e., 1.5 - 2 Gyr. The large discrepancy between their age estimate and ours originates from the difference in the model integrated spectra: their model spectra are significantly bluer than ours. Such a large difference in the models for an apparently simple population is unreasonable. A simple test seems to favor our models. If we adopt our age estimate, the continuum of this galaxy suggests zf ≈ 2 - 3. This anaysis, when performed to a larger sample, would efficiently constrain the global galaxy formation epoch. Title: Non-LTE Model Atmosphere Analysis of the Large Magellanic Cloud Supersoft X-Ray Source CAL 83 Authors: Lanz, Thierry; Telis, Gisela A.; Audard, Marc; Paerels, Frits; Rasmussen, Andrew P.; Hubeny, Ivan Bibcode: 2005ApJ...619..517L Altcode: 2004astro.ph.10093L We present a non-LTE (NLTE) model atmosphere analysis of Chandra High Resolution Camera (HRC-S) and Low Energy Transmission Grating (LETG) and XMM-Newton Reflection Grating Spectrometer (RGS) spectroscopy of the prototypical supersoft source CAL 83 in the Large Magellanic Cloud. Taken with a 16 month interval, the Chandra and XMM-Newton spectra are very similar. They reveal a very rich absorption-line spectrum from the hot white dwarf photosphere but no spectral signatures of a wind. We also report a third X-ray off-state during a later Chandra observation, demonstrating the recurrent nature of CAL 83. Moreover, we found evidence of short-timescale variability in the soft X-ray spectrum. We completed the analysis of the LETG and RGS spectra of CAL 83 with new NLTE line-blanketed model atmospheres that explicitly include 74 ions of the 11 most abundant species. We successfully matched the Chandra and XMM-Newton spectra assuming a model composition with LMC metallicity. We derived the basic stellar parameters of the hot white dwarf, but the current state of atomic data in the soft X-ray domain precludes a detailed chemical analysis. We have obtained the first direct spectroscopic evidence that the white dwarf is massive (MWD>~1 Msolar). The short timescale of the X-ray off-states is consistent with a high white dwarf mass. Our analysis thus provides direct support for supersoft sources as likely progenitors of Type Ia supernovae (SNe Ia). Title: Mid-Ultraviolet Spectral Diagnostics of Galaxy Evolution Authors: Heap, Sara R.; Lanz, Thierry Bibcode: 2005mmgf.conf..210H Altcode: We describe how optical spectra of z=1-2 galaxies can help test the hypothesis of hierarchical galaxy formation. Title: Blue Straggler Stars in 4 Globular Clusters: Masses, Rotation Rates and Disks Authors: De Marco, O.; Zurek, D.; Ouellette, J. A.; Lanz, T.; Shara, M. M.; Saffer, R. A.; Sepinsky, J. F. Bibcode: 2004AAS...20515905D Altcode: 2004BAAS...36.1611D We present an analysis of optical HST/STIS and HST/FOS spectroscopy of blue stragglers found in the globular clusters 47Tuc, M3, NGC6752 and NGC6397. Spectroscopic masses, and rotation rates are presented which impose constraints on their evolutionary history. 6 blue stragglers in our sample cannot be fit with state of the art non-LTE stellar atmosphere models. The 6 misfits possess Balmer jumps which are too large for the effective temperatures implied by their Paschen continua. We find that our data for these stars are consistent with models only if we account for extra absorption of stellar Balmer photons by an ionized circumstellar disk. Column densities of HI and CaII are derived as are the the disks' thicknesses. This is the first time that a circumstellar disk is detected around blue stragglers. The presence of magnetically-locked disks attached to the stars has been suggested as a mechanism to lose the large angular momentum imparted by the collision event at the birth of these stars. The disks implied by our study might not be massive enough to constitute such an angular momentum sink, but they could be the leftovers of once larger disks. Title: NLTE Analysis of Chandra and XMM-Newton Spectroscopy of CAL 83 Authors: Lanz, T.; Telis, G. A.; Audard, M.; Paerels, F.; Hubeny, I. Bibcode: 2004HEAD....8.1703L Altcode: 2004BAAS...36..936L We present a new NLTE model atmosphere analysis of Chandra HRC-S/LETG and XMM-Newton RGS spectroscopy of the supersoft source CAL 83 in the LMC. The two spectra are very similar, and reveal a very rich absorption line spectrum from the hot white dwarf photosphere. We also report a third X-ray off-state during a later Chandra observation, showing the recurrent nature of CAL 83. Moreover, we found evidence of short-timescale variability in the soft X-ray spectrum. The Chandra and XMM-Newton spectra have been successfully matched with our new NLTE line-blanketed model atmospheres that explicitly include 74 ions of the 11 most abundant species. We derived the basic stellar parameters of the hot white dwarf, obtaining the first direct spectroscopic evidence that the white dwarf is massive. The short timescale of the X-ray off-states is consistent with a high WD mass. Our analysis, therefore, provides direct support for supersoft sources as likely progenitors of SN Ia.

This work was supported by a grant from the NASA Astrophysics Theory Program. The Columbia group acknowledges support from NASA to Columbia University for XMM-Newton mission support and data analysis Title: Chandra High Resolution Spectroscopy of the Burst Spectrum of EXO 0748-67 Authors: Telis, G.; Paerels, F.; Audard, M.; Lanz, T.; Cottam, J.; Mendez, M.; Bildsten, L.; Chang, P.; Marshall, H. Bibcode: 2004HEAD....8.2503T Altcode: 2004BAAS...36..954T We have observed EXO0748-67 for approximately 300 ksec with the High Energy Transmission Grating Spectrometer on Chandra. A total of 35 Type I X-ray bursts occurred during our observation, and from these we obtained a composite burst spectrum with high sensitivity in the Fe K band. Along with the spectrum of GS1826-238, this is the highest sensitivity short-wavelength photospheric spectrum of a neutron star ever obtained. The total summed spectrum does not appear to exhibit a clear, narrow Fe Ly alpha absorption line; a provisional upper limit on the equivalent width of an unresolved feature is approximately 5 mA (11 eV). We present the spectroscopy and briefly discuss the physical implications of our analysis in the context of attempts to measure the fundamental properties of neutron stars by photospheric spectroscopic techniques. Analysis of the broad-band characteristics of the short-wavelength spectrum suggests that the majority of the bursts shows significant photospheric radius expansion. We will present the results of a spectroscopic analysis performed on subsets of the data selected by the apparent state of the photosphere, to allow for the fact that any narrow Fe absorption spectrum strongly depends on the physical parameters of the atmosphere.

This research was supported by NASA, through Chandra grant GO4-5045A. Title: NGC 1535 : UV Observations and Models Authors: Koesterke, L.; Werner, K.; Kruk, J. W.; Lanz, T. Bibcode: 2004ASPC..313..123K Altcode: 2004apnw.conf..123K; 2004astro.ph..2049K We re-investigate the UV spectrum of NGC 1535 by means of recently developed fully line-blanketed non-LTE models. These new models account for the wind in spherical geometry while handling the atomic data in a very similar way to the Tlusty code. This approach ensures at the same time realistic predictions of the photospheric absorption lines and of the emission lines formed in the wind. Our analysis confirms the results of previous studies. We derive T*=70 kK, dot M =10-7.8 M⊙/yr}, and v∞=2000 km/s. Title: Stellar activity and magnetism studied by optical interferometry Authors: Rousselet-Perraut, K.; Stehlé, C.; Lanz, T.; Le Bouquin, J. B.; Boudoyen, T.; Kilbinger, M.; Kochukhov, O.; Jankov, S. Bibcode: 2004A&A...422..193R Altcode: By means of numerical simulations, we investigate the ability of optical interferometry, via the fringe phase observable, to address stellar activity and magnetism. To derive abundance maps and stellar rotation axes, we use color differential interferometry which couples high angular resolution to high spectral resolution. To constrain magnetic field topologies, we add to this spectro-interferometer a polarimetric mode. Two cases of well-known Chemically Peculiar (CP) stars (βCrB and α2CVn) are simulated to derive instrumental requirements to obtain 2D-maps of abundance inhomogeneities and magnetic fields. We conclude that the near-infrared instrument AMBER of the VLTI will allow us to locate abundance inhomogeneities of CP stars larger than a fraction of milliarcsecond whereas the polarimetric mode of the French GI2T/REGAIN interferometer would permit one to disentangle various magnetic field topologies on CP stars. We emphasize the crucial need for developing and validating inversion algorithms so that future instruments on optical aperture synthesis arrays can be optimally used. Title: Effects of Rotation in Low-Metallicity Stars Authors: Heap, S. R.; Lanz, T. Bibcode: 2004IAUS..215..220H Altcode: No abstract at ADS Title: Evidence for Flash Mixing in He-rich sdB Stars Authors: Sweigart, Allen V.; Lanz, Thierry; Brown, Thomas M.; Hubeny, Ivan; Landsman, Wayne B. Bibcode: 2004Ap&SS.291..367S Altcode: 2003astro.ph..8517S We present FUSE spectra of three He-rich sdB stars. Two of these stars, PG1544+488 and JL87, reveal extremely strong C III lines, suggesting that they have mixed triple-α carbon from the deep interior out to their surfaces. Using TLUSTY NLTE line-blanketed model atmospheres, we find that PG1544+488 has a surface composition of 96% He, 2% C, and 1% N. JL87 shows a similar surface enrichment of C and N but still retains a significant amount of hydrogen. In contrast, the third star, LB1766, is devoid of hydrogen and strongly depleted of carbon, indicating that its surface material has undergone CN-cycle processing. Title: First Evidence of Circumstellar Disks around Blue Straggler Stars Authors: De Marco, Orsola; Lanz, Thierry; Ouellette, John A.; Zurek, David; Shara, Michael M. Bibcode: 2004ApJ...606L.151D Altcode: 2004astro.ph..4065D We present an analysis of optical Hubble Space Telescope Space Telescope Imaging Spectrograph and Faint Object Spectrograph spectroscopy of six blue stragglers found in the globular clusters M3, NGC 6752, and NGC 6397. These stars are a subsample of a set of ~50 blue stragglers and stars above the main-sequence turnoff in four globular clusters that will be presented in a forthcoming paper. All but the six stars presented here can be well fitted with non-LTE model atmospheres. The six misfits, on the other hand, possess Balmer jumps that are too large for the effective temperatures implied by their Paschen continua. We find that our data for these stars are consistent with models only if we account for extra absorption of stellar Balmer photons by an ionized circumstellar disk. Column densities of H I and Ca II are derived as are the the disks' thicknesses. This is the first time that a circumstellar disk is detected around blue stragglers. The presence of magnetically locked disks attached to the stars has been suggested as a mechanism to lose the large angular momentum imparted by the collision event at the birth of these stars. The disks implied by our study might not be massive enough to constitute such an angular momentum sink, but they could be the leftovers of once larger disks.

Based on observations 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 NAS5-2655. Title: Flash Mixing on the White Dwarf Cooling Curve: Far Ultraviolet Spectroscopic Explorer Observations of Three He-rich sdB Stars Authors: Lanz, Thierry; Brown, Thomas M.; Sweigart, Allen V.; Hubeny, Ivan; Landsman, Wayne B. Bibcode: 2004ApJ...602..342L Altcode: 2003astro.ph..8440L We present Far-Ultraviolet Spectroscopic Explorer (FUSE) spectra of three He-rich sdB stars. Two of these stars, PG 1544+488 and JL 87, reveal extremely strong C III lines at 977 and 1176 Å, while the carbon lines are quite weak in the third star, LB 1766. We have analyzed the FUSE data using TLUSTY non-LTE line-blanketed model atmospheres and find that PG 1544+488 has a surface composition of 96% He, 2% C, and 1% N. JL 87 shows a similar surface enrichment of carbon and nitrogen, but some significant fraction of hydrogen still remains in its atmosphere. Finally, LB 1766 has a surface composition devoid of hydrogen and strongly depleted of carbon, indicating that its surface material has undergone CN-cycle processing. We interpret these observations with new evolutionary calculations which suggest that He-rich sdB stars with C-rich compositions are the progeny of stars which underwent a delayed helium-core flash on the white-dwarf cooling curve. During such a flash the interior convection zone will penetrate into the hydrogen envelope, thereby mixing the envelope with the He- and C-rich core. Such ``flash-mixed'' stars will arrive on the extreme horizontal branch (EHB) with He- and C-rich surface compositions and will be hotter than the hottest canonical (i.e., unmixed) EHB stars. Two types of flash mixing are possible: ``deep'' and ``shallow,'' depending on whether the hydrogen envelope is mixed deeply into the site of the helium flash or only with the outer layers of the core. Based on both their stellar parameters and surface compositions, we suggest that PG 1544+488 and JL 87 are examples of ``deep'' and ``shallow'' flash mixing, respectively. Flash mixing may therefore represent a new evolutionary channel for producing the hottest EHB stars. However, flash mixing cannot explain the abundance pattern in LB 1766, which remains a challenge to current evolutionary models.

Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. FUSE is operated for NASA by the Johns Hopkins University under NASA contract NAS5-32985. Title: STIS Coronagraphic Observations of β Pictoris Authors: Heap, S. R.; Lindler, D. J.; Lanz, T. M. Bibcode: 2004IAUS..202..338H Altcode: No abstract at ADS Title: Quantitative Spectroscopy of O Stars at Low Metallicity: O Dwarfs in NGC 346 Authors: Bouret, J. -C.; Lanz, T.; Hillier, D. J.; Heap, S. R.; Hubeny, I.; Lennon, D. J.; Smith, L. J.; Evans, C. J. Bibcode: 2003ApJ...595.1182B Altcode: 2003astro.ph..1454B We present the results of a detailed analysis of the properties of dwarf O-type stars in a metal-poor environment. High-resolution, high-quality ultraviolet and optical spectra of six O-type stars in the H II region NGC 346 have been obtained from a spectroscopic survey of O stars in the SMC. Stellar parameters and chemical abundances have been determined using non-LTE (NLTE) line-blanketed photospheric models calculated with TLUSTY. Additionally, we have modeled the spectra with the NLTE line-blanketed wind code CMFGEN to derive wind parameters. Stellar parameters, chemical abundances, and in particular iron abundances obtained with the two NLTE codes compare quite favorably. This consistency demonstrates that basic photospheric parameters of main-sequence O stars can be reliably determined using NLTE static model atmospheres. With the two NLTE codes, we need to introduce a microturbulent velocity to match the observed spectra. Our results hint at a decrease of the required microturbulent velocity from a value close to the sonic velocity in early O stars to a low value in late O stars. As in several recent studies of Galactic, LMC, and SMC stars, we derive effective temperatures lower than predicted from the widely used relation between spectral type and Teff, resulting in lower stellar luminosities and lower ionizing fluxes. From evolutionary tracks in the H-R diagram, we find the age 3×106 yr for NGC 346. A majority of the stars in our sample reveal CNO cycle-processed material at their surface during the main-sequence stage, thus indicating fast stellar rotation and/or very efficient mixing processes. We obtain an overall metallicity Z=0.2Zsolar, in good agreement with other recent analyses of SMC stars. We study the dependence of the mass-loss rate on the stellar metallicity and find a satisfactory agreement with recent theoretical predictions for the three most luminous stars of the sample. The wind momentum-luminosity relation for our sample stars derived for these stars agrees with previous studies. However, the three other stars of our sample reveal very weak signatures of mass loss. We obtain mass-loss rates that are significantly lower than 10-8 Msolar yr-1, below the predictions of radiative line-driven wind theory by an order of magnitude or more. Furthermore, evidence of clumping in the wind of main-sequence O stars is provided by O V λ1371. As in previous studies of O star winds, we are unable to reproduce this line with homogeneous-wind models, but we have achieved very good fits with clumped models. Clumped-wind models systematically yield lower mass-loss rates than theoretical predictions.

Based on observations 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 5-2655. Also based on observations obtained at the European Southern Observatory (La Silla) and at the Anglo-Australian Observatory (Siding Spring). Title: Non-LTE Model Atmospheres for Late-Type Stars. I. A Collection of Data for Light Neutral and Singly Ionized Atoms Authors: Allende Prieto, Carlos; Lambert, David L.; Hubeny, Ivan; Lanz, Thierry Bibcode: 2003ApJS..147..363A Altcode: 2003astro.ph..3559A; 2003astro.ph..3559P With the goal of producing a reliable set of model atoms and singly ionized ions for use in building NLTE model atmospheres, we have combined measured energy levels, critically compiled line transition probabilities, and resonance-averaged calculations of photoionization cross sections.

A majority of the elements from Li to Ca are considered, covering most of the important species in late-type atmospheres. These include elements that contribute free electrons and/or continuous opacity in the ultraviolet (e.g., Mg and Si), as well as trace elements whose abundance determinations rely on ultraviolet lines (e.g., B from B I lines). The new data complement and, for the species in common, supersede a previous collection of model atoms originally designed for use in studies of early-type stars. Title: Erratum: ``A Grid of Non-LTE Line-blanketed Model Atmospheres of O-Type Stars'' (ApJS, 146, 417 [2003]) Authors: Lanz, Thierry; Hubeny, Ivan Bibcode: 2003ApJS..147..225L Altcode: We have constructed a comprehensive grid of 680 metal line-blanketed, non-LTE, plane-parallel, hydrostatic model atmospheres for the basic parameters appropriate to O-type stars. The OSTAR2002 grid considers 12 values of effective temperatures, 27,500K<=Teff<=55,000 K with 2500 K steps, eight surface gravities, 3.0<=logg<=4.75 with 0.25 dex steps, and 10 chemical compositions, from metal-rich relative to the Sun to metal-free. The lower limit of logg for a given effective temperature is set by an approximate location of the Eddington limit. The selected chemical compositions have been chosen to cover a number of typical environments of massive stars: the Galactic center, the Magellanic Clouds, blue compact dwarf galaxies like I Zw 18, and galaxies at high redshifts. The paper contains a description of the OSTAR2002 grid and some illustrative examples and comparisons. The complete OSTAR2002 grid is available at our Web site at ApJS, 146, 417 [2003].

Laboratory for Astronomy and Solar Physics, NASA Goddard Space Flight Center, Code 681, Greenbelt, MD 20771. Title: A Grid of Non-LTE Line-blanketed Model Atmospheres of O-Type Stars Authors: Lanz, Thierry; Hubeny, Ivan Bibcode: 2003ApJS..146..417L Altcode: 2002astro.ph.10157L We have constructed a comprehensive grid of 680 metal line-blanketed, non-LTE, plane-parallel, hydrostatic model atmospheres for the basic parameters appropriate to O-type stars. The OSTAR2002 grid considers 12 values of effective temperatures, 27,500K<=Teff<=55,000 K with 2500 K steps, eight surface gravities, 3.0<=logg<=4.75 with 0.25 dex steps, and 10 chemical compositions, from metal-rich relative to the Sun to metal-free. The lower limit of logg for a given effective temperature is set by an approximate location of the Eddington limit. The selected chemical compositions have been chosen to cover a number of typical environments of massive stars: the Galactic center, the Magellanic Clouds, blue compact dwarf galaxies like I Zw 18, and galaxies at high redshifts. The paper contains a description of the OSTAR2002 grid and some illustrative examples and comparisons. The complete OSTAR2002 grid is available on-line. Title: Discovery of a Little Homunculus within the Homunculus Nebula of η Carinae Authors: Ishibashi, Kazunori; Gull, Theodore R.; Davidson, Kris; Smith, Nathan; Lanz, Thierry; Lindler, Don; Feggans, Keith; Verner, Ekaterina; Woodgate, Bruce E.; Kimble, Randy A.; Bowers, Charles W.; Kraemer, Steven; Heap, Sarah R.; Danks, Anthony C.; Maran, Stephen P.; Joseph, Charles L.; Kaiser, Mary Elizabeth; Linsky, Jeffrey L.; Roesler, Fred; Weistrop, Donna Bibcode: 2003AJ....125.3222I Altcode: We report long-slit spectroscopic mapping of the η Carinae nebula obtained using the Space Telescope Imaging Spectrograph. The observations reveal the presence of a previously unknown bipolar emission nebula (roughly +/-2" along its major axis) embedded within the well-known and larger Homunculus Nebula. A preliminary analysis suggests that this embedded nebula may have originated from a minor eruption event circa 1890, 50 years after the formation of the larger Homunculus.

Based on observations made with the NASA/ESA Hubble Space Telescope, and supported by GO grants 7302 and 8327 from the Space Telescope Science Institute and STIS GTO funding. This paper is a merger of the HST GO programs (PI: K. Davidson) and STIS IDT Key Project 8483, which used orbits allocated in the shared Guaranteed Telescope Observations. The STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. Title: A Tale of Two Stars: The Extreme O7 Iaf+ Supergiant AV 83 and the OC7.5 III((f)) star AV 69 Authors: Hillier, D. John; Lanz, T.; Heap, S. R.; Hubeny, I.; Smith, L. J.; Evans, C. J.; Lennon, D. J.; Bouret, J. C. Bibcode: 2003ApJ...588.1039H Altcode: We present a detailed study of AV 83, an O7 Iaf+ supergiant, and AV 69 [OC7.5 III((f))] in the SMC. The stars have similar effective temperatures and luminosities but show very different wind signatures. For our study we have used the non-LTE line-blanketed atmosphere code developed by Hillier and Miller, which explicitly allows for line blanketing by C, N, O, S, Ar, Ne, Fe, and other elements. Our study finds that AV 83 has an effective temperature of approximately 33,000 K and logg~3.25. It has an extended photosphere as a result of a ``low'' effective surface gravity and a much denser wind than main-sequence O stars. We can match the spectrum only by using a slow velocity law with β~2, a value that is much larger than the values of around 1 predicted by standard radiation wind theory. Further, we show that the Hα emission profile in AV 83 is sensitive to the adopted surface gravity. To fit the spectrum of AV 83, we have considered conventional models in which the wind is smooth and alternate models in which the winds are highly clumped. Both types of winds yield a satisfactory fit to the majority of lines in the observed spectrum; however, strong UV photospheric lines and the P V resonance transitions favor a clumped wind. If clumping is important, it must begin at relatively low velocities (i.e., 30 km s-1, not 300 km s-1). In the smooth wind, the line force is too small to drive the wind. In the clumped wind, the line force is generally sufficient to drive the wind, although there are still some discrepancies around the sonic point. In AV 83, the N abundance is substantially enhanced relative to normal SMC abundances, while both C and O are SMC-like, consistent with the presence of internally processed CNO material at the stellar surface. The N III λ4640 multiplet, which is known to be produced by dielectronic recombination, is well reproduced by the models. These lines, and the adjacent C III λ4649 multiplet, show a significant sensitivity to surface gravity, as well as the usual sensitivity to abundance and effective temperature. Incoherent electron scattering, occurring within the photosphere, can explain the broad wings seen on these lines. We have modeled the Fe spectrum (Fe IV-Fe VI) in the UV in both AV 83 and AV 69. For stars with an effective temperature around 33,000 K, the Fe IV-to-Fe V line ratios form a useful effective temperature diagnostic and give results consistent with those found from optical and UV line diagnostics. The derived iron abundance, which is sensitive to the adopted microturbulent velocity, is 0.2-0.4 times the solar iron abundance in AV 83, while 0.2 solar gives a good fit for AV 69. The wind of AV 69 is substantially less dense than that of AV 83. Because of the lack of suitable diagnostics, it is impossible to constrain the mass-loss rate and velocity law independently. Its spectrum indicates that it has a similar effective temperature to AV 83 (Teff~34,000 K), a substantially higher gravity (logg=3.5) than AV 83, and a CNO abundance pattern that has not been influenced by internal CNO processing. We show that the N/C abundance ratio is substantially below solar, in agreement with SMC nebular and stellar abundance studies. The differences between the spectra of AV 83 and AV 69, and between the derived masses and surface abundances, are striking. We have examined possible causes, and only one seems consistent with the observations and our current understanding of massive star evolution. AV 83 was most likely a fast rotator that experienced rotationally enhanced mass loss. The presence of enhanced N but almost normal C and O abundances is a direct indication of rotationally induced mixing. On the other hand, AV 69 is a slow rotator. As part of our analyses, we have systematically examined the influence of the H/He abundance ratio, the mass-loss rate, the velocity law, the Fe abundance, microturbulence, and clumping on the theoretical spectrum. We illustrate which lines provide useful diagnostics and highlight some of the difficulties associated with spectroscopic analyses of O stars. The spectrum of AV 83 shows the presence of photospheric absorption lines, the presence of lines formed at the base of the wind, and numerous wind lines. Since these lines sample the photosphere and the entire wind, extreme O If supergiants, such as AV 83, are ideal candidates to probe conditions in stellar winds and hence further our knowledge of O star winds.

Based on observations 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 NAS5-26555. Based in part, on observations with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. FUSE is operated by Johns Hopkins University under NASA contract NAS5-32985. Title: UV Spectral Templates for High-Redshift Galaxies Authors: Heap, Sara; Lindler, Don; Lanz, Thierry Bibcode: 2003AAS...202.1107H Altcode: 2003BAAS...35R.715H New instrumentation such as DEIMOS on Keck-II now enable deep spectral surveys, and thereby samples of galaxies at younger ages. At a redshift, z=1, all galaxies are less than 6 Gyr old and hence, have not yet formed horizontal-branch stars. Also, at z=1, the restframe-UV comes into view, and with it, a new set of spectral diagnostics. UV spectral features are especially important because most of the UV flux comes from stars at the main-sequence turnoff (MSTO). Hence, UV spectral diagnostics enable the ages of z=1 galaxies to be estimated directly from MSTO stars.

In preparation for these high-redshift spectral surveys, we are developing UV spectral templates for stellar populations younger than 6 Gyr using UV-optical spectra of stars observed by HST/STIS. We are also planning to supplement these observations with theoretical spectral grids of stars of various metallicities. In this paper, we present a progress report on the observation-based spectral templates and spectral diagnostics. Title: An analysis of the optical spectra of the post-asymptotic giant branch stars LSIV -12 111 and HD 341617 Authors: Ryans, R. S. I.; Dufton, P. L.; Mooney, C. J.; Rolleston, W. R. J.; Keenan, F. P.; Hubeny, I.; Lanz, T. Bibcode: 2003A&A...401.1119R Altcode: High spectral resolution and signal-to-noise observations of the absorption and emission line spectra in two post-asymptotic-giant-branch (PAGB) stellar candidates, LSIV -12 111 and HD 314617 are discussed. The absorption line spectra have been analysed using non-LTE model atmosphere techniques to determine stellar atmospheric parameters and chemical compositions, both in absolute terms and relative to a standard star, HD 13841. The atmospheric parameters differ from previous estimates based on LTE model atmospheres, probably due to non-LTE effects. In turn these imply stellar masses that are generally larger than have been previously estimated. Both PAGB candidates have relative uniform underabundances of metals with mean values of -0.35 dex for LSIV -12 111 and -0.50 dex for HD 314617.

Furthermore, their abundance patterns are remarkably similar to that observed for optically bright, F-type PAGBs. From the emission spectra, the plasma parameters and angular size of the circumstellar material are constrained, and these are consistent with previous estimates and with a PAGB evolutionary stage. Title: Mapping abundance inhomogeneities and magnetic fields of chemically peculiar (CP) stars with optical aperture synthesis arrays Authors: Rousselet-Perraut, Karine; Stehle, Chantal; Lanz, Thierry; Boudoyen, Thomas; Jankov, Slobodan; Vakili, Farrokh; Kilbinger, Martin; Lebouquin, Jean-Baptiste; Kochukhov, Oleg Bibcode: 2003SPIE.4838.1396R Altcode: Simultaneously mapping the abundance inhomogeneities and the magnetic fields of chemically peculiar (CP) stars is essential to improve our understanding of stellar magnetism and its key role in structuring stellar atmospheres, in particular relative to ion migration and chemical stratification. However, magnetic fields and chemical inhomogenities tend to have similar effects on classical observables. Magnetic and abundance maps have therefore to be reconstructed most often either independently or in making a priori assumptions. To overcome these difficulties, we propose to take benefit of optical aperture synthesis arrays to resolve local magnetic structures and patchy stellar surfaces. This requires ability to resolve polarimetrically magnetically-sensitive spectral lines, and thus to add a polarimetric device at the combined focus of an interferometric array. Within this instrumental context, it becomes possible to map magnetic fields with visibility and phase measurements in circularly polarized light and to map the chemical inhomogeneities thanks to "classical" interferometric measurements (i.e. without the polarimeter). In this paper, we show that the interference fringe phase is the suitable observable for polarimetric measurements and for mapping patchy surfaces (see also Jankov et al. in these proceedings). We present some illustrative cases of different magnetic topologies and abundance distributions. We focus on two well-known CP stars, βCrB and α2CVn, and we show observational predictions with different instruments currently in operation (GI2T, VLTI). Title: A New Formation Mechanism for the Hottest Horizontal-Branch Stars Authors: Sweigart, A. V.; Brown, T. M.; Lanz, T.; Landsman, W. B.; Hubeny, I. Bibcode: 2003ASPC..296..313S Altcode: 2003nhgc.conf..313S; 2002astro.ph..7343S Stars with very large mass loss on the red-giant branch can undergo the helium flash while descending the white-dwarf cooling curve. Under these conditions the flash convection zone will mix the hydrogen envelope with the hot helium- burning core. Such ``flash-mixed'' stars will arrive on the extreme horizontal branch (EHB) with helium- and carbon-rich envelopes and will lie at higher temperatures than the hottest canonical (i.e., unmixed) EHB stars. Flash mixing provides a new evolutionary channel for populating the hot end of the EHB and may explain the origin of the high gravity, helium-rich sdO and sdB stars. Title: Multidimensional ALI Radiative Transfer in Cartesian, Cylindrical and Spherical Coordinate Systems Authors: van Noort, M.; Hubeny, I.; Lanz, T. Bibcode: 2003ASPC..288..445V Altcode: 2003sam..conf..445V A new Radiative Transfer code that can calculate the non-LTE line transfer problem in a two-level atom formulation in Cartesian, cylindrical and spherical coordinate systems is presented. The transfer equation is solved using the ALI and the short characteristics methods, while allowing for an arbitrary 3-dimensional velocity field.

The code is modularised so that changing geometry can be accomplished by simply setting a switch, and parallelised for use on a networked PC cluster to increase computational speed. The spatial parallelization method is employed. It is found to be robust and efficient, while not relying heavily on fast communication.

The internal accuracy of the code is tested extensively in all three geometries and is shown to be in good agreement with appropriate 1-D solutions. Title: Quantitative analysis of O-type stars properties, at low metallicity Authors: Bouret, Jean-Claude; Lanz, Thierry M.; Heap, Sara R.; Hubeny, Ivan; Hillier, D. John; Lennon, Daniel J.; Evans, Christopher J.; Smith, Linda J. Bibcode: 2003IAUS..212..156B Altcode: We have investigated the properties of main-sequence O-type stars in the SMC. Mass-loss rates, luminosities and Teff are much smaller for these stars than for Galactic ones, resulting in a steeper wind-momentum relation. Title: Using Optical Interferometry for Studying Stellar Activity and Magnetism Authors: Le Bouquin, J. B.; Rousselet-Perraut, K.; Stehlé, C.; Lanz, T.; Jankov, S. Bibcode: 2003ASPC..305..215L Altcode: 2003mfob.conf..215L No abstract at ADS Title: NLTE in a Hot Hydrogen Star: Auer & Mihalas Revisited Authors: Wiersma, J.; Rutten, R. J.; Lanz, T. Bibcode: 2003ASPC..288..130W Altcode: 2003sam..conf..130W We pay tribute to two landmark papers published by Auer & Mihalas in 1969. They modeled hot-star NLTE-RE hydrogen-only atmospheres, using two simplified hydrogen atoms: ApJ 156, 157: H I levels 1, 2 and c, Lyman α the only line ApJ 156, 681: H I levels 1, 2, 3 and c, Balmer α the only line and computed LTE and NLTE models with the single line turned on and off. The results were extensively analyzed in the two papers.

Any student of stellar line formation should take these beautiful papers to heart. The final exercise in Rutten's lecture notes ``Radiative Transfer in Stellar Atmospheres'' asks the student to work through five pages of questions concerning diagrams from the first paper alone! That exercise led to the present work in which we recompute the Auer-Mihalas hot-hydrogen-star models with TLUSTY, adding results from a complete hydrogen atom for comparison.

Our motivation for this Auer-Mihalas re-visitation is twofold:

1. to add diagnostic diagrams to the ones published by Auer & Mihalas, in particular Bν, Jν, Sν graphs to illustrate the role of the radiation field, and radiative heating & cooling graphs to illustrate the radiative energy budget,

2. to see the effect of adding the rest of the hydrogen atom. Title: Nitrogen Enrichment in Massive Stars Authors: Heap, S. R.; Lanz, T. Bibcode: 2003ASPC..304...41H Altcode: 2003cnou.conf...41H No abstract at ADS Title: A Grid of Non-LTE Line-Blanketed Model Atmospheres of O Stars Authors: Lanz, T.; Hubeny, I. Bibcode: 2003ASPC..288..157L Altcode: 2003sam..conf..157L We have constructed a grid of over 300 NLTE fully-blanketed model atmospheres covering the parameter range of O-type stars at various metallicities. We have assumed a plane-parallel geometry, hydrostatic and radiative equilibria. The models incorporate about 100,000 NLTE atomic levels of over 40 ions of H, He, C, N, O, Ne, Si, P, S, Fe, and Ni, which are grouped into about 900 superlevels. The models will be made publicly available in the coming months. Title: Quantitative Spectroscopy of O stars at low metallicity. O Dwarfs in NGC 346 Authors: Bouret, J. -C.; Lanz, T.; Heap, S.; Hubeny, I.; Hillier, D.; Lennon, D.; Evans, C. J.; Smith, L. Bibcode: 2003sf2a.conf..499B Altcode: 2003sf2a.confE.214B Dwarf O-type stars have been observed in NGC 346, the largest HII region in the SMC. UV and optical spectra have been analysed with NLTE line-blanketed models. These stars reveal CNO- cycle processed material at their surface, indicating fast stellar rotation and/or very efficient mixing processes. We obtain an overall metallicity Z = 0.2 Zodot. The dependence of the mass loss rate with the stellar metallicity is in good agreement with recent theoretical predictions for three most luminous stars of the sample. However, three other stars reveal mass loss rates that are significantly lower than 10-8 msol/yr, which is below the predictions of radiative line-driven wind theory by an order of magnitude or more. Clumping in the wind is evidenced by OV at 1371. Clumped wind models systematically yield lower mass loss rates than theoretical predictions. Title: NLTE Line Formation in Late-Type Stellar Atmospheres Authors: Allende Prieto, C.; Hubeny, I.; Lambert, D. L.; Lanz, T. Bibcode: 2003IAUS..210P.A24A Altcode: No abstract at ADS Title: Observing stellar activity with optical interferometers Authors: Lebouquin, J. -B.; Rousselet-Perraut, K.; Stehlé, C.; Lanz, T.; Jankov, S. Bibcode: 2003sf2a.conf..539L Altcode: No abstract at ADS Title: Model Photospheres with Accelerated Lambda Iteration Authors: Hubeny, I.; Lanz, T. Bibcode: 2003ASPC..288...51H Altcode: 2003sam..conf...51H No abstract at ADS Title: Multidimensional radiative effects in supercritical shocks Authors: Leygnac, S.; Lanz, T.; Stehlé, C.; Michaut, C.; Korĉáková, D. Bibcode: 2003sf2a.conf..595L Altcode: Recent radiative shocks experiments performed on the LULI laser at Ecole Polytechnique in France (Fleury et al., Lasers and Particle Beams 20, 263, 2002) put in evidence a supercritical shock wave in a xenon gas cell. The structure of these shocks is quite similar to those of accretion shock wave in the case of stellar formation, as indicated in Stehlé and Chieze (SF2A - Paris proceedings, 2002).

Some points require further studies like the contribution of the gas excitation/ionization energy to the compression ratio and the understanding of the discrepancy, which was noted between the velocity of the radiative precursor in the experiment and in the 1D simulation.

Thus, to understand the physics of the radiative shock waves, the academic case of the stationary shock is particularly interesting. We have thus studied the structure of a radiative shock wave which propagates in an ionized gas. We study the extended Rankine Hugoniot equations in various media with inclusion of radiation pressure and energy and study also the extension of the radiative precursor in the diffusion approximation.

We also study the equations of multidimensional radiative transfer for a snapshot of the experimental shock in xenon in order to quantify the radiative losses in the finite experimental cell.

This academic approach will help to improve the knowledge of the physical processes which take place in radiative shocks of astrophysical interest, like in the birth and death of stars, and prepare ourselves to define appropriate experiments on future high power lasers like LIL and LMJ in Bordeaux. Title: Atomic Data in Non-LTE Model Stellar Atmospheres Authors: Lanz, T.; Hubeny, I. Bibcode: 2003ASPC..288..117L Altcode: 2003sam..conf..117L Extensive sources of atomic data are required to calculate NLTE line-blanketed model atmospheres. I will discuss their implementation in our NLTE model atmosphere code, TLUSTY, and in our spectrum synthesis code, SYNSPEC, with a particular attention to the statistical methods required to incorporate the opacity of iron-peak elements. A few typical results and comparisons to other codes will be shown. Title: NLTE Line-Blanketed Model Stellar Atmospheres Authors: Lanz, T.; Hubeny, I.; Heap, S. R. Bibcode: 2003IAUS..210...67L Altcode: No abstract at ADS Title: FUSE Observations of He-rich sdB Stars Authors: Lanz, T.; Brown, T. M.; Sweigart, A. V.; Hubeny, I.; Landsman, W. B. Bibcode: 2002AAS...20111308L Altcode: 2002BAAS...34.1285L Subdwarf B stars all show significant abundance anomalies. Most are extremely deficient in helium and selected light elements, but a minority are helium-rich. Deficiencies in helium and heavier elements have been attributed to gravitational settling, but the helium-enriched members of the class present a puzzling exception, because radiative levitation should be too weak to prevent gravitational settling. New evolutionary calculations suggest that these helium-rich sdB stars are the result of a delayed helium-core flash on the white dwarf cooling curve. The convective zone produced by this flash will penetrate the hydrogen envelope, mixing hydrogen into the hot helium-burning interior, where it is rapidly consumed. The resulting star should show greatly enhanced helium and carbon with respect to the other heavy elements. This phenomenon is analogous to the born again scenario for producing hydrogen-deficient R CrB stars following a very late helium-shell flash. We have recently obtained FUSE spectra of two helium-rich sdB stars, revealing huge C III lines at 977 and 1176 Å. Our preliminary analysis yields a surface composition of 97% He and 3% C, in agreement with the new evolutionary scenario. This work is supported in part by NASA grant NAG5-12383. Title: Erratum: ``Isolating Clusters with Wolf-Rayet Stars in I Zw 18'' (ApJ, 579, L75 [2002]) Authors: Brown, Thomas M.; Heap, Sara R.; Hubeny, Ivan; Lanz, Thierry; Lindler, Don Bibcode: 2002ApJ...581L.129B Altcode: During publication, the software used to process Figure 1 corrupted the Y-axis label. The label should read ``Flux (10-16 ergs s-1 cm-2 Å-1)'' instead of the published ``Flux (1016 ergs s1 cm2 Å1).'' The Press sincerely regrets this error. Title: Abundance of Trace Elements in the Atmospheres of Three Hot White Dwarfs in Post-CE Binaries Authors: Dupuis, J.; Vennes, S.; Lanz, T.; Chayer, P. Bibcode: 2002AAS...20111908D Altcode: 2002BAAS...34.1301D We present FUSE observations of the hot white dwarfs in the post-CE binaries BPM 6502, EUVEJ2013+400, and EUVEJ0720-317. The abundance pattern observed in these stars is possibly affected by accretion from the cool star companion. In order to better understand the effect of accretion on the atmospheric composition of these stars, we measure the abundances of C, N, O, Si, P, and Fe from lines detected in the FUSE spectra. We report the detection of CIII, OVI, PV, and SiIV in the spectra of EUVEJ2013+400 (Teff = 48,000K) and EUVEJ0720-317 (Teff = 53,000K). BPM 6502, a cooler DA (Teff = 21,000K), shows photospheric absorption lines of CIII, NII, SiIII, SiIV, and FeIII. We measure the following abundances in EUVEJ0720-317 and EUVEJ2013+400 respectively: log (C/H) = -6.0, -5.8; log (O/H) = -6.1, -5.6; log (Si/H) = -6.1, -6.5; log (P/H) = -7.0, -7.7. For BPM 6502, we measure log (C/H) = -6.2, log (N/H) = -6.6, log (Si/H) = -6.1, and log (Fe/H) = -7.5. It is remarkable that although the carbon abundance is quite similar in the three stars, we see a decrease in the silicon and phosphorus abundances with decreasing effective temperature (as predicted by the radiative levitation theory) and an increase in the oxygen abundance. The presence of iron in BPM 6502 is surprising since it should not be supported by radiative levitation and is a possible evidence of ongoing accretion. This research is funded by NASA grants NAG5-11717 and NAG5-6551. Title: The Helium and Nitrogen-Rich UV-Bright Star UIT-644 in Omega Cen Authors: Landsman, W. B.; Crotts, A. P. S.; Heap, S.; Lanz, T.; Sweigart, A. Bibcode: 2002AAS...201.0715L Altcode: 2002BAAS...34.1103L; 2002AAS...201..715L We report on STIS medium-resolution echelle (E230M) observations of the UV-Bright star UIT-644, located near the center of the globular cluster Omega Centauri. The observations cover the wavelength region 1625 -- 2364 Å with a resolution R=30,000. Earlier low-resolution optical spectra with the CTIO-4m have been used to derive the temperature (20,000 K), gravity (log g = 3.5) and bolometric luminosity (log L/LSun = 2.9), while revealing the star to be helium-rich (Y=0.68). The STIS spectra are used to show that the star is also extremely nitrogen-rich ([N/H] ~ 10), while having a typical cluster iron abundance ([Fe/H] = -1.5). The large nitrogen abundance probably requires that the products of helium burning be mixed to the surface, with the carbon subsequently converted to nitrogen. We speculate on a scenario in which UIT-644 ejected its envelope subsequent to a helium flash on the AGB. Title: The Role of Rotation in the Evolution of Massive Stars Authors: Heap, S. R.; Lanz, T. M. Bibcode: 2002AAS...20111309H Altcode: 2002BAAS...34.1285H Recent evolutionary models of massive stars (Maeder & Meynet 2000, 2001; Heger & Langer 2000) predict important effects of rotation including: increasing the rate of mass-loss; lowering the effective gravity; altering the evolutionary track on the HRD; extending the main-sequence phase (both on the HR diagram and in time); and mixing of CNO-processed elements up to the stellar surface. Observations suggest that rotation is a more important factor at lower metallicities because of higher initial rotational velocities and weaker winds. This makes the SMC, a low-metallicity galaxy (Z=0.2Zsun), an excellent environment for discerning the role of rotation in massive stars. We report on a FUSE+STIS+optical spectral analysis of 17 O-type stars in the SMC, where we found an enormous range in N abundances. Three stars in the sample have the same (low) CN abundances as the nebular material out of which they formed, namely C=0.085 Csun and N=0.034 Nsun. However, more than half show N ~Nsun, an enrichment factor of 30X! Such unexpectedly high levels of N have ramifications for the evolution of massive stars including precursors to supernovae. They also raise questions about the sources of nitrogen in the early universe. This study was supported in part by grants from NASA's ADP, HST GO-7437, and FUSE B134. Title: Laboratory Astrophysics: Study of Radiative Shocks Authors: Leygnac, S.; Lanz, T.; Stehlé, C.; Michaut, C. Bibcode: 2002AAS...201.5415L Altcode: 2002BAAS...34.1201L Radiative shocks are high Mach number shocks with a strong coupling between radiation and hydrodynamics which leads to a structure governed by a radiative precursor. They might be encountered in various astrophysical systems: stellar accretion shocks, pulsating stars, interaction of supernovae with the intestellar medium etc. A numerical one dimensional (1D) stationary study of the coupling between hydrodynamics and radiative transfer is being performed. An estimate of the error made by the 1D approach in the radiative transfer treatment is done by an approximate short characteristics approach. It shows, for exemple, how much of the radiation escapes from the medium in the configuration of the experiment. The experimental study of these shocks has been performed with the high energy density laser of the LULI, at the École Polytechnique (France). We have observed several shocks identified as radiative shocks. The shock waves propagate at about 50 km/s in a tiny 10 mm3 shock tube filled with gaz. From the measurements, it is possible to infer several features of the shock such as the speed and the electronic density. Title: Isolating Clusters with Wolf-Rayet Stars in I Zw 18 Authors: Brown, Thomas M.; Heap, Sara R.; Hubeny, Ivan; Lanz, Thierry; Lindler, Don Bibcode: 2002ApJ...579L..75B Altcode: 2002astro.ph.10089B We present UV images and spectra of the starburst galaxy I Zw 18, taken with the Space Telescope Imaging Spectrograph. The high spatial resolution of these data allows us to isolate clusters containing Wolf-Rayet stars of the subtype WC. Our far-UV spectra clearly show C IV λλ1548, 1551 and He II λ1640 emission of WC stars in two clusters: one within the bright (northwest) half of I Zw 18 and one on the outskirts of this region. The latter spectrum is unusual because the C IV is seen only in emission, indicating a spectrum dominated by WC stars. These data also demonstrate that the H I column in I Zw 18 is strongly peaked in the fainter (southeast) half of I Zw 18, with a column depth far larger than that reported in previous analyses. 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 proposal 9054. Title: A chromospheric scenario for the activity of beta Pictoris, as revealed by FUSE Authors: Bouret, J. -C.; Deleuil, M.; Lanz, T.; Roberge, A.; Lecavelier des Etangs, A.; Vidal-Madjar, A. Bibcode: 2002A&A...390.1049B Altcode: We investigate a chromospheric scenario to explain the characteristics of the far-UV emission lines of beta Pictoris, revealed by FUSE spectra of this famous circumstellar disk system. The model assumes a thin region heated up to a few 105 K located close to the stellar photosphere. The resonance lines of C III at 977 Å and O VI at 1032-1037 Å, seen in emission, are produced in this chromosphere-transition region complex. Using complementary data in the mid and near UV (specifically, a resonance doublet of C IV at 1548-1550 Å and Mg II h & k), we show that the whole dataset is remarquably well reproduced by the model. In addition, we investigate the properties of the C III* multiplet at 1176 Å and conclude that this line likely forms in a weak warm wind, originating from the prominent circumstellar disk of beta Pictoris. Finally, radiative losses have been calculated and have provided estimates of the amount of non radiative energy dissipated in the external atmosphere of beta Pictoris, which is a measure of the star's activity level. Such behavior for an A5 V star presents a challenge for both evolution and activity models since the former predict that main-sequence A stars should not be active, while the latter are unable to quantitatively account for the characteristics of the chromospheric heating of beta Pictoris. Title: Young, Massive Stars at Low Metallicity Authors: Heap, S. R.; Lanz, T. Bibcode: 2002AAS...200.0713H Altcode: 2002BAAS...34..652H We report on an extensive analysis of O-type stars in the Small Magellanic Cloud that have been observed by HST and FUSE. The analysis, which involves detailed NLTE atmospheric models recently constructed by Lanz & Hubeny, has the following goals: (1) calibration of spectral properties in terms of fundamental parameters; (2) resolution of the discrepancy between spectro- scopic masses and evolutionary masses; and (3) evidence for mixing of nuclear-processed elements. We derive a significantly lower temperature scale for O-type stars than previously assumed; the lower temperatures help to resolve the ``mass discrepancy''. We describe the results of our analysis and discuss their implications for stellar evolution at low metallicity. This work was supported by NASA ADP, HST GO and FUSE grants. Title: Multidimensional Non-LTE Radiative Transfer. I. A Universal Two-dimensional Short-Characteristics Scheme for Cartesian, Spherical, and Cylindrical Coordinate Systems Authors: van Noort, Michiel; Hubeny, Ivan; Lanz, Thierry Bibcode: 2002ApJ...568.1066V Altcode: We have developed an efficient and robust two-dimensional non-LTE radiation transfer solver appropriate for line transfer in the equivalent two-level atom formalism. The numerical method applies the accelerated lambda iteration technique together with the short-characteristics scheme. The code presented in this paper incorporates all three standard geometries (Cartesian, cylindrical, and spherical) in a transparent way while allowing for arbitrary (three-dimensional) velocity fields. The geometry-specific parts of the radiative transfer solver are modularized so that the change of geometry is accomplished by simply setting the appropriate switch. We have also developed a parallel version of the code, in which we use a parallelization in spatial subdomains, and showed that such a scheme is sufficiently robust. We have performed a number of tests of the performance of the solver in all three geometries. Finally, we discuss the internal accuracy of the transfer solutions depending on the number of spatial, angular, and frequency grid points. Title: The Origin of Hot Subluminous Horizontal-Branch Stars in ω Centauri and NGC 2808 Authors: Sweigart, A. V.; Brown, T. M.; Lanz, T.; Landsman, W. B.; Hubeny, I. Bibcode: 2002ASPC..265..261S Altcode: 2002ocuw.conf..261S; 2002astro.ph..3063S Hot subluminous stars lying up to 0.7 mag below the extreme horizontal branch (EHB) are found in the UV color-magnitude diagrams of omega Cen and NGC 2808. Such stars are unexplained by canonical HB theory. In order to explore the origin of these subluminous stars, we evolved a set of low-mass stars from the main sequence through the helium-core flash to the HB for a wide range in the mass loss along the red-giant branch (RGB). Stars with the largest mass loss evolve off the RGB to high effective temperatures before igniting helium in their cores. Our results indicate that the subluminous EHB stars, as well as the gap within the EHB of NGC 2808, can be explained if these stars undergo a late helium-core flash on the white-dwarf cooling curve. Under these conditions the flash convection will penetrate into the stellar envelope, thereby mixing most, if not all, of the envelope hydrogen into the hot helium- burning interior. This phenomenon is analogous to the "born-again" scenario for producing hydrogen-deficient stars during a very late helium-shell flash. "Flash mixing" greatly enhances the envelope helium and carbon abundances and, as a result, leads to an abrupt increase in the HB effective temperature. We argue that the EHB gap in NGC 2808 is caused by this theoretically predicted dichotomy in the HB morphology. Using new helium- and carbon-rich stellar atmospheres, we show that the flash-mixed stars have the same reduced UV flux as the subluminous EHB stars. Moreover, we demonstrate that models without flash mixing lie, at most, ~0.1 mag below the EHB and hence fail to explain the observations. Flash mixing may also provide a new evolutionary channel for producing the high gravity, He-rich sdO and sdB stars. Title: FUSE Observations of Key DBA and DAB White Dwarfs Authors: Lanz, Thierry Bibcode: 2002fuse.prop.C026L Altcode: We propose FUSE observations of three key white dwarfs having hybrid helium and hydrogen spectra the DBA and DAB stars. These stars are relatively rare, but may hold the key for understanding the chemical evolution of white dwarf atmospheres along the cooling sequence, more particularly the so-called DB gap. One possible explanation for these white dwarfs with mixed atmospheres is convective dredge up, where convection mixes the hydrogen and helium layers. Potentially other elements such as carbon and nitrogen may also be dredged up. Strong C and N lines are present in the FUSE band, and can serve as sensitive measures of the presence of these elements. Alternatively, some DAB stars have been found to be spectral composites consisting of DA and DB double degenerate binary systems. The FUSE band is uniquely sensitive to the spectral energy distribution of stars in the 20,000 to 30,000 K range and will make it possible to distinguish two white dwarfs, even if their effective temperatures are similar. Other possible models, including interstellar accretion, stratified atmospheres and surface inhomogeneities, can be tested with FUSE spectroscopy. Title: The D/H Abundance Ratio in Local Interstellar Gas Authors: Sahu, M. S.; Landsman, W.; Bruhweiler, F. C.; Holberg, J.; Hubeny, I.; Barstow, M.; Linsky, J.; Gull, T.; Lindler, D.; Lanz, T.; Feggans, K. Bibcode: 2001AAS...199.1101S Altcode: 2001BAAS...33.1323S Variations of the D/H ratio on scales of 100 pc are important for two reasons: (1) they complicate the use of deuterium as a cosmological probe, and (2) they imply non-uniform deuterium production/destruction and an inefficient mixing of interstellar gas. The Local Interstellar Medium (LISM) is an ideal location to test whether the D/H abundance ratio varies or not, because the heating sources and radiation fields are well-studied and we have a detailed knowledge of the three-dimensional (3D) structure of the diffuse clouds in the LISM. This detailed knowledge of the number, structure and velocities of the absorbing clouds greatly helps in reducing errors in the derived D/H ratios. We are working on a project to obtain high-precision D/H abundance ratios in the interstellar gas within 100 pc. For this purpose, we primarily use HST-STIS data towards nearby hot, white dwarfs (WDs). We have also begun an HST archival research project to study about 20 WD sightlines. In this poster, we present new STIS observations for GD 153 and HZ 43 and report the results of our HST Archival project. Title: The Origin of Hot Subluminous Horizontal-Branch Stars in ω Cen and NGC 2808 Authors: Sweigart, A. V.; Brown, T. M.; Moehler, S.; Lanz, T.; Landsman, W. B.; Hubeny, I.; Dreizler, S.; Napiwotzki, R. Bibcode: 2001AAS...19913704S Altcode: 2001BAAS...33.1512S Ultraviolet (UV) observations of the globular clusters ω Cen and NGC 2808 have revealed an unexpected population of hot subluminous stars lying up to 0.7 mag below the extreme horizontal branch (EHB) in the UV, which are not explained by canonical stellar models. In order to explore the evolutionary status of these stars, we have evolved a set of low-mass stars from the main sequence through the helium flash to the horizontal branch (HB) for a wide range in the mass loss along the red-giant branch (RGB). Stars with the largest mass loss evolve off the RGB to high effective temperatures before igniting helium in their cores. Our results indicate that the subluminous stars can be explained if these stars undergo a late helium flash while descending the white dwarf cooling curve. Under these conditions the convection zone produced by the helium flash will penetrate into the stellar envelope, thereby mixing the envelope hydrogen into the hot helium-burning interior, where it is rapidly consumed. Such ``flash-mixed'' stars will have helium- and carbon-rich envelopes and will lie at higher effective temperatures than the hottest canonical (i.e., unmixed) EHB stars. Using new stellar atmospheres, we show that these changes in the envelope abundances will suppress the UV flux in the spectra of the flash-mixed stars by the amount needed to explain the hot subluminous stars in ω Cen and NGC 2808. To test this evolutionary scenario, we have obtained medium resolution spectra of a sample of the hottest HB stars in ω Cen. We find that these stars are indeed helium-rich compared to classical EHB stars and also considerably hotter than the hottest EHB models without flash mixing. Title: NLTE Models of Vertical structure of Accretion Disks around Stellar Mass Black Holes Authors: Hubeny, I.; Blaes, O.; Krolik, J. H.; Agol, E.; Lanz, T. Bibcode: 2001AAS...19915908H Altcode: 2002BAAS...34Q.569H Recent upgrades of our computer program TLUSDISK are briefly described. These include a self-consistent treatment of Compton scattering, and the effects of X-ray continuum opacities of the most important metal species (C, N, O, Ne, Mg, Si, S, Ar, Ca, Fe, Ni). In the case the central degenerate object is a neutron star or a black hole, we allow for a full general relativistic treatment. We show the effects of Comptonization and metal opacities on the structure of disk under various conditions. We also present a simple analytic prescription for the vertical temperature structure of the disk in the presence of Comptonization, and show under what conditions a hot outer layer (a corona) is formed. Title: Solar UV Radiation and the Origin of Life on Earth Authors: Heap, S. R.; Gaidos, E.; Hubeny, I.; Lanz, T. M. Bibcode: 2001AAS...199.0314H Altcode: 2001BAAS...33.1305H We have embarked on a program aimed at understanding the atmosphere of the early Earth, because of its importance as a greenhouse, radiation shield, and energy source for life. Here, we give a progress report on the first phase of this program: to establish the UV radiation from the early Sun. We are presently obtaining ultraviolet spectra (STIS, FUSE, EUVE) of carefully selected nearby, young solar-type stars, which act as surrogates for the early Sun. We are currently making detailed non-LTE analyses of the spectra and constructing models of their photospheres + chromospheres. Once validated, these models will allow us to extrapolate our theoretical spectra to unobserved spectral regions, and to proceed to the next step: to develop photochemical models of the pre-biotic and Archean atmosphere of the Earth. Title: NLTE Model Atmospheres of Supersoft X-Ray Sources Authors: Lanz, T.; Hubeny, I. Bibcode: 2001AAS...199.1712L Altcode: 2001BAAS...33.1336L Supersoft X-Ray Sources are most probably very hot white dwarfs undergoing steady nuclear burning in their envelopes. They are high luminosity (L > 0.1 L Edd), soft (kT≈ 50 eV) low mass binary with a white dwarf primary accreting at a sub-Eddington rate. High-resolution spectra in the soft X-rays are now becoming available with Chandra and XMM-Newton. Photospheric line spectroscopy has the potential to provide accurate diagnostics of the white dwarf properties. We have therefore embarked on a project aimed at providing new NLTE model atmospheres of supersoft sources. We are building a series of model atmospheres of hot white dwarfs (20<= kT<= 100 eV; 8<= log g<= 10) with various surface composition. The models incorporate the most abundant species, H, He, C, N, O, Ne, Mg, Si, S, and Fe, which are allowed to depart from Saha-Boltzmann statitics. Our models include the influence of multi-level model atoms, metal line blanketing, and Auger ionization on the atmospheric structure. A preliminary comparison of models calculated assuming coherent (Thomson) and non-coherent (Compton) scattering is presented. This work was supported through a NASA ATP grant (NRA 00-01-ATP-153). Title: Flash Mixing on the White Dwarf Cooling Curve: Understanding Hot Horizontal Branch Anomalies in NGC 2808 Authors: Brown, Thomas M.; Sweigart, Allen V.; Lanz, Thierry; Landsman, Wayne B.; Hubeny, Ivan Bibcode: 2001ApJ...562..368B Altcode: 2001astro.ph..8040B We present an ultraviolet color-magnitude diagram (CMD) spanning the hot horizontal branch (HB), blue straggler, and white dwarf populations of the globular cluster NGC 2808. These data were obtained with the far-UV and near-UV cameras on the Space Telescope Imaging Spectrograph (STIS). Although previous optical CMDs of NGC 2808 show a high-temperature gap within the hot HB population, no such gap is evident in our UV CMD. Instead, we find a population of hot subluminous HB stars, an anomaly only previously reported for the globular cluster ω Cen. Our theoretical modeling indicates that the location of these subluminous stars in the UV CMD, as well as the high-temperature gap along the HB in optical CMDs, can be explained if these stars underwent a late helium-core flash while descending the white dwarf cooling curve. We show that the convection zone produced by such a late helium flash will penetrate into the hydrogen envelope, thereby mixing hydrogen into the hot helium-burning interior, where it is rapidly consumed. This phenomenon is analogous to the ``born again'' scenario for producing hydrogen-deficient stars following a late helium-shell flash. The flash mixing of the envelope greatly enhances the envelope helium and carbon abundances, and leads, in turn, to a discontinuous increase in the HB effective temperatures at the transition between canonical and flash-mixed stars. We argue that the hot HB gap is associated with this theoretically predicted dichotomy in the HB properties. Moreover, the changes in the emergent spectral energy distribution caused by these abundance changes are primarily responsible for explaining the hot subluminous HB stars. Although further evidence is needed to confirm that a late helium-core flash can account for the subluminous HB stars and the hot HB gap, we demonstrate that an understanding of these stars requires the use of appropriate theoretical models for their evolution, atmospheres, and spectra. Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555. Title: Stellar Atmospheres Authors: Lanz, T. Bibcode: 2001eaa..bookE1873L Altcode: The atmosphere of a star is the boundary between the stellar interior and the interstellar medium. It spans the layers from which photons can escape to the surrounding space. Direct information about the physical state of the stellar atmosphere can therefore be derived from the emergent radiation. This is, however, not the case for deeper layers, because photons that are emitted in these layers a... Title: A Comparative Study of the Atmospheric Composition of the DA White Dwarfs Feige 24 and G191-B2B Authors: Vennes, Stéphane; Lanz, Thierry Bibcode: 2001ApJ...553..399V Altcode: We analyze Hubble Space Telescope Imaging Spectrograph spectra of two white dwarfs, Feige 24 and G191-B2B, and we compare the measured abundance patterns. Except for carbon, which appears overabundant in G191-B2B by 0.3-0.5 dex relative to Feige 24, the average heavy-element abundance in Feige 24 is 0.17 dex larger than in the cooler, hence older, G191-B2B, indicating a slow but perceptible decline of metallicity with time. We also found that the C IV λ1550 doublet in G191-B2B is composed of a photospheric component and a second component, either interstellar or circumstellar, separated by only Δv=15 km s-1. We find a simple correspondence between the measured element abundance, its solar abundance value, and its atomic weight from which we deduce the likely presence of additional elements such as neon and magnesium in the photospheres of Feige 24 and G191-B2B. Title: Understanding Horizontal Branch Anomalies in NGC 2808 Authors: Brown, T. M.; Sweigart, A. V.; Lanz, T.; Landsman, W. B.; Hubeny, I. Bibcode: 2001AAS...198.4302B Altcode: 2001BAAS...33..844B We present an ultraviolet color-magnitude diagram (CMD) spanning the hot horizontal branch (HB), blue straggler, and white dwarf populations of the globular cluster NGC 2808. These data, obtained with the Space Telescope Imaging Spectrograph (STIS), demonstrate that NGC 2808 harbors a significant population of hot subluminous HB stars, an anomaly only previously reported for the globular cluster Omega Cen. Our theoretical modeling indicates that the location of these subluminous stars in the CMD, as well as the high temperature gap along the HB of NGC 2808, can be explained if these stars underwent a late helium-core flash while descending the white dwarf cooling curve. We show that the convective zone produced by such a late helium flash will penetrate into the hydrogen envelope, thereby mixing hydrogen into the hot helium-burning interior, where it is rapidly consumed. This phenomenon is analogous to the "born again" scenario for producing hydrogen-deficient stars following a late helium-shell flash. The flash mixing of the envelope greatly enhances the envelope helium and carbon abundances that, in turn, leads to a discontinuous increase in the HB effective temperatures. We argue that the hot HB gap is associated with this theoretically predicted dichotomy in the HB properties. Moreover, the changes in the emergent spectral energy distribution caused by these abundance changes are primarily responsible for explaining the hot subluminous HB stars. Although further evidence is needed to confirm that a late helium-core flash can account for the subluminous HB stars and the hot HB gap, we demonstrate that an understanding of these stars requires use of sophisticated models for the stellar evolution, atmospheres, and synthetic spectra. Title: Determination of magnetic fields in stars based on polarized hydrogen lines Authors: Brillant, Stéphane; Stehlé, Chantal; Mathys, Gautier; Lanz, Thierry Bibcode: 2001AIPC..559..205B Altcode: The study of polarized light is a powerful tool for the determination of magnetic fields in stellar objects. The most used methods are the so-called ``photographic'' method [1] and Balmer line photopolarimetry [2]. However, in various cases these two methods give different results for the same star. To assess the validity of the interpretation of photopolarimetric measurements of Balmer lines, we have developed a new model of the formation of these lines in a magnetized plasma. This model is based on the Simplified Unified Theory (SUT) for the computation of the line shape [3-5]. We present this model, some numerical results computed with it, and conclusions drawn from consideration of there results [6]. We discuss the prospects for future developments in continuation of this work. . Title: CMFGEN: A non-LTE Line-Blanketed Radiative Transfer Code for Modeling Hot Stars with Stellar Winds Authors: Hillier, D. John; Lanz, Thierry Bibcode: 2001ASPC..247..343H Altcode: 2001scpp.conf..343H No abstract at ADS Title: NLTE Model Atmospheres for OB Stars Authors: Lanz, Thierry; Hubeny, Ivan Bibcode: 2001ASPC..247..351L Altcode: 2001scpp.conf..351L No abstract at ADS Title: Recent Developments for Balmer Line Photopolarimetry Authors: Stehlé, C.; Mathys, G.; Brillant, S.; Lanz, T. Bibcode: 2001ASPC..248..357S Altcode: 2001mfah.conf..357S No abstract at ADS Title: Simulation of Interfero-Polarimetric Observations for Magnetic Stars Authors: Rousselet-Perraut, K.; Stehlé, C.; Kilbinger, M.; Chesneau, O.; Vakili, F.; Lanz, T.; Mathys, G. Bibcode: 2001ASPC..248..629R Altcode: 2001mfah.conf..629R No abstract at ADS Title: Properties of the z = 2.73 galaxy, MS1512-cB58 Authors: Heap, Sara R.; Hubeny, Ivan; Lanz, Thierry M. Bibcode: 2001ApSSS.277..263H Altcode: MS1512-cB58 is an z=2.73 galaxy whose apparent brightness is amplified by30-50X due to gravitationally lensing. The restframe far-UV spectrum of cB58 that was obtained by Keck-I/LRIS is `one of the best ultraviolet spectra of a starburst galaxy obtained at any redshift' (Pettini et al., 2000). We have analyzed and modeled the spectrum of this galaxy in order to learn the properties of high-redshift galaxies. We find that our model spectrum is a near match to the observed spectrum of cB58 if the galaxy has a SMC-like metallicity, and has a Salpeter IMF extending up to ~ 100 M. The spectrum of cB58 also shows many absorption lines formed in a giant, expanding gas shell surrounding the star-forming complex. We show preliminary findings of the properties of the giant HII region based on our CLOUDSPEC model. Title: GALSPEC: A Tool for Computing the Spectra of Star-Forming Galaxies of Low Metallicity Authors: Heap, S. R.; Hubeny, I.; Lanz, T. M.; Lindler, D. J. Bibcode: 2000AAS...197.7813H Altcode: 2000BAAS...32R1531H The far-ultraviolet spectral region is rich in diagnostics of star formation in galaxies; it gives information not only about the star-forming complex, but also about the dust and circumstellar gas. We have therefore developed a tool, called GALSPEC, for computing far-UV spectra of star-forming galaxies. The user controls the process through a graphical user interface (GUI) by specifying the input parameters for the stars, dust, and gas. GALSPEC computes the integrated spectrum of a star-forming complex, given the rate of star formation, metallicity (0.2 Zsun or 0.5 Zsun), and IMF upper mass limit. To compute the integrated spectrum, GALSPEC makes use of a library of stellar spectra composed of observed spectra from HST and model spectra calculated with the TLUSTY/SYNSPEC program (Hubeny & Lanz 1995). GALSPEC then applies wavelength-dependent extinction according to Calzetti's starburst extinction law. The mechanical luminosity from massive stars and supernovae sweeps up interstellar material thereby forming an expanding shell with a central cavity. GALSPEC computes the absorption spectrum of the shell using the CLOUDSPEC program (Hubeny 2000), a combination of Ferland's CLOUDY program and SYNSPEC. This work was supported by the STIS GTO program at Goddard and by STScI grants (GO 7437, AR 7985). Title: Ultraviolet Spectroscopy of O and B Stars at 1/30 Solar Metallicity: The Stars of the Local Group Galaxy Sextans A Authors: Bruhweiler, F.; Miskey, C.; de Koter, A.; Smith Neubig, M.; Walborn, N.; Lanz, T. Bibcode: 2000AAS...197.7805B Altcode: 2000BAAS...32.1530B We have obtained slitless two-dimensional ultraviolet spectroscopy using the Space Telescope Imaging Spectrograph (STIS) aboard the HST of the brightest concentration of OB stars in the faint irregular galaxy, Sextans A. The spectral image obtained with the G140L grating samples a 25'x25" region spanning a wavelength range approximately from 1170 to 1730 A. Sextans A lies at the edge of the Local Group at a distance of 1.4 Mpc and has an extremely low metallicity, roughly 1/30 solar or about 1/3 that of the Small Magellanic Cloud. This is within a factor of two of the most metal deficient galaxy known, I Zw 18. The STIS dataset is unique in that it represents spectra of individual O and B stars at the lowest metallicity ever obtained. The earliest and one of the UV-brightest stars we classify as O6 V based upon comparisons with UV spectra of SMC stars. This star exhibits a definite N V 1240 P Cygni profile that appears only slightly weaker, but with a lower terminal velocity than its SMC analogues. Both C IV and Si IV are also very weak. Absorption from Fe V is also detectable near 1430 and 1450 Angstroms. Other very UV-luminous stars are also present. Several early B-supergiants are also detected as evidenced by the pronounced Si IV 1400 A absorption. Further detailed modeling of the UV spectra using sophisticated model atmosphere codes is planned. Title: Ultraviolet Spectra of Hot Horizontal Branch Stars in NGC 6752 Authors: Landsman, W. B.; Catelan, M.; Grundahl, F.; Lanz, T.; Proffitt, C.; Sweigart, A. V. Bibcode: 2000AAS...197.4120L Altcode: 2000BAAS...32Q1464L We have obtained STIS ultraviolet low-resolution spectra (1140 -- 3100 Å) of eight hot horizontal branch (HB) stars with 10,500 < Teff (K) < 20,000 in the globular cluster NGC 6752 ([Fe/H = -1.6). The two coolest targets have temperatures just coolward of Teff ~ 11,500 K, where a sudden jump in the iron abundance has been observed in optical spectra of HB stars in NGC 6752 (Moehler et al. 2000, A&A, 360, 120) presumably due to the onset of radiative levitation. The spectra of the remaining six targets are consistent with an iron abundance near solar. Because this dramatic increase in the iron abundance appears to be characteristic of the HB in all globular clusters (Grundahl et al. 1999, ApJ, 524, 242), our data set can be used as a template for the ultraviolet spectral synthesis of the HB of metal-poor populations. An unexpected result is the apparent detection of the 1485 Å autoionizing feature of Si II in four of the iron-rich stars. Title: A New Grid of NLTE Model Atmospheres of Hot Stars Authors: Hubeny, I.; Lanz, T. Bibcode: 2000AAS...197.7812H Altcode: 2000BAAS...32Q1531H We briefly describe improvements in our non-LTE model atmosphere code, TLUSTY. The upgrades include opacity sampling, improved Fe collisional strengths, resonance-averaged photoionization cross-sections, and a number of numerical enhancements. Using TLUSTY, version 200, we have calculated a grid of NLTE, fully-blanketed, model atmospheres covering the range of O to early B-type stars ( 20000 <= T eff <= 55000 K, 3.0 <= log g <= 4.75) with a solar or 1/5 solar metallicity. The models incorporate over 70,000 NLTE atomic levels of about 30 ions of H, He, C, N, O, Ne, Mg, Si, S, Fe and Ni, which are grouped into about 700 superlevels. We compare our predicted EUV fluxes to other existing grids of model atmospheres (Atlas9, CoStar models), as well as to actual UV and optical observations of representative stars. Title: The D/H Abundance Ratio in Local Interstellar Gas Authors: Sahu, M. S.; Landsman, W.; Bruhweiler, F. C.; Linsky, J.; Hubeny, I.; Barstow, M.; Holberg, J.; Gull, T.; Bowers, C.; Lindler, D.; Lanz, T.; Feggans, K. Bibcode: 2000AAS...197.0703S Altcode: 2000AAS...197..703S; 2000BAAS...32.1401S Does the D/H ratio vary within ~ 100 pc of the Sun? If it does, what is the range of these variations? Are the D/H variations correlated to any physical property of the absorbing cloud? To answer these questions we have begun a project to obtain high-precision D/H abundance ratios in the Local Interstellar Medium within ~ 100 pc (Sahu et al. 1999, ApJ 523, L159). For this purpose, we primarily use HST-Space Telescope Imaging Spectrograph (STIS) data towards nearby hot, white dwarfs (WDs). The advantages of using STIS for this study are: (1) The high velocity resolution of STIS in the UV ( ~ 2 km s-1 in the E140H and E230H modes) allows the velocity structure in the sightline to be resolved and reliable component-to-component variations can be studied (2) WDs provide a strong, smooth UV continuum against which other diagnostic interstellar absorption lines can be measured. The wider wavelength coverage ( ~ 200Å per setting) of STIS in the high-resolution E140H mode allows important interstellar lines such as N i, Si ii, O i and S ii to be measured in addition to D i and H i, at no extra cost in HST time (3) The superior echelle scatter and background corrections possible with the 2-dimensional STIS-MAMA detectors allow more accurate D/H determinations from WD spectra. In this poster, we present a status report of our project along with some recent results. Title: NGC 346-12, a Rapidly Rotating O9.5V Star in the SMC: Test Case of Weak Winds Authors: Lanz, T.; Bouret, J. -C.; Heap, S. R.; Hubeny, I.; Hillier, D. J.; Lennon, D. J.; Smith, L. J.; Evans, C. J.; Owocki, S. P. Bibcode: 2000AAS...197.7811L Altcode: 2000BAAS...32R1531L We have analyzed the UV and optical spectrum of the O9.5V Star, NGC 346-12, using sophisticated, NLTE line-blanketed model atmospheres calculated with our code TLUSTY. The following stellar parameters were derived: T eff=30,000 K, log g=3.5. An abundance analysis yields a metallicity, [Fe/H]=-1.0. The N/C abundance ratio is 25 times the solar ratio, indicating that material processed through the CNO-cycle has been brought up to the surface. Assuming a distance modulus, (m-M)=18.9, for the SMC, we have derived the luminosity, radius, and mass of the star. We found, similarly to higher luminosity galactic stars, a discrepancy between the mass derived from the spectroscopic analysis and from stellar evolution theory. We conclude that it is very likely that NGC 346-12 is a fast rotator, whose evolution has been affected by rotation. Furthermore, the wind of NGC 346-12 appears abnormal: while the C IV resonance lines do not reveal any indication of a wind, a weak P-Cygni profile is observed in N 5 1240. Various possible explanations for the low inferred ion density in the stellar wind, including an enhanced degree of wind ionization associated with ion frictional heating, or ion runaway due to frictional decoupling from the hydrogen-helium bulk plasma, are examined. This work was supported through a NASA/NRC RA award and STScI grants (GO 7437, AR 7985). Title: NLTE Model Atmosphere Analysis of Main-Sequence O Stars in NGC 346 Authors: Bouret, J. -C.; Heap, S. R.; Hubeny, I.; Lanz, T.; Hillier, D. J.; Lennon, D. J.; Smith, L. J.; Evans, C. J. Bibcode: 2000AAS...197.7810B Altcode: 2000BAAS...32.1531B To serve as spectral templates of young, low-metallicity populations, high-resolution, high-quality, ultraviolet (STIS) and optical spectra of several main-sequence O stars in NGC 346 have been obtained. Stellar parameters and abundance of light elements are derived from the optical spectrum using NLTE, line-blanketed, photospheric models calculated with TLUSTY. In addition, the UV spectrum is analyzed with the NLTE, line-blanketed wind model code, CMFGEN, to derive wind parameters (mass-loss rate, clumping properties, terminal velocity). Iron abundances, derived from fitting Fe 4 and Fe 5 lines with the two NLTE codes, compare favorably. We achieve good matches of the observed spectra, thus supporting our modeling work of spectra of starburst galaxies at high redshift. We finally discuss the dependence of the mass-loss rate in terms of metallicity and luminosity. This work was supported through a NASA/NRC RA award and STScI grants (GO 7437, AR 7985). Title: Hubble Space Telescope Imaging Spectrograph Observations of the Hot White Dwarf in the Close Binary Feige 24 Authors: Vennes, Stéphane; Polomski, Elisha F.; Lanz, Thierry; Thorstensen, John R.; Chayer, Pierre; Gull, Ted R. Bibcode: 2000ApJ...544..423V Altcode: We obtained and analyzed two Hubble Space Telescope Imaging Spectrograph spectra of the white dwarf in the DA plus dMe binary Feige 24. The spectra, obtained at orbital quadratures, provide new estimates of the white dwarf motion and gravitational redshift resulting in revised white dwarf parameters. An analysis of interstellar absorption lines reveals the presence of two clouds (+3.1 km s-1, +17.6 km s-1) in the line of sight toward Feige 24; one of these clouds (+17.6 km s-1) is identified with the ``local cloud.'' A study of the Lyα H I and D I interstellar medium lines shows that the deuterium-to-hydrogen abundance ratio (D/H=1.3×10-5) is consistent with other measurements supporting a relative constancy of this ratio throughout the local interstellar medium. The total hydrogen column density measured with Lyα (lognH=2.95×1018 cm-2) is in agreement with EUV Lyman continuum flux measurements. Finally, we present a complete abundance pattern for the white dwarf, demonstrating the predominance of iron and nickel over lighter elements. Residual ionization imbalance in the case of several elements, most notably in the case of O IV/O V, which cannot be explained by temperature or surface gravity variations, may indicate the presence of other atmospheric constituents, inhomogeneous stratification of oxygen in the photosphere, and/or remaining inaccuracies in the treatment of model atoms. The abundance patterns in Feige 24 and in the hot DA white dwarf G191-B2B are remarkably similar, indicating that the same processes are operating equally in both stars. Title: Space Telescope Imaging Spectrograph Coronagraphic Observations of β Pictoris Authors: Heap, Sara R.; Lindler, Don J.; Lanz, Thierry M.; Cornett, Robert H.; Hubeny, Ivan; Maran, S. P.; Woodgate, Bruce Bibcode: 2000ApJ...539..435H Altcode: 1999astro.ph.11363H We present new coronagraphic images of β Pictoris obtained with the Space Telescope Imaging Spectrograph (STIS) in 1997 September. The high-resolution images (0.1") clearly detect the circumstellar disk as close to the star as 0.75", corresponding to a projected radius of 15 AU. The images define the warp in the disk with greater precision and at closer radii to β Pic than do previous observations. They show that the warp can be modeled by the projection of two components: the main disk and a fainter component, which is inclined to the main component by 4°-5° and extends only as far as ~4" from the star. We interpret the main component as arising primarily in the outer disk and the tilted component as defining the inner region of the disk. The observed properties of the warped inner disk are inconsistent with a driving force from stellar radiation. However, warping induced by the gravitational potential of one or more planets is consistent with the data. Using models of planet-warped disks constructed by Larwood & Papaloizou, we derive possible masses of the perturbing object. Based on observations 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 NAS5-26555. Title: The physical foundations of stellar magnetic field diagnosis from polarimetric observations of hydrogen lines Authors: Mathys, G.; Stehlé, C.; Brillant, S.; Lanz, T. Bibcode: 2000A&A...358.1151M Altcode: We have completed a new study of the formation of hydrogen lines in dense magnetized plasmas. We have developed a global formalism including the effects of the magnetic field and of the motional electric field. The latter, resulting from the motion of the radiating hydrogen atom in the magnetic field, modifies the structure of the matrices appearing in the equation of transfer of the Stokes vector and introduces a coupling between microscopic (``intrinsic'') and macroscopic (Doppler) contributions to the line profiles. We have used this new treatment of line formation to revisit the interpretation of photopolarimetric (Stokes I and V) observations of Balmer lines in terms of mean longitudinal magnetic field. Title: The D/H Abundance Ratio in Local Interstellar Gas Authors: Sahu, M. S.; Landsman, W.; Bruhweiler, F. C.; Linsky, J. L.; Hubeny, I.; Barstow, M.; Holberg, J.; Gull, T.; Bowers, C.; Lindler, D.; Lanz, T.; Feggans, K. Bibcode: 2000AAS...196.2620S Altcode: 2000BAAS...32..713S Does the D/H ratio vary within ~ 100 pc of the Sun? If it does, what is the range of these variations? Are the D/H variations correlated to any physical property of the absorbing cloud? To answer these questions we have begun a project to obtain high-precision D/H abundance ratios in the Local Interstellar Medium within ~ 100 pc (Sahu et al. 1999, ApJ 523, L159). For this purpose, we primarily use HST-Space Telescope Imaging Spectrograph (STIS) data towards nearby hot, white dwarfs (WDs). The advantages of using STIS for this study are: (1) The high velocity resolution of STIS in the UV ( ~ 2 km s-1 in the E140H and E230H modes) allows the velocity structure in the sightline to be resolved and reliable component-to-component variations can be studied (2) WDs provide a strong, smooth UV continuum against which other diagnostic interstellar absorption lines can be measured. The wider wavelength coverage ( ~ 200 Angstroms per setting) of STIS in the high-resolution E140H mode allows important interstellar lines such as N i, Si ii, O i and S ii to be measured in addition to D i and H i, at no extra cost in HST time (3) The superior echelle scatter and background corrections possible with the 2-dimensional STIS-MAMA detectors allow more accurate D/H determinations from WD spectra. In this poster, we present a status report of our project along with some recent results. Title: Analysis of the Spectrum of the z=2.73 Galaxy, MS1512-cB58: A Unified Model of the Stellar and Interstellar Contributions Authors: Hubeny, I.; Heap, S. R.; Lanz, T. M. Bibcode: 2000AAS...196.2914H Altcode: 2000BAAS...32R.718H We perform a detailed analysis of the restframe far-UV spectrum of cB58 that was obtained by Pettini et al. (ApJ 528,96, 2000) using the KECK-I/LRIS. In the previous paper (AAS 195, 9.19), we modeled the spectrum of the galaxy and found that the observed spectrum is well described by a starburst model having a SMC-like metallicity and a constant star formation rate. Here, we extend the analysis to model the total spectrum that includes stellar as well as interstellar contributions. The latter is modeled using a newly developed program CLOUDSPEC that combines the photoionization program CLOUDY (Ferland, http://www.pa.uky.edu/ gary/cloudy) with our spectrum synthesis program SYNSPEC (Hubeny & Lanz, http://tlusty.gsfc.nasa.gov). With the ionization structure of the interstellar medium given by CLOUDY, CLOUDSPEC solves the radiative transfer along the line-of-sight to the starburst and yields the profiles of the interstellar absorption lines. We show that such a ``unified'' model of the stellar + interstellar spectrum provides an excellent match to the observed spectrum of cB58. Title: On the Age Estimation of LBDS 53W091 Authors: Yi, Sukyoung; Brown, Thomas M.; Heap, Sara; Hubeny, Ivan; Landsman, Wayne; Lanz, Thierry; Sweigart, Allen Bibcode: 2000ApJ...533..670Y Altcode: 1999astro.ph.11067Y The recent spectral analysis of LBDS 53W091 by Spinrad and his collaborators has suggested that this red galaxy at z=1.552 is at least 3.5 Gyr old. This imposes an important constraint on cosmology, suggesting that this galaxy formed at z>~6.5, assuming recent estimates of cosmological parameters. While their analysis was heavily focused on the use of some UV spectral breaks as age indicators, we have performed χ2 tests to the continuum of this galaxy using its UV spectrum and photometric data (R, J, H, and K: 2000-9000 Å in rest frame). We have used the updated Yi models that are based on the Yale tracks. We find it extremely difficult to reproduce such large age estimates, under the assumption of the most probable input parameters. Using the same configuration as in Spinrad et al. (conventional solar abundance models), our analysis suggests an age of approximately 1.4-1.8 Gyr. We have improved our models over conventional ones by taking into account convective core overshoot in the stellar model calculations and realistic metallicity distributions in the galaxy population synthesis. Overshoot affects the visible continuum normalized to the UV and raises the photometry-based age estimates by 25%. The use of metallicity mixtures affects the whole spectrum and raises all continuum-based age estimates by up to a factor of 2. If the mean metallicity of the stars in this galaxy is assumed to be twice solar, the models including these two effects match the UV spectrum and photometric data of LBDS 53W091 near the age of 1.5-2.0 Gyr. Our results cannot be easily reconciled with that of Spinrad et al. The discrepancy between Spinrad et al.'s age estimate (based on the Jimenez models) and ours originates from the large difference in the model integrated spectrum: the Jimenez models are much bluer than the Yi models and the Bruzual & Charlot models. We propose to apply some viable tests to them for verification and search for the origin of the difference through a more thorough investigation. Considering the significance of the age estimates of distant galaxies as probes of cosmology, it would be an urgent task. Title: VizieR Online Data Catalog: Ages of main-sequence stars (Lachaume+, 1999) Authors: Lachaume, R.; Dominik, C.; Lanz, T.; Habing, H. J. Bibcode: 2000yCat..33480897L Altcode: We have determined the age of a sample of nearby main-sequence stars with spectral types B9-K9. We have derived the stellar ages from five different age estimators: the location in the HR diagram compared to theoretical isochrones, the rotational velocity, the strength of chromospheric calcium emission lines, the stellar metallicity, and their space velocity. New calibrations consistent with recent theoretical isochrones are provided for the last four indicators. For hot stars, isochrones are the best indicator, while stellar rotation is best for cool stars. However, many stars require in fact a combination of different methods to properly bracket their actual age. We also discuss the uncertainties involved, in particular those in using isochrones, and we find that these uncertainties are often underestimated in the literature.

(6 data files). Title: Model Atmospheres and Radiatively Driven Winds of Metal-Poor Stars Authors: Hubeny, Ivan; Lanz, Thierry; Heap, Sally Bibcode: 2000fist.conf..133H Altcode: We present a grid of non-LTE metal-line-blanketed hydrostatic model atmospheres for various effective temperatures, surface gravities (luminosities), and metallicities. The metallicities range from an essentially metal-free, primordial composition to a solar composition. For each model, we calculate the exact radiation force and compare to gravity acceleration. We thus find for each metallicity, the maximum surface gravity as a function of effective temperature required for the radiation force to exceed gravity and thus to initiate a radiatively-driven wind. Title: Properties of the z=2.73 Galaxy, MS1512-cB58 Authors: Heap, S. R.; Bouret, J. -C.; Hubeny, I.; Lanz, T. M. Bibcode: 1999AAS...195.0919H Altcode: 1999BAAS...31.1387H MS1512-cB58 is an z=2.73 galaxy whose apparent brightness is amplified by 30X due to gravitationally lensing. The restframe far-UV spectrum of cB58 that was obtained by Keck-I/LRIS is ``one of the best ultraviolet spectra of a starburst galaxy obtained at any redshift'' (Pettini et al. 1999, astro-ph/9908007). We have therefore analyzed and modelled the spectrum of this galaxy in order to obtain information about the stellar content of high-redshift galaxies. We find that our model spectrum is a near match to the observed spectrum of cB58 if we assume a constant rate of star formation, a SMC-like metallicity, and a Salpeter IMF extending up to 100 Msun. In our paper, we will present the details of the spectral synthesis (based on both observed and model spectra of OB stars), and we will investigate the reliability of current evolutionary tracks (based on single, non-rotating stars) for interpreting the spectra of star-forming galaxies. Title: Age determinations of main-sequence stars: combining different methods Authors: Lachaume, R.; Dominik, C.; Lanz, T.; Habing, H. J. Bibcode: 1999A&A...348..897L Altcode: We have determined the age of a sample of nearby main-sequence stars with spectral types B9-K9. We have derived the stellar ages from five different age estimators: the location in the HR diagram compared to theoretical isochrones, the rotational velocity, the strength of chromospheric calcium emission lines, the stellar metallicity, and their space velocity. New calibrations consistent with recent theoretical isochrones are provided for the last four indicators. For hot stars, isochrones are the best indicator, while stellar rotation is best for cool stars. However, many stars require in fact a combination of different methods to properly bracket their actual age. We also discuss the uncertainties involved, in particular those in using isochrones, and we find that these uncertainties are often underestimated in the literature. Title: UV Spectroscopic Dating of Stars and Galaxies Authors: Heap, Sara; Brown, Thomas; Lanz, Thierry; Yi, Sukyoung Bibcode: 1999Ap&SS.265..531H Altcode: 1997astro.ph.11074H An echelle spectrogram (R = 30,000) of the 2300-3100 A region in the ultraviolet spectrum of the F8V star 9 Comae is presented. The observation is used to calibrate features in the mid-ultraviolet spectra of similar stars according to age and metal content. In particular, the spectral break at 2640 A is interpreted using the spectral synthesis code SYNSPEC. We use this feature to estimate the time since the last major star formation episode in the z=1.55 early-type galaxy LBDS 53W091, whose rest frame mid-ultraviolet spectrum, observed with the Keck Telescope, is dominated by the flux from similar stars that are at or near the main-sequence turnoff in that system (Spinrad et al. 1997). Our result, 1 Gyr if the flux-dominating stellar population has a metallicity twice solar, or 2 Gyr for a more plausible solar metallicity, is significantly lower than the previous estimate and thereby relaxes constraints on cosmological parameters that were implied by the earlier work. Title: Hot stellar population synthesis from the UV spectrum: the globular cluster M79 (NGC 1904) Authors: Vink, Jorick S.; Heap, Sara R.; Sweigart, Allen V.; Lanz, Thierry; Hubeny, Ivan Bibcode: 1999A&A...345..109V Altcode: 1999astro.ph..3258V We have analyzed the far UV-spectrum of the globular cluster M79. We show that the nearly Gaussian mass distribution of zero-age horizontal-branch stars, as derived by Dixon et al. (1996), is able to reproduce the far-UV Hopkins Utraviolet Telescope (HUT) spectrum, if there is a luminous UV-bright star of about T_eff\ = 9,500 K within the HUT entrance slit, or, more likely, if the horizontal branch morphology becomes considerably redder in the core of M79, as observed in some other centrally condensed globular clusters. Agreement between the synthetic and observed far-UV spectra for M79 would also be improved if the surface abundances of the heavy elements in the hot horizontal-branch stars were enhanced by radiative diffusion. Contrary to Dixon et al. (1996) we do not need extremely low gravities to reproduce the width of Ly alpha . Title: Detailed non-LTE Analysis of the High-S/N STIS Spectrum of the Hot Subdwarf, BD+28{(deg}4211) Authors: Hubeny, I.; Lanz, T.; Haas, S.; Heap, S. R.; Lindler, D. J.; Kaiser, M. E. Bibcode: 1999AAS...194.6702H Altcode: 1999BAAS...31..930H During the course of commissioning the Space Telescope Imaging Spectrograph (STIS), observations were obtained to demonstrate the feasibility of achieving very high S/N spectra in the ultraviolet with the MAMA detectors. The hot subdwarf, BD+28{(deg}4211) , was selected as the test case. The two series of FP-SPLIT observations (E140M and E230M) have been shown to provide spectra with S/N=370 per resolution element in both the far-UV and near-UV. The observations were reduced by an iterative procedure that uses models of all relevant scatter sources including echelle scatter, MAMA detector halo and OTA scatter. We have performed a detailed analysis of the spectrum, using sophisticated non-LTE model atmospheres including several million metal lines in non-LTE. First, we derive basic parameters of this star (effective temperature, surface gravity, chemical composition) and compare to earlier results. We present a detailed comparison of predicted and observed spectra in order to provide a benchmark for future detailed spectroscopic studies of hot compact objects. Finally, we present a preliminary analysis of the effects of elemental abundance stratification on the selected line profiles with the aim of studying the detectability of non-homogeneous abundance patterns. Title: UV Spectroscopic Dating of Stars and Galaxies Authors: Heap, S.; Brown, T.; Lanz, T.; Yi, S. Bibcode: 1999gecd.conf..531H Altcode: We use STIS high-resolution spectra (R ~30,000) of well-studied F-type stars to calibrate ultraviolet spectral features in terms of age and metallicity. We apply our results to Keck spectra of "Extremely Red Objects" such as the early-type galaxy LBDS 53W091 at a redshift of z = 1.55. Title: Dating intermediate-age populations with main-sequence A and F-type stars Authors: Lanz, T.; Heap, S.; Brown, T. M.; Hubeny, I.; Yi, S. Bibcode: 1999ASPC..192..106L Altcode: 1999sdsg.conf..106L No abstract at ADS Title: On the Age Estimation of High Redshift Galaxies Authors: Yi, S.; Brown, T. M.; Heap, S.; Hubeny, I.; Landsman, W.; Lanz, T.; Sweigart, A. Bibcode: 1999ASPC..192..126Y Altcode: 1999sdsg.conf..126Y No abstract at ADS Title: On the Age Estimation of LBDS 53W091 Authors: Yi, S.; Brown, T.; Heap, S.; Hubeny, I.; Landsman, W.; Lanz, T.; Sweigart, A. Bibcode: 1999ASPC..193..185Y Altcode: 1999hrug.conf..185Y No abstract at ADS Title: Mid-UV Spectroscopic Dating of LBDS 53W091 Authors: Heap, S. R.; Lanz, T. M.; Brown, T.; Hubeny, I. Bibcode: 1999ASPC..193..167H Altcode: 1999hrug.conf..167H No abstract at ADS Title: Non-LTE model atmospheres of hot DA white dwarfs Authors: Hubeny, I.; Barstow, M. A.; Lanz, T.; Holberg, J. B. Bibcode: 1999ASPC..169..445H Altcode: 1999ewwd.conf..445H No abstract at ADS Title: New insight in the diagnosis of stellar magnetic fields by polarimetry of hydrogen lines Authors: Brillant, S.; Lanz, T.; Mathys, G.; Stehl, S. Bibcode: 1999AAS...19312402B Altcode: 1999BAAS...31..668B The study of hydrogen lines in polarized light is a powerful tool for the detection and the study of stellar magnetic fields. In the case of fast rotating stars it is even the only method (Landstreet, 1982, ApJ, 258, 639). However, some discrepancies are observed between the values of the field deduced from such observations and those obtained from the observation of the polarization in metal lintes (Mathys, 1989, Fundam. Cosmic. Phys., 13, 143). In order to explain these discrepancies, we used the formalism for the formation of hydrogen lines in magnetized plasmas we had developed (Brillant et al., 1998, A&A, 339, 286), to compute synthetic line profiles in all the four Stokes parameters for a typical magnetic Ap star model. We present here the preliminary conclusions of the application of this method for the diagnosis of stellar magnetic fields. Title: Age and Metallicity of F-type Stars Authors: Lanz, T.; Heap, S. R.; Brown, T. M.; Hubeny, I.; Yi, S. Bibcode: 1998AAS...193.0302L Altcode: 1998BAAS...30.1246L The rest-frame ultraviolet spectrum of intermediate-age stellar populations (1 to few Gyr) is dominated by the UV flux of F-type stars at the main sequence turn-off. We have obtained high-resolution spectra of four well-studied F stars with STIS aboard HST to calibrate ultraviolet spectral features in terms of age and metallicity. The UV spectrum is mostly degenerate in terms of effective temperature and metallicity, but we found that two spectral indices, [2850/2770] and [2310/3040], allow a discrimination between these two parameters. The first index measures mainly the strength of Mg I 2852, while the second index is the mid-UV color. We find that the Mg I index is sensitive to the effective temperature, but insensitive to metallicity; the UV color is sensitive to both metallicity and T_eff. We discuss the importance of departures from LTE and chromospheric activity on these two spectral indices. We apply our results to estimate the time since the last major star formation episode in the early-type galaxy LBDS 53W091 at redshift z=1.55. Title: Multidimensional, non-LTE Radiation Transport in Astrophysical Media Authors: Hubeny, I.; van Noort, M.; Lanz, T. Bibcode: 1998AAS...193.6918H Altcode: 1998BAAS...30.1352H We present first results of our long-term program whose ultimate goal is to develop multidimensional, self-consistent, non-LTE radiation hydrodynamic and magnetohydrodynamic spectroscopic diagnostics of astronomical objects. As the first step, we have developed an efficient and robust 2-D and 3-D radiation transfer solver, based on the short characteristics method. We assume the source function either to be fully specified (i.e., the so-called formal solution of the transfer equation), or to be given in terms of the equivalent-two-level atom formalism. In the latter case, we have adopted the idea of the Accelerated Lambda Iteration technique to treat iteratively the non-LTE coupling of radiation and matter. We compare the efficiency and numerical accuracy of several variants of the short-characteristics scheme. Finally, we present some illustrative examples of radiative transfer for 2-D and 3-D media with the specified temperature, density, and velocity structure. Title: Fe II emission lines in the UV spectrum of Sirius-A and VEGA Authors: van Noort, M.; Lanz, T.; Lamers, H. J. G. L. M.; Kurucz, R. L.; Ferlet, R.; Hebrard, G.; Vidal-Madjar, A. Bibcode: 1998A&A...334..633V Altcode: 1998astro.ph..3127V We present high-quality HST/GHRS spectra in the Hydrogen Lalpha spectral region of Vega and Sirius-A. Thanks to the signal-to-noise ratio achieved in these observations and to the similarity of the two spectra, we found clear evidence of emission features in the low flux region, lambda lambda 1190-1222 Angstroms. These emission lines can be attributed unambiguously to Fe Ii and Cr Ii transitions. In this spectral range, silicon lines are observed in absorption. We built a series of non-LTE model atmospheres with different, prescribed temperature stratification in the upper atmosphere and treating Fe Ii with various degrees of sophistication in non-LTE. Emission lines are produced by the combined effect of the Schuster mechanism and radiative interlocking, and can be explained without the presence of a chromosphere. Silicon absorption lines and the Lalpha profile set constraints on the presence of a chromosphere, excluding a strong temperature rise in layers deeper than tau_R ~ 10(-4) . Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555. Title: Non-LTE Line-Blanketed Model Atmospheres of O Stars Authors: Hubeny, I.; Heap, S. R.; Lanz, T. Bibcode: 1998ASPC..131..108H Altcode: 1998phls.conf..108H No abstract at ADS Title: Ultraviolet Spectral Dating of Stars and Galaxies Authors: Heap, S. R.; Brown, T. M.; Hubeny, I.; Landsman, W.; Yi, S.; Fanelli, M.; Gardner, J. P.; Lanz, T.; Maran, S. P.; Sweigart, A.; Kaiser, M. E.; Linsky, J.; Timothy, J. G.; Lindler, D.; Beck, T.; Bohlin, R. C.; Clampin, M.; Grady, J.; Loiacono, J.; Krebs, C. Bibcode: 1998ApJ...492L.131H Altcode: An echelle spectrogram (R = 30,000) of the 2300-3100 Å region in the ultraviolet spectrum of the F8 V star 9 Comae is presented. The observation is used to calibrate features in the mid-ultraviolet spectra of similar stars according to age and metal content. In particular, the spectral break at 2640 Å is interpreted using the spectral synthesis code SYNSPEC. We use this feature to estimate the time since the last major star formation episode in the early-type galaxy LBDS 53W091 at redshift z=1.55, whose rest-frame mid-ultraviolet spectrum, observed with the Keck Telescope, is dominated by the flux from similar stars that are at or near the main-sequence turnoff in that system (Spinrad et al.). Our result, 1 Gyr if the flux-dominating stellar population has a metallicity twice solar, or 2 Gyr for a more plausible solar metallicity, is significantly lower than the previous estimate and thereby relaxes constraints on cosmological parameters that were implied by the earlier work.

Based on observations 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 NAS5-2655. Title: Axisymmetric Model Atmospheres of B[e] Stars Authors: Lanz, T. Bibcode: 1998ASSL..233..229L Altcode: 1998best.work..229L No abstract at ADS Title: Non-LTE Line-blanketed Model Atmospheres of Hot Stars. III. Hot Subdwarfs: The sdO Star BD +75°325 Authors: Lanz, Thierry; Hubeny, Ivan; Heap, Sara R. Bibcode: 1997ApJ...485..843L Altcode: We have made a detailed comparison of results of spectroscopic analysis using three differents types of model atmospheres: classical non-LTE H-He models; approximate non-LTE line-blanketed models (with only a subset of Fe and Ni lines: those originating from transitions between levels with measured energies); and non-LTE fully blanketed models. The three models were applied to the sdO star BD +75°325, adopted as a test case.

We demonstrate that the effects of line blanketing are very important: the best fit of the observed H and He lines is achieved for Teff = 58,000 K when using H-He models, Teff = 55,000 K for approximate non-LTE line-blanketed models, and Teff = 52,000 K for fully blanketed non-LTE models.

Using the high-resolution Goddard High Resolution Spectrograph spectrum of BD +75°325 and our final fully blanketed model, we have derived reliable abundances of He, C, N, O, Si, Fe, and Ni. We find that BD +75°325 is an He-enriched star (He/H = 1 by number), whose surface exhibits CNO-cycle products, i.e., N-rich (AN = 1.5 × 10-3 by number, or 4.2 × 10-3 by mass fraction), and C and O deficient (by factor of about 100 with respect to the solar value). We also find a significant surface depletion of silicon and an enhancement of iron and nickel. We argue that these anomalous abundances reflect some mixing with processed material from the core, with subsequent modification at the surface by diffusion processes. Finally, BD +75°325 possesses a weak wind. Using a simplified description of the wind, we have derived a preliminary value of the mass loss rate: Ṁ=1.5×10-11 M⊙ yr-1.

Based on observations with the ESA/NASA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555. Title: Hubble Space Telescope Ultraviolet Spectroscopy of Two Hot White Dwarfs Authors: Holberg, J. B.; Barstow, M. A.; Lanz, T.; Hubeny, I. Bibcode: 1997ApJ...484..871H Altcode: We have obtained Hubble Space Telescope high-dispersion spectroscopy of two hot white dwarfs using the Goddard High-Resolution Spectrograph. The stars were selected on the basis of EUV flux as examples of H-rich DA stars containing high and low levels of short-wavelength opacity. The high-opacity star, REJ 1614-085, possesses strong features due to C IV, Si IV, and N V. Carbon and silicon in this star are underabundant with respect to the predictions of radiative levitation; nitrogen, however, is superabundant. This is in sharp contrast to the apparently similar DA star GD 394, where silicon is in extreme overabundance. The low-opacity star PG 1057+719, as expected, shows no clear evidence of these or any other heavy ions. We find evidence of weak blueshifted components in the REJ 1614-085 C IV and Si IV lines, which may indicate the star is experiencing ongoing mass loss.

Based on observations 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, Incorporated under NASA contract NAS 5-26555.

Observations reported here have been obtained in part with the Multiple Mirror Telescope, a joint facility of the University of Arizona and Smithsonian Institution. Title: The variations of the BP star HD 137509. Authors: Mathys, G.; Lanz, T. Bibcode: 1997A&A...323..881M Altcode: A refined value P=(4.4916+/-0.0002) d is derived for the rotation period of the Bp star HD 137509, from the simultaneous consideration of photometric data recorded in the Geneva system and of measurements of the mean longitudinal magnetic field and of the crossover. The variations of the magnetic field (one of the most intense fields known in an Ap or Bp star) show the signature of a strong quadrupolar component. This remarkable property is reflected in the brightness and line equivalent width variations. Title: The mean magnetic field modulus of AP stars Authors: Mathys, G.; Hubrig, S.; Landstreet, J. D.; Lanz, T.; Manfroid, J. Bibcode: 1997A&AS..123..353M Altcode: We present new measurements of the mean magnetic field modulus of a sample of Ap stars with spectral lines resolved into magnetically split components. We report the discovery of 16 new stars having this property. This brings the total number of such stars known to 42. We have performed more than 750 measurements of the mean field modulus of 40 of these 42 stars, between May 1988 and August 1995. The best of them have an estimated accuracy of 25 - 30 G. The availability of such a large number of measurements allows us to discuss for the first time the distribution of the field modulus intensities. A most intriguing result is the apparent existence of a sharp cutoff at the low end of this distribution, since no star with a field modulus (averaged over the rotation period) smaller than 2.8 kG has been found in this study. For more than one third of the studied stars, enough field determinations well distributed throughout the stellar rotation cycle have been achieved to allow us to characterize at least to some extent the variations of the field modulus. These variations are often significantly anharmonic, and it is not unusual for their extrema not to coincide in phase with the extrema of the longitudinal field (for the few stars for which enough data exist about the latter). This, together with considerations on the distribution of the relative amplitude of variation of the studied stars, supports the recently emerging evidence for markedly non-dipolar geometry and fine structure of the magnetic fields of most Ap stars. New or improved determinations of the rotation periods of 9 Ap stars have been achieved from the analysis of the variations of their mean magnetic field modulus. Tentative values of the period have been derived for 5 additional stars, and lower limits have been established for 10 stars. The shortest definite rotation period of an Ap star with magnetically resolved lines is 3.4 deg, while those stars that rotate slowest appear to have periods in excess of 70 or 75 years. As a result of this study, the number of known Ap stars with rotation periods longer than 30 days is almost doubled. We briefly rediscuss the slow-rotation tail of the period distribution of Ap stars. This study also yielded the discovery of radial velocity variations in 8 stars. There seems to be a deficiency of binaries with short orbital periods among Ap stars with magnetically resolved lines. Based on observations collected at the European Southern Observatory (La Silla, Chile; ESO programmes Nos. 43.7-004, 44.7-012, 49.7-030, 50.7-067, 51.7-041, 52.7-063, 53.7-028, 54.E-0416, and 55.E-0751), at the Observatoire de Haute-Provence (Saint-Michel-l'Observatoire, France), at Kitt Peak National Observatory, and at the Canada-France-Hawaii Telescope. Tables 2, 3, and 4 are also 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/Abstract.html. Title: Interstellar and photospheric opacity from EUV spectroscopy of DA white dwarfs Authors: Barstow, M. A.; Dobbie, P. D.; Holberg, J. B.; Hubeny, I.; Lanz, T. Bibcode: 1997MNRAS.286...58B Altcode: We present a detailed analysis of the extreme-ultraviolet (EUV) spectra of 13 hydrogen-rich DA white dwarfs, observed by the Extreme Ultraviolet Explorer (EUVE) satellite, paying attention to the possible sources of absorbing material along the lines of sight both in the local interstellar medium (ISM) and in the photospheres of the stars themselves. The range of interstellar column densities seen are consistent with our previous understanding of the local distribution of material. Absorption from interstellar Heii is found in the direction of five stars, allowing us to measure directly the He ionization fraction and estimate, indirectly, that of H. The weighted mean ionization fractions along these lines of sight are 0.27+/-0.04 and 0.35+/-0.1 respectively. Where Heii is directly detected, the observed ionization fractions are not correlated with direction or with the volume/column density of material along the line of sight. Furthermore, the limits on the amount of Heii established in all other directions completely encompass the range of observed values. Indeed, all the data can be consistent with more or less constant He and H ionization fractions throughout the local ISM. It is clear that there is little photospheric opacity, from either He or heavier elements, in the majority of the stars we have studied. This poses further difficulties in explaining the observed division of white dwarfs into H- and He-rich groups, the temperature gap in the He-rich sequence and the detailed spectral evolution of the H-rich DA white dwarfs as they cool. A striking observational result is that our spectroscopic evidence indicates that radiative levitation effects are only important at temperatures above 50000K, rather than the 40000K suggested by broad-band photometry. There is clearly an urgent need for further theoretical work on the mechanisms that determine the photospheric composition of white dwarf stars. Title: NLTE Line Blanketed Model Atmospheres of A-Type Stars Authors: Hubeny, I.; Lanz, T. Bibcode: 1997IAUJD..16E.102H Altcode: We present new non-LTE metal line-blanketed model atmospheres of A-type stars. We show that thanks to the recent progress in developing fast and robust numerical schemes, such as the hybrid complete linearization/accelerated lambda iteration method, one may now construct model atmospheres of unprecedented degree of complexity and realism. Other basic ingredient of the recent progress are the new atomic data calculations, like the Opacity and Iron Projects, which provide a substantial quantity of high-accuracy data. In this review we discuss the influence of the non-LTE approach, the influence of metal line blanketing, and the impact of new atomic data on model atmospheres and predicted spectra of A-type stars. Title: VizieR Online Data Catalog: Variations of HD 137509 (Mathys+ 1997) Authors: Mathys, G.; Lanz, T. Bibcode: 1997yCat..33230881M Altcode: A refined value P=(4.4916+/-0.0002) d is derived for the rotation period of the Bp star HD 137509, from the simultaneous consideration of photometric data recorded in the Geneva system and of measurements of the mean longitudinal magnetic field and of the crossover. The photometric data are given in Table 1. For a description of the Geneva photometric system, see e.g. <GCPD/13> (1 data file). Title: Heavy elements in white dwarf envelopes Authors: Barstow, M. A.; Holberg, J. B.; Hubeny, I.; Lanz, T. Bibcode: 1997ASSL..214..237B Altcode: 1997whdw.work..237B No abstract at ADS Title: VizieR Online Data Catalog: Mean magnetic field modulus of Ap stars (Mathys, 1997) Authors: Mathys, G.; Hubrig, S.; Landstreet, J. D.; Lanz, T.; Manfroid, J. Bibcode: 1997yCat..41230353M Altcode: The paper reports about a systematic study of the mean magnetic field modulus of Ap stars with magnetically resolved split lines. Tables 2a and 2b present the main properties of all such stars presently known. The results about their mean magnetic field modulus derived in this study are summarized in Table 3, while the individual field measurements are given in Table 4.

(4 data files). Title: A Self-consistent Optical, Ultraviolet, and Extreme-Ultraviolet Model for the Spectrum of the Hot White Dwarf G191-B2B Authors: Lanz, T.; Barstow, M. A.; Hubeny, I.; Holberg, J. B. Bibcode: 1996ApJ...473.1089L Altcode: The star G191-B2B is one of a number of hot DA white dwarfs whose atmospheres have been found to contain significant quantities of heavy elements, including C, N, 0, Si, Fe, and Ni. Several earlier studies have measured their abundances using IUE echelle data in conjunction with synthetic spectra derived from theoretical model atmosphere calculations of varying degrees of sophistication. However, predictions of the EUV spectrum based on these observations failed completely to match either its shape or absolute flux level. We present here the results of new non-LTE model calculations including the effects of line blanketing from more than 9 million (mainly Fe and Ni) transitions. For the first time, we are able to find an effective temperature and composition that can consistently match the optical, FUV and EUV data. However, to maintain this agreement below the He II λ228 Lyman limit, it is necessary to incorporate additional He II opacity in the form of photo spheric, circumstellar or interstellar material. Title: Toward Resolving the ``Mass Discrepancy'' in O-Type Stars Authors: Lanz, Thierry; de Koter, Alex; Hubeny, Ivan; Heap, Sara R. Bibcode: 1996ApJ...465..359L Altcode: We show that metal line blanketing has an important effect on the atmospheres of hot stars, and we argue that the omission of metal line blanketing in previous non-LTE model atmospheres of 0-type stars has led to underestimates of surface gravity and stellar mass. In addition to wind emission, metal line blanketing contributes to the solution of the long-standing discrepancy between spectroscopic and evolutionary masses for 0-type stars.

To support our argument, we calculated a series of non-LTE stellar atmospheres for 0 and Of-type stars. We compared the predicted profiles of hydrogen and helium lines produced by (1) a static plane- parallel H-He model, (2) a metal line-blanketed static model, and (3) an extended expanding model atmosphere. We find that simple H-He models produce stronger lines than do our metal line-blanketed models. Consequently, they lead to underestimated gravities. Wind emission is more effective in filling in the wings of H or He lines in the case of large mass-loss rates typical of extreme Of stars. These findings apply equally to young, massive 0-type stars and to highly evolved stars, i.e., central stars of planetary nebulae. The lower gravities derived from H-He models yield underestimated masses for young 0-type stars, and overestimated masses for highly evolved stars. Title: Photoionization resonances of SiII in stellar spectra. Authors: Lanz, T.; Artru, M. -C.; Le Dourneuf, M.; Hubeny, I. Bibcode: 1996A&A...309..218L Altcode: We have studied the influence of the Si^+^ photoionization on stellar atmospheres. Recent calculations using the R-matrix code provide detailed cross-sections with autoionization resonances for the first 50 energy levels of Si^+^. These photoionization cross-sections are included in the calculation of the far ultraviolet synthetic spectrum of A and late B-type stars. The opacity from the six lowest excited states of Si^+^ provides observable features in the emergent spectrum of a solar-composition A0 star. The photoionization from higher states of Si^+^ should be included when modeling hotter or silicon-rich stars. These new cross-sections increase dramatically the opacity in the far UV spectrum of Ap Si stars and allow to reproduce the most characteristic UV features of these stars, as illustrated by a comparison with IUE data for the hot Ap Si star HD 34452. Broad features in its spectrum are definitely assigned to Si ii autoionization resonances, and the flux deficiency below 130nm is well matched with a predicted continuum edge near this wavelength. The successful stellar spectrum synthesis brings a strong support to the R-matrix photoionization calculations, which cannot otherwise be compared to laboratory data. Title: Solving the mystery of the heavy-element opacity in the DA white dwarf GD 394. Authors: Barstow, M. A.; Holberg, J. B.; Hubeny, I.; Lanz, T.; Bruhweiler, F. C.; Tweedy, R. W. Bibcode: 1996MNRAS.279.1120B Altcode: The white dwarf GD 394 is one of a very small number of bright hot DAs which have detectable quantities of highly ionized heavy elements in their spectra. Whether this material resides in the photosphere of GD 394 or is in the form of a circumstellar shell has been the subject of much debate, with the large discrepancy between the published radial velocity of the star and the measured velocities of the Si III/IV lines having apparently settled the argument in favour of the latter idea. However, GD 394 is also the coolest DA white dwarf in the ROSAT sky survey showing convincing evidence for the presence of absorbing material which strongly suppresses the EUV continuum. This result has reopened the argument about the nature of the silicon lines. From a combined analysis of the IUE, HST and EUVE spectra, coupled with new measurements of the photospheric radial velocity from Hα data, the authors have established that the Si III and Si IV lines are indeed photospheric. Furthermore, other heavy elements must be present to account for the EUV opacity, but at levels below the thresholds for detecting absorption lines in either far-UV or EUV bands. Title: Non-LTE spectroscopic analysis of the wind of the central star of NGC 6543 Authors: de Koter, A.; Hubeny, I.; Heap, S. R.; Lanz, T. Bibcode: 1996ASPC...96..141D Altcode: 1996hds..conf..141D No abstract at ADS Title: Spectroscopic Constraints on the Helium Abundance in Globular Cluster Stars Authors: Landsman, W. B.; Crotts, A. P. S.; Hubeny, I.; Lanz, T.; O'Connell, R. W.; Whitney, J.; Stecher, T. P. Bibcode: 1996ASPC...99..199L Altcode: 1996coab.proc..199L No abstract at ADS Title: EUV Radiation from Hot Star Photoepheres: Theory Versus Observations Authors: Hubeny, Ivan; Lanz, Thierry Bibcode: 1996aeu..conf..381H Altcode: 1996IAUCo.152..381H No abstract at ADS Title: Fully line-blanketed NLTE model atmospheres for hot hydrogen-deficient stars Authors: Hubeny, I.; Lanz, T. Bibcode: 1996ASPC...96..249H Altcode: 1996hds..conf..249H No abstract at ADS Title: The Composition and Structure of White Dwarf Atmospheres Revealed by Extreme Ultraviolet Spectroscopy Authors: Barstow, Martin A.; Hubeny, Ivan; Lanz, Thierry; Holberg, Jay B.; Sion, Edward M. Bibcode: 1996aeu..conf..203B Altcode: 1996IAUCo.152..203B No abstract at ADS Title: Atomic data for non-LTE model stellar atmospheres Authors: Lanz, T.; Hubeny, I.; de Koter, A. Bibcode: 1996PhST...65..144L Altcode: We briefly review the state-of-the-art in non-LTE modeling of hot star atmospheres. During the last decade, the development of a new class of radiation transfer methods, based on the Accelerated Lambda Iteration method, have brought considerable progress in this field. Severe limitations of the size of model atoms used in non-LTE model atmospheres have been removed, so that realistic model atoms can now be considered. In recent years, the problem of non-LTE line blanketing, which aims to include the influence of thousands to millions of lines on the atmospheric structure, has been attacked, and first non-LTE line-blanketed models have been calculated. We illustrate the importance of non-LTE line blanketing with the example of a hot subdwarf, BD +75° 325.

As a consequence of these developments, there is a need for extensive radiative and collisional atomic databases to build non-LTE models. Recent projects, like the Opacity Project, provide a substantial quantity of the necessary data. We discuss the current needs of atomic data for the purpose of building reliable non-LTE line-blanketed model atmospheres. A critical evaluation of the quality of such extensive sets of atomic data is essential. Finally, we present a new, IDL-based, graphic tool intended to facilitate the manipulation of large amounts of data needed to build the realistic model atoms that we use. Title: Support for the Primordial Helium Abundance Derived from Observation of Globular Cluster Stars Authors: Stecher, T. P.; Landsman, W.; Crotts, A. P. S.; Whitney, J.; O'Connell, R. W.; Lanz, T.; Hubeny, I.; Sweigart, A. Bibcode: 1995AAS...187.8606S Altcode: 1995BAAS...27.1414S The UIT on Astro-1 found nearly 2000 hot stars in the globular cluster Omega Centauri (Whitney et al. 1994, AJ, 108, 1350), of which 28 are at least 1 mag brighter than the horizontal branch (HB). We have obtained CTIO 4m and/or IUE low-dispersion spectra of eleven of the brightest of these stars. All seven stars observed at CTIO are radial velocity members of the clusters. Three of these stars have log Teff > 60,000 K and show only Balmer lines and He II lines. These stars, which appear to be in the post-asymptotic giant branch phase, are the hottest stars ever found in a globular cluster. The other four stars have log Teff ~ 20,000 K and show a B-type spectrum with numerous He I lines but none of He II. We have estimated the helium abundance by comparison with hydrogen-helium NLTE model stellar atmospheres. The He II lines in the hottest three stars in our sample appear to be best fit by models with a He/H ratio of about 0.1. This helium abundance should reflect the primordial helium abundance of the gas from which Omega Cen formed nearly 13 x 10(9) yr ago, except for the modest helium enrichment caused by the first dredge-up during the red giant branch phase. By analyzing these stars we have been able to derive a helium abundance for a globular cluster which is independent of the well-known R-method and which should provide an estimate of the helium abundance produced by the Big Bang. The four cooler stars in our sample show a quite different pattern of helium abundances. The three faintest of these stars have a substantially lower helium abundance which we attribute to the effects of diffusion during the preceding HB phase. These stars appear to have recently evolved off of the blue end of the HB. In contrast, the most luminous of these stars is very helium rich (He/H = 0.55), indicating that some UV-bright stars undergo interior mixing during their preceding AGB evolution. Title: VizieR Online Data Catalog: Photoelectric Photometry in Johnson UBVRI System (Lanz 1986) Authors: Lanz, T. Bibcode: 1995yCat.2116....0L Altcode: The catalog is a compilation of photoelectric photometry in the Johnson UBVRI system published prior to mid-1984. The catalog contains a total of 6849 stars in the original Johnson system (Morel and Magnenat 1978), 467 stars in the system of Kunkel and Rydgren (1979), 1150 stars in the system of Moffett and Barnes III (1979), and 252 stars in the system of Neckel and Chini (1980). The catalog is in 14 files: The first 12 files list observational data, mean values, and references for each of the four standard systems. An additional two files contain the numbering scheme of Mermilliod (1978) used to identify objects, and introductory information. (9 data files). Title: HST/GHRS Observations of the beta Pictoris System: Basic Parameters of the Age of the System Authors: Lanz, Thierry; Heap, Sara R.; Hubeny, Ivan Bibcode: 1995ApJ...447L..41L Altcode: We have reevaluated the basic parameters and age of the A5 IV--V star, beta Pictoris, making use of new observations obtained by the Goddard High-Resolution Spectrograph aboard the Hubble Space Telescope and using modern methods of atmospheric modeling and spectral synthesis. We derive stellar parameters appropriate to its spectral type (Teff = 8200 +/- 150 K, log g = 4.25 +/- 0.1) including a normal (solar) metallicity. When dated by comparison with current evolutionary tracks, we find that (1) the star is apparently "underluminous"---a situation we attribute to (gray) extinction by the disk---and (2) the star is either a pre--main-sequence (PMS) star nearing the zero-age main sequence (ZAMS), or it is a main-sequence star older than 0.3 Gyr. Given the apparent youth of the disk as inferred from its high density, we judge the PMS interpretation as most likely. Title: The UV-Bright Stars of Omega Centauri Authors: Landsman, W. B.; Crotts, A.; O'Connell, R. W.; Whitney, J. H.; Lanz, T.; Stecher, T. P. Bibcode: 1995AAS...186.1203L Altcode: 1995BAAS...27..828L Images of the globular cluster Omega Centauri obtained with the Ultraviolet Imaging Telescope (UIT) in 1990 revealed numerous hot stars more luminous than zero-age horizontal branch (Whitney et al. 1994, AJ, 108, 1350). We have obtained CTIO 4m and IUE low-dispersion spectra of seven of the brightest stars in the Whitney et al. catalog. The target stars include UIT-1 and UIT-2 in the core of Omega Cen (Landsman et al. 1992 ApJL, 395, L21), as well as ROA 5342, Dk 3873, and Dk 3089 from the catalog of Dickens (1988). All of the target stars are found to be radial velocity members of the cluster. Three of the stars (ROA 5342, UIT-151, Dk 3873) show strong He II lines in their spectra and are probably very hot (> 50,000 K) post-AGB stars. The remaining four stars show strong He I lines, and UIT-1 also shows numerous nitrogen lines. We present results of an atmospheric analysis to constrain the reddening, effective temperatures, and helium abundances. Title: Eclipse observations of an accretion disc wind Authors: Mason, K. O.; Drew, J. E.; Cordova, F. A.; Horne, Keith; Hilditch, R.; Knigge, C.; Lanz, T.; Meylan, T. Bibcode: 1995MNRAS.274..271M Altcode: The Goddard High Resolution Spectrometer on the Hubble Space Telescope has been used to monitor, through eclipse, the C iv 1550-A and He ii 1640-A lines in the spectrum of the nova-like cataclysmic variable UX UMa. Previous work has suggested that the Civ line arises in an accretion disc wind. The HST data confirm this, in that the line shows a broad, asymmetric emission profile. However, superimposed on the emission are previously unsuspected narrow absorption components at the rest wavelengths of the doublet. The absorption disappears during the continuum eclipse, causing the net continuum-subtracted flux near line centre to increase by about 20 per cent during this time. The mid-eclipse line profile shows the classic asymmetric shape expected for a bipolar wind viewed edge-on. The various wavelength segments within the C iv line exhibit different eclipse light curves. The flux at line centre declines gradually, beginning at least 0.05 in orbital phase before the continuum eclipse, then rising sharply coincident with continuum ingress due to loss of the absorption components. Flux is also lost from the red wing of the C iv line beginning just before (Deltaphi ~ 0.02) the onset of continuum eclipse. The red wing flux declines more slowly than the continuum itself, reaching a minimum after the time of continuum mid-eclipse. This suggests that the emitting gas may have a significant rotational component about the white dwarf. However, the blue wing shows no comparable signature of rotation. These data show that the shallowness of the line flux eclipse is primarily due to occultation of regions of net absorption, with the implication that the line-forming region can be smaller than the companion star. To explain the appearance and eclipse evolution of the C iv line profile solely in terms of outflow will require a highly collimated disc wind. Alternatively the narrow, superposed absorption components may originate elsewhere in the system. These data also provide the first clear demonstration that He ii 1640 A is mainly a wind-formed feature. Title: Non-LTE Line-blanketed Model Atmospheres of Hot Stars. II. Hot, Metal-rich White Dwarfs Authors: Lanz, T.; Hubeny, I. Bibcode: 1995ApJ...439..905L Altcode: We present several model atmospheres for a typical hot metal-rich DA white dwarf, Teff = 60,000 K, log g = 7.5. We consider pure hydrogen models, as well as models with various abundances of two typical 'trace' elements-carbon and iron. We calculte a number of Local Thermodynamic Equilibrium (LTE) and non-LTE models, taking into account the effect of numerous lines of these elements on the atmospheric structure. We demostrate that while the non-LTE effects are not very significant for pure hydrogen models, except for describing correctly the central emission in H-alpha they are essential for predicting correctly the ionization balance of metals, such as carbon and iron. Previously reported discrepancies in LTE abundances determinations using C III and C IV lines are easily explained by non-LTE effects. We show that if the iron abundance is larger than 10-5, the iron line opacity has to be considered not only for the spectrum synthesis, but also in the model construction itself. For such metal abundances, non-LTE metal line-blanketed models are needed for detailed abundance studies of hot, metal-rich white dwarfs. We also discuss the predicted Extreme Ultraviolet (EUV) spectrum and show that it is very sensitive to metal abundances, as well as to non-LTE effects. Title: Non-LTE Line-blanketed Model Atmospheres of Hot Stars. I. Hybrid Complete Linearization/Accelerated Lambda Iteration Method Authors: Hubeny, I.; Lanz, T. Bibcode: 1995ApJ...439..875H Altcode: A new munerical method for computing non-Local Thermodynamic Equilibrium (non-LTE) model stellar atmospheres is presented. The method, called the hybird complete linearization/accelerated lambda iretation (CL/ALI) method, combines advantages of both its constituents. Its rate of convergence is virtually as high as for the standard CL method, while the computer time per iteration is almost as low as for the standard ALI method. The method is formulated as the standard complete lineariation, the only difference being that the radiation intensity at selected frequency points is not explicity linearized; instead, it is treated by means of the ALI approach. The scheme offers a wide spectrum of options, ranging from the full CL to the full ALI method. We demonstrate that the method works optimally if the majority of frequency points are treated in the ALI mode, while the radiation intensity at a few (typically two to 30) frequency points is explicity linearized. We show how this method can be applied to calculate metal line-blanketed non-LTE model atmospheres, by using the idea of 'superlevels' and 'superlines' introduced originally by Anderson (1989). We calculate several illustrative models taking into accont several tens of thosands of lines of Fe III to Fe IV and show that the hybrid CL/ALI method provides a robust method for calculating non-LTE line-blanketed model atmospheres for a wide range of stellar parameters. The results for individual stellar types will be presented in subsequent papers in this series. Title: Non-LTE Line Blanketed Model Atmospheres of Hot, Metal-Rich White Dwarfs Authors: Hubeny, I.; Lanz, T. Bibcode: 1995LNP...443...98H Altcode: 1995whdw.conf...98H No abstract at ADS Title: Interpreting the spectra of chemically peculiar stars Authors: Lanz, T. Bibcode: 1995HiA....10..582L Altcode: No abstract at ADS Title: The Atmospheres of Chemically Peculiar Stars: A Laboratory for Atomic Physics Authors: Lanz, T. Bibcode: 1995ASPC...78..423L Altcode: 1995aapn.conf..423L No abstract at ADS Title: Magnetic Splitting and Identification of Spectral Lines in AP Stars Authors: Mathys, G.; Lanz, T. Bibcode: 1995ASPC...81..531M Altcode: 1995lahr.conf..531M No abstract at ADS Title: The UV-bright stars of Omega Centauri. Authors: Landsman, W. B.; Crotts, A.; O'Connell, R. W.; Whitney, J. H.; Lanz, T.; Stecher, T. P. Bibcode: 1995BAAS...27S.828L Altcode: No abstract at ADS Title: EUVE Spectroscopy of epsilon Canis Majoris (B2 II) from 70 to 730 Angstrom Authors: Cassinelli, J. P.; Cohen, D. H.; Macfarlane, J. J.; Drew, J. E.; Lynas-Gray, A. E.; Hoare, M. G.; Vallerga, J. V.; Welsh, B. Y.; Vedder, P. W.; Hubeny, I.; Lanz, T. Bibcode: 1995ApJ...438..932C Altcode: We present spectra of the brightest stellar source of extreme ultraviolet (EUV) radiation longward of 400 A, the B2 II star, epsilon CMa. These data were taken with the three spectrometers aboard the NASA Extreme Ultraviolet Explorer satellite (EUVE) during the first cycle of pointed observations. We report on our initial studies of the continuum and line spectrum of the stellar photosphere in the 320 to 730 A region, and on the wind emission lines observed in the 170-375 A region. This is the first EUV spectrum of an early-type star, and thus makes epsilon CMa the most comprehensively observed B star from the X-ray to infrared regimes. The radiation in both the H Lyman continuum and He I continuum (shortward of 504 A) are found to be significantly greater than predicted by both Local Thermodynamic Equilibrium (LTE) and non-LTE model atmospheres. Since epsilon CMa also exhibits a mid-infrared excess, this points to the outer layers being warmer than the models indicate. The anomalously large Lyman continuum flux, combined with the very low column density measured in the direction toward this star implies that it is the dominant source of hydrogen ionization of the local interstellar medium in the immediate vicinity of the sun. All of the lines predicted to be strong from model atmospheres are present and several wind absorption features are also identified. We have detected emission lines from highly ionized iron that are consistent with the ROSAT Position Sensitive Proportional Counter (PSPC) observations if a multi-temperature emission model is used, and the assumption is made that there is significant absorption beyond that of the neutral phase of the ISM. The spectrum shows strong O III 374 A line emission produced by the Bowen flourescence mechanism, which has not previously been observed in the spectra of hot stars. Title: HST/GHRS Observations of beta Pictoris Authors: Heap, S. R.; Lanz, T.; Hubeny, I.; Lindler, D. Bibcode: 1994AAS...185.4812H Altcode: 1994BAAS...26.1389H In September 1994, we obtained HST/GHRS spectra and maps of beta Pic (HD 39060) and its surrounding disk. In this paper, we report our preliminary findings concerning the star and circumstellar gas disk. Both Stromgren photometry and Geneva photometry (as listed by SIMBAD) yield the following parameters for the star: T_eff=8200 +/- 50 K log g = 4.25 +/- 0.05 [Fe/H] = -0.1 +/- 0.2 To refine the estimated metallicity, we have compared the GHRS ultraviolet spectra with model spectra. Our model of beta Pic incorporates both a LTE stellar atmosphere plus a circumstellar gas disk, treated in a first approximation as a homogeneous, absorbing slab at T=5,000 K with a column density, N_H=2x10(19) cm(-2) . We confirm that the star has a near-solar metallicity. We have compared the derived stellar parameters with evolutionary models for Z=0.017. For reference, a ZAMS star having a T_eff=8,200 K has a mass, M ~1.72 M_sun, and a gravity, log g= 4.37. The somewhat low surface gravity of beta Pic suggests that the star has evolved slightly off the ZAMS or is still in the pre-main sequence phase. In either case, it should be somewhat more luminous than a 1.72M_sun ZAMS star. However, its absolute magnitude, M_V= 2.70 (for a distance of 17 pc) is 0.21 mag fainter than a M=1.72 M_sun star on the ZAMS. We therefore conclude that (1) the dust disk must diminish light from the star, and (2) it is entirely possible that beta Pic is a very young star. Both conclusions raise problems for interpreting observations of the dust disk (c.f. Paresce, 1991, A&A, 247, L25). Title: HST/FOS Spectroscopy of the Central Star of NGC 7027 Authors: Hubeny, I.; Heap, S. R.; Lanz, T. Bibcode: 1994AAS...185.4710H Altcode: 1994BAAS...26.1385H With its rich UV-optical-IR spectrum, NGC 7027 has been the target of choice for study of important physical processes including charge-exchange, dust-quenching of nebular lines, PDR's, extended-red emission, etc. Surprisingly little, though, is known about the central star, because the star is embedded in such a bright nebula. With the success of the HST servicing mission, it is now possible to carry out direct studies of the central star. Here, we report on HST/FOS spectroscopy of the central star. In August 1994, we obtained FOS/G400H, G570H spectra of NGC 7027 with the central star centered in the 1farcs 0 aperture. The spectra cover the wavelength range, 3250-6750 Angstroms. The spectra are of excellent quality. For example, at lambda 4800 Angstroms, the S/N=76 per resolution element, and the star contributes 94% of the observed continuum flux. Other very hot stars, i.e. PG 1159 stars, are known to show high-ionization lines, such as O VI lambda lambda 3811,3834, C IV lambda 4441, or C V lambda 4945, etc. To our surprise, these lines are absent in the spectrum of the central star. In fact, the stellar spectrum is apparently featureless. We have therefore used the absence of spectral features to put limits on effective temperature and chemical composition of the central star. We are constructing a small grid of line-blanketed NLTE model atmospheres for the effective temperatures range 100,000 K < T_eff < 250,000 K, 5.8 < log g < 7.5, and for two compositions: (i) a H-rich (solar) composition, and (ii) a C-O enriched atmosphere, typical of PG 1159 stars. We will report on the results of this analysis. Title: NLTE Line-blanketed Model Atmosphere Analysis of the hot Subdwarf BD +75° 325 Authors: Lanz, T.; Hubeny, I.; Heap, S. R. Bibcode: 1994AAS...185.8005L Altcode: 1994BAAS...26.1447L With the Goddard High Resolution Spectrograph aboard HST, we have recorded a high-resolution, high signal-to-noise ratio, far ultraviolet spectra of the hot subdwarf, BD +75° 325. Because of the low apparent rotational velocity (V sin i < 20 km/s) of the star, the high quality of the spectra and our newly developed NLTE line-blanketed model atmospheres, we have an unprecedented opportunity to derive accurate stellar parameters. With our hybrid CL/ALI method, we have constructed a small grid of NLTE line-blanketed model atmospheres for hot subdwarfs. These model atmospheres include consistently about 200 NLTE levels (or superlevels) of H, He, C, N, O, Si, Fe and Ni, and all transitions between them. Individual atomic levels of iron and nickel are merged into a relatively small number of superlevels. The cross-sections of the transitions between superlevels are represented by NLTE Opacity Distribution Functions. This grid of NLTE model atmospheres is used for a thorough analysis of BD +75° 325, which then provides in addition a ``real'' check of our methodology. Our results show that BD +75° 325 is a He enriched star, whose surface exhibit CNO-cycle products (N-rich and C-deficient). The very rich Fe v and Fe vi line spectra can only be reproduced by a significant overabundance of iron (about 5 to 10 times solar). With our metal line blanketed NLTE model atmospheres, we are now able to reproduce the deep line cores in the ultraviolet spectrum assuming a microturbulence of 5-10 km/s. Moreover, the line blanketed models are necessary to match the H i/He ii line cores in the visual spectrum (models including only H and He produce too shallow lines). Finally, we have found a weak asymmetry of the N v resonance line profiles, which can be attributed to a weak wind with M = 9+/- 2times 10(-11) M_sun /yr. Title: (Erratum) The nature of the F STR λ4077 stars. III. Spectroscopy of the barium dwarfs and other CP stars. Authors: North, P.; Berthet, S.; Lanz, T. Bibcode: 1994A&A...292..350N Altcode: Erratum to Astron. Astrophys. 281, 775-796 (1994) Title: Theoretical Modeling of GHRS Observations of the Of/WN-Type Star R136a5 Authors: de Koter, Alex; Hubeny, Ivan; Heap, Sara R.; Lanz, Thierry Bibcode: 1994ApJ...435L..71D Altcode: We present a theoretical analysis of the Hubble Space Telescope (HST) Goddard High Resolution Spectrograph (GHRS) spectrum of R136a5, an O3fWN star in the R136a cluster in 30 Doradus. Using non-LTE extended and expanding model atmospheres, we find a surprisingly high mass-loss rate dot-M = 1.8 +/- 0.5 x 10-5 solar mass/yr and an effective temperature Teff = 42.5 +/- 2.5 kK. With the observed visual magnitude, this implies a radius R* = 17 +/- 2 solar radius and a solar luminosity L = 8.5 +/- 1 x 105 solar luminosity. We discuss possible sources of the uncertainties in the derived stellar parameters and conclude that the effective temperature may be underestimated; however, if so, the value of the mass-loss rate would not be affected. Title: Dynamic Processes in Be Star Atmospheres. II. He i 2P--nD Line Formation in lambda Eridani (Outburst) Authors: Smith, Myron A.; Hubeny, Ivan; Lanz, Thierry; Meylan, Thomas Bibcode: 1994ApJ...432..392S Altcode: The He I lambda 6678 line of early Be stars generally shows violet (V) and red (R) emission whenever hydrogen alpha emission is present, but its use as a diagnostic has been handicapped by a poor understanding of the processes that drive it into emission. In an attempt to address this problem we obtained three series of eschelle spectra of the first two members of the singlet and triplet 2P-nD series of lambda Eri (B2e) during 1992 November 3-5 at Kitt Peak. During these observations lambda 6678 showed substantial emission variability in both the wings and central profile, providing an opportunity to compare its behavior with that of the lambda 4922, lambda 5876, and lambda 4471 lines. We found that the responses of the lines were different in several respects. Whereas the emissions in the V wings of all four lines scaled together, the R wing of the lambda 4922 line invariably responded with increased absorption whenever the R wing of lambda 6678 line showed increased emission. These same trends occurred within the central photospheric profiles. The R-wing behavior shows that much, but not all of the emission in lambda 6678 is caused by matter projected against the stellar disk. The excitation temperatures of the neighboring 21 P transitions, lambda 6678 and lambda 4922 must be greater than and less than the photospheric continuum temperature, respectively. We have investigated departures from local thermodynamic equilibrium (LTE) for the He I spectrum in a variety of ad hoc, perturbed model atmospheres. We have found only one way to cause the source function of lambda 6678 to increase so strongly, namely, by increasing the atmospheric temperature in the line formation region to 30,000 - 40,000 K. This effect was discovered by Auer and Mihalas for O3-O4 atmospheric models, but it has not been applied to active B stars. Our models suggest that lambda 6678 emission in Be stars can be used as a sensitive monitor of localized hot spots on these stars' surfaces. The energies involved in heating the active portions of the atmosphere are too high to be produced by gravitational infall. This leaves magnetically induced flares among the few known processes on the surfaces of stars capable of sustaining this energy level. Title: NLTE Model Stellar Atmospheres: The Hybrid CL/ALI Method Authors: Lanz, T.; Hubeny, I. Bibcode: 1994AAS...184.4011L Altcode: 1994BAAS...26..924L A new numerical method for computing sophisticated NLTE model stellar atmospheres is presented. The method, called the hybrid Complete Linearization/Accelerated Lambda Iteration (CL/ALI) method, combines advantages of both its constituents. Its rate of convergence is virtually as high as for the standard CL method, while the computer time per iteration is almost as low as for the standard ALI method. The method is formulated as the standard complete linearization; the only difference being that the radiation intensity at selected frequency points is not explicitly linearized; instead, it is treated by means of the ALI approach. The scheme offers a wide spectrum of options, ranging from the full CL to the full ALI method. We demonstrate that the method works optimally if the majority of frequency points are treated in the ALI mode, while the radiation intensity at a few (typically 2 - 30) frequency points is explicitly linearized. We show how this method can be applied to calculate metal line blanketed NLTE model atmospheres of hot stars. We use the idea of ``superlevels'' and ``superlines'' introduced originally by Anderson (1989). We calculate several illustrative models taking into account several tens of thousand of lines of Fe iii to Fe vi, and show that the hybrid CL/ALI method provides a robust method for calculating NLTE line blanketed model atmospheres for a wide range of stellar parameters. This work was supported in part by NASA grants NAGW-3025 and NAGW-3834. Title: GHRS observations and theoretical modeling of early type stars in R136a Authors: de Koter, A.; Heap, S.; Hubeny, I.; Lanz, T.; Hutchings, J.; Lamers, H. J. G. L. M.; Maran, S.; Schmutz, W. Bibcode: 1994AAS...184.3106D Altcode: 1994BAAS...26..909D We present the first spectroscopic observations of individual stars in R136a, the most dense part of the starburst cluster 30 Doradus in the LMC. Spectra of two stars are scheduled to be obtained with the GHRS on board the HST: R136a5, the brightest of the complex and R136a2, a Wolf-Rayet star of type WN. The 30 Doradus cluster is the only starburst region in which individual stars can be studied. Therefore, quantitative knowledge of the basic stellar parameters will yield valuable insight into the formation of massive stars in starbursts and into their subsequent evolution. Detailed modeling of the structure of the atmosphere and wind of these stars will also lead to a better understanding of the mechanism(s) that govern their dynamics. We present the first results of our detailed quantitative spectral analysis using state-of-the-art non-LTE model atmospheres for stars with extended and expanding atmospheres. The models are computed using the Improved-Sobolev Approximation wind code (ISA-WIND) of de Koter, Schmutz & Lamers (1993, A&A 277, 561), which has been extended to include C, N and Si. Our model computations are not based on the core-halo approximation, but use a unified treatment of the photosphere and wind. This approach is essential for Wolf-Rayet stars. Our synthetic spectra, dominated by the P Cygni profiles of the UV resonance lines, also account for the numerous weak metal lines of photospheric origin. Title: The Discovery of NI V in the Photospheres of the Hot DA White Dwarfs RE 2214-492 and G191-B2B Authors: Holberg, J. B.; Hubeny, I.; Barstow, M. A.; Lanz, T.; Sion, E. M.; Tweedy, R. W. Bibcode: 1994ApJ...425L.105H Altcode: We have co-added six recently obtained International Ultraviolet Explorer (IUE) echelle spectra of the hot DA white dwarf RE 2214-492 and 10 existing archive spectra of the well-known hot DA, G191-B2B. We find that both stars contain numerous weak features due to Ni V. Nickel is thus the second iron-group element to be found in the spectra of the very hottest DA white dwarfs. In addition to Ni V, we also observe Al III in both stars and present evidence for the possible presence of Ni IV and Fe IV in RE 2214-492. The presence of Ni and Al, together with previously reported elements, will contribute significantly to both the EUV opacity and to the apparent complexity of the UV spectra of these stars. Using Non-Local Thermodynamic Equilibrium (NLTE) model atmospheres we estimate the Ni abundances in RE 2214-492 the G191-B2B to be log(Ni/H) = -5.5 +/- 0.3 and -6.0 +/- 0.3, respectively. Title: The nature of the F STR λ4077 stars. V. Spectroscopic data Authors: North, P.; Berthet, S.; Lanz, T. Bibcode: 1994A&AS..103..321N Altcode: We report first high-resolution spectroscopic observations of 17 stars classified F str λ4077 by Bidelman. They are compared to different types of stars, which have been observed at the same time, including 3 CH subgiants, 3 barium stars, 5 λ Bootis stars, Ap/Am stars, and normal main-sequence stars. The detailed equivalent widths and abundances are given for all measured spectral lines, as well as the mean abundance per species for each object. These data and their significance are described in detail and discussed in an article published in the Main Journal. Title: NLTE model stellar atmospheres with line blanketing near the series limits. Authors: Hubeny, I.; Hummer, D. G.; Lanz, T. Bibcode: 1994A&A...282..151H Altcode: In this paper we study the influence of line-merging regions at the intermediate long-wavelength side of a continuum threshold on the computed model atmosphere structure and predicted spectrum. In order to model these regions sufficiently accurately, we have developed two concepts. First, we have extended the occupation probability formalism of Hummer and Mihalas to non-local thermodynamic equilibrium (non-LTE) plasmas. Second, in order to treat the very complicated opacity in the line merging region, we have generalized the concept of opacity distribution functions to treat non-LTE situations. All Rydberg states are consistently included within this framework, so that no arbitrary cutoff of high (LTE) levels is made. We have calculated several pure hydrogen models atmospheres for two effective temperatures, Teff = 20000 and 35000 K, and discussed the differences between models calculated with various treatments of the line merging. In particular, we have shown that the error in the predicted profiles of Balmer lines resulting from the neglect of line merging is typically of the order of 3-4%, while the errors in the far-UV portion of the Balmer continuum reaches 15-35%. The errors generally decrease with increasing effective temperature. At the same time, the internal accuracy of the models is shown to be about or below 0.5% for all predicted spectral features. We conclude that for interpreting current high-accuracy spectrophotometric observations models including the line merging are necessary, and that the formalism developed in this paper is capable of providing a sufficiently accurate and robust modeling technique. Title: Puzzling problems of He I line formation in early B stars Authors: Smith, M. A.; Hubeny, I.; Lanz, T. Bibcode: 1994IAUS..162..273S Altcode: No abstract at ADS Title: HST Eclipse Mapping of the Dwarf Nova OY Carinae in Quiescence: an "Fe II Curtain" with Mach approximately equals 6 Velocity Dispersion Veils the White Dwarf Authors: Horne, K.; Cheng, F. H.; Marsh, T.; Hubeny, I.; Lanz, T. Bibcode: 1994ASPC...56..283H Altcode: 1994ibs..conf..283H No abstract at ADS Title: Iron-Peak Elements in Hot Subdwarfs: CYCLE4 Medium Authors: Lanz, Thierry Bibcode: 1994hst..prop.5305L Altcode: 1994hst..prop.1753L Hot evolved subdwarfs are the immediate progenitors of white dwarfs, and are therefore of great importance for our understanding of the late stages of stellar evolution. Recently, significant abundances of heavy metals have been found in some hot DA white dwarfs, formely thought of having hydrogen-pure atmospheres. Iron and nickel now appear to be the major opacity sources in the FUV and EUV spectrum of these white dwarfs as well as in the hot evolved sdO subdwarfs. We will observe one sdO stars to redetermine most accurately its stellar parameters. We will analyze the spectrum using our new most sophisticated NLTE line-blanketed model atmospheres, including the blanketing of tens of thousands of Fe IV, Fe V, Fe VI, Ni IV and Ni V lines. We will derive consistently the abundances of the iron-peak elements. Contrary to the abundances of light elements which are primarily signatures of nucleosynthetic processes, the iron-peak elements may shed light on the relative importance of the individual mixing processes in the photospheres of sdO stars, and give us a better insight on the connection between these subdwarfs and the white dwarfs. Title: HST/GHRS Observations of β Pictoris Authors: Heap, S.; Lanz, T.; Hubeny, Y. Bibcode: 1994cddp.conf..367H Altcode: No abstract at ADS Title: The nature of the F STR lambda 4077 stars. III. Spectroscopy of the barium dwarfs and other CP stars. Authors: North, P.; Berthet, S.; Lanz, T. Bibcode: 1994A&A...281..775N Altcode: The abundances of C, O, Al, Ca, iron-peak and s-process elements have been derived from high-resolution spectra for a sample of stars classified as F str lambda 4077 by Bidelman. Among the 20 stars mentioned by Bidelman, we have discovered 8 barium dwarfs (or CH subgiants, according to Bond's terminology), while a 9th star, HD 182274, was already known as a CH subgiant. In addition, we have analyzed three barium stars taken from the list of Lu et al. (1983) which are probably dwarfs rather than giants, and three CH subgiants. The other 11 F str lambda 4077 stars resemble either the delta Delphini stars, since their iron abundance is enhanced while Ca is normal, or are probably spectrum composites. A few Am, Ap, lambda Bootis and normal stars have been analyzed for comparison. In particular, we have included three lambda Boo candidates, selected from their photometric properties, and their iron deficiency is confirmed. The spectroscopic, photometric and statistical evidences concerning the Ba dwarfs, support the idea that these stars may be the main sequence counterparts, and possibly the progenitors of the Ba giants. The C/O ratio varies in these stars from normal values to a maximum of 1.5, but mostly within 0.6 and 1.2. Some of these objects may therefore be considered, in this sense, as carbon stars. On the other hand, the abundances of carbon and s-process elements relative to iron are inversely correlated with metallicity, and may even exceed significantly those of typical, solar-metallicity carbon stars. Metal-deficient C stars must therefore have (C/Fe) greater than or approximately equal to 1 and (s/Fe) greater than or approximately equal to 1.5 as soon as (Fe/H) less than or approximately equal to -1. The neutron exposure is shown to increase when the metallicity decreases, which is compatible with the C-13 (alpha, n) O-16 neutron source, but not with the Ne-22 (alpha, n) Mg-25 one. The evolutionary state (within the main sequence) of the Ba dwarfs, is rediscussed in relation with their photometric and spectroscopic surface gravity, but it remains unclear. Title: A search for magnetic fields in AM stars. Authors: Lanz, T.; Mathys, G. Bibcode: 1993A&A...280..486L Altcode: Although Am stars have been assumed to be nonmagnetic stars, we found recently that the hot Am stars omicron Peg has a magnetic field of the order of 2 kG, with a complex structure. Therefore, we reconsidered the question of magnetism among upper main sequence chemically peculiar stars. We report here new observations of a pair of magnetically sensitive Fe II lines in the red spectrum of Am stars and of HgMn stars. Two other hot Am stars, HD 29173 and HD 195479A, are likely to possess also a magnetic field of about the same strength. It is striking that 3 out of 4 hot Am stars observed up to now could be magnetic. On the other hand, this fast method is limited by blends and cannot provide a satisfactory diagnosis for classical Am stars and for HgMn stars. A multi-line approach must be the next step towards our understanding of magnetism among these chemically peculiar stars. Title: Erratum: The nature of the F STR A4077 stars. IV. Search for white dwarfs around barium dwarfs Authors: North, P.; Lanz, T. Bibcode: 1993A&A...273..720N Altcode: No abstract at ADS Title: NLTE Line Blanketed Model Atmospheres for Hot, Metal-rich White Dwarfs Authors: Hubeny, I.; Lanz, T. Bibcode: 1993AAS...182.4701H Altcode: 1993BAAS...25..878H Recent observations of some hot DA white dwarfs (Feige 24, G191 B2B - Sion et al. 1992, Ap.J. 391, L29; Vennes et al. 1992, Ap.J. 392, L27) and subsequent analyses have demonstrated that their atmospheres are contaminated with heavy metal species with appreciable abundances. So far, modeling was mostly limited to calculating synthetic spectra (usually in LTE), based on previously calculated model atmospheres that were constructed assuming a simplified chemical composition. However, in reality the metal lines may also significantly influence the temperature structure, and consequently the ionization balance of some important species. Since the effective temperature is rather high (55000 to 60000 K), the NLTE effects may be important despite the high gravity. Therefore, in order to settle the question on metal abundances of hot DA white dwarfs, we need to calculate NLTE fully line blanketed model atmospheres. Using our previously developed hybrid complete linearization/accelerated lambda iteration method, we have calculated a set of NLTE models including H, He, C, N, O, and Fe, with some 13000 lines of Fe IV, Fe V and Fe VI taken into account explicitly in model construction. The NLTE departure coefficients for all levels of Fe IV, Fe V, and Fe VI are then employed in the spectrum synthesis program SYNSPEC. In the first part of this study, we present a theoretical comparison between models computed with adding more and more opacity sources, and show how the temperature structure and synthetic spectra develop. We also discuss possible errors arising from using an inconsistent spectrum synthesis (the metals considered only in the spectrum synthesis, not in the model construction). In the second part, we compare the computed UV spectra with with available observations of Feige 24 and G191 B2B, and deduce limits for the iron abundance of these stars. Title: The GA II lines in the red spectrum of AP stars. Authors: Lanz, T.; Artru, M. -C.; Didelon, P.; Mathys, G. Bibcode: 1993A&A...272..465L Altcode: We report first observations of the Ga II λ633.4 line in the red spectrum of Ap stars. This line is present in the spectrum of 16 out of 45 Ap stars of our sample, translating to large gallium overabundances. While most HgMn stars are gallium-rich, we have observed strong gallium lines in several magnetic stars. We have derived LTE abundances from this line after reviewing all needed atomic data and stellar parameters. In particular, a new calibration of the Geneva photometry for the effective temperature of the magnetic Ap stars is presented. LTE abundances arc compared to previous analyses of the ultraviolet resonance lines, and they arc about 1 dex larger. This large discrepancy might possibly be explained by departures from LTE or by the stratification of gallium predicted by the radiative diffusion theory. We present the variations of the Ga II line strength during the rotation period of three magnetic Ap stars; from the variations of silicon and of the longitudinal component of the magnetic field it appears that gallium accumulates around magnetic poles. Moreover, we suggest that secular changes in the gallium surface abundance on time-scales of about 1-2 108 yr might explain why some Ap HgMn stars arc not gallium-rich. Title: Observations of Magnetically Split Lines in Ap-Stars Authors: Mathys, G.; Landstreet, J. D.; Lanz, T. Bibcode: 1993ASPC...44..300M Altcode: 1993IAUCo.138..300M; 1993pvnp.conf..300M No abstract at ADS Title: Similarities and Differences Between Magnetic and Non-Magnetic Cp-Stars Authors: Lanz, T. Bibcode: 1993ASPC...44...60L Altcode: 1993IAUCo.138...60L; 1993pvnp.conf...60L No abstract at ADS Title: Modelling A-Type Atmospheres - NLTE Models Authors: Hubeny, I.; Lanz, T. Bibcode: 1993ASPC...44...98H Altcode: 1993IAUCo.138...98H; 1993pvnp.conf...98H No abstract at ADS Title: Spectral Diagnoses of Chromospheres and Winds in A-Type Stars Authors: Lanz, T.; Hubeny, I. Bibcode: 1993ASPC...44..517L Altcode: 1993IAUCo.138..517L; 1993pvnp.conf..517L No abstract at ADS Title: Line Blanketed Model Atmospheres for Metal-rich White Dwarfs Authors: Lanz, T.; Hubeny, I.; Cheng, F. -H.; Horne, K. Bibcode: 1992AAS...181.5004L Altcode: 1992BAAS...24.1203L In many cases of a close binary system composed of a white dwarf, a companion main-sequence or giant star, and an accretion disk around the white dwarf, the UV spectrum of the system contains contributions from both the white dwarf and the disk. In order to derive information about the system from its UV spectrum, the white dwarf atmosphere has to be accurately modeled. The chemical composition of the white dwarf is very likely to be quite unusual, however, because it may have nearly solar composition due to accretion. Due to its high surface gravity, the white dwarf's lines are generally very broad, and thus metal line blanketing may play a very important role. We have therefore begun a project of calculating a grid of non-LTE, fully line-blanketed model atmospheres for white dwarfs, for a large range of metal abundances, from zero (pure DA) to solar composition. The method is described in a companion paper (Hubeny and Lanz) at this meeting. We consider a representative model atmosphere with T_eff=15000 K and log g = 8. In this preliminary study, only hydrogen and iron (Fe II and Fe III) opacity was considered. The models take into account 45815 Fe II and 23059 Fe III lines (all lines between the levels with measured energies). As expected, the NLTE effects are rather small for this temperature, but the effect of metal line blanketing is very important. In particular, we demonstrate how the metal line blanketing influences the UV spectrum and the hydrogen Lyman and Balmer lines profiles. The work on the project was supported in part by the NASA grant No.65 under program NRA 91-OSSA-12. Title: Non-LTE Line Blanketed Model Atmospheres: Application to A-stars Authors: Hubeny, I.; Lanz, T. Bibcode: 1992AAS...181.1911H Altcode: 1992BAAS...24.1152H We present a new method for calculating realistic line blanketed model atmospheres without the assumption of local thermodynamic equilibrium (LTE). The method is based on complete linearization, with the following two important modifications with respect to the standard variant. i) Instead of dealing separately with individual energy levels of a complicated metal species (e.g. Fe II), several levels with the same parity and close enough energy are grouped together to form a ``superlevel". Transitions between superlevels, the so-called ``superlines", are treated by resampling the detailed absorption cross-section and forming a partial ``NLTE opacity distribution function", which is representable by a relatively small number of frequency points (typically 15 - 30 points per superline). ii) The radiative rates (together with the heating/cooling rates, and the contributions to the rates in other important transitions - e.g. the hydrogen and carbon continua) are not fully linearized. However, in contrast to the earlier approaches, they are not held fixed. The only fixed quantity here is the approximate lambda operator. The new method is therefore a hybrid combining the complete linearization and the class of modern methods called accelerated lambda iteration (ALI). We have contructed several non-LTE model atmospheres for T_eff = 10000 K, log g = 4, with H, C I, C II, Mg II, and Fe II treated in NLTE. A variable number of Fe II lines, up to 45815 (i.e. all lines originating between the levels with measured energies), have been included, to form 221 superlines. The most interesting preliminary result is that the first 8 Fe II superlevels (about 13000 lines) produce most of the total blanketing effect. The work on the project was supported by the NASA grant No.65 under program NRA 91-OSSA-12. Title: Non-LTE Analysis of the Hot Subdwarf: BD 75 325 Authors: Heap, S. R.; Hubeny, I.; Lanz, T.; Altner, B. Bibcode: 1992AAS...181.5003H Altcode: 1992BAAS...24.1203H No abstract at ADS Title: Stratification of chemical elements in the atmosphere of the AP star 53 Camelopardalis. Authors: Babel, J.; Lanz, T. Bibcode: 1992A&A...263..232B Altcode: A detailed abundance analysis of Cr, Fe, Mn, Ca, and Ti in the Ap star 53 Cam from IUE high-resolution spectra has been performed. It is found that UV lines give significantly lower abundance than visible lines for Cr and Fe. Theoretical spectra of Cr II line calculated with the stratified chromium abundance distributions predicted a priori by the diffusion-mass loss model match well both visible and UV lines. The spectral shape of the Ca II K line and its variation are well accounted for by the diffusion model with mass loss. A spectrum synthesis with a stratified abundance well reproduces the observations. These findings provide convincing evidence for stratification in the photosphere of 53 Cam. Title: Accelerated complete-linearization method for calculating NLTE model stellar atmospheres Authors: Hubeny, I.; Lanz, T. Bibcode: 1992A&A...262..501H Altcode: Two approaches to accelerating the method of complete linearization for calculating NLTE model stellar atmospheres are suggested. The first one, the so-called Kantorovich variant of the Newton-Raphson method, consists of keeping the Jacobi matrix of the system fixed, which allows us to calculate the costly matrix inversions only a few times and then keep them fixed during the subsequent computations. The second method is an application of the Ng acceleration. Both methods are extremely easy to implement with any model atmosphere code based on complete linearization. It is demonstrated that both methods, and especially their combination, yield a rapidly and globally convergent algorithm, which takes 2 to 5 times less computer time, depending on the model at hand and the required accuracy, than the ordinary complete linearization. Generally, the time gain is more significant for more complicated models. The methods were tested for a broad range of atmospheric parameters, and in all cases they exhibited similar behavior. Ng acceleration applied on the Kantorovich variant thus offers a significant improvement of the standard complete-linearization method, and may now be used for calculating relatively involved NLTE model stellar atmospheres. Title: Mass loss in main-sequence A-type stars? Authors: Lanz, T.; Catala, C. Bibcode: 1992A&A...257..663L Altcode: We present new observations of the H-alpha line profile for five main-sequence A-type stars, where very high SNRs were achieved. A search for weak asymmetries was carried out to detect a stellar wind, but the many telluric absorption lines in this range prevent us from taking the full benefit of the high SNRs. This situation was improved by modeling the telluric absorptions to remove them from the observed spectra, but the H-alpha profiles were nevertheless found to be quite symmetric. We calculated the profile of the H-alpha line for a grid of model atmospheres of an A-type star including a weak wind, in order to assess an upper limit on the mass loss rate. The asymmetry is found to be sensitive to the velocity law and to the turbulent velocity of the wind, but its first moment depends much less on them. The upper limits deduced on the mass loss rate are between 1 and 2 x 10 exp -10 solar mass/yr. An improvement up to a factor 10 could be anticipated if new observations could be secured from a much drier location, achieving similar SNRs. Title: Ap stars with resolved magnetically split lines. Authors: Mathys, G.; Lanz, T. Bibcode: 1992A&A...256..169M Altcode: New high-resolution spectroscopic observations of Ap stars with resolved magnetically split spectral lines are presented. Six new such stars have been discovered: HD 2453, HD 9996, HD 18078, HD 50169, HD 137949, and HD 192678. This raises the total number of known Ap stars with resolved magnetically split lines to 21. The spectral line profiles observed in two more stars, HD 111133 and HD 147010, are strongly affected by the magnetic field, but they are not fully resolved, due to non-negligible rotational Doppler effect. Observations of fairly sharp-lined Ap stars whose lines are unresolved are also presented. The relative difference between the equivalent widths of the Fe II lines λ6147.7 and λ6149.2 is shown to increase with the mean magnetic field modulus. The existence of a simple, unambiguous correlation between the mean magnetic field modulus and the index Z of Geneva photometry, which had previously been advocated, is not confirmed. At most, stars with large negative values of Z may be more prone to have strong magnetic fields, but the evidence for this is not statistically strong. Title: Compilation of atomic oscillator strengths for carbon, nitrogen and oxygen ions. II - Addendum for the beryllium isoelectronic sequence Authors: Allard, N.; Le Dourneuf, M.; Artru, M. -C.; Lanz, T. Bibcode: 1991A&AS...91..399A Altcode: This addendum complements the recent compilation of oscillator strengths for C III, N IV and O V by Allard et al. (1990). It concerns high-excitation multiplets of C III and N W Adopted g f-values are proposed, following the same scheme of averaging. Additional references are given to recent theoretical results. In particular we present a comparative analysis of the data of Nussbaumer & Storey (1984) which were not previously considered. Title: The Rotation Period of the A0p Star HD 133652 Authors: Lanz, T.; Bohlender, D. A.; Landstreet, J. D. Bibcode: 1991IBVS.3678....1L Altcode: No abstract at ADS Title: The nature of the F STR lambda 4077 stars. IV. Search for white dwarfs around barium dwarfs. Authors: North, P.; Lanz, T. Bibcode: 1991A&A...251..489N Altcode: The energy distribution of three stars classified as F str 4077-A by Bidelman has been observed in the ultraviolet with the IUE satellite. Although the stars examined have the same kind of abundance anomalies as the Ba giants and Bond's CH subgiants (i.e. an overabundance of s-process elements), no UV excess related with the presence of the expected white dwarf companion was found. The implications for the mass-transfer scenario are discussed. Title: Reduced upper limits on the equivalent width of interstellar Li I 670.8 towards SN 1987A. Authors: Baade, D.; Cristiani, S.; Lanz, T.; Malaney, R. A.; Sahu, K. S.; Vladilo, G. Bibcode: 1991A&A...251..253B Altcode: A new search for interstellar Li I 670.8 along the line of sight to SN 1987A has been carried out using all 34 known (and previously published) high-resolution Reticon spectra obtained at the European Southern Observatory. In spite of the large number and high quality of the observations, the line remains undetected even in the carefully weighted and filtered mean spectrum. The most probable 3-σ detection limit in equivalent width is 7.3 10^-6^ nm. For the cloud at radial velocity +286 km s^-1^, standard relations then imply a maximum abundance, Li/H, of 1.7 10^-11^ in the gaseous interstellar medium of the LMC. If depletion to dust grains is the same for both lithium and potassium, this figure has to be increased by 1.1 dex. Accordingly, conventional present-day reasoning suggests that the elemental abundance of lithium does not exceed 2.7 10^-10^. However, the cumulative uncertainties are such that a primordial abundance which is an order of magnitude higher is not firmly ruled out. Constraints on primordial nucleosynthesis models are, therefore, only weak. Title: Photometric variations of some magnetic AP stars. Authors: Lanz, T.; Mathys, G. Bibcode: 1991A&AS...90..365L Altcode: New photometric observations of five magnetic Ap stars in the Geneva system are reported. New or improved parameters of the light curves are presented. Magnetic data confirm the adopted rotation periods and make it possible in some cases to remove ambiguities. Title: A New Look at the Standard Teff = 10000K, log g=4, Non-LTE Model Atmosphere Authors: Lanz, T.; Hubeny, I.; Altner, B. Bibcode: 1991BAAS...23R1381L Altcode: No abstract at ADS Title: The Photometric Variability of the B9p Star HD 137509 Authors: Lanz, T.; Mathys, G. Bibcode: 1991IBVS.3655....1L Altcode: No abstract at ADS Title: Compilation of atomic oscillator strengths for carbon, nitrogen and oxygen ions - The beryllium isoelectronic sequence (C III, N IV, and O V) Authors: Allard, N.; Artru, M. -C.; Lanz, T.; Le Dourneuf, M. Bibcode: 1990A&AS...84..563A Altcode: Oscillator strengths for the observed lines of C III, N IV, and O V are compiled. All the data published between 1966 and 1989 are reported and critically analyzed. Recent exhaustive results of the OPACITY project are included. The level lifetimes measured for these ions are also compiled and compared to the theoretical lifetimes. Adopted gf-values are proposed from a weighted mean of the best theoretical results. These data concern 204, 190, and 140 multiplets of C III, N IV, and O V respectively, with an accuracy in the 10 percent range in most cases. Title: Nitrogen abundances in chemically peculiar stars Authors: Lanz, T.; Artru, M. -C. Bibcode: 1990ESASP.310..275L Altcode: 1990eaia.conf..275L No abstract at ADS Title: The magnetic field of the AM star omicron Pegasi. Authors: Mathys, G.; Lanz, T. Bibcode: 1990A&A...230L..21M Altcode: Evidence is presented that the hot Am star Omicron Peg (= HD 214994) has a magnetic field of the order of 2 kG, with a complex structure. Two independent approaches support this conclusion. The first one is an analysis by the Stenflo-Lindegren (1977) technique of the widths of a statistical sample of lines. The other relies on a line-intensity anomaly that is found in magnetic stars. The implications of this probable detection of a field in a member of a class of stars that were not previously known as magnetic is discussed. Title: A detailed study of a 6° diameter field in Scutum Authors: Reichen, M.; Lanz, T.; Golay, M.; Huguenin, D. Bibcode: 1990Ap&SS.163..275R Altcode: A 6-deg diameter field centered on the star ɛ Scutum has been selected from the SCAP-2000 balloon-borne UV survey for its high apparent stellar density contrast and relatively good transparency on a small projected portion of the galactic plane. In view of a detailed study of the absorption and interstellar matter distribution in this field, a large collection of data has been made and is presented here. Ultraviolet magnitudes at 2000 Å have been obtained from the reduction of three SCAP frames. In addition 322 stars among the 487 stars detected on the UV frames have been mesured in the Geneva photometric system. For a small number of photometrically distant objects, medium resolution spectra have been obtained in order to determine spectral types and confirm the distances determination. An overall description of the distribution of interstellar material in the ultraviolet is made using star counting methods, and is confronted to similar studies in the visible range. A discussion of the variations of the extinction law in different parts of the field is made. A new distance determination of the Sct OB2 asociation is proposed, as well as a list of additional probable members of the association. A certain number of distant hot stars seem to indicate the presence of an unknown OB association at the edge of the Scutum spiral, arm at a distance of over 3 kpc from the Sun. Title: The IUE spectral atlas of two normal B stars : pi Ceti and nu Capricorni (125-198 nm). Authors: Artu, M. -C.; Borsenberger, J.; Lanz, T. Bibcode: 1989A&AS...80...17A Altcode: The ultraviolet spectra of two main-sequence B stars, Pi Cet (HD 17081) and Nu Capricorni (HD 193432) recorded by the IUE satellite have been used to elaborate a reference atlas, with line identification, in the low wavelength range 125-198 nm. To obtain realistic identifications and to select the dominant contributors in blends, a synthetic calculation of most of the detected lines has been performed in the LTE approximation. About 80 percent of the 1460 measured stellar lines have a definite identification, 57 percent of them corresponding to a single assignment. The presence of 20 different elements (33 atoms or ions) is firmly established and the elimination of spurious identifications is justified. The observed spectra are displayed between 125 and 198 nm, with comparison to the calculated ones. Title: Modelling of Magnetic Field Geometries and Chemical Abundance Distributions of Ap Si Stars Authors: Landstreet, J. D.; Lanz, T. Bibcode: 1989BAAS...21.1199L Altcode: No abstract at ADS Title: The chemically peculiar magnetic stars. Authors: Mégessier, C.; Lanz, T.; Artru, M. C. Bibcode: 1989musi.work...61M Altcode: No abstract at ADS Title: Les étoiles magnetiques chimiquement particulières. Authors: Megessier, C.; Artru, M. -C.; Lanz, T. Bibcode: 1989JAF....34R...7M Altcode: No abstract at ADS Title: The IUE spectral atlas of two normal B stars: πCet and νCap Authors: Artru, M. -C.; Borsenberger, J.; Lanz, T. Bibcode: 1988ESASP.281a.389A Altcode: 1988IUE88...1..389A; 1988uvai....1..389A An atlas for B stars pi Cet (HD17081, B7V) and nu Cap (HD192432, B9.5V) was prepared in the range 125 to 200 nm. By means of an improved software to process the IUE images and by the coaddition of 10 high-resolution images, the best possible resolution and signal to noise ratio are obtained. The identification of the lines is based on updated laboratory lists with a selection of the dominant contributors for each absorption line. About 80 percent of the measured lines have a definite identification. Title: Stark broadening of visible SI II lines in stellar atmospheres Authors: Lanz, T.; Dimitrijevic, M. S.; Artru, M. -C. Bibcode: 1988A&A...192..249L Altcode: The Stark broadening of visible Si II lines is studied and its consequence on the intensity of the stellar absorption is analyzed. A complete set of atomic data concerning the Stark widths, is elaborated for 19 multiplets of Si II of astrophysical importance. Previous available determinations are reviewed and new Stark widths are calculated by means of the semi-classical impact theory. The accuracy of different theoretical and empirical methods are discussed. The new adopted Stark widths are applied, in several examples, to the analysis of stellar absorption lines. These widths are generally smaller than the previous theoretical estimates for the low-excitation transitions of Si II. Title: The Period of Two Bp Si Mg Stars: HD 60431 and CoD -51 3378 Authors: North, P.; Babel, J.; Lanz, T. Bibcode: 1988IBVS.3155....1N Altcode: No abstract at ADS Title: Magnetic Field and Silicon Diffusion in Bp-Si Stars Authors: Megessier, C.; Lanz, T.; Landstreet, J. D. Bibcode: 1988IAUS..132..329M Altcode: Si II lines of magnetic Bp-Si stars in open clusters have been observed with the CAT (ESO) in order to get a mapping of the silicon abundance distribution over the stellar surface, in the frame of the oblique rotator model. The authors point out the influence of the Zeeman splitting and of the abundance inhomogeneities on the line profiles. Title: Critical Overview of Atomic Data for Stellar Abundance Analyses Authors: Lanz, T.; Artru, M. -C. Bibcode: 1988eaa..conf..156L Altcode: The atomic data which enter into LTE abundance analyses are reviewed. Improvements permitted by the current progress in physics are examined, with emphasis on oscillator strengths and line widths. Different compilation efforts are reviewed. The set of available oscillator strengths is shown to be satisfactory for light elements, but some examples of the many problems which remain for the iron sequence ions are detailed. The need for theoretical or empirical evaluations of many Stark widths is pointed out. Title: Radiation in moving gaseous media Authors: Chmielewski, Y.; Lanz, T. Bibcode: 1988rmgm.conf.....C Altcode: 1988SAAS...18.....C No abstract at ADS Title: Abundance Analysis of Phi Herculis Authors: Lanz, T. Bibcode: 1988eaa..conf...82L Altcode: Contents: 1. Computer programs. 2. Measurements of spectrograms. 3. Line identifications. 4. Atomic data. 5. Photospheric parameters. 6. Microturbulent velocity. 7. Metal abundances. Title: The Measurement of Equivalent Widths: A Comparison for 100 Selected Lines Authors: Lanz, T. Bibcode: 1988eaa..conf..120L Altcode: Contents: 1. Introduction. 2. The selected lines. 3. The methods. 4. The comparison. 5. Conclusion. Title: Elemental abundance analyses Authors: Adelman, Saul J.; Lanz, T. Bibcode: 1988eaa..conf.....A Altcode: No abstract at ADS Title: Silicon absorption in UV spectra of AP SI stars. Authors: Artru, M. -C.; Lanz, T. Bibcode: 1987A&A...182..273A Altcode: The authors investigate the role of silicon UV absorption occurring in the atmosphere of early-type stars. A detailed analysis of the λλ1250 - 1850 Å range is presented. High-resolution spectra are obtained from the IUE database for typical silicon stars and for a few normal main-sequence stars. The strongest absorption in these spectra are the resonance lines of Si II, and three large features (λ1400, λ1560 and λ1770 Å) which are strongly enhanced in the spectra of ApSi stars. For the hottest silicon stars, a discontinuity at 1310 Å is observed. The atomic data are carefully reviewed for Si II to perform spectrum synthesis calculations showing out more precisely the silicon absorption. Title: The interstellar spectrum toward SN 1987A. Authors: Vidal-Madjar, A.; Andreani, P.; Cristiani, S.; Ferlet, R.; Lanz, T.; Vladilo, G. Bibcode: 1987A&A...177L..17V Altcode: The supernova 1987A offers a unique opportunity to investigate the visible interstellar absorption spectrum toward the Large Magellanic Cloud at high resolution and high signal to noise ratio. The authors report in this letter the first results of these observations. A Ca II "forest" is detected through all the velocity range between the Galaxy and the LMC and, in particular, a component is seen at a velocity of ≡215 km s-1 which may correspond to cooler gas in the halo. According to the Na I/Ca II ratio, the galactic (and LMC) gas is clearly different from the "halo" one at intermediate velocities. For the first time, neutral lithium, calcium and potassium are detected in an extragalactic interstellar medium. Six diffuse interstellar bands are also seen in both the Galaxy and the LMC.