Author name code: antia ADS astronomy entries on 2022-09-14 author:"Antia, H.M." ------------------------------------------------------------------------ Title: Lessons learnt from AstroSat observations of cyclotron resonance scattering features from accretion powered pulsars Authors: Mukerjee, Kallol; Dey, Prithwitosh; Antia, H. M. Bibcode: 2022cosp...44.2231M Altcode: AstroSat observed and studied many accretion powered pulsars over the period and detected prominent cyclotron line features in their spectrum covering 0.7-60 keV energy band. Some of the interesting results obtained from our studies of cyclotron resonance scattering features from a few of these candidates, 4U 1626-67, Vela X-1, GX 301-2, GRO J1008-57, Cep X-4 and GRO J2058+42 would be presented along with their dependencies on source luminosity, pulse-phase and time. The physical interpretation of observed behaviors along with their implications on our current understanding would be discussed briefly along with future scope of these studies in the context of AstroSat. Title: Improved Background Model for the Large Area X-Ray Proportional Counter (LAXPC) Instrument on board AstroSat Authors: Antia, H. M.; Agrawal, P. C.; Katoch, Tilak; Manchanda, R. K.; Mukerjee, Kallol; Shah, Parag Bibcode: 2022ApJS..260...40A Altcode: 2022arXiv220503136A We present an improved background model for the Large Area X-ray Proportional Counter (LAXPC) detectors on board AstroSat. Because of the large collecting area and high pressure, the LAXPC instrument has a large background count rate, which varies during the orbit. Apart from the variation with latitude and longitude during the orbit there is a prominent quasi-diurnal variation which has not been previously modeled. Using over 5 yr of background observations, we determined the period of the quasi-diurnal variation to be 84,495 s and using this period it is possible to account for the variation and also identify time intervals where the fit is not good. These lead to a significant improvement in the background model. The quasi-diurnal variation can be ascribed to the changes in charged particle flux in the near-Earth orbit. Title: Changes in the Near-surface Shear Layer of the Sun Authors: Antia, H. M.; Basu, Sarbani Bibcode: 2022ApJ...924...19A Altcode: 2021arXiv211013952A We use helioseismic data obtained over two solar cycles to determine whether there are changes in the near-surface shear layer (NSSL). We examine this by determining the radial gradient of the solar rotation rate. The radial gradient itself shows a solar-cycle dependence, and the changes are more pronounced in the active latitudes than at adjoining higher latitudes; results at the highest latitudes (≳70°) are unreliable. The pattern changes with depth, even within the NSSL. We find that the near-surface shear layer is deeper at lower latitudes than at high latitudes and that the extent of the layer also shows a small solar-cycle-related change. Title: Large Area X-ray Proportional Counter (LAXPC) in orbit performance: Calibration, background, analysis software Authors: Antia, H. M.; Agrawal, P. C.; Dedhia, Dhiraj; Katoch, Tilak; Manchanda, R. K.; Misra, Ranjeev; Mukerjee, Kallol; Pahari, Mayukh; Roy, Jayashree; Shah, P.; Yadav, J. S. Bibcode: 2021JApA...42...32A Altcode: 2021arXiv210107514A The Large Area X-ray Proportional Counter (LAXPC) instrument on-board AstroSat has three nominally identical detectors for timing and spectral studies in the energy range of 3-80 keV. The performance of these detectors during the five years after the launch of AstroSat is described. Currently, only one of the detector is working nominally. The variation in pressure, energy resolution, gain and background with time are discussed. The capabilities and limitations of the instrument are described. A brief account of available analysis software is also provided. Title: Studies of Cepheus X-4 during the 2018 Outburst Observed with AstroSat Authors: Mukerjee, Kallol; Antia, H. M. Bibcode: 2021ApJ...920..139M Altcode: 2021arXiv210703608M We present timing and spectral results for the 2018 outburst of Cepheus X-4, observed twice by AstroSat at luminosities of 2.04 × 1037 erg s-1 and 1.02 × 1037 erg s-1. The light curves showed strong pulsation and co-related X-ray intensity variation in the SXT (0.5-8.0 keV) and LAXPC (3-60 keV) energy bands. The spin period and spin-down rate of the pulsar were determined from two observations to be 65.35080 ± 0.00014 s and (-2.10 ± 0.8) × 10-12 Hz s-1 at epoch MJD 58301.61850, and 65.35290 ± 0.00017 s and (-1.6 ± 0.8) × 10-12 Hz s-1 at epoch MJD 58307.40211. Pulse shape studies with AstroSat showed energy- and intensity-dependent variations. The pulsar showed an overall continuous spin-down over 30 yr at an average rate of (-2.455 ± 0.004) × 10-14 Hz s-1, attributed to the propeller effect in the subsonic regime of the pulsar, in addition to variations during its outburst activities. Spectra between the 0.7 keV and 55 keV energy bands were well fitted by two continuum models, an absorbed compTT model and an absorbed power law with a Fermi-Dirac cutoff (FD-cutoff) model with a blackbody. These were combined with an iron emission line and a cyclotron absorption line. The prominent cyclotron resonance scattering features with a peak absorption energy of ${30.48}_{-0.34}^{+0.33}$ keV and ${30.68}_{-0.44}^{+0.45}$ keV for the FD-cutoff model and ${30.46}_{-0.28}^{+0.32}$ keV and ${30.30}_{-0.34}^{+0.36}$ keV for the compTT model were detected during two AstroSat observations. When compared with earlier results, these showed long-term stability of an average value of 30.23 ± 0.22 keV with wide variation in source luminosity. The pulsar showed pulse phase as well as luminosity dependent variations in the cyclotron line energy and width and in the plasma optical depth of its spectral continuum. Title: Accretion Flow Properties of GRS 1915+105 During Its θ Class Using AstroSat Data Authors: Banerjee, Anuvab; Bhattacharjee, Ayan; Chatterjee, Debjit; Debnath, Dipak; Chakrabarti, Sandip Kumar; Katoch, Tilak; Antia, H. M. Bibcode: 2021ApJ...916...68B Altcode: 2020arXiv200705273B The Galactic microquasar GRS 1915+105 shows rich variability that is categorized into different classes. In this paper, we report the temporal and spectral analysis of GRS 1915+105 to study the properties of the accretion flow when the light curve shows θ class variability. For this purpose, we use the Large Area X-ray Proportional Counter data from the Target of Opportunity observations of India's first multiwavelength astronomy satellite AstroSat. The θ class is marked by the recurrent appearance of U-shaped regions in the light curve, where the photon count rate first decreases rapidly and then increases slowly. For our analysis, we use U-shaped regions of the first two orbits (02345 and 02346) on 2016 March 4. In both cases, the dynamic power-density spectra (PDS) showed significant power at around 4-5 Hz, suggesting the presence of a low-frequency quasi-periodic oscillation (QPO) around that frequency interval. The QPO frequency is found to increase with time when the energy flux is also enhanced. From the evolution of the spectra, we determine the evolution of the accretion flow parameters in these two observations. Fitting the spectra with the transonic flow solution-based two-component advective flow (TCAF) model in the 4-25 keV energy band shows that the Keplerian disk accretion rate increases with the increase in radiation intensity, while the location of the centrifugal pressure-driven shock front decreases. In both these data, a gradual increment of power-law photon index with intensity is observed, suggesting the progressive softening of the source. Title: AstroSat Detection of a soft X-ray source in HR6819 (QV Tel) Authors: Manchanda, R. K.; Katoch, T.; Antia, H. M. Bibcode: 2021ATel14739....1M Altcode: Proposed non-accreting stellar mass black hole in binary around the B-star, in the B-Be double star system HR 6819 (Rivinis et al. A & A, 637, L3, 2020) was observed with AstroSat observatory [Singh et al., SPIE 9905E..1E, 2016] on three separate occasions corresponding to MJD 59273, 59310 and 59318 for a combined SXT exposure of ~14250 sec. An ultrasoft X-ray flux was detected during all three observations in the SXT data.The spectral fits in the 0.3-3 keV band correspond to a black body temperature of 165 eV, 90 eV and 110 eV for a fixed n_H value of 5.0 x 10^20 cm^-2. The flux from source was observed to be 5e-12 ergs/cm^2/s in the energy range 0.3 - 3 keV. The observations correspond to binary phase value of 0.143, 0.06, 0.25 respectively (assuming T_conj = 53177.4). Preliminary analysis of the light curves from the large area X-ray proportional counters shows only a very weak emission at a level of ~2-4 counts/sec in the hard X-ray band of 20-80 keV. PDS analysis of the data do not show any short time variations in the source. Soft X-ray data is consistent with those observed from bright stars however, present observations do not preclude the presence of a dormant BH, which may mimic a very low black body temperature. Hard X-ray spectral features in BH sources arise mainly due to large mass accretion and during quiescence emission and the absence of any jet, the observed flux is very weak. Detail analysis of the data from LAXPC along with other co-aligned instruments is in progress. Title: AstroSat view of IGR J17091-3624 and GRS 1915 + 105: decoding the 'pulse' in the 'Heartbeat State' Authors: Katoch, Tilak; Baby, Blessy E.; Nandi, Anuj; Agrawal, Vivek K.; Antia, H. M.; Mukerjee, Kallol Bibcode: 2021MNRAS.501.6123K Altcode: 2020MNRAS.tmp.3544K; 2020arXiv201113282K IGR J17091-3624 is a transient galactic black hole which has a distinct quasi-periodic variability known as 'heartbeat', similar to the one observed in GRS 1915 + 105. In this paper, we report the results of ∼125 ks AstroSat observations of this source during the 2016 outburst. For the first time, a double-peaked QPO (DPQ) is detected in a few time segments of this source with a difference of δf ∼ 12 mHz between the two peaks. The nature of the DPQ was studied based on hardness ratios and using the static as well as the dynamic power spectrum. Additionally, a low-frequency (25-48 mHz) 'heartbeat' single-peak QPO (SPQ) was observed at different intervals of time along with harmonics (50-95 mHz). Broad-band spectra in the range 0.7-23 keV, obtained with Soft X-ray Telescope and Large Area X-ray Proportional Counter, could be fitted well with combination of a thermal Comptonization and a multicolour disc component model. During AstroSat observation, the source was in the soft-intermediate state (SIMS) as observed with Swift/XRT. We present a comparative study of the 'heartbeat' state variability in IGR J17091-3624 with GRS 1915 + 105. Significant difference in the timing properties is observed although spectral parameters (Γ ∼ 2.1-2.4 and Tmax ∼ 0.6-0.8 keV) in the broad energy band remain similar. Spectral properties of segments exhibiting SPQ and DPQ are further studied using simple phase-resolved spectroscopy which does not show a significant difference. Based on the model parameters, we obtain the maximum ratio of mass accretion rate in GRS 1915 + 105 to that in IGR J17091-3624 as ∼25: 1. We discuss the implications of our findings and comment on the physical origin of these exotic variabilities. Title: GRB 210116: AstroSat LAXPC detection Authors: Antia, H. M.; Katoch, Tilak; Shah, Parag; Dedhia, Dhiraj Bibcode: 2021GCN.29340....1A Altcode: Analysis of AstroSat LAXPC data showed the detection of a long GRB 210116. The light curve showed a peak profile triggered at T0 = 05h 53m 33s UT on 16 Jan 2021. The light curve showed a peak profile with T90 = 9 sec. The measured peak count rate above the background associated with the burst is 867 +/- 37 cts/s in LAXPC10 and 773 +/- 31 cts/s in LAXPC20 at T0+4 sec. The total counts in the peak are 3219 +/- 85 cts in LAXPC10 and 2683 +/- 68 in LAXPC 20. Both LAXPC instruments (LAXPC10 and LAXPC20) have registered this burst profile in the light curve. For LAXPC20 the nominal energy range is 3-100 keV, but due to lower gain in LAXPC10 the energy range is about 30-400 keV. The background subtracted light curve with 1 sec time-bin is available at the web-site: https://www.tifr.res.in/~astrosat_laxpc/grb210116lc.jpg LAXPC was built by TIFR in collaboration with the Indian Space Research Organisation. The Indian Space Research Organisation funded, managed and facilitated the project. Title: Studying the Properties of MAXI J1535-571 using Swift, MAXI and AstroSat data Authors: Chatterjee, Debjit; Debnath, Dipak; Chakrabarti, Sandip Kumar; Jana, Arghajit; Banerjee, Anuvab; Bhattacharjee, Ayan; Chang, Hsiang-Kuang; Shang, Jie-Rou; Katoch, Tilak; Antia, H. M. Bibcode: 2021cosp...43E1712C Altcode: Galactic X-ray transient black hole candidate MAXI J1535-571 was discovered on 2017 September 2 simultaneously by MAXI/GSC and Swift/BAT instruments. We use archival data of Swift (XRT and BAT), MAXI (GSC) and AstroSat (LAXPC) satellite instruments to make a detailed study on the accretion flow dynamics of the source during the initial period of the outburst. The daily average fluxes of MAXI/GSC and Swift/BAT and their ratio as HR (BAT/GSC fluxes) are used to understand the nature of source during the entire 2017-18 outburst. Low frequency quasi-periodic oscillations (QPOs) are observed during our detailed spectral and temporal analysis period of the outburst. Spectral analysis is done using two types of models: with phenomenological power-law or disk blackbody plus power-law model and with physical two component advective flow (TCAF) model. Studying the spectral and temporal properties of MAXI J1535-571, we categorized the spectral states during the outburst. The state transitions are also explained in a more physical point of views. The dynamic variation of QPOs and spectra are studied using AstroSat/LAXPC data. We have also estimated the mass of the black hole from the spectral fitted results. Title: AstroSat and MAXI view of the black hole binary 4U 1630-472 during 2016 and 2018 outbursts Authors: Baby, Blessy E.; Agrawal, V. K.; Ramadevi, M. C.; Katoch, Tilak; Antia, H. M.; Mandal, Samir; Nandi, Anuj Bibcode: 2020MNRAS.497.1197B Altcode: 2020MNRAS.tmp.2075B; 2020arXiv200700928B We present an in-depth spectral and timing analysis of the black hole binary 4U 1630-472 during 2016 and 2018 outbursts as observed by AstroSat and MAXI. The extensive coverage of the outbursts with MAXI is used to obtain the hardness intensity diagram (HID). The source follows a 'c'-shaped profile in agreement with earlier findings. Based on the HIDs of previous outbursts, we attempt to track the evolution of the source during a 'super'-outburst and 'mini'-outbursts. We model the broad-band energy spectra (0.7-20.0 keV) of AstroSat observations of both outbursts using phenomenological and physical models. No Keplerian disc signature is observed at the beginning of 2016 outburst. However, the disc appears within a few hours after which it remains prominent with temperature (Tin) ~ 1.3 keV and increase in photon index (Γ) from 1.8 to 2.0, whereas the source was at a disc dominant state throughout the AstroSat campaign of 2018 outburst. Based on the HIDs and spectral properties, we classify the outbursts into three different states - the 'canonical' hard and soft states along with an intermediate state. Evolution of rms along different states is seen although no quasi-periodic oscillations are detected. We fit the observed spectra using a dynamical accretion model and estimate the accretion parameters. Mass of the black hole is estimated using inner disc radius, bolometric luminosity, and two-component flow model to be 3-9 M. Finally, we discuss the possible implications of our findings. Title: AstroSat Observations of GRO J2058+42 during the 2019 Outburst Authors: Mukerjee, Kallol; Antia, H. M.; Katoch, Tilak Bibcode: 2020ApJ...897...73M Altcode: 2020arXiv200514044M We present results from AstroSat observations of the recent outburst of GRO J2058+42, an X-ray pulsar in a Be-binary system. The source was observed by the LAXPC and SXT instruments on AstroSat on 2019 April 10 during the declining phase of its latest giant outburst. Light curves showed a strong pulsation of the pulsar with a period of 194.2201 ± 0.0016 s and a spin-up rate of (1.65 ± 0.06) × 10-11 Hz s-1. Intermittent flaring was detected in light curves between the 3 and 80 keV energy bands, with an increase in intensity of up to 1.8 times its average intensity. Pulse profiles obtained between the 3 and 80 keV energy bands of the pulsar showed strong dependence on energy. During AstroSat observations, a broad peak was consistently observed in the power density spectrum of the source with a peak oscillation frequency of 0.090 Hz along with its higher harmonics, which may be due to quasi-periodic oscillations, a commonly observed phenomenon in transient X-ray pulsars during their outburst. AstroSat observations also detected cyclotron absorption features in its spectrum corresponding to (9.7-14.4) keV, (19.3-23.8) keV, and (37.8-43.1) keV. The pulse-phase-resolved spectroscopy of the source showed a phase-dependent variation in its energy and the relative strength of these features. The spectrum was well fitted with an absorbed blackbody, Fermi-Dirac cutoff model and alternatively with an absorbed CompTT model. Both of these models were combined with an Fe line and three Gaussian absorption lines to account for the observed cyclotron resonance scattering features in the spectrum. Title: GRB200210A AstroSat observations Authors: Antia, H. M.; Katoch, Tilak; Shah, Parag; Dedhia, Dhiraj; Gupta, S.; Gaikwad, R.; Sharma, V.; Vibhute, A.; Bhattacharya, D.; Laxpc, Astrosat; Czti Collaboration Bibcode: 2020GCN.27313....1A Altcode: Analysis of AstroSat LAXPC and CZTI data showed the detection of a long GRB 2002010A. The source was clearly detected in the 3-400 keV energy range. The light curve showed multiple peaks of emission with the strongest peak at 2020-02-10 08:03:40.00 UT. The measured peak count rate above the background associated with the burst is 354 +/- 29 cts/s in LAXPC10 and 197 +/- 21 in LAXPC20, with a total of 5524 +/- 143 cts in LAXPC10 and 3384 +/- 99 in LAXPC20. LAXPC20 has a nominal energy range of 3-100 keV but due to lower gain in LAXPC10 the energy range is about 30-400 keV. The background subtracted light curve with 1 sec time-bin in LAXPC detectors is available at the web-site: https://www.tifr.res.in/~astrosat_laxpc/grb200210lc.jpg The measured peak count rate associated with the burst in CZTI is 288 +/- 29 cts/s above the background in the combined data of four quadrants, with a total of 6338 +/- 67 cts. The local mean background count rate was 533 +/- 15 cts/s. Using cumulative rates, we measure a T90 of 29.2 +/- 0.01 s. CZTI operates in the energy range of 40-200 keV. It was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. CZTI GRB detections are reported regularly on the payload site at http://astrosat.iucaa.in/czti/?q=grb. LAXPC is built by TIFR in collaboration with the Indian Space Research Organisation. CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, ISAC, IUCAA, SAC and PRL. The Indian Space Research Organisation funded, managed and facilitated the project. Title: GRB 200326B: AstroSat LAXPC detection Authors: Antia, H. M.; Katoch, Tilak; Shah, Parag; Dedhia TIFR, Dhiraj; Mumbai; India. Bibcode: 2020GCN.27454....1A Altcode: Analysis of AstroSat LAXPC data showed the detection of a long GRB 200326B. The light curve showed a double peak profile triggered at T0 = 21h 13m 52s UT on 26 Mar 2020, with T90=10 sec for the first peak followed by a second smaller peak of similar duration about 10 s later. The measured peak count rate above the background associated with the burst is 158 +/- 24 cts/s in LAXPC10 and 171 +/- 20 cts/s in LAXPC20 at T0+3 sec. The total counts in the first peak are 542 +/- 71 cts in LAXPC10 and 773 +/- 54 in LAXPC20. Both LAXPC instruments (LAXPC10 and LAXPC20) have registered this burst profile in the light curve. For LAXPC20 the nominal energy range is 3-100 keV, but due to lower gain in LAXPC10 the energy range is about 30-400 keV. The background subtracted light curve with 1 sec time-bin is available at the web-site: https://www.tifr.res.in/~astrosat_laxpc/grb200326blc.jpg LAXPC was built by TIFR in collaboration with the Indian Space Research Organisation. The Indian Space Research Organisation funded, managed and facilitated the project. Title: Time-Distance Helioseismology of Deep Meridional Circulation Authors: Rajaguru, S. P.; Antia, H. M. Bibcode: 2020ASSP...57..107R Altcode: 2020arXiv200412708R A key component of solar interior dynamics is the meridional circulation (MC), whose poleward component in the surface layers has been well observed. Time-distance helioseismic studies of the deep structure of MC, however, have yielded conflicting inferences. Here, following a summary of existing results we show how a large center-to-limb systematics (CLS) in the measured travel times of acoustic waves affects the inferences through an analysis of frequency dependence of CLS, using data from the Helioseismic and Doppler Imager (HMI) onboard Solar Dynamics Observatory (SDO). Our results point to the residual systematics in travel times as a major cause of differing inferences on the deep structure of MC. Title: Age dating of an early Milky Way merger via asteroseismology of the naked-eye star ν Indi Authors: Chaplin, William J.; Serenelli, Aldo M.; Miglio, Andrea; Morel, Thierry; Mackereth, J. Ted; Vincenzo, Fiorenzo; Kjeldsen, Hans; Basu, Sarbani; Ball, Warrick H.; Stokholm, Amalie; Verma, Kuldeep; Mosumgaard, Jakob Rørsted; Silva Aguirre, Victor; Mazumdar, Anwesh; Ranadive, Pritesh; Antia, H. M.; Lebreton, Yveline; Ong, Joel; Appourchaux, Thierry; Bedding, Timothy R.; Christensen-Dalsgaard, Jørgen; Creevey, Orlagh; García, Rafael A.; Handberg, Rasmus; Huber, Daniel; Kawaler, Steven D.; Lund, Mikkel N.; Metcalfe, Travis S.; Stassun, Keivan G.; Bazot, Michäel; Beck, Paul G.; Bell, Keaton J.; Bergemann, Maria; Buzasi, Derek L.; Benomar, Othman; Bossini, Diego; Bugnet, Lisa; Campante, Tiago L.; Orhan, Zeynep çelik; Corsaro, Enrico; González-Cuesta, Lucía; Davies, Guy R.; Di Mauro, Maria Pia; Egeland, Ricky; Elsworth, Yvonne P.; Gaulme, Patrick; Ghasemi, Hamed; Guo, Zhao; Hall, Oliver J.; Hasanzadeh, Amir; Hekker, Saskia; Howe, Rachel; Jenkins, Jon M.; Jiménez, Antonio; Kiefer, René; Kuszlewicz, James S.; Kallinger, Thomas; Latham, David W.; Lundkvist, Mia S.; Mathur, Savita; Montalbán, Josefina; Mosser, Benoit; Bedón, Andres Moya; Nielsen, Martin Bo; Örtel, Sibel; Rendle, Ben M.; Ricker, George R.; Rodrigues, Thaíse S.; Roxburgh, Ian W.; Safari, Hossein; Schofield, Mathew; Seager, Sara; Smalley, Barry; Stello, Dennis; Szabó, Róbert; Tayar, Jamie; Themeßl, Nathalie; Thomas, Alexandra E. L.; Vanderspek, Roland K.; van Rossem, Walter E.; Vrard, Mathieu; Weiss, Achim; White, Timothy R.; Winn, Joshua N.; Yıldız, Mutlu Bibcode: 2020NatAs...4..382C Altcode: 2020NatAs.tmp....7C; 2020arXiv200104653C Over the course of its history, the Milky Way has ingested multiple smaller satellite galaxies1. Although these accreted stellar populations can be forensically identified as kinematically distinct structures within the Galaxy, it is difficult in general to date precisely the age at which any one merger occurred. Recent results have revealed a population of stars that were accreted via the collision of a dwarf galaxy, called Gaia-Enceladus1, leading to substantial pollution of the chemical and dynamical properties of the Milky Way. Here we identify the very bright, naked-eye star ν Indi as an indicator of the age of the early in situ population of the Galaxy. We combine asteroseismic, spectroscopic, astrometric and kinematic observations to show that this metal-poor, alpha-element-rich star was an indigenous member of the halo, and we measure its age to be 11.0 ±0.7 ? (stat) ±0.8 ? (sys) billion years. The star bears hallmarks consistent with having been kinematically heated by the Gaia-Enceladus collision. Its age implies that the earliest the merger could have begun was 11.6 and 13.2 billion years ago, at 68% and 95% confidence, respectively. Computations based on hierarchical cosmological models slightly reduce the above limits. Title: TESS Asteroseismology of the Known Red-giant Host Stars HD 212771 and HD 203949 Authors: Campante, Tiago L.; Corsaro, Enrico; Lund, Mikkel N.; Mosser, Benoît; Serenelli, Aldo; Veras, Dimitri; Adibekyan, Vardan; Antia, H. M.; Ball, Warrick; Basu, Sarbani; Bedding, Timothy R.; Bossini, Diego; Davies, Guy R.; Delgado Mena, Elisa; García, Rafael A.; Handberg, Rasmus; Hon, Marc; Kane, Stephen R.; Kawaler, Steven D.; Kuszlewicz, James S.; Lucas, Miles; Mathur, Savita; Nardetto, Nicolas; Nielsen, Martin B.; Pinsonneault, Marc H.; Reffert, Sabine; Silva Aguirre, Víctor; Stassun, Keivan G.; Stello, Dennis; Stock, Stephan; Vrard, Mathieu; Yıldız, Mutlu; Chaplin, William J.; Huber, Daniel; Bean, Jacob L.; Çelik Orhan, Zeynep; Cunha, Margarida S.; Christensen-Dalsgaard, Jørgen; Kjeldsen, Hans; Metcalfe, Travis S.; Miglio, Andrea; Monteiro, Mário J. P. F. G.; Nsamba, Benard; Örtel, Sibel; Pereira, Filipe; Sousa, Sérgio G.; Tsantaki, Maria; Turnbull, Margaret C. Bibcode: 2019ApJ...885...31C Altcode: 2019arXiv190905961C The Transiting Exoplanet Survey Satellite (TESS) is performing a near all-sky survey for planets that transit bright stars. In addition, its excellent photometric precision enables asteroseismology of solar-type and red-giant stars, which exhibit convection-driven, solar-like oscillations. Simulations predict that TESS will detect solar-like oscillations in nearly 100 stars already known to host planets. In this paper, we present an asteroseismic analysis of the known red-giant host stars HD 212771 and HD 203949, both systems having a long-period planet detected through radial velocities. These are the first detections of oscillations in previously known exoplanet-host stars by TESS, further showcasing the mission’s potential to conduct asteroseismology of red-giant stars. We estimate the fundamental properties of both stars through a grid-based modeling approach that uses global asteroseismic parameters as input. We discuss the evolutionary state of HD 203949 in depth and note the large discrepancy between its asteroseismic mass (M * = 1.23 ± 0.15 M if on the red-giant branch or M * = 1.00 ± 0.16 M if in the clump) and the mass quoted in the discovery paper (M * = 2.1 ± 0.1 M ), implying a change >30% in the planet’s mass. Assuming HD 203949 to be in the clump, we investigate the planet’s past orbital evolution and discuss how it could have avoided engulfment at the tip of the red-giant branch. Finally, HD 212771 was observed by K2 during its Campaign 3, thus allowing for a preliminary comparison of the asteroseismic performances of TESS and K2. We estimate the ratio of the observed oscillation amplitudes for this star to be {A}\max {TESS}/{A}\max K2=0.75+/- 0.14, consistent with the expected ratio of ∼0.85 due to the redder bandpass of TESS. Title: Hemispheric asymmetry in meridional flow and the sunspot cycle Authors: Lekshmi, B.; Nandy, Dibyendu; Antia, H. M. Bibcode: 2019MNRAS.489..714L Altcode: Magnetohydrodynamic dynamo modelling shows that the large-scale solar meridional plasma flow plays an important role in governing the dynamics of the sunspot cycle. Observations indicate that meridional flow velocities at each solar latitude and depth vary over time and are asymmetric across the equator. Here, using helioseismic observations we explore the temporal variation in the hemispherical asymmetry of near-surface residual (time-varying) component of the Sun's meridional flow velocity. The meridional flow velocities obtained from Global Oscillation Network Group (GONG) and Helioseismic and Magnetic Imager (HMI) onboard Solar Dynamics Observatory (SDO) ring-diagram pipelines are used in this work. Our data set covers the declining phase of cycle 23 and cycle 24 (from July 2001 till December 2018) and the flow velocities are poleward for the observed depth range. We observe a time delayed anticorrelation between the hemispherical asymmetry in near-surface meridional flow velocities and the sunspot cycle quantified in terms of magnetic flux and sunspot number. Interestingly, asymmetry in meridional flow velocity precedes the asymmetry in sunspot cycle by 3.1-3.5 yr. We propose that meridional flow asymmetry is a precursor of asymmetry in hemispherical cycle strength. The symmetric component of meridional flow is observed to be positively correlated with the corresponding symmetric components of the magnetic cycle, also with a time delay. Our analysis sets important constraints on theories for the origin of meridional plasma flow asymmetries and its temporal variations and is relevant for understanding the role of plasma flux transport processes in determining hemispheric asymmetry in the sunspot cycle. Title: Changes in Solar Rotation over Two Solar Cycles Authors: Basu, Sarbani; Antia, H. M. Bibcode: 2019ApJ...883...93B Altcode: 2019arXiv190805282B We use helioseismic data from ground- and space-based instruments to analyze how solar rotation has changed since the beginning of solar Cycle 23 with emphasis on studying the differences between Cycles 23 and 24. We find that the nature of solar rotation is indeed different for the two cycles. While the changes in the latitudinally independent component follows solar-cycle indices, some of the other components have a more complicated behavior. There is a substantial change in the behavior of the solar zonal flows and their spatial gradients too. While the zonal flows in Cycle 24 are weaker in general than those in Cycle 23, there are clear signs of the emergence of Cycle 25. We have also investigated the properties of the solar tachocline, in particular, its position, width, and the change (or jump) in the rotation rate across it. We find significant temporal variation in the change of the rotation rate across the tachocline. We also find that the changes in solar Cycle 24 were very different from those of Cycle 23. We do not find any statistically significant change in the position or the width of the tachocline. Title: Broad-band reflection spectroscopy of MAXI J1535-571 using AstroSat: estimation of black hole mass and spin Authors: Sridhar, Navin; Bhattacharyya, Sudip; Chandra, Sunil; Antia, H. M. Bibcode: 2019MNRAS.487.4221S Altcode: 2019arXiv190509253S; 2019MNRAS.tmp.1413S We report the results from AstroSat observations of the transient Galactic black hole X-ray binary MAXI J1535-571 during its hard-intermediate state of the 2017 outburst. We systematically study the individual and joint spectra from two simultaneously observing AstroSat X-ray instruments, and probe and measure a number of parameter values of accretion disc, corona, and reflection from the disc in the system using models with generally increasing complexities. Using our broad-band (1.3-70 keV) X-ray spectrum, we clearly show that a soft X-ray instrument, which works below 10-12 keV, alone cannot correctly characterize the Comptonizing component from the corona, thus highlighting the importance of broad-band spectral analysis. By fitting the reflection spectrum with the latest version of the RELXILL family of relativistic reflection models, we constrain the black hole's dimensionless spin parameter to be 0.67^{+0.16}_{-0.04}. We also jointly use the reflection spectral component (RELXILL) and a general relativistic thin disc component (Kerrbb), and estimate the black hole's mass and distance to be 10.39_{-0.62}^{+0.61} M_{\odot } and 5.4_{-1.1}^{+1.8} kpc, respectively. Title: VizieR Online Data Catalog: High-precision radial velocities for HD 221416 (Huber+, 2019) Authors: Huber, D.; Chaplin, W. J.; Chontos, A.; Kjeldsen, H.; Christensen-Dalsgaard, J.; Bedding, T. R.; Ball, W.; Brahm, R.; Espinoza, N.; Henning, T.; Jordan, A.; Sarkis, P.; Knudstrup, E.; Albrecht, S.; Grundahl, F.; Andersen, M. F.; Palle, P. L.; Crossfield, I.; Fulton, B.; Howard, A. W.; Isaacson, H. T.; Weiss, L. M.; Handberg, R.; Lund, M. N.; Serenelli, A. M.; Rorsted Mosumgaard, J.; Stokholm, A.; Bieryla, A.; Buchhave, L. A.; Latham, D. W.; Quinn, S. N.; Gaidos, E.; Hirano, T.; Ricker, G. R.; Vanderspek, R. K.; Seager, S.; Jenkins, J. M.; Winn, J. N.; Antia, H. M.; Appourchaux, T.; Basu, S.; Bell, K. J.; Benomar, O.; Bonanno, A.; Buzasi, D. L.; Campante, T. L.; Celik Orhan, Z.; Corsaro, E.; Cunha, M. S.; Davies, G. R.; Deheuvels, S.; Grunblatt, S. K.; Hasanzadeh, A.; di Mauro, M. P.; Garcia, R. A.; Gaulme, P.; Girardi, L.; Guzik, J. A.; Hon, M.; Jiang, C.; Kallinger, T.; Kawaler, S. D.; Kuszlewicz, J. S.; Lebreton, Y.; Li, T.; Lucas, M.; Lundkvist, M. S.; Mann, A. W.; Mathis, S.; Mathur, S.; Mazumdar, A.; Metcalfe, T. S.; Miglio, A.; Monteiro, M. J. P. F. G.; Mosser, B.; Noll, A.; Nsamba, B.; Ong, J. M. J.; Ortel, S.; Pereira, F.; Ranadive, P.; Regulo, C.; Rodrigues, T. S.; Roxburgh, I. W.; Aguirre, V. S.; Smalley, B.; Schofield, M.; Sousa, S. G.; Stassun, K. G.; Stello, D.; Tayar, J.; White, T. R.; Verma, K.; Vrard, M.; Yildiz, M.; Baker, D.; Bazot, M.; Beichmann, C.; Bergmann, C.; Bugnet, L.; Cale, B.; Carlino, R.; Cartwright, S. M.; Christiansen, J. L.; Ciardi, D. R.; Creevey, O.; Dittmann, J. A.; Do Nascimento, J. -D., Jr.; van Eylen, V.; Furesz, G.; Gagne, J.; Gao, P.; Gazeas, K.; Giddens, F.; Hall, O. J.; Hekker, S.; Ireland, M. J.; Latouf, N.; Lebrun, D.; Levine, A. M.; Matzko, W.; Natinsky, E.; Page, E.; Plavchan, P.; Mansouri-Samani, M.; McCauliff, S.; Mullally, S. E.; Orenstein, B.; Soto, A. G.; Paegert, M.; van Saders, J. L.; Schnaible, C.; Soderblom, D. R.; Szabo, R.; Tanner, A.; Tinney, C. G.; Teske, J.; Thomas, A.; Trampedach, R.; Wright, D.; Yuan, T. T.; Zohrabi, F. Bibcode: 2019yCat..51570245H Altcode: We obtained high-resolution spectra of HD 221416 using several facilities within the TESS Follow-up Observation Program (TFOP), including HIRES (Vogt et al. 1994SPIE.2198..362V) on the 10 m telescope at Keck Observatory (Maunakea, Hawai'i); the Hertzsprung SONG Telescope at Teide Observatory (Tenerife; Grundahl et al. 2017ApJ...836..142G); HARPS (Mayor et al. 2003Msngr.114...20M), FEROS (Kaufer et al. 1999Msngr..95....8K), Coralie (Queloz et al. 2001Msngr.105....1Q), and FIDEOS (Vanzi et al. 2018MNRAS.477.5041V) on the MPG/ESO 3.6 m, 2.2 m, 1.2 m, and 1 m telescopes at La Silla Observatory (Chile); Veloce (Gilbert et al. 2018SPIE10702E..0YG) on the 3.9 m Anglo-Australian Telescope at Siding Spring Observatory (Australia); TRES (Furesz 2008, PhD thesis Univ. Szeged) on the 1.5 m Tillinghast reflector at the F. L. Whipple Observatory (Mt. Hopkins, Arizona); and iSHELL (Rayner et al. 2012SPIE.8446E..2CR) on the NASA IRTF Telescope (Maunakea, Hawai'i). All spectra used in this paper were obtained between 2018 November 11 and December 30 and have a minimum spectral resolution of R~44000.

(1 data file). Title: Survey of Li-rich Giants among Kepler and LAMOST Fields: Determination of Li-rich Giants’ Evolutionary Phase Authors: Singh, Raghubar; Reddy, Bacham E.; Bharat Kumar, Yerra; Antia, H. M. Bibcode: 2019ApJ...878L..21S Altcode: 2019arXiv190603198S In this Letter, we report the discovery of 24 new super Li-rich (A(Li) ≥ 3.2) giants of He-core burning phase at the red clump region. Results are based on a systematic search of a large sample of about 12,500 giants common to the LAMOST spectroscopic and Kepler time-resolved photometric surveys. The two key parameters derived from Kepler data are an average period spacing (Δp) between l = 1 mixed gravity-dominated g-modes and average large frequency-separation (Δν) l = 0 acoustic p-modes, which suggest all the Li-rich giants are in the He-core burning phase. This is the first unbiased survey subjected to a robust technique of asteroseismic analysis to unambiguously determine the evolutionary phase of Li-rich giants. The results provide strong evidence that the Li enhancement phenomenon is associated with giants in the He-core burning phase post He-flash, rather than any other phase on the red giant branch with an inert He-core surrounded by a H-burning shell. Title: A Hot Saturn Orbiting an Oscillating Late Subgiant Discovered by TESS Authors: Huber, Daniel; Chaplin, William J.; Chontos, Ashley; Kjeldsen, Hans; Christensen-Dalsgaard, Jørgen; Bedding, Timothy R.; Ball, Warrick; Brahm, Rafael; Espinoza, Nestor; Henning, Thomas; Jordán, Andrés; Sarkis, Paula; Knudstrup, Emil; Albrecht, Simon; Grundahl, Frank; Fredslund Andersen, Mads; Pallé, Pere L.; Crossfield, Ian; Fulton, Benjamin; Howard, Andrew W.; Isaacson, Howard T.; Weiss, Lauren M.; Handberg, Rasmus; Lund, Mikkel N.; Serenelli, Aldo M.; Rørsted Mosumgaard, Jakob; Stokholm, Amalie; Bieryla, Allyson; Buchhave, Lars A.; Latham, David W.; Quinn, Samuel N.; Gaidos, Eric; Hirano, Teruyuki; Ricker, George R.; Vanderspek, Roland K.; Seager, Sara; Jenkins, Jon M.; Winn, Joshua N.; Antia, H. M.; Appourchaux, Thierry; Basu, Sarbani; Bell, Keaton J.; Benomar, Othman; Bonanno, Alfio; Buzasi, Derek L.; Campante, Tiago L.; Çelik Orhan, Z.; Corsaro, Enrico; Cunha, Margarida S.; Davies, Guy R.; Deheuvels, Sebastien; Grunblatt, Samuel K.; Hasanzadeh, Amir; Di Mauro, Maria Pia; García, Rafael A.; Gaulme, Patrick; Girardi, Léo; Guzik, Joyce A.; Hon, Marc; Jiang, Chen; Kallinger, Thomas; Kawaler, Steven D.; Kuszlewicz, James S.; Lebreton, Yveline; Li, Tanda; Lucas, Miles; Lundkvist, Mia S.; Mann, Andrew W.; Mathis, Stéphane; Mathur, Savita; Mazumdar, Anwesh; Metcalfe, Travis S.; Miglio, Andrea; Monteiro, Mário J. P. F. G.; Mosser, Benoit; Noll, Anthony; Nsamba, Benard; Ong, Jia Mian Joel; Örtel, S.; Pereira, Filipe; Ranadive, Pritesh; Régulo, Clara; Rodrigues, Thaíse S.; Roxburgh, Ian W.; Silva Aguirre, Victor; Smalley, Barry; Schofield, Mathew; Sousa, Sérgio G.; Stassun, Keivan G.; Stello, Dennis; Tayar, Jamie; White, Timothy R.; Verma, Kuldeep; Vrard, Mathieu; Yıldız, M.; Baker, David; Bazot, Michaël; Beichmann, Charles; Bergmann, Christoph; Bugnet, Lisa; Cale, Bryson; Carlino, Roberto; Cartwright, Scott M.; Christiansen, Jessie L.; Ciardi, David R.; Creevey, Orlagh; Dittmann, Jason A.; Do Nascimento, Jose-Dias, Jr.; Van Eylen, Vincent; Fürész, Gabor; Gagné, Jonathan; Gao, Peter; Gazeas, Kosmas; Giddens, Frank; Hall, Oliver J.; Hekker, Saskia; Ireland, Michael J.; Latouf, Natasha; LeBrun, Danny; Levine, Alan M.; Matzko, William; Natinsky, Eva; Page, Emma; Plavchan, Peter; Mansouri-Samani, Masoud; McCauliff, Sean; Mullally, Susan E.; Orenstein, Brendan; Garcia Soto, Aylin; Paegert, Martin; van Saders, Jennifer L.; Schnaible, Chloe; Soderblom, David R.; Szabó, Róbert; Tanner, Angelle; Tinney, C. G.; Teske, Johanna; Thomas, Alexandra; Trampedach, Regner; Wright, Duncan; Yuan, Thomas T.; Zohrabi, Farzaneh Bibcode: 2019AJ....157..245H Altcode: 2019arXiv190101643H We present the discovery of HD 221416 b, the first transiting planet identified by the Transiting Exoplanet Survey Satellite (TESS) for which asteroseismology of the host star is possible. HD 221416 b (HIP 116158, TOI-197) is a bright (V = 8.2 mag), spectroscopically classified subgiant that oscillates with an average frequency of about 430 μHz and displays a clear signature of mixed modes. The oscillation amplitude confirms that the redder TESS bandpass compared to Kepler has a small effect on the oscillations, supporting the expected yield of thousands of solar-like oscillators with TESS 2 minute cadence observations. Asteroseismic modeling yields a robust determination of the host star radius (R = 2.943 ± 0.064 R ), mass (M = 1.212 ± 0.074 M ), and age (4.9 ± 1.1 Gyr), and demonstrates that it has just started ascending the red-giant branch. Combining asteroseismology with transit modeling and radial-velocity observations, we show that the planet is a “hot Saturn” (R p = 9.17 ± 0.33 R ) with an orbital period of ∼14.3 days, irradiance of F = 343 ± 24 F , and moderate mass (M p = 60.5 ± 5.7 M ) and density (ρ p = 0.431 ± 0.062 g cm-3). The properties of HD 221416 b show that the host-star metallicity-planet mass correlation found in sub-Saturns (4-8 R ) does not extend to larger radii, indicating that planets in the transition between sub-Saturns and Jupiters follow a relatively narrow range of densities. With a density measured to ∼15%, HD 221416 b is one of the best characterized Saturn-size planets to date, augmenting the small number of known transiting planets around evolved stars and demonstrating the power of TESS to characterize exoplanets and their host stars using asteroseismology. Title: LAXPC/AstroSat Study of ∼1 and ∼2 mHz Quasi-periodic Oscillations in the Be/X-Ray Binary 4U 0115+63 during Its 2015 Outburst Authors: Roy, Jayashree; Agrawal, P. C.; Iyer, N. K.; Bhattacharya, D.; Yadav, J. S.; Antia, H. M.; Chauhan, J. V.; Choudhury, M.; Dedhia, D. K.; Katoch, T.; Madhavani, P.; Manchanda, R. K.; Misra, R.; Pahari, M.; Paul, B.; Shah, P. Bibcode: 2019ApJ...872...33R Altcode: 2019arXiv190109382R The Be/X-ray binary 4U 0115+63 was observed by the Large Area X-ray Proportional Counter (LAXPC) instrument on AstroSat on 2015 October 24 during the peak of a giant Type II outburst. Prominent intensity oscillations at ∼1 and ∼2 mHz frequency were detected during the outburst. Nuclear Spectroscopic Telescope Array (NuSTAR) observations made during the same outburst also show millihertz quasi-periodic oscillations (QPOs). Details of the oscillations and their characteristics deduced from LAXPC/AstroSat and NuSTAR observations are reported in this paper. Analysis of the archival Rossi X-ray Timing Explorer/Proportional Counter Array data during 2001-2011 also show the presence of millihertz QPOs during some of the outbursts, and details of these QPOs are also reported. Possible models to explain the origin of the millihertz oscillations are examined. Similar QPOs, albeit at higher frequencies, have been reported from other neutron star and black hole sources, and both may have a common origin. Current models to explain the instability in the inner accretion disk causing the intense oscillations are discussed. Title: Thermonuclear X-ray bursts in rapid succession in 4U 1636-536 with AstroSat-LAXPC Authors: Beri, Aru; Paul, Biswajit; Yadav, J. S.; Antia, H. M.; Agrawal, P. C.; Manchanda, R. K.; Dedhia, Dhiraj; Chauhan, Jai Verdhan; Pahari, Mayukh; Misra, Ranjeev; Katoch, Tilak; Madhwani, P.; Shah, Parag; Varun, Mate, Sujay Bibcode: 2019MNRAS.482.4397B Altcode: 2018MNRAS.tmp.2837B; 2018arXiv181110030B We present results from an observation of the low-mass X-ray binary 4U 1636-536 obtained with the LAXPC instrument aboard AstroSat. The observations of 4U 1636-536 made during the performance verification phase of AstroSat showed seven thermonuclear X-ray bursts in a total exposure of ∼65 ks over a period of about two consecutive days. Moreover, the light curve of 4U 1636-536 revealed the presence of a rare triplet of X-ray bursts, having a wait time of about 5.5 min between the second and the third bursts. We also present results from time-resolved spectroscopy performed during these seven X-ray bursts. In addition, we have also detected a transient quasi-periodic oscillation at ∼5 Hz. However, we did not find any evidence of kilo-hertz quasi-periodic oscillations and/or X-ray burst oscillations, perhaps due to the hard spectral state of the source during this observation. Title: Helioseismic Inversion to Infer the Depth Profile of Solar Meridional Flow Using Spherical Born Kernels Authors: Mandal, K.; Hanasoge, S. M.; Rajaguru, S. P.; Antia, H. M. Bibcode: 2018ApJ...863...39M Altcode: 2018arXiv180700314M Accurate inferences of solar meridional flow are crucial for understanding solar dynamo processes. Wave travel times, as measured on the surface, will change if the waves encounter perturbations, e.g., in the sound speed or flows, as they propagate through the solar interior. Using functions called sensitivity kernels, we can image the underlying anomalies that cause measured shifts in travel times. The inference of large-scale structures, e.g., meridional circulation, requires computing sensitivity kernels in spherical geometry. Mandal et al. have computed such spherical kernels in the limit of the first-Born approximation. In this work, we perform an inversion for meridional circulation using travel-time measurements obtained from 6 years of Solar Dynamics Observatory/Helioseismic and Magnetic Imager data and those sensitivity kernels. We enforce mass conservation by inverting for a stream function. The number of free parameters is reduced by projecting the solution onto cubic B-splines in radius and derivatives of the Legendre-polynomial basis in latitude, thereby improving the condition number of the inverse problem. We validate our approach for synthetic observations before performing the actual inversion. The inversion suggests a single-cell profile with a return flow occurring at depths below 0.78 R . Title: Asymmetry in Solar Torsional Oscillation and the Sunspot Cycle Authors: Lekshmi, B.; Nandy, Dibyendu; Antia, H. M. Bibcode: 2018ApJ...861..121L Altcode: 2018arXiv180703588B; 2018ApJ...861..121B; 2018arXiv180703588L Solar torsional oscillations are migrating bands of slower- and faster-than-average rotation, which are strongly related to the Sun’s magnetic cycle. We perform a long-term study (16 yr) of hemispherical asymmetry in solar torsional oscillation velocity using helioseismic data for the first time. We study the north-south asymmetry in the velocity using the zonal flow velocities obtained by ring diagram analysis of the Global Oscillation Network Group (GONG) Doppler images. We find significant hemispherical asymmetry in the torsional oscillation velocity and explore its variation with respect to depth, time, and latitude. We also calculate the hemispherical asymmetry in the surface velocity measurements from the Mount Wilson Observatory and the zonal flow velocities obtained from the Helioseismic and Magnetic Imager ring diagram pipeline. These asymmetries are found to be consistent with the asymmetry obtained from GONG observations. We show that the asymmetry in near-surface torsional oscillation velocity is correlated with the asymmetry in magnetic flux and sunspot number at the solar surface, with the velocity asymmetry preceding the flux and sunspot number asymmetries. We speculate that the asymmetry in torsional oscillation velocity may help in predicting the hemispherical asymmetry in sunspot cycles. Title: Effects of Thermonuclear X-Ray Bursts on Non-burst Emissions in the Soft State of 4U 1728-34 Authors: Bhattacharyya, Sudip; Yadav, J. S.; Sridhar, Navin; Verdhan Chauhan, Jai; Agrawal, P. C.; Antia, H. M.; Pahari, Mayukh; Misra, Ranjeev; Katoch, Tilak; Manchanda, R. K.; Paul, Biswajit Bibcode: 2018ApJ...860...88B Altcode: 2018arXiv180505393B It has recently been shown that the persistent emission of a neutron star low-mass X-ray binary (LMXB) evolves during a thermonuclear (type-I) X-ray burst. The reason of this evolution, however, is not fully known. This uncertainty can introduce significant systematics in the neutron star radius measurement using burst spectra, particularly if an unknown but significant fraction of the burst emission, which is reprocessed, contributes to the changes in the persistent emission during the burst. Here, by analyzing individual burst data of AstroSat/LAXPC from the neutron star LMXB 4U 1728-34 in the soft state, we show that the burst emission is not significantly reprocessed by a corona covering the neutron star. Rather, our analysis suggests that the burst emission enhances the accretion disk emission, possibly by increasing the accretion rate via disk. This enhanced disk emission, which is Comptonized by a corona covering the disk, can explain an increased persistent emission observed during the burst. This finding provides an understanding of persistent emission components and their interaction with the thermonuclear burst emission. Furthermore, as burst photons are not significantly reprocessed, non-burst and burst emissions can be reliably separated, which is required to reduce systematic uncertainties in the stellar radius measurement. Title: Torsional Oscillations in the Suns rotation contribute to the Waldmeier-effect in Solar Cycles Authors: Mahajan, Sushant S.; Nandy, Dibyendu; Antia, H. M.; Dwivedi, B. N. Bibcode: 2018arXiv180307758M Altcode: Temporal variations in the Suns internal velocity field with a periodicity of about 11 years have been observed over the last four decades. The period of these torsional oscillations and their latitudinal propagation roughly coincides with the period and equatorward propagation of sunspots which originate from a magnetohydrodynamic dynamo mechanism operating in the Suns interior. While the solar differential rotation plays an important role in this dynamo mechanism by inducting the toroidal component of magnetic field, the impact of torsional oscillations on the dynamo mechanism and hence the solar cycle is not well understood. Here, we include the observed torsional oscillations into a flux transport dynamo model of the solar cycle to investigate their effect. We find that the overall amplitude of the solar cycle does not change significantly on inclusion of torsional oscillations. However, all the characteristics of the Waldmeier effect in the sunspot cycle are qualitatively reproduced by varying only the amplitude of torsional oscillations. The Waldmeier effect, first noted in 1935, includes the important characteristic that the amplitude of sunspot cycles is anti-correlated to their rise time; cycles with high initial rise rate tend to be stronger. This has implications for solar cycle predictions. Our results suggest that the Waldmeier effect could be a plausible outcome of cycle to cycle modulation of torsional oscillations and provides a physical basis for sunspot cycle forecasts based on torsional oscillation observations. We also provide a theoretical explanation based on the magnetic induction equation thereby connecting two apparently disparate phenomena. Title: Asymmetry in Solar Torsional Oscillation Authors: Lekshmi, B.; Nandy, Dibyendu; Antia, H. M. Bibcode: 2018IAUS..340...11L Altcode: Solar torsional oscillations are migrating bands of slower and faster than average rotation, which are thought to be related to the Sun's magnetic cycle. We perform the first long-term study (16 years) of hemispherical asymmetry in solar torsional oscillation velocity using helioseismic data. We explore the spatial and temporal variation of North-South asymmetry using zonal flow velocities obtained from ring diagram analysis of the Global Oscillation Network Group (GONG) Doppler images. We find a strong correlation between the asymmetries of near-surface torsional oscillation with magnetic flux and sunspot number, with the velocity asymmetry preceding in both the cases. We speculate that the asymmetry in torsional oscillation velocity may help in predicting the hemispherical asymmetry in the sunspot cycle. Title: VizieR Online Data Catalog: 61 main-sequence and subgiant oscillations (Appourchaux+, 2012) Authors: Appourchaux, T.; Chaplin, W. J.; Garcia, R. A.; Gruberbauer, M.; Verner, G. A.; Antia, H. M.; Benomar, O.; Campante, T. L.; Davies, G. R.; Deheuvels, S.; Handberg, R.; Hekker, S.; Howe, R.; Regulo, C.; Salabert, D.; Bedding, T. R.; White, T. R.; Ballot, J.; Mathur, S.; Silva Aguirre, V.; Elsworth, Y. P.; Basu, S.; Gilliland, R. L.; Christensen-Dalsgaard, J.; Kjeldsen, H.; Uddin, K.; Stumpe, M. C.; Barclay, T. Bibcode: 2017yCat..35430054A Altcode: Kepler observations are obtained in two different operating modes: long cadence (LC) and short cadence (SC) (Gilliland et al., 2010ApJ...713L.160G; Jenkins et al., 2010ApJ...713L..87J). This work is based on SC data. For the brightest stars (down to Kepler magnitude, Kp~=12), SC observations can be obtained for a limited number of stars (up to 512 at any given time) with a faster sampling cadence of 58.84876s (Nyquist frequency of ~8.5mHz), which permits a more precise transit timing and the performance of asteroseismology. Kepler observations are divided into three-month-long quarters (Q). A subset of 61 solar-type stars observed during quarters Q5-Q7 (March 22, 2010 to December 22, 2010) were chosen because they have oscillation modes with high signal-to-noise ratios. This length of data gives a frequency resolution of about 0.04uHz.

(2 data files). Title: Erratum: “Standing on the Shoulders of Dwarfs: The Kepler Asteroseismic LEGACY Sample. I. Oscillation Mode Parameters” (2017, ApJ, 835, 172) Authors: Lund, Mikkel N.; Silva Aguirre, Víctor; Davies, Guy R.; Chaplin, William J.; Christensen-Dalsgaard, Jørgen; Houdek, Günter; White, Timothy R.; Bedding, Timothy R.; Ball, Warrick H.; Huber, Daniel; Antia, H. M.; Lebreton, Yveline; Latham, David W.; Handberg, Rasmus; Verma, Kuldeep; Basu, Sarbani; Casagrande, Luca; Justesen, Anders B.; Kjeldsen, Hans; Mosumgaard, Jakob R. Bibcode: 2017ApJ...850..110L Altcode: No abstract at ADS Title: X-Ray Timing Analysis of Cyg X-3 Using AstroSat/LAXPC: Detection of Milli-hertz Quasi-periodic Oscillations during the Flaring Hard X-Ray State Authors: Pahari, Mayukh; Antia, H. M.; Yadav, J. S.; Verdhan Chauhan, Jai; Agrawal, P. C.; Misra, Ranjeev; Chitnis, V. R.; Dedhia, Dhiraj; Katoch, Tilak; Madhwani, P.; Manchanda, R. K.; Paul, B.; Shah, Parag Bibcode: 2017ApJ...849...16P Altcode: 2017arXiv170906353P We present here results from the X-ray timing and spectral analysis of the X-ray binary Cyg X-3 using observations from the Large Area X-ray proportional Counter on board AstroSat. Consecutive light curves observed over a period of one year show the binary orbital period of 17253.56 ± 0.19 s. Another low-amplitude, slow periodicity of the order of 35.8 ± 1.4 days is observed, which may be due to the orbital precession as suggested earlier by Molteni et al. During the rising binary phase, power density spectra from different observations during the flaring hard X-ray state show quasi-periodic oscillations (QPOs) at ∼5-8 mHz, ∼12-14 mHz, and ∼18-24 mHz frequencies at the minimum confidence of 99%. However, during the consecutive binary decay phase, no QPO is detected up to 2σ significance. Energy-dependent time-lag spectra show soft lag (soft photons lag hard photons) at the mHz QPO frequency and the fractional rms of the QPO increases with the photon energy. During the binary motion, the observation of mHz QPOs during the rising phase of the flaring hard state may be linked to the increase in the supply of the accreting material in the disk and corona via stellar wind from the companion star. During the decay phase, the compact source moves in the outer wind region causing the decrease in supply of material for accretion. This may cause weakening of the mHz QPOs below the detection limit. This is also consistent with the preliminary analysis of the orbital phase-resolved energy spectra presented in this paper. Title: Sensitivity of helioseismic measurements of normal-mode coupling to flows and sound-speed perturbations Authors: Hanasoge, Shravan M.; Woodard, Martin; Antia, H. M.; Gizon, Laurent; Sreenivasan, Katepalli R. Bibcode: 2017MNRAS.470.1404H Altcode: 2017arXiv170508204H In this article, we derive and compute the sensitivity of measurements of coupling between normal modes of oscillation in the Sun to underlying flows. The theory is based on first-born perturbation theory, and the analysis is carried out using the formalism described by Lavely & Ritzwoller (1992). Albeit tedious, we detail the derivation and compute the sensitivity of specific pairs of coupled normal modes to anomalies in the interior. Indeed, these kernels are critical for the accurate inference of convective flow amplitudes and large-scale circulations in the solar interior. We resolve some inconsistencies in the derivation of Lavely & Ritzwoller (1992) and reformulate the fluid-continuity condition. We also derive and compute sound-speed kernels, paving the way for inverting for thermal anomalies alongside flows. Title: Large Area X-ray Proportional Counter instrument on AstroSat Authors: Yadav, J. S.; Agrawal, P. C.; Antia, H. M.; Manchanda, R. K.; Paul, B.; Misra, Ranjeev Bibcode: 2017CSci..113..591Y Altcode: 2017arXiv170506440Y Large Area X-ray Proportional Counter (LAXPC) is one of the major AstroSat payloads. LAXPC instrument will provide high time resolution X-ray observations in 3 to 80 keV energy band with moderate energy resolution. A cluster of three co-aligned identical LAXPC detectors is used in AstroSat to provide large collection area of more than 6000 cm2 . The large detection volume (15 cm depth) filled with xenon gas at about 2 atmosphere pressure, results in detection efficiency greater than 50%, above 30 keV. With its broad energy range and fine time resolution (10 microsecond), LAXPC instrument is well suited for timing and spectral studies of a wide variety of known and transient X-ray sources in the sky. We have done extensive calibration of all LAXPC detectors using radioactive sources as well as GEANT4 simulation of LAXPC detectors. We describe in brief some of the results obtained during the payload verification phase along with LXAPC capabilities. Title: VizieR Online Data Catalog: Kepler asteroseismic LEGACY sample. I. Oscillations (Lund+, 2017) Authors: Lund, M. N.; Silva Aguirre, V.; Davies, G. R.; Chaplin, W. J.; Christensen-Dalsgaard, J.; Houdek, G.; White, T. R.; Bedding, T. R.; Ball, W. H.; Huber, D.; Antia, H. M.; Lebreton, Y.; Latham, D. W.; Handberg, R.; Verma, K.; Basu, S.; Casagrande, L.; Justesen, A. B.; Kjeldsen, H.; Mosumgaard, J. R. Bibcode: 2017yCat..18350172L Altcode: The advent of space-based missions like Kepler has revolutionized the study of solar-type stars, particularly through the measurement and modeling of their resonant modes of oscillation. Here we analyze a sample of 66 Kepler main-sequence stars showing solar-like oscillations as part of the Kepler seismic LEGACY project. We use Kepler short-cadence data, of which each star has at least 12 months, to create frequency-power spectra optimized for asteroseismology. For each star, we identify its modes of oscillation and extract parameters such as frequency, amplitude, and line width using a Bayesian Markov chain Monte Carlo "peak-bagging" approach. We report the extracted mode parameters for all 66 stars, as well as derived quantities such as frequency difference ratios, the large and small separations Δν and δν02; the behavior of line widths with frequency and line widths at νmax with Teff, for which we derive parametrizations; and behavior of mode visibilities. These average properties can be applied in future peak-bagging exercises to better constrain the parameters of the stellar oscillation spectra. The frequencies and frequency ratios can tightly constrain the fundamental parameters of these solar-type stars, and mode line widths and amplitudes can test models of mode damping and excitation.

(4 data files). Title: VizieR Online Data Catalog: Kepler asteroseismic LEGACY sample. II. (Silva Aguirre+, 2017) Authors: Silva Aguirre, V.; Lund, M. N.; Antia, H. M.; Ball, W. H.; Basu, S.; Christensen-Dalsgaard, J.; Lebreton, Y.; Reese, D. R.; Verma, K.; Casagrande, L.; Justesen, A. B.; Mosumgaard, J. R.; Chaplin, W. J.; Bedding, T. R.; Davies, G. R.; Handberg, R.; Houdek, G.; Huber, D.; Kjeldsen, H.; Latham, D. W.; White, T. R.; Coelho, H. R.; Miglio, A.; Rendle, B. Bibcode: 2017yCat..18350173S Altcode: The 66 stars comprising the LEGACY sample were chosen from more than 500 main-sequence and subgiant targets in which Kepler detected oscillations (Chaplin+ 2014, J/ApJS/210/1). We selected all targets that had more than one year of short-cadence observations, and where inspection of the power spectrum did not reveal any clear signature of bumped l=1 modes.

(3 data files). Title: Calibration of the Large Area X-Ray Proportional Counter (LAXPC) Instrument on board AstroSat Authors: Antia, H. M.; Yadav, J. S.; Agrawal, P. C.; Verdhan Chauhan, Jai; Manchanda, R. K.; Chitnis, Varsha; Paul, Biswajit; Dedhia, Dhiraj; Shah, Parag; Gujar, V. M.; Katoch, Tilak; Kurhade, V. N.; Madhwani, Pankaj; Manojkumar, T. K.; Nikam, V. A.; Pandya, A. S.; Parmar, J. V.; Pawar, D. M.; Pahari, Mayukh; Misra, Ranjeev; Navalgund, K. H.; Pandiyan, R.; Sharma, K. S.; Subbarao, K. Bibcode: 2017ApJS..231...10A Altcode: 2017arXiv170208624A We present the calibration and background model for the Large Area X-ray Proportional Counter (LAXPC) detectors on board AstroSat. The LAXPC instrument has three nominally identical detectors to achieve a large collecting area. These detectors are independent of each other, and in the event analysis mode they record the arrival time and energy of each photon that is detected. The detectors have a time resolution of 10 μs and a dead-time of about 42 μs. This makes LAXPC ideal for timing studies. The energy resolution and peak channel-to-energy mapping were obtained from calibration on the ground using radioactive sources coupled with GEANT4 simulations of the detectors. The response matrix was further refined from observations of the Crab after launch. At around 20 keV the energy resolution of the detectors is 10%-15%, while the combined effective area of the three detectors is about 6000 cm2. Title: Large Area X-Ray Proportional Counter (LAXPC) Instrument on AstroSat and Some Preliminary Results from its Performance in the Orbit Authors: Agrawal, P. C.; Yadav, J. S.; Antia, H. M.; Dedhia, Dhiraj; Shah, P.; Chauhan, Jai Verdhan; Manchanda, R. K.; Chitnis, V. R.; Gujar, V. M.; Katoch, Tilak; Kurhade, V. N.; Madhwani, P.; Manojkumar, T. K.; Nikam, V. A.; Pandya, A. S.; Parmar, J. V.; Pawar, D. M.; Roy, Jayashree; Paul, B.; Pahari, Mayukh; Misra, Ranjeev; Ravichandran, M. H.; Anilkumar, K.; Joseph, C. C.; Navalgund, K. H.; Pandiyan, R.; Sarma, K. S.; Subbarao, K. Bibcode: 2017JApA...38...30A Altcode: 2017arXiv170506446A Large area X-ray propositional counter (LAXPC) instrument on AstroSat is aimed at providing high time resolution X-ray observations in 3-80 keV energy band with moderate energy resolution. To achieve large collecting area, a cluster of three co-aligned identical LAXPC detectors, is used to realize an effective area in access of {∼ }6000 cm2 at 15 keV. The large detection volume of the LAXPC detectors, filled with xenon gas at {∼ }2 atmosphere pressure, results in detection efficiency greater than 50%, above 30 keV. In this article, we present salient features of the LAXPC detectors, their testing and characterization in the laboratory prior to launch and calibration in the orbit. Some preliminary results on timing and spectral characteristics of a few X-ray binaries and other type of sources, are briefly discussed to demonstrate that the LAXPC instrument is performing as planned in the orbit. Title: AstroSat/LAXPC Detection of Millisecond Phenomena in 4U 1728-34 Authors: Verdhan Chauhan, Jai; Yadav, J. S.; Misra, Ranjeev; Agrawal, P. C.; Antia, H. M.; Pahari, Mayukh; Sridhar, Navin; Dedhia, Dhiraj; Katoch, Tilak; Madhwani, P.; Manchanda, R. K.; Paul, B.; Shah, Parag Bibcode: 2017ApJ...841...41V Altcode: 2017arXiv170404931V The low-mass X-ray binary 4U 1728-24 was observed with AstroSat/LAXPC on 2016 March 8th. Data from a randomly chosen orbit of over 3 ks was analyzed for detection of rapid intensity variations. We found that the source intensity was nearly steady but, toward the end of the observation, a typical Type-1 burst was detected. Dynamical power spectrum of the data in the 3-20 keV band, reveals the presence of a kHz Quasi-Periodic Oscillation (QPO) for which the frequency drifted from ∼815 Hz at the beginning of the observation to about 850 Hz just before the burst. The QPO is also detected in the 10-20 keV band, which was not obtainable by earlier RXTE observations of this source. Even for such a short observation with a drifting QPO frequency, the time lag between the 5-10 and 10-20 keV bands can be constrained to be less than 100 microseconds. The Type-1 burst that lasted for about 20 s had a typical profile. During the first four seconds, dynamic power spectra reveal a burst oscillation for which the frequency increased from ∼361.5 to ∼363.5 Hz. This is consistent with the earlier results obtained with RXTE/PCA, showing the same spin frequency of the neutron star. The present results demonstrate the capability of the LAXPC instrument for detecting millisecond variability even from short observations. After RXTE ceased operation, LAXPC on AstroSat is the only instrument at present with the capability of detecting kHz QPOs and other kinds of rapid variations from 3 keV to 20 keV and possibly at higher energies as well. Title: Kepler observations of the asteroseismic binary HD 176465 Authors: White, T. R.; Benomar, O.; Silva Aguirre, V.; Ball, W. H.; Bedding, T. R.; Chaplin, W. J.; Christensen-Dalsgaard, J.; Garcia, R. A.; Gizon, L.; Stello, D.; Aigrain, S.; Antia, H. M.; Appourchaux, T.; Bazot, M.; Campante, T. L.; Creevey, O. L.; Davies, G. R.; Elsworth, Y. P.; Gaulme, P.; Handberg, R.; Hekker, S.; Houdek, G.; Howe, R.; Huber, D.; Karoff, C.; Marques, J. P.; Mathur, S.; McQuillan, A.; Metcalfe, T. S.; Mosser, B.; Nielsen, M. B.; Régulo, C.; Salabert, D.; Stahn, T. Bibcode: 2017A&A...601A..82W Altcode: 2016arXiv160909581W; 2016A&A...601A..82W Binary star systems are important for understanding stellar structure and evolution, and are especially useful when oscillations can be detected and analysed with asteroseismology. However, only four systems are known in which solar-like oscillations are detected in both components. Here, we analyse the fifth such system, HD 176465, which was observed by Kepler. We carefully analysed the system's power spectrum to measure individual mode frequencies, adapting our methods where necessary to accommodate the fact that both stars oscillate in a similar frequency range. We also modelled the two stars independently by fitting stellar models to the frequencies and complementaryparameters. We are able to cleanly separate the oscillation modes in both systems. The stellar models produce compatible ages and initial compositions for the stars, as is expected from their common and contemporaneous origin. Combining the individual ages, the system is about 3.0 ± 0.5 Gyr old. The two components of HD 176465 are young physically-similar oscillating solar analogues, the first such system to be found, and provide important constraints for stellar evolution and asteroseismology. Title: Seismic Measurement of the Locations of the Base of Convection Zone and Helium Ionization Zone for Stars in the Kepler Seismic LEGACY Sample Authors: Verma, Kuldeep; Raodeo, Keyuri; Antia, H. M.; Mazumdar, Anwesh; Basu, Sarbani; Lund, Mikkel N.; Silva Aguirre, Víctor Bibcode: 2017ApJ...837...47V Altcode: 2017arXiv170108987V Acoustic glitches are regions inside a star where the sound speed or its derivatives change abruptly. These leave a small characteristic oscillatory signature in the stellar oscillation frequencies. With the precision achieved by Kepler seismic data, it is now possible to extract these small amplitude oscillatory signatures, and infer the locations of the glitches. We perform glitch analysis for all the 66 stars in the Kepler seismic LEGACY sample to derive the locations of the base of the envelope convection zone (CZ) and the helium ionization zone. The signature from helium ionization zone is found to be robust for all stars in the sample, whereas the CZ signature is found to be weak and problematic, particularly for relatively massive stars with large errorbars on the oscillation frequencies. We demonstrate that the helium glitch signature can be used to constrain the properties of the helium ionization layers and the helium abundance. Title: Standing on the Shoulders of Dwarfs: the Kepler Asteroseismic LEGACY Sample. I. Oscillation Mode Parameters Authors: Lund, Mikkel N.; Silva Aguirre, Víctor; Davies, Guy R.; Chaplin, William J.; Christensen-Dalsgaard, Jørgen; Houdek, Günter; White, Timothy R.; Bedding, Timothy R.; Ball, Warrick H.; Huber, Daniel; Antia, H. M.; Lebreton, Yveline; Latham, David W.; Handberg, Rasmus; Verma, Kuldeep; Basu, Sarbani; Casagrande, Luca; Justesen, Anders B.; Kjeldsen, Hans; Mosumgaard, Jakob R. Bibcode: 2017ApJ...835..172L Altcode: 2016arXiv161200436L The advent of space-based missions like Kepler has revolutionized the study of solar-type stars, particularly through the measurement and modeling of their resonant modes of oscillation. Here we analyze a sample of 66 Kepler main-sequence stars showing solar-like oscillations as part of the Kepler seismic LEGACY project. We use Kepler short-cadence data, of which each star has at least 12 months, to create frequency-power spectra optimized for asteroseismology. For each star, we identify its modes of oscillation and extract parameters such as frequency, amplitude, and line width using a Bayesian Markov chain Monte Carlo “peak-bagging” approach. We report the extracted mode parameters for all 66 stars, as well as derived quantities such as frequency difference ratios, the large and small separations {{Δ }}ν and δ {ν }02; the behavior of line widths with frequency and line widths at {ν }\max with {T}{eff}, for which we derive parametrizations; and behavior of mode visibilities. These average properties can be applied in future peak-bagging exercises to better constrain the parameters of the stellar oscillation spectra. The frequencies and frequency ratios can tightly constrain the fundamental parameters of these solar-type stars, and mode line widths and amplitudes can test models of mode damping and excitation. Title: AstroSat/LAXPC Observation of Cygnus X-1 in the Hard State Authors: Misra, Ranjeev; Yadav, J. S.; Verdhan Chauhan, Jai; Agrawal, P. C.; Antia, H. M.; Pahari, Mayukh; Chitnis, V. R.; Dedhia, Dhiraj; Katoch, Tilak; Madhwani, P.; Manchanda, R. K.; Paul, B.; Shah, Parag Bibcode: 2017ApJ...835..195M Altcode: 2016arXiv161208793M We report the first analysis of data from AstroSat/LAXPC observations of Cygnus X-1 in 2016 January. LAXPC spectra reveals that the source was in the canonical hard state, represented by a prominent thermal Comptonization component having a photon index of ∼1.8 and high temperature of kTe > 60 keV along with weak reflection and possible disk emission. The power spectrum can be characterized by two broad lorentzian functions centered at ∼0.4 and ∼3 Hz. The rms of the low-frequency component decreases from ∼15% at around 4 keV to ∼10% at around 50 keV, while that of the high-frequency one varies less rapidly from ∼13.5% to ∼11.5% in the same energy range. The time lag between the hard (20-40 keV) and soft (5-10 keV) bands varies in a step-like manner being nearly constant at ∼50 milliseconds from 0.3 to 0.9 Hz, decreasing to ∼8 milliseconds from 2 to 5 Hz and finally dropping to ∼2 milliseconds for higher frequencies. The time lags increase with energy for both the low and high-frequency components. The event mode LAXPC data allows for flux resolved spectral analysis on a timescale of 1 s, which clearly shows that the photon index increased from ∼1.72 to ∼1.80 as the flux increased by nearly a factor of two. We discuss the results in the framework of the fluctuation propagation model. Title: Standing on the Shoulders of Dwarfs: the Kepler Asteroseismic LEGACY Sample. II.Radii, Masses, and Ages Authors: Silva Aguirre, Víctor; Lund, Mikkel N.; Antia, H. M.; Ball, Warrick H.; Basu, Sarbani; Christensen-Dalsgaard, Jørgen; Lebreton, Yveline; Reese, Daniel R.; Verma, Kuldeep; Casagrande, Luca; Justesen, Anders B.; Mosumgaard, Jakob R.; Chaplin, William J.; Bedding, Timothy R.; Davies, Guy R.; Handberg, Rasmus; Houdek, Günter; Huber, Daniel; Kjeldsen, Hans; Latham, David W.; White, Timothy R.; Coelho, Hugo R.; Miglio, Andrea; Rendle, Ben Bibcode: 2017ApJ...835..173S Altcode: 2016arXiv161108776S We use asteroseismic data from the Kepler satellite to determine fundamental stellar properties of the 66 main-sequence targets observed for at least one full year by the mission. We distributed tens of individual oscillation frequencies extracted from the time series of each star among seven modeling teams who applied different methods to determine radii, masses, and ages for all stars in the sample. Comparisons among the different results reveal a good level of agreement in all stellar properties, which is remarkable considering the variety of codes, input physics, and analysis methods employed by the different teams. Average uncertainties are of the order of ∼2% in radius, ∼4% in mass, and ∼10% in age, making this the best-characterized sample of main-sequence stars available to date. Our predicted initial abundances and mixing-length parameters are checked against inferences from chemical enrichment laws ΔY/ΔZ and predictions from 3D atmospheric simulations. We test the accuracy of the determined stellar properties by comparing them to the Sun, angular diameter measurements, Gaia parallaxes, and binary evolution, finding excellent agreement in all cases and further confirming the robustness of asteroseismically determined physical parameters of stars when individual frequencies of oscillation are available. Baptised as the Kepler dwarfs LEGACY sample, these stars are the solar-like oscillators with the best asteroseismic properties available for at least another decade. All data used in this analysis and the resulting stellar parameters are made publicly available for the community. Title: Astrosat/LAXPC Reveals the High-energy Variability of GRS 1915+105 in the X Class Authors: Yadav, J. S.; Misra, Ranjeev; Verdhan Chauhan, Jai; Agrawal, P. C.; Antia, H. M.; Pahari, Mayukh; Dedhia, Dhiraj; Katoch, Tilak; Madhwani, P.; Manchanda, R. K.; Paul, B.; Shah, Parag; Ishwara-Chandra, C. H. Bibcode: 2016ApJ...833...27Y Altcode: 2016arXiv160807023Y We present the first quick look analysis of data from nine AstroSat's Large Area X-ray Proportional Counter (LAXPC) observations of GRS 1915+105 during 2016 March when the source had the characteristics of being in the Radio-quiet χ class. We find that a simple empirical model of a disk blackbody emission, with Comptonization and a broad Gaussian Iron line can fit the time-averaged 3-80 keV spectrum with a systematic uncertainty of 1.5% and a background flux uncertainty of 4%. A simple dead time corrected Poisson noise level spectrum matches well with the observed high-frequency power spectra till 50 kHz and as expected the data show no significant high-frequency (\gt 20 {Hz}) features. Energy dependent power spectra reveal a strong low-frequency (2-8 Hz) quasi-periodic oscillation and its harmonic along with broadband noise. The QPO frequency changes rapidly with flux (nearly 4 Hz in ∼5 hr). With increasing QPO frequency, an excess noise component appears significantly in the high-energy regime (\gt 8 keV). At the QPO frequencies, the time-lag as a function of energy has a non-monotonic behavior such that the lags decrease with energy till about 15-20 keV and then increase for higher energies. These first-look results benchmark the performance of LAXPC at high energies and confirms that its data can be used for more sophisticated analysis such as flux or frequency-resolved spectro-timing studies. Title: Oscillation mode linewidths and heights of 23 main-sequence stars observed by Kepler (Corrigendum) Authors: Appourchaux, T.; Antia, H. M.; Benomar, O.; Campante, T. L.; Davies, G. R.; Handberg, R.; Howe, R.; Régulo, C.; Belkacem, K.; Houdek, G.; García, R. A.; Chaplin, W. J. Bibcode: 2016A&A...595C...2A Altcode: No abstract at ADS Title: Asteroseismic determination of fundamental parameters of Sun-like stars using multilayered neural networks Authors: Verma, Kuldeep; Hanasoge, Shravan; Bhattacharya, Jishnu; Antia, H. M.; Krishnamurthi, Ganapathy Bibcode: 2016MNRAS.461.4206V Altcode: 2016arXiv160200902V The advent of space-based observatories such as Convection, Rotation and planetary Transits (CoRoT) and Kepler has enabled the testing of our understanding of stellar evolution on thousands of stars. Evolutionary models typically require five input parameters, the mass, initial helium abundance, initial metallicity, mixing length (assumed to be constant over time), and the age to which the star must be evolved. Some of these parameters are also very useful in characterizing the associated planets and in studying Galactic archaeology. How to obtain these parameters from observations rapidly and accurately, specifically in the context of surveys of thousands of stars, is an outstanding question, one that has eluded straightforward resolution. For a given star, we typically measure the effective temperature and surface metallicity spectroscopically and low-degree oscillation frequencies through space observatories. Here we demonstrate that statistical learning, using artificial neural networks, is successful in determining the evolutionary parameters based on spectroscopic and seismic measurements. Our trained networks show robustness over a broad range of parameter space, and critically, are entirely computationally inexpensive and fully automated. We analyse the observations of a few stars using this method and the results compare well to inferences obtained using other techniques. This method is both computationally cheap and inferentially accurate, paving the way for analysing the vast quantities of stellar observations from past, current, and future missions. Title: AstroSat/LAXPC observed 4U 1630-472 on very first day of a new outburst Authors: Yadav, J. S.; Chauhan, Jai Verdhan; Agrawal, P. C.; Antia, H. M.; Pahari, Mayukh; Chitnis, V. R.; Misra, Ranjeev; Dedhia, Dhiraj; Katoch, Tilak; Madhwani, P.; Manchanda, R. K.; Paul, B.; Shah, Parag; Altamirano, Diego; Gandhi, P.; Navalgund, K. H.; Sarma, K. S.; Seetha, S.; Subbarao, K. Bibcode: 2016ATel.9515....1Y Altcode: We have observed 4U 1630-472 on 1st day of outburst during 01:07(UT) 27th August to 01.20(UT) 28 August, 2016 with AstroSat/LAXPC. 4U 1630-472 is a transient black hole X-ray binary. Title: Large Area X-ray Proportional Counter (LAXPC) instrument onboard ASTROSAT Authors: Yadav, J. S.; Agrawal, P. C.; Antia, H. M.; Chauhan, Jai Verdhan; Dedhia, Dhiraj; Katoch, Tilak; Madhwani, P.; Manchanda, R. K.; Misra, Ranjeev; Pahari, Mayukh; Paul, B.; Shah, Parag Bibcode: 2016SPIE.9905E..1DY Altcode: ASTROSAT, India's first dedicated astronomy space mission was launched on September 28, 2015. The Large Area X-ray Proportional Counter (LAXPC) is one of the major payloads on ASTROSAT. A cluster of three co-aligned identical LAXPC detectors provide large area of collection .The large detection volume (15 cm depth) filled with mixture of xenon gas (90(%) and methane (10%) at 2 atmospheres pressure, results in detection efficiency greater than 50%, above 30 keV. The LAXPC instrument is best suited for X-ray timing and spectral studies. It will provide the largest effective area in 3-80 keV range among all the satellite missions flown so far worldwide and will remain so for the next 5-10 years. The LAXPC detectors have been calibrated using radioactive sources in the laboratory. GEANT4 simulation for LAXPC detectors was carried out to understand detector background and its response. The LAXPC instrument became fully operational on 19th October 2015 for the first time in space. We have performed detector calibration in orbit. The LAXPC instrument is functioning well and has achieved all detector parameters proposed initially. In this paper, we will describe LAXPC detector calibration in lab as well as in orbit along with first results. Title: Timing and spectral properties of Vela X-1 with ASTROSAT-LAXPC Authors: Pradhan, Pragati; Paul, Biswajit; Manchanda, R. K.; Jain, Chetana; Islam, Nazma; Maitra, Chandreyee; Pahari, Mayukh; Singh Yadav, Jagdish; Katoch, Tilak; Antia, H. M.; Beri, Aru; Madhwani, Pankaj; Raman, Gayathri; Bahal, Varun; Mate, Sujay; Agrawal, P. C.; Dedhia, Dhiraj K.; Chauhan, Jai V.; Shah, Parag Bibcode: 2016cosp...41E1588P Altcode: Vela X-1 is an eclipsing and persistent yet highly variable HMXB. It's variability, which is often attributed to the presence of clumpy winds around it, places it as a link between classical HMXBs and supergiant fast X-ray transients. We present a detailed timing and spectral analysis of the persistent yet highly variable HMXB 'Vela X-1' from LAXPC observations onboard ASTROSAT over wide energy band of 3-80 keV. The X-ray spectrum of Vela X-1 hosts several interesting features like the evidence of a cyclotron line at 25 keV. It is for the first time that this object has been studied with a single instrument in such a wide energy range. We report significant variations in the spectral parameters with different pulse phases and discuss them in terms of the accretion mechanism and stellar wind properties of this accreting pulsar. In addition, for the first time, we also detect pulsations upto 80 keV for this source. Title: SpaceInn hare-and-hounds exercise: Estimation of stellar properties using space-based asteroseismic data Authors: Reese, D. R.; Chaplin, W. J.; Davies, G. R.; Miglio, A.; Antia, H. M.; Ball, W. H.; Basu, S.; Buldgen, G.; Christensen-Dalsgaard, J.; Coelho, H. R.; Hekker, S.; Houdek, G.; Lebreton, Y.; Mazumdar, A.; Metcalfe, T. S.; Silva Aguirre, V.; Stello, D.; Verma, K. Bibcode: 2016A&A...592A..14R Altcode: 2016arXiv160408404R Context. Detailed oscillation spectra comprising individual frequencies for numerous solar-type stars and red giants are either currently available, e.g. courtesy of the CoRoT, Kepler, and K2 missions, or will become available with the upcoming NASA TESS and ESA PLATO 2.0 missions. The data can lead to a precise characterisation of these stars thereby improving our understanding of stellar evolution, exoplanetary systems, and the history of our galaxy.
Aims: Our goal is to test and compare different methods for obtaining stellar properties from oscillation frequencies and spectroscopic constraints. Specifically, we would like to evaluate the accuracy of the results and reliability of the associated error bars, and to see where there is room for improvement.
Methods: In the context of the SpaceInn network, we carried out a hare-and-hounds exercise in which one group, the hares, simulated observations of oscillation spectra for a set of ten artificial solar-type stars, and a number of hounds applied various methods for characterising these stars based on the data produced by the hares. Most of the hounds fell into two main groups. The first group used forward modelling (I.e. applied various search/optimisation algorithms in a stellar parameter space) whereas the second group relied on acoustic glitch signatures.
Results: Results based on the forward modelling approach were accurate to 1.5% (radius), 3.9% (mass), 23% (age), 1.5% (surface gravity), and 1.8% (mean density), as based on the root mean square difference. Individual hounds reached different degrees of accuracy, some of which were substantially better than the above average values. For the two 1M stellar targets, the accuracy on the age is better than 10% thereby satisfying the requirements for the PLATO 2.0 mission. High stellar masses and atomic diffusion (which in our models does not include the effects of radiative accelerations) proved to be sources of difficulty. The average accuracies for the acoustic radii of the base of the convection zone, the He II ionisation, and the Γ1 peak located between the two He ionisation zones were 17%, 2.4%, and 1.9%, respectively. The results from the forward modelling were on average more accurate than those from the glitch fitting analysis as the latter seemed to be affected by aliasing problems for some of the targets.
Conclusions: Our study indicates that forward modelling is the most accurate way of interpreting the pulsation spectra of solar-type stars. However, given its model-dependent nature, this method needs to be complemented by model-independent results from, e.g. glitch analysis. Furthermore, our results indicate that global rather than local optimisation algorithms should be used in order to obtain robust error bars. Title: ASTROSAT/LAXPC observations of X-Ray binaries: A new window to hard X-Ray science aspects Authors: Pahari, Mayukh; Singh Yadav, Jagdish; Antia, H. M.; Misra, Ranjeev Bibcode: 2016cosp...41E1495P Altcode: In this presentation I would like to discuss some of the recent results obtained from ASTROSAT/LAXPC observations of X-Ray binaries. With the specific focus on hard X-Ray regime, where LAXPC performs much better than RXTE due to large effective area, we obtained interesting results which has never observed before from high energy timing instrument. In this talk, we will revisit RXTE results in terms of power density spectra, time-lag, fractional rms as a function of photon energy and compare them to LAXPC results and will discuss the nature of the same above 30 keV which was totally unexplored area in timing domain. We will compare broadband spectral and timing properties of accretion flow in different X-ray binaries as observed from LAXPC. Title: Fast Fourier transform to measure pressure coefficient of muons in the GRAPES-3 experiment Authors: Mohanty, P. K.; Ahmad, S.; Antia, H. M.; Arunbabu, K. P.; Chandra, A.; Dugad, S. R.; Gupta, S. K.; Hariharan, B.; Hayashi, Y.; Jagadeesan, P.; Jain, A.; Kawakami, S.; Kojima, H.; Morris, S. D.; Nayak, P. K.; Oshima, A.; Rao, B. S.; Reddy, L. V.; Shibata, S. Bibcode: 2016APh....79...23M Altcode: The GRAPES-3 large area (560 m2) tracking muon telescope is operating at Ooty in India since 2001. It records 4 × 109 muons of energy ≥ 1 GeV every day. These high statistics data have enabled extremely sensitive measurements of solar phenomena, including the solar anisotropies, Forbush decreases, coronal mass ejections etc. to be made. However, prior to such studies, the variation in observed muon rate caused by changes in atmospheric pressure needs to be corrected. Traditionally, the pressure coefficient (β) for the muon rate was derived from the observed data. But the influence of various solar effects makes the measurement of β somewhat difficult. In the present work, a different approach to circumvent this difficulty was used to measure β, almost independent of the solar activity. This approach exploits a small amplitude (∼1 hPa) periodic (12 h) variation of atmospheric pressure at Ooty that introduces a synchronous variation in the muon rate. By using the fast Fourier transform technique the spectral power distributions at 12 h from the atmospheric pressure, and muon rate were used to measure β. The value of pressure coefficient was found to be β =(- 0.128 ± 0.005) % hPa-1. Title: The Impact Of Torsional Oscillations On The Solar Cycle: The Waldmeier-effect As An Outcome Authors: Mahajan, Sushant S.; Nandy, Dibyendu; Dwivedi, Bhola N.; Antia, H. M. Bibcode: 2016SPD....47.0718M Altcode: Temporal variations in the Sun’s internal velocity field with a periodicity of about 11 years have been observed in the last three decades. The period of these torsional oscillations and their latitudinal propagation roughly coincide with the period and equatorward propagation of sunspots which originate from a magnetohydrodynamic dynamo mechanism operating in the Sun’s interior. While the solar differential rotation plays an important role in this dynamo mechanism by inducting the toroidal component of magnetic field, the impact of torsional oscillations on the dynamo mechanism - and hence the solar cycle - is not well understood. Here, we include the observed torsional oscillations into a flux transport dynamo model of the solar cycle to inves- tigate their effect. Although the overall amplitude of the solar cycle does not change significantly on inclusion of torsional oscillations we find that all the characteristics of the Waldmeier effect inthe sunspot cycle are qualitatively reproduced by varying only the amplitude of torsional oscillations. The Waldmeier effect, first noted in 1935, includes the important characteristic that the amplitude of sunspot cycles is anti-correlated to their rise time; cycles with high initial rise rate tend to be stronger. This has implications for solar cycle predictions. Our result suggests that the Waldmeier effect is a plausible outcome of cycle-to-cycle modulation of torsional oscillations and provides a physical basis for sunspot cycle forecasts based on torsional oscillation observations. Title: Frequency shifts of resonant modes of the Sun due to near-surface convective scattering Authors: Bhattacharya, J.; Hanasoge, S. M.; Antia, H. M. Bibcode: 2016IAUFM..29B.614B Altcode: Measurements of oscillation frequencies of the Sun and stars can provide important independent constraints on their internal structure and dynamics. Seismic models of these oscillations are used to connect structure and rotation of the star to its resonant frequencies, which are then compared with observations, the goal being that of minimizing the difference between the two. Even in the case of the Sun, for which structure models are highly tuned, observed frequencies show systematic deviations from modeled frequencies, a phenomenon referred to as the ``surface term.'' The dominant source of this systematic effect is thought to be vigorous near-surface convection, which is not well accounted for in both stellar modeling and mode-oscillation physics. Here we bring to bear the method of homogenization, applicable in the asymptotic limit of large wavelengths (in comparison to the correlation scale of convection), to characterize the effect of small-scale surface convection on resonant-mode frequencies in the Sun. We show that the full oscillation equations, in the presence of temporally stationary 3D flows, can be reduced to an effective ``quiet-Sun'' wave equation with altered sound speed, Brünt-Väisäla frequency, and Lamb frequency. We derive the modified equation and relations for the appropriate averaging of 3D flows and thermal quantities to obtain the properties of this effective medium. Using flows obtained from 3D numerical simulations of near-surface convection, we quantify their effect on solar oscillation frequencies and find that they are shifted systematically and substantially. We argue therefore that consistent interpretations of resonant frequencies must include modifications to the wave equation that effectively capture the impact of vigorous hydrodynamic convection. Title: Meridional Circulation in the Solar Convection Zone: Time-Distance Helioseismic Inferences from Four Years of HMI/SDO Observations Authors: Rajaguru, S. P.; Antia, H. M. Bibcode: 2015ApJ...813..114R Altcode: 2015arXiv151001843R We present and discuss results from time-distance helioseismic measurements of meridional circulation (MC) in the solar convection zone using 4 yr of Doppler velocity observations by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. Using a built-in mass conservation constraint in terms of the stream function, we invert helioseismic travel times to infer the MC in the solar convection zone. We find that the return flow that closes the MC is possibly beneath the depth of 0.77 R. We discuss the significance of this result in relation to other helioseismic inferences published recently and possible reasons for the differences in the results. Our results show clearly the pitfalls involved in the measurements of material flows in the deep solar interior given the current limits on the signal-to-noise ratio and our limited understanding of systematics in the data. We also discuss the implications of our results for the dynamics of solar interior and popular solar dynamo models. Title: A seismic and gravitationally bound double star observed by Kepler. Implication for the presence of a convective core Authors: Appourchaux, T.; Antia, H. M.; Ball, W.; Creevey, O.; Lebreton, Y.; Verma, K.; Vorontsov, S.; Campante, T. L.; Davies, G. R.; Gaulme, P.; Régulo, C.; Horch, E.; Howell, S.; Everett, M.; Ciardi, D.; Fossati, L.; Miglio, A.; Montalbán, J.; Chaplin, W. J.; García, R. A.; Gizon, L. Bibcode: 2015A&A...582A..25A Altcode: Context. Solar-like oscillations have been observed by Kepler and CoRoT in many solar-type stars, thereby providing a way to probe stars using asteroseismology.
Aims: The derivation of stellar parameters has usually been done with single stars. The aim of the paper is to derive the stellar parameters of a double-star system (HIP 93511), for which an interferometric orbit has been observed along with asteroseismic measurements.
Methods: We used a time series of nearly two years of data for the double star to detect the two oscillation-mode envelopes that appear in the power spectrum. Using a new scaling relation based on luminosity, we derived the radius and mass of each star. We derived the age of each star using two proxies: one based upon the large frequency separation and a new one based upon the small frequency separation. Using stellar modelling, the mode frequencies allowed us to derive the radius, the mass, and the age of each component. In addition, speckle interferometry performed since 2006 has enabled us to recover the orbit of the system and the total mass of the system.
Results: From the determination of the orbit, the total mass of the system is 2.34-0.33+0.45 M. The total seismic mass using scaling relations is 2.47 ± 0.07 M. The seismic age derived using the new proxy based upon the small frequency separation is 3.5 ± 0.3 Gyr. Based on stellar modelling, the mean common age of the system is 2.7-3.9 Gyr. The mean total seismic mass of the system is 2.34-2.53 M consistent with what we determined independently with the orbit. The stellar models provide the mean radius, mass, and age of the stars as RA = 1.82-1.87R, MA = 1.25-1.39 M, AgeA = 2.6-3.5 Gyr; RB = 1.22-1.25 R, MB = 1.08-1.14 M, AgeB = 3.35-4.21 Gyr. The models provide two sets of values for Star A: [1.25-1.27] M and [1.34-1.39] M. We detect a convective core in Star A, while Star B does not have any. For the metallicity of the binary system of Z ≈ 0.02, we set the limit between stars having a convective core in the range [1.14-1.25] M.

Appendices are available in electronic form at http://www.aanda.org Title: Asteroseismology of Solar-Type Stars with K2: Detection of Oscillations in C1 Data Authors: Chaplin, W. J.; Lund, M. N.; Handberg, R.; Basu, S.; Buchhave, L. A.; Campante, T. L.; Davies, G. R.; Huber, D.; Latham, D. W.; Latham, C. A.; Serenelli, A.; Antia, H. M.; Appourchaux, T.; Ball, W. H.; Benomar, O.; Casagrande, L.; Christensen-Dalsgaard, J.; Coelho, H. R.; Creevey, O. L.; Elsworth, Y.; García, R. A.; Gaulme, P.; Hekker, S.; Kallinger, T.; Karoff, C.; Kawaler, S. D.; Kjeldsen, H.; Lundkvist, M. S.; Marcadon, F.; Mathur, S.; Miglio, A.; Mosser, B.; Régulo, C.; Roxburgh, I. W.; Silva Aguirre, V.; Stello, D.; Verma, K.; White, T. R.; Bedding, T. R.; Barclay, T.; Buzasi, D. L.; Dehuevels, S.; Gizon, L.; Houdek, G.; Howell, S. B.; Salabert, D.; Soderblom, D. R. Bibcode: 2015PASP..127.1038C Altcode: 2015arXiv150701827C We present the first detections by the NASA K2 Mission of oscillations in solar-type stars, using short-cadence data collected during K2 Campaign\,1 (C1). We understand the asteroseismic detection thresholds for C1-like levels of photometric performance, and we can detect oscillations in subgiants having dominant oscillation frequencies around $1000\,\rm \mu Hz$. Changes to the operation of the fine-guidance sensors are expected to give significant improvements in the high-frequency performance from C3 onwards. A reduction in the excess high-frequency noise by a factor of two-and-a-half in amplitude would bring main-sequence stars with dominant oscillation frequencies as high as ${\simeq 2500}\,\rm \mu Hz$ into play as potential asteroseismic targets for K2. Title: Asteroseismic estimate of helium abundance of 16 Cyg A, B Authors: Verma, Kuldeep; Faria, João P.; Antia, H. M.; Basu, Sarbani; Mazumdar, Anwesh; Monteiro, Mário J. P. F. G.; Appourchaux, Thierry; Chaplin, William J.; García, Rafael A.; Metcalfe, Travis S. Bibcode: 2015EPJWC.10106066V Altcode: The helium ionization zone in a star leaves a characteristic signature on its oscillation frequencies, which can be used to estimate the helium content in the envelope of the star. We use the oscillation frequencies of 16 Cyg A and B, obtained using 2.5 years of Kepler data, to estimate the envelope helium abundance of these stars. We find the envelope helium abundance to lie in the range 0.231-0.251 for 16 Cyg A and 0.218-0.266 for 16 Cyg B. Title: How are Forbush decreases related to interplanetary magnetic field enhancements? Authors: Arunbabu, K. P.; Antia, H. M.; Dugad, S. R.; Gupta, S. K.; Hayashi, Y.; Kawakami, S.; Mohanty, P. K.; Oshima, A.; Subramanian, P. Bibcode: 2015A&A...580A..41A Altcode: 2015arXiv150406473A
Aims: A Forbush decrease (FD) is a transient decrease followed by a gradual recovery in the observed galactic cosmic ray intensity. We seek to understand the relationship between the FDs and near-Earth interplanetary magnetic field (IMF) enhancements associated with solar coronal mass ejections (CMEs).
Methods: We used muon data at cutoff rigidities ranging from 14 to 24 GV from the GRAPES-3 tracking muon telescope to identify FD events. We selected those FD events that have a reasonably clean profile, and magnitude >0.25%. We used IMF data from ACE/WIND spacecrafts. We looked for correlations between the FD profile and that of the one-hour averaged IMF. We wanted to find out whether if the diffusion of high-energy protons into the large scale magnetic field is the cause of the lag observed between the FD and the IMF.
Results: The enhancement of the IMF associated with FDs occurs mainly in the shock-sheath region, and the turbulence level in the magnetic field is also enhanced in this region. The observed FD profiles look remarkably similar to the IMF enhancement profiles. The FDs typically lag behind the IMF enhancement by a few hours. The lag corresponds to the time taken by high-energy protons to diffuse into the magnetic field enhancement via cross-field diffusion.
Conclusions: Our findings show that high-rigidity FDs associated with CMEs are caused primarily by the cumulative diffusion of protons across the magnetic field enhancement in the turbulent sheath region between the shock and the CME.

Appendices are available in electronic form at http://www.aanda.org Title: Relation of Forbush decrease with interplanetary magnetic fields. Authors: Arunbabu, K. P.; Antia, H. M.; Dugad, S.; Gupta, S. K.; Hayashi, Y.; Kawakami, S.; Mohanty, P. K.; Oshima, A.; Subramanian, P. Bibcode: 2015ICRC...34...43A Altcode: 2015PoS...236...43A; 2015ICRC...34...43K No abstract at ADS Title: Rigidity dependence of the intensity variations of galactic cosmic rays Authors: Kojima, H.; Shibata, S.; Oshima, A.; Hayashi, Y.; Kawakami, S.; Morishita, I.; T; NAkamura; Nonaka, T.; Ogio, S.; Takamaru, H.; Tanaka, K.; Ito, N.; Matsuya, T.; Yamazaki, K.; Tokumaru, M.; Gupta, S. K.; Antia, H. M.; Dugad, S. R.; Mohanty, P. K.; Jain, A.; Rao, B. S. Bibcode: 2015ICRC...34...62K Altcode: 2015PoS...236...62K No abstract at ADS Title: Forbush decrease precursors observed using GRAPES-3 Authors: Arunbabu, K. P.; Antia, H. M.; Dugad, S.; Gupta, S. K.; Hayashi, Y.; Kawakami, S.; Mohanty, P. K.; Oshima, A.; Subramanian, P. Bibcode: 2015ICRC...34...44A Altcode: 2015PoS...236...44A; 2015ICRC...34...44K No abstract at ADS Title: An estimation of the diffusion coefficient of galactic cosmic rays in the heliosphere near the Earth. Authors: Kojima, H.; Shibata, S.; Oshima, A.; Hayashi, Y.; Kawakami, S.; Morishita, I.; T; NAkamura; Nonaka, T.; Ogio, S.; Takamaru, H.; Tanaka, K.; Ito, N.; Matsuya, T.; Yamazaki, K.; Tokumaru, M.; Gupta, S. K.; Antia, H. M.; Dugad, S. R.; Mohanty, P. K.; Jain, A.; Rao, B. S. Bibcode: 2015ICRC...34...61K Altcode: 2015PoS...236...61K No abstract at ADS Title: Measurements of solar diurnal ansiotropy with GRAPES-3 experiment Authors: Mohanty, P. K.; Antia, H. M.; Arunbabu, K. P.; Dugad, S.; Gupta, S. K.; Balakrishnan, H. H.; Hayashi, Y.; Jagadeesan, P.; Jain, A.; Kawakami, S.; Kojima, H.; Morris, S.; Nayak, P.; Nonaka, T.; Oshima, A.; Rao, B. S.; Shibata, S.; Subramanian, P.; Tanaka, K. Bibcode: 2015ICRC...34...42M Altcode: 2015PoS...236...42M No abstract at ADS Title: Dependence of cosmic ray intensity on variation of solar wind velocity measured by the GRAPES-3 experiment for space weather studies Authors: Kojima, H.; Antia, H. M.; Dugad, S. R.; Gupta, S. K.; Jagadeesan, P.; Jain, A.; Mohanty, P. K.; Rao, B. S.; Hayashi, Y.; Kawakami, S.; Nonaka, T.; Oshima, A.; Shibata, S.; Tanaka, K. Bibcode: 2015PhRvD..91l1303K Altcode: The space weather impacts propagation of galactic cosmic rays (CRs) in the heliosphere as explained by the diffusion-convection mechanism which predicts that the variation in CR intensity should be anticorrelated with changes in solar wind velocity (VSW). Several unrelated solar phenomena, including periodic ones such as 27 d solar rotation, annual, 11 yr solar activity, 22 yr solar magnetic cycle etc., and transient ones, for example, Forbush decreases (FDs), and ground level enhancements generally caused by solar flares or coronal mass ejections also affect CR intensity on Earth. These solar phenomena make a quantitative study of dependence of CR intensity on VSW rather difficult. Here, the high statistics muon data of six years (2000-2005) from the large GRAPES-3 muon telescope have been used to study the correlation between VSW and CR intensity. Data gathered during these six years were used after minimizing the contribution of various unrelated solar phenomena outlined above. We observed a strong anticorrelation between the variations in VSW and CR intensity at a significance of 19 σ . Title: Frequency Shifts of Resonant Modes of the Sun due to Near-Surface Convective Scattering Authors: Bhattacharya, J.; Hanasoge, S.; Antia, H. M. Bibcode: 2015ApJ...806..246B Altcode: 2015arXiv150504048B Measurements of oscillation frequencies of the Sun and stars can provide important independent constraints on their internal structure and dynamics. Seismic models of these oscillations are used to connect structure and rotation of the star to its resonant frequencies, which are then compared with observations, the goal being that of minimizing the difference between the two. Even in the case of the Sun, for which structure models are highly tuned, observed frequencies show systematic deviations from modeled frequencies, a phenomenon referred to as the “surface term.” The dominant source of this systematic effect is thought to be vigorous near-surface convection, which is not well accounted for in both stellar modeling and mode-oscillation physics. Here we bring to bear the method of homogenization, applicable in the asymptotic limit of large wavelengths (in comparison to the correlation scale of convection), to characterize the effect of small-scale surface convection on resonant-mode frequencies in the Sun. We show that the full oscillation equations, in the presence of temporally stationary three-dimensional (3D) flows, can be reduced to an effective “quiet-Sun” wave equation with altered sound speed, Brünt-Väisäla frequency, and Lamb frequency. We derive the modified equation and relations for the appropriate averaging of 3D flows and thermal quantities to obtain the properties of this effective medium. Using flows obtained from 3D numerical simulations of near-surface convection, we quantify their effect on solar oscillation frequencies and find that they are shifted systematically and substantially. We argue therefore that consistent interpretations of resonant frequencies must include modifications to the wave equation that effectively capture the impact of vigorous hydrodynamic convection. Title: Measurement of the radial density gradient of cosmic ray in the heliosphere by the GRAPES-3 experiment Authors: Kojima, H.; Antia, H. M.; Dugad, S. R.; Gupta, S. K.; Hayashi, Y.; Jagadeesan, P.; Jain, A.; Kawakami, S.; Mohanty, P. K.; Nonaka, T.; Oshima, A.; Rao, B. S.; Shibata, S.; Grapes-3 Collaboration Bibcode: 2015APh....62...21K Altcode: A radial anisotropy in the flux of cosmic rays in heliosphere was theoretically predicted by Parker and others within the framework of the diffusion-convection mechanism. The solar wind is responsible for sweeping out the galactic cosmic rays, creating a radial density gradient within the heliosphere. This gradient coupled with the interplanetary magnetic field induces a flow of charged particles perpendicular to the ecliptic plane which was measured and correctly explained by Swinson, and is hereafter referred as 'Swinson flow'. The large area GRAPES-3 tracking muon telescope offers a powerful probe to measure the Swinson flow and the underlying radial density gradient of the galactic cosmic rays at a relatively high rigidity of ∼100 GV. The GRAPES-3 data collected over a period of six years (2000-2005) were analyzed and the amplitude of the Swinson flow was estimated to be (0.0644 ± 0.0008)% of cosmic ray flux which was an ∼80σ effect. The phase of the maximum flow was at a sidereal time of (17.70 ± 0.05) h which was 18 min earlier than the expected value of 18 h. This small 18 min phase difference had a significance of ∼6σ indicating the inherent precision of the GRAPES-3 measurement. The radial density gradient of the galactic cosmic rays at a median rigidity of 77 GV was found to be 0.65% AU-1. Title: A Theoretical Study of Acoustic Glitches in Low-mass Main-sequence Stars Authors: Verma, Kuldeep; Antia, H. M.; Basu, Sarbani; Mazumdar, Anwesh Bibcode: 2014ApJ...794..114V Altcode: 2014arXiv1408.4284V There are regions in stars, such as ionization zones and the interface between radiative and convective regions, that cause a localized sharp variation in the sound speed. These are known as "acoustic glitches". Acoustic glitches leave their signatures on the oscillation frequencies of stars, and hence these signatures can be used as diagnostics of these regions. In particular, the signatures of these glitches can be used as diagnostics for the position of the second helium ionization zone and that of the base of the envelope convection zone. With the help of stellar models, we study the properties of these acoustic glitches in main-sequence stars. We find that the acoustic glitch due to the helium ionization zone does not correspond to the dip in the adiabatic index Γ1 caused by the ionization of He II, but to the peak in Γ1 between the He I and He II ionization zones. We find that it is easiest to study the acoustic glitch that is due to the helium ionization zone in stars with masses in the range 0.9-1.2 M . Title: Asteroseismic Estimate of Helium Abundance of a Solar Analog Binary System Authors: Verma, Kuldeep; Faria, João P.; Antia, H. M.; Basu, Sarbani; Mazumdar, Anwesh; Monteiro, Mário J. P. F. G.; Appourchaux, Thierry; Chaplin, William J.; García, Rafael A.; Metcalfe, Travis S. Bibcode: 2014ApJ...790..138V Altcode: 2014arXiv1405.7512V 16 Cyg A and B are among the brightest stars observed by Kepler. What makes these stars more interesting is that they are solar analogs. 16 Cyg A and B exhibit solar-like oscillations. In this work we use oscillation frequencies obtained using 2.5 yr of Kepler data to determine the current helium abundance of these stars. For this we use the fact that the helium ionization zone leaves a signature on the oscillation frequencies and that this signature can be calibrated to determine the helium abundance of that layer. By calibrating the signature of the helium ionization zone against models of known helium abundance, the helium abundance in the envelope of 16 Cyg A is found to lie in the range of 0.231 to 0.251 and that of 16 Cyg B lies in the range of 0.218 to 0.266. Title: ASTROSAT mission Authors: Singh, Kulinder Pal; Tandon, S. N.; Agrawal, P. C.; Antia, H. M.; Manchanda, R. K.; Yadav, J. S.; Seetha, S.; Ramadevi, M. C.; Rao, A. R.; Bhattacharya, D.; Paul, B.; Sreekumar, P.; Bhattacharyya, S.; Stewart, G. C.; Hutchings, J.; Annapurni, S. A.; Ghosh, S. K.; Murthy, J.; Pati, A.; Rao, N. K.; Stalin, C. S.; Girish, V.; Sankarasubramanian, K.; Vadawale, S.; Bhalerao, V. B.; Dewangan, G. C.; Dedhia, D. K.; Hingar, M. K.; Katoch, T. B.; Kothare, A. T.; Mirza, I.; Mukerjee, K.; Shah, H.; Shah, P.; Mohan, R.; Sangal, A. K.; Nagabhusana, S.; Sriram, S.; Malkar, J. P.; Sreekumar, S.; Abbey, A. F.; Hansford, G. M.; Beardmore, A. P.; Sharma, M. R.; Murthy, S.; Kulkarni, R.; Meena, G.; Babu, V. C.; Postma, J. Bibcode: 2014SPIE.9144E..1SS Altcode: ASTROSAT is India's first astronomy satellite that will carry an array of instruments capable of simultaneous observations in a broad range of wavelengths: from the visible, near ultraviolet (NUV), far-UV (FUV), soft X-rays to hard X-rays. There will be five principal scientific payloads aboard the satellite: (i) a Soft X-ray Telescope (SXT), (ii) three Large Area Xenon Proportional Counters (LAXPCs), (iii) a Cadmium-Zinc-Telluride Imager (CZTI), (iv) two Ultra-Violet Imaging Telescopes (UVITs) one for visible and near-UV channels and another for far-UV, and (v) three Scanning Sky Monitors (SSMs). It will also carry a charged particle monitor (CPM). Almost all the instruments have qualified and their flight models are currently in different stages of integration into the satellite structure in ISRO Satellite Centre. ASTROSAT is due to be launched by India's Polar Satellite Launch Vehicle (PSLV) in the first half of 2015 in a circular 600 km orbit with inclination of ~6 degrees, from Sriharikota launching station on the east coast of India. A brief description of the design, construction, capabilities and scientific objectives of all the main scientific payloads is presented here. A few examples of the simulated observations with ASTROSAT and plans to utilize the satellite nationally and internationally are also presented. Title: How are Forbush decreases related with IP magnetic field enhancements ? Authors: Arunbabu K., P.; Subramanian, P.; Gupta, Sunil; Antia, H. M. Bibcode: 2014arXiv1406.4967A Altcode: Cosmic ray Forbush decreases (FDs) are usually thought to be due to Earth-directed coronal mass ejections (CMEs) from the Sun and their associated shocks. When CMEs and their shocks reach the Earth, they cause magnetic field compressions. We seek to understand the relation between these magnetic field compressions and FDs at rigidities between 12 and 42 GV using data from the GRAPES-3 instrument at Ooty. We find that the shapes of the Forbush decrease profiles show a startling similarity to that of the magnetic field compression in the near-Earth IP medium. We seek to understand the implications of this interesting result. Title: Oscillation mode linewidths and heights of 23 main-sequence stars observed by Kepler Authors: Appourchaux, T.; Antia, H. M.; Benomar, O.; Campante, T. L.; Davies, G. R.; Handberg, R.; Howe, R.; Régulo, C.; Belkacem, K.; Houdek, G.; García, R. A.; Chaplin, W. J. Bibcode: 2014A&A...566A..20A Altcode: 2014arXiv1403.7046A Context. Solar-like oscillations have been observed by Kepler and CoRoT in many solar-type stars, thereby providing a way to probe the stars using asteroseismology.
Aims: We provide the mode linewidths and mode heights of the oscillations of various stars as a function of frequency and of effective temperature.
Methods: We used a time series of nearly two years of data for each star. The 23 stars observed belong to the simple or F-like category. The power spectra of the 23 main-sequence stars were analysed using both maximum likelihood estimators and Bayesian estimators, providing individual mode characteristics such as frequencies, linewidths, and mode heights. We study the source of systematic errors in the mode linewidths and mode heights, and we present a way to correct these errors with respect to a common reference fit.
Results: Using the correction, we can explain all sources of systematic errors, which could be reduced to less than ±15% for mode linewidths and heights, and less than ±5% for amplitude, when compared to the reference fit. The effect of a different estimated stellar background and a different estimated splitting will provide frequency-dependent systematic errors that might affect the comparison with theoretical mode linewidth and mode height, therefore affecting the understanding of the physical nature of these parameters. All other sources of relative systematic errors are less dependent upon frequency. We also provide the dependence of the so-called linewidth dip in the middle of the observed frequency range as a function of effective temperature. We show that the depth of the dip decreases with increasing effective temperature. The dependence of the dip on effective temperature may imply that the mixing length parameter α or the convective flux may increase with effective temperature.

Tables 4-27 and Appendices are available in electronic form at http://www.aanda.org Title: Measurement of Acoustic Glitches in Solar-type Stars from Oscillation Frequencies Observed by Kepler Authors: Mazumdar, A.; Monteiro, M. J. P. F. G.; Ballot, J.; Antia, H. M.; Basu, S.; Houdek, G.; Mathur, S.; Cunha, M. S.; Silva Aguirre, V.; García, R. A.; Salabert, D.; Verner, G. A.; Christensen-Dalsgaard, J.; Metcalfe, T. S.; Sanderfer, D. T.; Seader, S. E.; Smith, J. C.; Chaplin, W. J. Bibcode: 2014ApJ...782...18M Altcode: 2013arXiv1312.4907M For the very best and brightest asteroseismic solar-type targets observed by Kepler, the frequency precision is sufficient to determine the acoustic depths of the surface convective layer and the helium ionization zone. Such sharp features inside the acoustic cavity of the star, which we call acoustic glitches, create small oscillatory deviations from the uniform spacing of frequencies in a sequence of oscillation modes with the same spherical harmonic degree. We use these oscillatory signals to determine the acoustic locations of such features in 19 solar-type stars observed by the Kepler mission. Four independent groups of researchers utilized the oscillation frequencies themselves, the second differences of the frequencies and the ratio of the small and large separation to locate the base of the convection zone and the second helium ionization zone. Despite the significantly different methods of analysis, good agreement was found between the results of these four groups, barring a few cases. These results also agree reasonably well with the locations of these layers in representative models of the stars. These results firmly establish the presence of the oscillatory signals in the asteroseismic data and the viability of several techniques to determine the location of acoustic glitches inside stars. Title: High-rigidity Forbush decreases: due to CMEs or shocks? Authors: Arunbabu, K. P.; Antia, H. M.; Dugad, S. R.; Gupta, S. K.; Hayashi, Y.; Kawakami, S.; Mohanty, P. K.; Nonaka, T.; Oshima, A.; Subramanian, P. Bibcode: 2013A&A...555A.139A Altcode: 2013arXiv1304.5343B
Aims: We seek to identify the primary agents causing Forbush decreases (FDs) in high-rigidity cosmic rays observed from the Earth. In particular, we ask if these FDs are caused mainly by coronal mass ejections (CMEs) from the Sun that are directed towards the Earth, or by their associated shocks.
Methods: We used the muon data at cutoff rigidities ranging from 14 to 24 GV from the GRAPES-3 tracking muon telescope to identify FD events. We selected those FD events that have a reasonably clean profile, and can be reasonably well associated with an Earth-directed CME and its associated shock. We employed two models: one that considers the CME as the sole cause of the FD (the CME-only model) and one that considers the shock as the only agent causing the FD (the shock-only model). We used an extensive set of observationally determined parameters for both models. The only free parameter in these models is the level of MHD turbulence in the sheath region, which mediates cosmic ray diffusion (into the CME for the CME-only model, and across the shock sheath for the shock-only model).
Results: We find that good fits to the GRAPES-3 multi-rigidity data using the CME-only model require turbulence levels in the CME sheath region that are only slightly higher than those estimated for the quiescent solar wind. On the other hand, reasonable model fits with the shock-only model require turbulence levels in the sheath region that are an order of magnitude higher than those in the quiet solar wind.
Conclusions: This observation naturally leads to the conclusion that the Earth-directed CMEs are the primary contributors to FDs observed in high-rigidity cosmic rays.

Appendix A is available in electronic form at http://www.aanda.org Title: Position and velocity sensitivities at the triangular libration points in the restricted problem of three bodies when the bigger primary is an oblate body Authors: Hassan, M. R.; Antia, H. M.; Bhatnagar, K. B. Bibcode: 2013Ap&SS.346...71H Altcode: 2013Ap&SS.tmp..176H In this paper we have examined the stability of triangular libration points in the restricted problem of three bodies when the bigger primary is an oblate spheroid. Here we followed the time limit and computational process of Tuckness (Celest. Mech. Dyn. Mech. 61, 1-19, 1995) on the stability criteria given by McKenzie and Szebehely (Celest. Mech. 23, 223-229, 1981). In this study it was found that in comparison to other studies the value of the critical mass μ c has been reduced due to oblateness of the bigger primary, i.e. the range of stability of the equilateral triangular libration points reduced with the increase of the oblateness parameter I and hence the order of commensurability was increased. Title: Contrasting the solar rotation rate of cycles 23 and 24 Authors: Antia, H. M.; Basu, Sarbani Bibcode: 2013JPhCS.440a2018A Altcode: The minimum between solar cycles 23 and 24 was quite unusual compared with other minima for which detailed data are available and this pointed to the possibility that cycle 24 will be unusual. Cycle 24 is almost at its maximum now and we take this opportunity to compare and contrast the solar rotation rate and zonal flows between the two cycles. We find that the rotation rate during cycle 24 is slightly lower than that during cycle 23. Additionally we find that the poleward branch of the zonal flow that is believed to be the harbinger of the next solar cycle is very week in cycle 24. Title: Revisiting the Issue of Solar Abundances Authors: Basu, Sarbani; Antia, H. M. Bibcode: 2013JPhCS.440a2017B Altcode: We revisit the issue of solar abundances and examine whether the updated abundances result in solar models that have structures that agree with the structure of the Sun as determined by helioseismology. We quantify the changes in opacity required to bring the models constructed with the newer solar abundances in agreement with the Sun. Title: An Anisotropy of Galactic Cosmic Rays Observed with GRAPES-3 Authors: Oshima, A.; Kojima, H.; Shibata, S.; Hayashi, Y.; Antia, H.; Dugad, S.; Fujii, T.; Gupta, S. K.; Kawakami, S.; Minamino, M.; Mohanty, P. K.; Morishita, I.; Nakamura, T.; Nonaka, T.; Ogio, S.; Takamaru, H.; Tanaka, H.; Tanaka, K.; Ito, N.; Jain, A.; Matsuyama, T.; Rao, B.; Yamazaki, K.; Yoshida, N. Bibcode: 2013ICRC...33.1848O Altcode: Anisotropy in arrival direction of galactic cosmic rays were reported by several group Nagashima et al. and Hall et al., or more recently by ground based experiments such as Milagro and Tibet AS-gamma in subTeV energy region, and also IceCube in a few hundreds TeV region. A large scale anisotropy could be caused in several ways; the motion of the earth, large scale magnetic field structures, discrete distribution of cosmic ray sources, and so on. We have also reported a sidereal anisotropy of low energy cosmic rays in GeV energy observed with the large tracking muon detector of GRAPES-3 [1] [2]. Here we report a galactic cosmic ray anisotropy observed with GRAPES-3 air shower array in high energy region, specially in combination with the muon detectors for suppression of the contamination of primary hadrons. Title: How are Forbush decreases related with IP magnetic field enhancements ? Authors: Arunbabu, K. P.; Subramanian, P.; Gupta, Sunil; Antia, H. M. Bibcode: 2013ASInC..10...95A Altcode: Cosmic ray Forbush decreases (FDs) are usually thought to be due to Earth-directed coronal mass ejections (CMEs) from the Sun and their associated shocks. When CMEs and their shocks reach the Earth, they cause magnetic field compressions. We investigate the relation between these magnetic field compressions and FDs at rigidities between 12 and 42 GV using data from the GRAPES-3 instrument at Ooty. We find that the shapes of the Forbush decrease profiles show a startling similarity to that of the magnetic field compression in the near-Earth IP medium. We seek to understand the implications of this interesting result. Title: Swinson Flow and the Tilt Angle of the Neutral Current Sheet Authors: Kojima, H.; Shibata, S.; Oshima, A.; Hayashi, Y.; Antia, H.; Dugad, S.; Fujii, T.; Gupta, S. K.; Kawakami, S.; Minamino, M.; Mohanty, P. K.; Morishita, I.; Nakamura, T.; Nonaka, T.; Ogio, S.; Takamaru, H.; Tanaka, H.; Ito, N.; Jain, A.; Matsuyama, T.; Rao, B.; Yamazaki, K.; Yoshida, N.; ICRC Collaboration Bibcode: 2013ICRC...33.3557K Altcode: No abstract at ADS Title: On the magnetic field required for driving the observed angular-velocity variations in the solar convection zone Authors: Antia, H. M.; Chitre, S. M.; Gough, D. O. Bibcode: 2013MNRAS.428..470A Altcode: 2012MNRAS.tmp...41A; 2012arXiv1210.6606A A putative temporally varying circulation-free magnetic-field configuration is inferred in an equatorial segment of the solar convection zone from the helioseismologically inferred angular-velocity variation, assuming that the predominant dynamics is an angular acceleration produced by the azimuthal Maxwell stress exerted by a field whose surface values are consistent with photospheric line-of-sight measurements. Title: A Proposal of a Multi Directional Neutron Telescope for Observations of Galactic Cosmic Rays Authors: Kojima, H.; Oshima, A.; Shibata, S.; Hayashi, Y.; Antia, H.; Dugad, S.; Fujii, T.; Gupta, S. K.; Kawakkami, S.; Minamino, M.; Mohanty, P. K.; Morishita, I.; Nakamura, T.; Nonaka, T.; Ogio, S.; Takamaru, H.; Tanaka, H.; Tanaka, K.; Ito, N.; Jain, A.; Matsuyama, T.; Rao, B.; Yamazaki, K.; Yoshida, N. Bibcode: 2013ICRC...33.3649K Altcode: No abstract at ADS Title: Acoustic glitches in solar-type stars from Kepler Authors: Mazumdar, A.; Monteiro, M. J. P. F. G.; Ballot, J.; Antia, H. M.; Basu, S.; Houdek, G.; Mathur, S.; Cunha, M. S.; Silva Aguirre, V.; García, R. A.; Salabert, D.; Verner, G. A.; Christensen-Dalsgaard, J.; Metcalfe, T. S.; Chaplin, W. J. Bibcode: 2012AN....333.1040M Altcode: We report the measurement of the acoustic locations of layers of sharp variation in sound speed in the interiors of 19 solar-type stars observed by the Kepler mission. The oscillatory signal in the frequencies arising due to the acoustic glitches at the base of the convection zone and the second helium ionisation zone was utilised to determine their location by four independent methods. Despite the significantly different methods of analysis, remarkable agreement was found between the results of these four methods. Further, the extracted locations of these layers were found to be consistent with representative models of the stars. Title: Seismic Evidence for a Rapidly Rotating Core in a Lower-giant-branch Star Observed with Kepler Authors: Deheuvels, S.; García, R. A.; Chaplin, W. J.; Basu, S.; Antia, H. M.; Appourchaux, T.; Benomar, O.; Davies, G. R.; Elsworth, Y.; Gizon, L.; Goupil, M. J.; Reese, D. R.; Regulo, C.; Schou, J.; Stahn, T.; Casagrande, L.; Christensen-Dalsgaard, J.; Fischer, D.; Hekker, S.; Kjeldsen, H.; Mathur, S.; Mosser, B.; Pinsonneault, M.; Valenti, J.; Christiansen, J. L.; Kinemuchi, K.; Mullally, F. Bibcode: 2012ApJ...756...19D Altcode: 2012arXiv1206.3312D Rotation is expected to have an important influence on the structure and the evolution of stars. However, the mechanisms of angular momentum transport in stars remain theoretically uncertain and very complex to take into account in stellar models. To achieve a better understanding of these processes, we desperately need observational constraints on the internal rotation of stars, which until very recently was restricted to the Sun. In this paper, we report the detection of mixed modes—i.e., modes that behave both as g modes in the core and as p modes in the envelope—in the spectrum of the early red giant KIC 7341231, which was observed during one year with the Kepler spacecraft. By performing an analysis of the oscillation spectrum of the star, we show that its non-radial modes are clearly split by stellar rotation and we are able to determine precisely the rotational splittings of 18 modes. We then find a stellar model that reproduces very well the observed atmospheric and seismic properties of the star. We use this model to perform inversions of the internal rotation profile of the star, which enables us to show that the core of the star is rotating at least five times faster than the envelope. This will shed new light on the processes of transport of angular momentum in stars. In particular, this result can be used to place constraints on the angular momentum coupling between the core and the envelope of early red giants, which could help us discriminate between the theories that have been proposed over the last few decades. Title: Oscillation mode frequencies of 61 main-sequence and subgiant stars observed by Kepler Authors: Appourchaux, T.; Chaplin, W. J.; García, R. A.; Gruberbauer, M.; Verner, G. A.; Antia, H. M.; Benomar, O.; Campante, T. L.; Davies, G. R.; Deheuvels, S.; Handberg, R.; Hekker, S.; Howe, R.; Régulo, C.; Salabert, D.; Bedding, T. R.; White, T. R.; Ballot, J.; Mathur, S.; Silva Aguirre, V.; Elsworth, Y. P.; Basu, S.; Gilliland, R. L.; Christensen-Dalsgaard, J.; Kjeldsen, H.; Uddin, K.; Stumpe, M. C.; Barclay, T. Bibcode: 2012A&A...543A..54A Altcode: 2012arXiv1204.3147A Context. Solar-like oscillations have been observed by Kepler and CoRoT in several solar-type stars, thereby providing a way to probe the stars using asteroseismology
Aims: We provide the mode frequencies of the oscillations of various stars required to perform a comparison with those obtained from stellar modelling.
Methods: We used a time series of nine months of data for each star. The 61 stars observed were categorised in three groups: simple, F-like, and mixed-mode. The simple group includes stars for which the identification of the mode degree is obvious. The F-like group includes stars for which the identification of the degree is ambiguous. The mixed-mode group includes evolved stars for which the modes do not follow the asymptotic relation of low-degree frequencies. Following this categorisation, the power spectra of the 61 main-sequence and subgiant stars were analysed using both maximum likelihood estimators and Bayesian estimators, providing individual mode characteristics such as frequencies, linewidths, and mode heights. We developed and describe a methodology for extracting a single set of mode frequencies from multiple sets derived by different methods and individual scientists. We report on how one can assess the quality of the fitted parameters using the likelihood ratio test and the posterior probabilities.
Results: We provide the mode frequencies of 61 stars (with their 1-σ error bars), as well as their associated échelle diagrams.

Appendices are available in electronic form at http://www.aanda.org Title: Seismic detection of acoustic sharp features in the CoRoT target HD 49933 Authors: Mazumdar, A.; Michel, E.; Antia, H. M.; Deheuvels, S. Bibcode: 2012A&A...540A..31M Altcode: 2012arXiv1202.2692M The technique of determining the acoustic location of layers of sharp changes in the sound speed inside a star from the oscillatory signal in its frequencies is applied to a solar-type star, the CoRoT target, HD 49933. We are able to determine the acoustic depth of the second helium ionisation zone of HD 49933 to be 794-68+55 s. The acoustic depth of the base of the convective zone is found to be 1855-412+173 s where the large error bars reflect the ambiguity in the result, which is difficult to determine with present precision on the frequencies because of the intrinsically weak nature of the signal. The positions of both these layers are consistent with those in a representative stellar model of HD 49933. Title: Asteroseismology of the Solar Analogs 16 Cyg A and B from Kepler Observations Authors: Metcalfe, T. S.; Chaplin, W. J.; Appourchaux, T.; García, R. A.; Basu, S.; Brandão, I.; Creevey, O. L.; Deheuvels, S.; Doǧan, G.; Eggenberger, P.; Karoff, C.; Miglio, A.; Stello, D.; Yıldız, M.; Çelik, Z.; Antia, H. M.; Benomar, O.; Howe, R.; Régulo, C.; Salabert, D.; Stahn, T.; Bedding, T. R.; Davies, G. R.; Elsworth, Y.; Gizon, L.; Hekker, S.; Mathur, S.; Mosser, B.; Bryson, S. T.; Still, M. D.; Christensen-Dalsgaard, J.; Gilliland, R. L.; Kawaler, S. D.; Kjeldsen, H.; Ibrahim, K. A.; Klaus, T. C.; Li, J. Bibcode: 2012ApJ...748L..10M Altcode: 2012arXiv1201.5966M The evolved solar-type stars 16 Cyg A and B have long been studied as solar analogs, yielding a glimpse into the future of our own Sun. The orbital period of the binary system is too long to provide meaningful dynamical constraints on the stellar properties, but asteroseismology can help because the stars are among the brightest in the Kepler field. We present an analysis of three months of nearly uninterrupted photometry of 16 Cyg A and B from the Kepler space telescope. We extract a total of 46 and 41 oscillation frequencies for the two components, respectively, including a clear detection of octupole (l = 3) modes in both stars. We derive the properties of each star independently using the Asteroseismic Modeling Portal, fitting the individual oscillation frequencies and other observational constraints simultaneously. We evaluate the systematic uncertainties from an ensemble of results generated by a variety of stellar evolution codes and fitting methods. The optimal models derived by fitting each component individually yield a common age (t = 6.8 ± 0.4 Gyr) and initial composition (Z i = 0.024 ± 0.002, Y i = 0.25 ± 0.01) within the uncertainties, as expected for the components of a binary system, bolstering our confidence in the reliability of asteroseismic techniques. The longer data sets that will ultimately become available will allow future studies of differential rotation, convection zone depths, and long-term changes due to stellar activity cycles. Title: Oscillation mode linewidths of main-sequence and subgiant stars observed by Kepler Authors: Appourchaux, T.; Benomar, O.; Gruberbauer, M.; Chaplin, W. J.; García, R. A.; Handberg, R.; Verner, G. A.; Antia, H. M.; Campante, T. L.; Davies, G. R.; Deheuvels, S.; Hekker, S.; Howe, R.; Salabert, D.; Bedding, T. R.; White, T. R.; Houdek, G.; Silva Aguirre, V.; Elsworth, Y. P.; van Cleve, J.; Clarke, B. D.; Hall, J. R.; Kjeldsen, H. Bibcode: 2012A&A...537A.134A Altcode: 2011arXiv1112.3295A Context. Solar-like oscillations have been observed by Kepler and CoRoT in several solar-type stars.
Aims: We study the variations in the stellar p-mode linewidth as a function of effective temperature.
Methods: We study a time series of nine months of Kepler data. We analyse the power spectra of 42 cool main-sequence stars and subgiants using both maximum likelihood estimators and Bayesian estimators to recover individual mode characteristics such as frequencies, linewidths, and mode heights.
Results: We report on the mode linewidth at both maximum power and maximum mode height for these 42 stars as a function of effective temperature.
Conclusions: We show that the mode linewidth at either maximum mode height or maximum amplitude follows a scaling relation with effective temperature, which is a combination of a power law and a lower bound. The typical power-law index is about 13 for the linewidth derived from the maximum mode height, and about 16 for the linewidth derived from the maximum amplitude, while the lower bound is about 0.3 μHz and 0.7 μHz, respectively. We stress that this scaling relation is only valid for cool main-sequence stars and subgiants, and does not have any predictive power outside the temperature range of these stars. Title: Revisiting the Solar Tachocline: Average Properties and Temporal Variations Authors: Antia, H. M.; Basu, Sarbani Bibcode: 2011ApJ...735L..45A Altcode: 2011arXiv1107.1004A The tachocline is believed to be the region where the solar dynamo operates. With over a solar cycle's worth of data available from the Michelson Doppler Imager and Global Oscillation Network Group instruments, we are in a position to investigate not merely the average structure of the solar tachocline, but also its time variations. We determine the properties of the tachocline as a function of time by fitting a two-dimensional model that takes latitudinal variations of the tachocline properties into account. We confirm that if we consider the central position of the tachocline, it is prolate. Our results show that the tachocline is thicker at latitudes higher than the equator, making the overall shape of the tachocline more complex. Of the tachocline properties examined, the transition of the rotation rate across the tachocline, and to some extent the position of the tachocline, show some temporal variations. Title: Are recent solar heavy element abundances consistent with helioseismology? Authors: Antia, H. M.; Basu, Sarbani Bibcode: 2011JPhCS.271a2034A Altcode: During the last decade the abundances of heavy elements in the Sun have been revised downwards leading to serious discrepancy between solar models constructed using these abundances and the available seismic data. Much of these downward revision of abundances of Oxygen and other light elements was attributed to use of improved 3D solar atmospheric models. Recently, independent 3D models have been used to calculate solar abundances of these elements and calculated values are higher than the earlier estimates also obtained using 3D atmospheric models. In this work we investigate if these revised abundances are consistent with seismic data. We also investigate whether an increase in Neon abundance can help in resolving the discrepancy. Title: Zonal Flows Throughout Cycle 23 Authors: Antia, H. M.; Basu, Sarbani Bibcode: 2011JPhCS.271a2072A Altcode: We use available GONG and MDI data to study how solar zonal flows evolved over cycle 23 and whether or not there were differences between the minima of cycles 23 and 24. An autocorrelation study shows that as far as the zonal-flows are concerned, cycle 23 lasted for about 11.7 years, considerably shorter than what is seen from magnetic indices. We also find that there were significant differences in the nature of the flows between the minima of cycles 23 and 24. Title: Characteristics of Solar Meridional Flows Authors: Basu, Sarbani; Antia, H. M. Bibcode: 2011JPhCS.271a2071B Altcode: We have done a ring-diagram analysis of MDI full-disc data to determine the properties of solar meridional flow in the outer 2% of the Sun over the solar cycle 23. The meridional flows show a migrating pattern with higher-velocity flows migrating toward the equator as activity increases. Additionally, we find that the migrating pattern of the meridional flow matches those of the sunspot butterfly diagram and the zonal flows in the shallow layers. A Legendre polynomial decomposition of the meridional flows shows that the latitudinal pattern of the flow was also different during the maximum as compared to that during the two minima. We also find that the dominant component of the meridional flows during solar maxima was much lower than that during the minima of solar cycles 23 and 24. Title: Internal magnetic fields inferred from helioseismic data Authors: Baldner, C. S.; Antia, H. M.; Basu, S.; Larson, T. P. Bibcode: 2010AN....331..879B Altcode: Measuring the internal solar magnetic fields and how they change over the course of a solar cycle is one of the key aims of helioseismology. We present the results of attempts to model the global mode splitting coefficients over solar cycle 23, assuming that the frequency splitting is only due to rotation and a large-scale magnetic field. The first results using only the a_2 coefficients show that the data are best fit by a combination of a poloidal field and a double-peaked near-surface toroidal field. The toroidal fields are centered at r_0 = 0.999 R_⊙ and r_0 = 0.996 R_⊙ and are confined to the near-surface layers. The poloidal field is a dipole field. The peak strength of the poloidal field is 124±17 G. The toroidal field peaks at 380 ± 30 G and 1.4 ± 0.2 kG for the shallower and deeper fields, respectively. The field strengths are highly correlated with surface activity. We also examine the differences between the minima at the beginning and the end of solar cycle 23. Title: Solar Rotation Rate During the Cycle 24 Minimum in Activity Authors: Antia, H. M.; Basu, Sarbani Bibcode: 2010ApJ...720..494A Altcode: 2010arXiv1008.1787A The minimum of solar cycle 24 is significantly different from most other minima in terms of its duration as well as its abnormally low levels of activity. Using available helioseismic data that cover epochs from the minimum of cycle 23 to now, we study the differences in the nature of the solar rotation between the minima of cycles 23 and 24. We find that there are significant differences between the rotation rates during the two minima. There are differences in the zonal-flow pattern too. We find that the band of fast rotating region close to the equator bifurcated around 2005 and recombined by 2008. This behavior is different from that during the cycle 23 minimum. By autocorrelating the zonal-flow pattern with a time shift, we find that in terms of solar dynamics, solar cycle 23 lasted for a period of 11.7 years, consistent with the result of Howe et al. (2009). The autocorrelation coefficient also confirms that the zonal-flow pattern penetrates through the convection zone. Title: Characteristics of Solar Meridional Flows during Solar Cycle 23 Authors: Basu, Sarbani; Antia, H. M. Bibcode: 2010ApJ...717..488B Altcode: 2010arXiv1005.3031B We have analyzed available full-disk data from the Michelson Doppler Imager on board SOHO using the "ring diagram" technique to determine the behavior of solar meridional flows over solar cycle 23 in the outer 2% of the solar radius. We find that the dominant component of meridional flows during solar maximum was much lower than that during the minima at the beginning of cycles 23 and 24. There were differences in the flow velocities even between the two minima. The meridional flows show a migrating pattern with higher-velocity flows migrating toward the equator as activity increases. Additionally, we find that the migrating pattern of the meridional flow matches those of sunspot butterfly diagram and the zonal flows in the shallow layers. A high-latitude band in meridional flow appears around 2004, well before the current activity minimum. A Legendre polynomial decomposition of the meridional flows shows that the latitudinal pattern of the flow was also different during the maximum as compared to that during the two minima. The different components of the flow have different time dependences, and the dependence is different at different depths. Title: Is the solar convection zone in strict thermal wind balance? Authors: Brun, A. S.; Antia, H. M.; Chitre, S. M. Bibcode: 2010A&A...510A..33B Altcode: 2009arXiv0910.4954B Context. The solar rotation profile is conical rather than cylindrical as it could be expected from classical rotating fluid dynamics (e.g. Taylor-Proudman theorem). Thermal coupling to the tachocline, baroclinic effects and latitudinal transport of heat have been suggested to explain this peculiar state of rotation.
Aims: To test the validity of thermal wind balance in the solar convection zone using helioseismic inversions for both the angular velocity and fluctuations in entropy and temperature.
Methods: Entropy and temperature fluctuations obtained from 3D hydrodynamical numerical simulations of the solar convection zone are compared with solar profiles obtained from helioseismic inversions.
Results: The temperature and entropy fluctuations in 3D numerical simulations have smaller amplitude in the bulk of the solar convection zone than those derived from seismic inversions. Seismic inversion provides variations of temperature from about 1 K at the surface to up to 100 K at the base of the convection zone while in 3D simulations they are of an order of 10 K throughout the convection zone up to 0.96 R. In 3D simulations, baroclinic effects are found to be important to tilt the isocontours of Ω away from a cylindrical profile in most of the convection zone, helped by Reynolds and viscous stresses at some locations. By contrast the baroclinic effect inverted by helioseismology is much larger than what is required to yield the observed angular velocity profile.
Conclusions: The solar convection does not appear to be in strict thermal wind balance, Reynolds stresses must play a dominant role in setting not only the equatorial acceleration but also the observed conical angular velocity profile. Title: Are Polar Faculae Generated by a Local Dynamo? Authors: Sivaraman, K. R.; Antia, H. M.; Chitre, S. M. Bibcode: 2010ASSP...19..386S Altcode: 2010mcia.conf..386S Polar faculae (PF) are bright, small-scale structures measuring a few seconds of arc, populating the polar zones at latitudes >50°. They possess magnetic fields ranging from 150 to 1,700 Gauss and largely constitute the polar magnetic fields. Where and how their fields are generated in the solar interior remain open questions. Using measurements of PF rotation rates, we show that their anchor depths probably lie in subsurface layers at radius r/R = 0:94-1.00. If so, the PF fields are possibly generated by a local dynamo in a subsurface shear layer extending to r/R > 0:94. Title: Changing Magnetic Fields in the Solar Interior Authors: Baldner, C. S.; Antia, H. M.; Basu, S.; Larson, T. P. Bibcode: 2009ASPC..416..289B Altcode: Even-order a-coefficients show changes that are correlated with solar activity. We analyze Michelson Doppler Imager (MDI) data that are available for solar cycle 23 to try and determine the change in solar magnetic fields, particularly in the upper part of the convection zone. This is the region where the even-order a-coefficients show the largest change. We use a forward calculation to determine the change in the magnetic fields between the last solar minimum and solar maximum. Title: Does the Selection of a Quiet Region Influence the Local Helioseismic Inferences? Authors: Tripathy, S. C.; Antia, H. M.; Jain, K.; Hill, F. Bibcode: 2009ASPC..416..139T Altcode: 2009arXiv0901.4939T We apply the ring-diagram technique to high resolution Dopplergrams in order to estimate the variation in oscillation mode parameters between active and quiet regions. We demonstrate that the difference in mode parameters between two quiet regions can be as large as those between a pair of active and quiet region. This leads us to conclude that the results derived on the basis of a single quiet region could be biased. Title: Solar Flows and Their Effect on Frequencies of Acoustic Modes Authors: Chatterjee, Piyali; Antia, H. M. Bibcode: 2009ApJ...707..208C Altcode: 2009arXiv0910.4137C We have calculated the effects of large-scale solar flows, such as the meridional circulation, giant convection cells, and solar rotation on the helioseismic splitting coefficients using quasi-degenerate perturbation theory (QDPT). Our investigation reveals that the effect of poloidal flows like the large-scale meridional circulation are difficult to detect in observational data of the global acoustic modes since the frequency shifts are much less than the errors. However, signatures of large-scale convective flows may be detected if their amplitude is sufficiently large by looking for frequency shifts due to nearly degenerate modes coupled by convection. In this comprehensive study, we attempt to put limits on the magnitude of flow velocities in giant cells by comparing the splitting coefficients obtained from the QDPT treatment with observational data. Title: What is New with Zonal Flows? Authors: Antia, H. M.; Basu, S.; Chitre, S. M. Bibcode: 2009ASPC..416..265A Altcode: We have used available helioseismic data for the solar cycle 23 to undertake a detailed study of temporal variations in solar rotation. We concentrate mainly on variation of the radial and latitudinal gradients of rotation. We find that the temporal variations of the rotation-rate gradients is about 20% or more than their average value, i.e., much larger than the relative variation in the rotation rate itself. These variations may play an important role in solar dynamo models. Title: The Evolution of the Sub-Surface Structure of Long-Lived Active Regions Authors: Baldner, C. S.; Bogart, R. S.; Basu, S.; Antia, H. M. Bibcode: 2009ASPC..416..119B Altcode: The strongest active regions on the Sun are usually very long lived and are ideal candidates to study the sub-surface evolution of active regions through ring-diagram analysis. We do a ring-diagram analysis of several long-lived active regions. All selected groups last for more than two Carrington rotations. When the data is available, we track the regions from before their first appearance to after their final disappearance. We determined mode frequencies and velocity parameters from the active regions. The frequencies are inverted to determine how the wave-speed and adiabatic index evolve and the velocity parameters are inverted to determine flows below the regions. We use the inversion results to examine the evolution of structure and dynamics below the selected active regions. Title: Solar Magnetic Field Signatures in Helioseismic Splitting Coefficients Authors: Baldner, Charles S.; Antia, H. M.; Basu, Sarbani; Larson, Timothy P. Bibcode: 2009ApJ...705.1704B Altcode: 2009arXiv0910.1597B Normal modes of oscillation of the Sun are useful probes of the solar interior. In this work, we use the even-order splitting coefficients to study the evolution of magnetic fields in the convection zone over solar cycle 23, assuming that the frequency splitting is only due to rotation and a large-scale magnetic field. We find that the data are best fit by a combination of a poloidal field and a double-peaked near-surface toroidal field. The toroidal fields are centered at r 0 = 0.999 R sun and r = 0.996 R sun and are confined to the near-surface layers. The poloidal field is a dipole field. The peak strength of the poloidal field is 124 ± 17 G. The toroidal field peaks at 380 ± 30 G and 1.4 ± 0.2 kG for the shallower and deeper fields, respectively. The field strengths are highly correlated with surface activity. The toroidal field strength shows a hysteresis-like effect when compared to the global 10.7 cm radio flux. The poloidal field strength shows evidence of saturation at high activity. Title: Internal Magnetic Fields Inferred From Helioseismic Data Authors: Baldner, Charles; Antia, H. M.; Basu, S.; Larson, T. P. Bibcode: 2009SPD....40.0721B Altcode: The splitting coefficients of helioseismic frequencies encode information about departures from spherical symmetry in the solar interior. In particular, magnetic fields in the solar interior will affect the even-order splitting coefficients. The effects of magnetic fields on solar structure and on wave propagation are treated as perturbations on an equilibrium spherically symmetric state, and the changes in splitting coefficients are computed using a variational principal. Using the splitting coefficients from the MDI instrument, we infer magnetic field strengths and configurations for several data sets over the course of solar cycle 23. Title: Solar oscillations Authors: Antia, H. M. Bibcode: 2009IAUS..257...83A Altcode: Study of solar oscillations has provided us detailed information about solar structure and dynamics. These in turn provide a test of theories of stellar structure and evolution as well as theories of angular momentum transfer and dynamo. Some of these results about the solar structure and its implication on the recent revision of heavy element abundances are described. Apart from these the solar cycle variations in the rotation rate and its gradients are also discussed. Title: Forbush decreases and turbulence levels at coronal mass ejection fronts Authors: Subramanian, P.; Antia, H. M.; Dugad, S. R.; Goswami, U. D.; Gupta, S. K.; Hayashi, Y.; Ito, N.; Kawakami, S.; Kojima, H.; Mohanty, P. K.; Nayak, P. K.; Nonaka, T.; Oshima, A.; Sivaprasad, K.; Tanaka, H.; Tonwar, S. C.; GRAPES-3 Collaboration Bibcode: 2009A&A...494.1107S Altcode: 2008arXiv0810.2851S Aims: We seek to estimate the average level of MHD turbulence near coronal mass ejection (CME) fronts as they propagate from the Sun to the Earth.
Methods: We examined the cosmic ray data from the GRAPES-3 tracking muon telescope at Ooty, together with the data from other sources for three closely observed Forbush decrease events. Each of these event is associated with frontside halo coronal mass ejections (CMEs) and near-Earth magnetic clouds. The associated Forbush decreases are therefore expected to have significant contributions from the cosmic-ray depressions inside the CMEs/ejecta. In each case, we estimate the magnitude of the Forbush decrease using a simple model for the diffusion of high-energy protons through the largely closed field lines enclosing the CME as it expands and propagates from the Sun to the Earth. The diffusion of high-energy protons is inhibited by the smooth, large-scale magnetic field enclosing the CME and aided by the turbulent fluctuations near the CME front. We use estimates of the cross-field diffusion coefficient Dperp derived from the published results of extensive Monte Carlo simulations of cosmic rays propagating through turbulent magnetic fields. We then compare our estimates with the magnitudes of the observed Forbush decreases.
Results: Our method helps constrain the ratio of energy density in the turbulent magnetic fields to that in the mean magnetic fields near the CME fronts. This ratio is found to be 2% for the 2001 April 11 Forbush decrease event, 6% for the 2003 November 20 Forbush decrease event and 249% for the much more energetic event of 2003 October 29. Title: Comparison of High-Degree Solar Acoustic Frequencies and Asymmetry Between Velocity and Intensity Data Authors: Tripathy, S. C.; Antia, H. M.; Jain, K.; Hill, F. Bibcode: 2009ApJ...691..365T Altcode: 2008arXiv0809.4486T Using the local helioseismic technique of ring diagram we analyze the frequencies of high-degree f- and p-modes derived from both velocity and continuum intensity data observed by Michelson Doppler Imager. Fitting the spectra with asymmetric peak profiles, we find that the asymmetry associated with velocity line profiles is negative for all frequency ranges, agreeing with previous observations, while the asymmetry of the intensity profiles shows a complex and frequency-dependent behavior. We also observe systematic frequency differences between intensity and velocity spectra at the high end of the frequency range, mostly above 4 mHz. We infer that this difference arises from the fitting of the intensity rather than the velocity spectra. We also show that the frequency differences between intensity and velocity do not vary significantly from the disk center to the limb when the spectra are fitted with the asymmetric profile and conclude that only a part of the background is correlated with the intensity oscillations. Title: How Do f-Mode Frequencies Change with Solar Radius? Authors: Chatterjee, Piyali; Antia, H. M. Bibcode: 2008ApJ...688L.123C Altcode: 2008arXiv0810.4213C We test the relation between relative f-mode frequency variation (δ ν/ν) and Lagrangian perturbation in the solar radius (δ r/r) obtained by Dziembowski and Goode using several pairs of solar models and show that it does not hold true for any of the model pairs we have used. We attempt to derive a better approximation for the kernel linking the relative frequency changes and the solar radius variation in the subsurface layers. Title: Zonal Velocity Bands and the Solar Activity Cycle Authors: Sivaraman, K. R.; Antia, H. M.; Chitre, S. M.; Makarova, V. V. Bibcode: 2008SoPh..251..149S Altcode: 2008arXiv0802.0907S; 2008SoPh..tmp...72S We compare the zonal-flow pattern in subsurface layers of the Sun with the distribution of surface magnetic features such as sunspots and polar faculae. We demonstrate that, in the activity belt, the butterfly pattern of sunspots coincides with the fast stream of zonal flows, although part of the sunspot distribution does spill over to the slow stream. At high latitudes, the polar faculae and zonal-flow bands have similar distributions in the spatial and temporal domains. Title: Probing the Subsurface Structures of Active Regions with Ring-Diagram Analysis Authors: Bogart, Richard S.; Basu, Sarbani; Rabello-Soares, Maria Cristina; Antia, H. M. Bibcode: 2008SoPh..251..439B Altcode: 2008SoPh..tmp..113B We analyze the variations in the near-surface profiles of sound speed and adiabatic constant between active regions and neighboring quiet-Sun areas using the technique of ring-diagram analysis and inversions of the frequency differences between the regions. This approach minimizes the systematic observational effects on the fitted spectral model parameters. The regions analyzed have been selected from a large sample of data available from both GONG and MDI and include a wide range of magnetic activity levels as measured in several respects. We find that the thermal-structure anomalies under active regions have a consistent depth profile, with only the magnitude of the effect varying with the intensity of the active regions. Both the sound speed and the first adiabatic index are depressed near the surface but enhanced at greater depths. The turnover for the sound speed occurs at a shallower depth than that for the adiabatic index. The amplitude of the thermal anomalies at all depths correlates more closely with the total magnetic flux of the active regions than with spot areas or flare activity levels. The depth of the turnover does not appear to depend on the strength of the region. Title: Solar Rotation Rate and Its Gradients During Cycle 23 Authors: Antia, H. M.; Basu, Sarbani; Chitre, S. M. Bibcode: 2008ApJ...681..680A Altcode: 2008arXiv0803.3619A Available helioseismic data now span almost the entire solar activity cycle 23, making it possible to study solar-cycle-related changes of the solar rotation rate in detail. In this paper we study how the solar rotation rate, in particular, the zonal flows, change with time. In addition to the zonal flows that show a well-known pattern in the solar convection zone, we also study changes in the radial and latitudinal gradients of the rotation rate, particularly in the shear layer that is present in the immediate subsurface layers of the Sun. In the case of the zonal flow pattern, we find that the band indicating fast rotating region close to the equator seems to have bifurcated around 2005. Our investigation of the rotation rate gradients shows that the relative variation in the rotation rate gradients is about 20% or more of their average values, which is much larger than the relative variation in the rotation rate itself. These results can be used to test predictions of various solar dynamo models. Title: Helioseismology and solar abundances Authors: Basu, Sarbani; Antia, H. M. Bibcode: 2008PhR...457..217B Altcode: 2007arXiv0711.4590B Helioseismology has allowed us to study the structure of the Sun in unprecedented detail. One of the triumphs of the theory of stellar evolution was that helioseismic studies had shown that the structure of solar models is very similar to that of the Sun. However, this agreement has been spoiled by recent revisions of the solar heavy-element abundances. Heavy-element abundances determine the opacity of the stellar material and hence, are an important input to stellar model calculations. The models with the new, low abundances do not satisfy helioseismic constraints. We review here how heavy-element abundances affect solar models, how these models are tested with helioseismology, and the impact of the new abundances on standard solar models. We also discuss the attempts made to improve the agreement of the low-abundance models with the Sun and discuss how helioseismology is being used to determine the solar heavy-element abundances. A review of current literature shows that attempts to improve agreement between solar models with low heavy-element abundances and seismic inference have been unsuccessful so far. The low-metallicity models that have the least disagreement with seismic data require changing all input physics to stellar models beyond their acceptable ranges. Seismic determinations of the solar heavy-element abundances yield results that are consistent with the older, higher values of the solar abundance, and hence, no major changes to the inputs to solar models are required to make higher-metallicity solar models consistent with the helioseismic data. Title: Seismic study of magnetic field in the solar interior Authors: Antia, H. M. Bibcode: 2008JApA...29...85A Altcode: No abstract at ADS Title: Temporal variations in the Sun's rotational kinetic energy Authors: Antia, H. M.; Chitre, S. M.; Gough, D. O. Bibcode: 2008A&A...477..657A Altcode: 2007arXiv0711.0799A Aims: We study the variation of the angular momentum and the rotational kinetic energy of the Sun, and associated variations in the gravitational multipole moments, on a timescale of the solar cycle.
Methods: These quantities are determined by inverting helioseismic rotational splitting data obtained by the Global Oscillation Network Group and by the Michelson Doppler Imager on the Solar and Heliospheric Observatory.
Results: The temporal variation in angular momentum and kinetic energy at high latitudes (>π/4) through the convection zone is positively correlated with the level of solar activity, whereas at low latitudes it is anticorrelated, except in the top 10% by radius where both are correlated positively.
Conclusions: The helioseismic data imply significant temporal variation in the angular momentum and the rotational kinetic energy, and in the gravitational multipole moments. The properties of that variation will help constrain dynamical theories of the solar cycle. Title: The Inconstant Sun Authors: Antia, H. M.; Chitre, S. M.; Gough, D. O. Bibcode: 2007AIPC..948..133A Altcode: We study temporal variation in the solar angular momentum, J, rotational kinetic energy, T, and the rotational contribution to the gravitational multipole moments J2k of the Sun, inferred from helioseismic data from GONG and MDI over the last 11 years, covering most of the cycle 23. We have found that the variations in J and T at high latitudes (>45°) through the convection zone are correlated positively with solar activity, while at low latitudes they are correlated negatively, except for the top 10% of solar radius, where the correlation is positive. Title: Seismic Study of the Chemical Composition of the Solar Convection Zone Authors: Lin, Chia-Hsien; Antia, H. M.; Basu, Sarbani Bibcode: 2007ApJ...668..603L Altcode: 2007arXiv0706.3046L Recent downward revision of solar heavy-element abundances using a three-dimensional atmospheric model has introduced serious discrepancies between standard solar models and helioseismic inferences about solar structure. In this paper we investigate the possibility of determining the heavy-element abundances using helioseismic inversion techniques with the hope of providing an independent estimate. We use the adiabatic index Γ1≡(∂lnP/∂lnρ)s as a probe to examine the effects of the total heavy-element abundance, as well as the effects due to the abundance of individual elements. Our inversion results show that the new, lower, abundance increases the discrepancy between the Sun and the solar models. Title: Local helioseismology using ring diagram analysis Authors: Antia, H. M.; Basu, S. Bibcode: 2007AN....328..257A Altcode: Ring diagram analysis is an extension of global helioseismology that is applied to small areas on the Sun. It can be used to infer the horizontal components of large scale flows as well as the structure, and variations thereof, in the outer convection zone. We describe below the ring-diagram analysis technique, and some results obtained using this technique. Title: Structure of the Near-Surface Layers of the Sun: Asphericity and Time Variation Authors: Basu, Sarbani; Antia, H. M.; Bogart, Richard S. Bibcode: 2007ApJ...654.1146B Altcode: 2006astro.ph..9505B We present results on the structure of the near-surface layers of the Sun obtained by inverting frequencies of high-degree solar modes from ``ring diagrams.'' We have results for eight epochs between 1996 June and 2003 October. The frequencies for each epoch were obtained from ring diagrams constructed from Michelson Doppler Imager (MDI) Dopplergrams spanning complete Carrington rotations. We find that there is a substantial latitudinal variation of both sound speed and the adiabatic index Γ1 in the outer 2% of the Sun. We find that both the sound-speed and Γ1 profiles change with changes in the level of solar activity. In addition, we also study differences between the northern and southern hemispheres of the Sun and find a small asymmetry that appears to reflect the difference in magnetic activity between the two hemispheres. Title: Possibility of Excitation of Low-ℓ P-Modes by Energetic Solar Transients Authors: Ambastha, Ashok; Antia, H. M. Bibcode: 2006SoPh..238..219A Altcode: 2006SoPh..tmp...63A We examine the temporal variation of power in low-ℓ modes using GONG data for the period of May 1995-October 2005 and compare this with disk-integrated flare and CME indices. A poor correlation between the running means of Flare Index and mode power is found. A similar result is found for CME Index also. Variations in the running mean mode power corresponding to ℓ = 0 modes with different radial orders are generally stochastic in nature. This behaviour is also reflected in the distribution of mode power. Title: A comparison of acoustic mode parameters using multi-spectral data Authors: Jain, K.; Hill, F.; Tripathy, S. C.; Antia, H. M.; Armstrong, J. D.; Jefferies, S. M.; Rhodes, E. J., Jr.; Rose, P. J. Bibcode: 2006ESASP.624E.103J Altcode: 2006soho...18E.103J No abstract at ADS Title: Comparison of local frequency shifts between MDI velocity and intensity data Authors: Tripathy, S. C.; Antia, H. M.; Hill, F.; Jain, K.; González Hernández, I. Bibcode: 2006ESASP.624E.104T Altcode: 2006soho...18E.104T No abstract at ADS Title: Seismic estimate of solar heavy element abundances Authors: Basu, S.; Antia, H. M. Bibcode: 2006ESASP.624E..80B Altcode: 2006soho...18E..80B No abstract at ADS Title: Examining the effect of the new chemical composition on solar structure by using the adiabatic index Authors: Lin, C. -H.; Antia, H. M.; Basu, S. Bibcode: 2006ESASP.624E..87L Altcode: 2006soho...18E..87L No abstract at ADS Title: Temporal variations of solar rotation during solar cycle 23 Authors: Basu, S.; Antia, H. M. Bibcode: 2006ESASP.624E.128B Altcode: 2006soho...18E.128B No abstract at ADS Title: Subsurface structure evolution associated with the rise and fall of intensely active regions Authors: Bogart, R. S.; Basu, S.; Rabello-Soares, M. C.; Antia, H. M. Bibcode: 2006ESASP.624E..62B Altcode: 2006soho...18E..62B No abstract at ADS Title: Seismic Determination of Solar Heavy Element Abundances Authors: Basu, S.; Antia, H. M. Bibcode: 2006ESASP.617E..36B Altcode: 2006soho...17E..36B No abstract at ADS Title: Temporal Variation in the Solar Rotation Rate During the Last One Decade Authors: Basu, S.; Antia, H. M. Bibcode: 2006ESASP.617E..49B Altcode: 2006soho...17E..49B No abstract at ADS Title: Examining the Effect of the New Chemical Composition on the Solar Structure by Using the Adiabatic Gradient Authors: Lin, C. -H.; Antia, H. M.; Basu, S. Bibcode: 2006ESASP.617E..44L Altcode: 2006soho...17E..44L No abstract at ADS Title: Asphericity and Time Variation of the Near-Surface Layers of the Sun Authors: Basu, S.; Antia, H. M.; Bogart, R. S. Bibcode: 2006ESASP.617E..50B Altcode: 2006soho...17E..50B No abstract at ADS Title: Determining Solar Abundances Using Helioseismology Authors: Antia, H. M.; Basu, Sarbani Bibcode: 2006ApJ...644.1292A Altcode: 2006astro.ph..3001A The recent downward revision of solar photospheric abundances of oxygen and other heavy elements has resulted in serious discrepancies between solar models and solar structure as determined through helioseismology. In this work we investigate the possibility of determining the solar heavy-element abundance without reference to spectroscopy by using helioseismic data. Using the dimensionless sound-speed derivative in the solar convection zone, we find that the heavy-element abundance Z=0.0172+/-0.002, which is closer to the older, higher value of the abundances. Title: Helioseismology Authors: Antia, H. M. Bibcode: 2005JApA...26..161A Altcode: The sun being the nearest star, seismic observations with high spatial resolution are possible, thus providing accurate measurement of frequencies of about half million modes of solar oscillations covering a wide range of degree. With these data helioseismology has enabled us to study the solar interior in sufficient detail to infer the large-scale structure and rotation of the solar interior. With the availability of high quality helioseismic data over a good fraction of a solar cycle it is also possible to study temporal variations in solar structure and dynamics. Some of these problems and recent results will be discussed. Title: The structure of the near-surface layers of the Sun: asphericity and time variation Authors: Basu, S.; Bogart, R. S.; Antia, H. M. Bibcode: 2005AGUSMSP11B..10B Altcode: We present results on the structure of the near-surface layers of the Sun. These results were obtained by inverting frequencies of high-degree solar modes obtained by using "ring diagrams". We have results for seven epochs from June 1996 to March 2002. The frequencies of each epoch were obtained from ring diagrams constructed from MDI Dopplergrams spanning one complete Carrington rotation. We find that there is a substantial latitudinal variation of both sound speed and the adiabatic index Γ1 in the outer 2% of the Sun. In addition, it appears that both the sound-speed and Γ1 profiles change with change in the level of solar activity. Title: The Discrepancy between Solar Abundances and Helioseismology Authors: Antia, H. M.; Basu, Sarbani Bibcode: 2005ApJ...620L.129A Altcode: 2005astro.ph..1129A There have been recent downward revisions of the solar photospheric abundances of oxygen and other heavy elements. These revised abundances along with OPAL opacities are not consistent with seismic constraints. In this work we show that the recently released Opacity Project opacity tables cannot resolve this discrepancy either. While the revision in opacities does not seem to resolve this conflict, an upward revision of neon abundance in the solar photosphere offers a possible solution to this problem. Title: Temporal Variations in the Solar Radius? Authors: Antia, H. M.; Basu, S. Bibcode: 2004ESASP.559..301A Altcode: 2004soho...14..301A No abstract at ADS Title: Temporal Variations in Zonal Flows Authors: Antia, H. M.; Basu, S. Bibcode: 2004ESASP.559..305A Altcode: 2004soho...14..305A No abstract at ADS Title: Variations in LOW-l Solar P-Modes with Flare Activity Authors: Ambastha, A.; Antia, H. M. Bibcode: 2004ESASP.559..289A Altcode: 2004soho...14..289A No abstract at ADS Title: Helioseismic Constraints on Photospheric Abundances Authors: Basu, S.; Antia, H. M. Bibcode: 2004ESASP.559..317B Altcode: 2004soho...14..317B No abstract at ADS Title: Asteroseismic Determination of Helium Abundance in Solar-Type Stars Authors: Basu, S.; Mazumdar, A.; Antia, H. M.; Demarque, P. Bibcode: 2004ESASP.559..313B Altcode: 2004soho...14..313B No abstract at ADS Title: Solar P-Mode Characteristics Associated with Superactive-Regions Observed during OCT NOV 2003 Authors: Ambastha, A.; Basu, S.; Antia, H. M.; Bogart, R. S. Bibcode: 2004ESASP.559..293A Altcode: 2004soho...14..293A No abstract at ADS Title: Ring-Diagram Analysis of the Structure of Solar Active Regions Authors: Basu, Sarbani; Antia, H. M.; Bogart, Richard S. Bibcode: 2004ApJ...610.1157B Altcode: We measure differences in structure between active and quiet regions of the Sun using the frequencies of high-degree modes determined from ring-diagram analyses. We find that both the speed of sound and the adiabatic index Γ1 differ in active regions as compared with quiet regions. In the immediate subsurface layers, the sound speed is lower in active regions, but below a depth of about 7 Mm the opposite is true. A comparison of sound-speed inversion results with those for Γ1 indicates that at least a part of the differences between active and quiet regions is likely to be due to the structural and thermal perturbations caused by magnetic fields in the active region. Title: Constraining Solar Abundances Using Helioseismology Authors: Basu, Sarbani; Antia, H. M. Bibcode: 2004ApJ...606L..85B Altcode: 2004astro.ph..3485B Recent analyses of solar photospheric abundances suggest that the oxygen abundance in the solar atmosphere needs to be revised downward. In this study, we investigate the consequence of this revision on helioseismic analyses of the depth of the solar convection zone and the helium abundance in the solar envelope and find no significant effect. We also find that the revised abundances along with the current OPAL opacity tables are not consistent with seismic data. A significant upward revision of the opacity tables is required to make solar models with lower oxygen abundance consistent with seismic observations. Title: Asteroseismic determination of helium abundance in stellar envelopes Authors: Basu, Sarbani; Mazumdar, Anwesh; Antia, H. M.; Demarque, Pierre Bibcode: 2004MNRAS.350..277B Altcode: 2004astro.ph..2360B Intermediate degree modes of the solar oscillations have previously been used to determine the solar helium abundance to a high degree of precision. However, we cannot expect to observe such modes in other stars. In this work we investigate whether low degree modes that should be available from space-based asteroseismology missions can be used to determine the helium abundance, Y, in stellar envelopes with sufficient precision. We find that the oscillatory signal in the frequencies caused by the depression in Γ1 in the second helium ionization zone can be used to determine the envelope helium abundance of low-mass main-sequence stars. For frequency errors of one part in 104, we expect errors σY in the estimated helium abundance to range from 0.03 for 0.8-Msolar stars to 0.01 for 1.2-Msolar stars. The task is more complicated in evolved stars, such as subgiants, but is still feasible if the relative errors in the frequencies are less than 10-4. Title: Helioseismic Constraints on Photospheric Abundances Authors: Basu, S.; Antia, H. M. Bibcode: 2004AAS...204.5302B Altcode: 2004BAAS...36..755B Recent analyses of solar photospheric abundances suggest that the oxygen abundance in the solar atmosphere needs to be revised downwards. We investigate if solar models constructed with lower oxygen and other heavy element abundances are consistent with helioseismic results. We find that lowered abundances along with the current OPAL opacity tables are not consistent with seismic data. A significant upward revision of the opacity tables is required to make solar models with lower heavy element abundances that are consistent with seismic constraints.

This work is supported by a grant from the NSF. Title: Determining the helium abundance of stellar envelopes Authors: Basu, S.; Antia, H. M.; Demarque, P. Bibcode: 2004ESASP.538..273B Altcode: 2004sshp.conf..273B Intermediate-degree mode solar oscillation have been used to determine the solar helium abundance to a high degree of precision. However, we cannot expect to observe these modes in other stars. We investigate whether low degree modes that should be available from space-based asteroseismology missions can be used to determine the helium abundance with sufficient precision. We find that the oscillatory signal in the frequencies caused by the depression in Γ1 in the second helium ionisation zone can be used to determine the envelope helium abundance of low mass stars. Title: Flare-Induced Excitation of Solar p modes Authors: Ambastha, Ashok; Basu, Sarbani; Antia, H. M. Bibcode: 2003SoPh..218..151A Altcode: Solar flares release large amounts of energy at different layers of the solar atmosphere, including at the photosphere in the case of exceptionally major events. Therefore, it is expected that large flares would be able to excite acoustic waves on the solar surface, thereby affecting the p-mode oscillation characteristics. We have applied the ring-diagram analysis technique to 3-D power spectra obtained for different flare regions in order to study how flares affect the amplitude, frequency and width of the acoustic modes. Data from the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO) has been used. We have used data obtained for several active regions of the current solar cycle that have produced flares. In most cases, during the period of high flare activity, power in p modes appears to be larger when compared to that in non-flaring regions of similar magnetic field strength. Title: Does the Sun Shrink with Increasing Magnetic Activity? Authors: Antia, H. M. Bibcode: 2003ApJ...590..567A Altcode: 2003astro.ph..4269A It has been demonstrated that frequencies of f-modes can be used to estimate the solar radius to a good accuracy. These frequencies have been used to study temporal variations in the solar radius with conflicting results. The variation in f-mode frequencies is more complicated than what is assumed in these studies. If a careful analysis is performed, then it turns out that there is no evidence for any variation in the solar radius. Title: Excitation of Solar p-mode Oscillations by Flares Authors: Ambastha, Ashok; Basu, Sarbani; Antia, H. M. Bibcode: 2003BASI...31..319A Altcode: Solar flares release large amounts of energy at different layers of the solar atmosphere. It is, therefore, expected that major flares would be able to excite waves, thereby affecting the p-mode oscillation characteristics. From the analysis of MDI data, we find that power in p-modes appears to increase for some flares, beyond the normal values expected from the influence of magnetic field. Title: Changes in Solar Dynamics from 1995 to 2002 Authors: Basu, Sarbani; Antia, H. M. Bibcode: 2003ApJ...585..553B Altcode: 2002astro.ph.11548B Data obtained by the GONG and MDI instruments over the last 7 years are used to study how solar dynamics-both rotation and other large scale flows-has changed with time. In addition to the well-known phenomenon of bands of faster and slower rotation moving toward the equator and pole, we find that the zonal flow pattern rises upward with time. Like the zonal flows, the meridional flows also show distinct solar activity-related changes. In particular, the antisymmetric component of the meridional flow shows a decrease in speed with activity. We do not see any significant temporal variations in the dynamics of the tachocline region where the solar dynamo is believed to be operating. Title: Temporal variations of solar structure Authors: Basu, Sarbani; Antia, H. M. Bibcode: 2003ESASP.517..231B Altcode: 2003soho...12..231B We have analysed GONG and MDI data for the past 7 years to determine if there are any changes in solar structure. We fail to find any change in the solar interior. In the process of investigations, we find that there are possible systematic differences between the pre- and post-recovery MDI data for the high degree (l >~ 120) modes. Title: Temporal variations in the rotation rate in the solar interior Authors: Basu, Sarbani; Antia, H. M. Bibcode: 2003ESASP.517..235B Altcode: 2003soho...12..235B The frequency splittings obtained from GONG and MDI observations over the last 7 years are used to study how the rotation rate of the solar convection zone has evolved with time. The bands of faster and slower than average rotation rate are found to move towards the equator at low latitudes, while at high latitudes they move towards the poles. The low latitude bands also move upwards with time, and they extend almost to the base of the convection zone. We find no significant temporal variation in the rotation rate in the tachocline region. Title: Effects of flares on solar oscillation characteristics Authors: Ambastha, Ashok; Basu, Sarbani; Antia, H. M. Bibcode: 2003ESASP.517..219A Altcode: 2003soho...12..219A We use ring diagram analysis to study the effects of solar flares on p-mode oscillation characteristics. We study the changes in the amplitude, frequency and width of acoustic modes using data before, during and after a few of the major flares during the current solar cycle. Mode power is found to be enhanced during and after some flares, though the enhancement is not seen in all flares. Title: On variation of the latitudinal structure of the solar convection zone Authors: Antia, H. M.; Chitre, S. M.; Thompson, M. J. Bibcode: 2003A&A...399..329A Altcode: 2002astro.ph.12095A The latitudinal sound-speed structure of the Sun's convection zone gives insight into the physical processes occurring there, specifically the cellular convection and possibly the presence of magnetic fields. Using helioseismic data from the GONG network and MDI instrument on SOHO, we map the latitudinal acoustic structure of the convection zone from 1995 to 2002. The temporally averaged structure confirms previous findings of an excess in sound speed at the 10-4 level at 60 degrees latitude. There also appear to be some variation with time, with the peak in sound-speed asphericity at 60 degrees growing towards the maximum of solar activity according to the MDI data. However, we present some evidence that such variation may be associated with instrumental variation between the epochs before and after SOHO was temporarily lost in 1998. Nonetheless, some genuine temporal variation may be present, and we discuss the possible physical causes of that. Title: Lectures on Solar Physics Authors: Antia, H. M.; Bhatnagar, A.; Ulmschneider, P. Bibcode: 2003LNP...619.....A Altcode: 2003lsp..conf.....A No abstract at ADS Title: Seismic Sun Authors: Chitre, S. M.; Antia, H. M. Bibcode: 2003dysu.book...36C Altcode: Helioseismology probes the internal structure and dynamics of the Sun with high precision. Frequencies of nearly half a million resonant modes of oscillations have been measured by the ground-based Global Oscillation Network Group (GONG) project and space-based Michelson Doppler Imager (MDI) on the SOHO spacecraft. Each of these modes is trapped in a different region of the solar interior and hence its frequency is sensitive to structure and dynamics in the corresponding region. Conversely, by combining the information from these large number of independent modes of solar oscillations, the inference is made of the structure and dynamics of the solar interior to unprecedented precision. These seismic data provide a test for solar models and theories of stellar structure and evolution. Title: Helioseismic Estimates of Solar Structure and Dynamics Authors: Basu, S.; Antia, H. M. Bibcode: 2003ASPC..293..250B Altcode: 2003tdse.conf..250B The only star whose structure and dynamics we know with confidence is the Sun. During the last decade, helioseismology has provided us a unique tool to study the solar interior. Helioseismology has been concerned with the radial structure and dynamics of the Sun for a long time. However, once high precision GONG and MDI data were available, the focus shifted to the latitudinal dependence, mainly of solar rotation. Attempts to determine the latitudinal dependence on structure came later. We discuss what helioseismology has revealed about the radial as well as the latitudinal dependence of the structure and dynamics of the Sun. Title: Solar Interior and Seismology Authors: Antia, H. M. Bibcode: 2003LNP...619...80A Altcode: 2003lsp..conf...80A Helioseismology is probing the internal structure and dynamics of the Sun with high precision. Frequencies of nearly half a million resonant modes of oscillations have been measured by the ground based Global Oscillation Network Group project and the space based Michelson Doppler Imager. Each of these modes is trapped in a different region of the solar interior and hence its frequency is sensitive to structure and dynamics in the corresponding region. Conversely, by combining the information from these large number of independent modes of solar oscillations it has become possible to infer the structure and dynamics of the solar interior to unprecedented precision. These seismic data have provided a test for solar models and theories of stellar structure, evolution and angular momentum transport. Interesting dynamical phenomena have been inferred from these data which are not understood. Some of these developments are described. Title: Seismic Study of Temporal Variations of Solar Magnetic Field Authors: Antia, H. M.; Chitre, S. M.; Thompson, M. J. Bibcode: 2003IAUJD..12E..30A Altcode: The temporal variations of acoustic frequencies of solar oscillations with the solar activity cycle are studied with a view to understanding the changes in the Sun's internal magnetic field. The data used are even a-coefficients from the GONG and MDI observations of the Sun's p-mode oscillations. These coefficients describe the modification of the frequencies by agents such as magnetic fields or latitudinal thermal perturbations that break the spherical symmetry of the Sun but that - unlike rotational advection and Coriolis forces - do not distinguish eastward- and westward-propagating acoustic waves. It is hoped that such a study will shed light on the mechanism of the solar dynamo. Title: Helioseismic limit on heavy element abundance Authors: Antia, H. M.; Chitre, S. M. Bibcode: 2002A&A...393L..95A Altcode: 2002astro.ph..9134A Primary inversions of accurately measured solar oscillation frequencies coupled with the equations of thermal equilibrium and other input physics, enable us to infer the temperature and hydrogen abundance profiles inside the Sun. These profiles also help in setting constraints on the input physics as well as on heavy element abundance in the solar core. Using different treatments of plasma screening for nuclear reaction rates, limits on the cross-section of proton-proton nuclear reaction as a function of heavy element abundance in the solar core are obtained and an upper limit on heavy element abundance in the solar core is also derived from these results. Title: Subsurface magnetic fields from helioseismology Authors: Antia, H. M. Bibcode: 2002ESASP.505...71A Altcode: 2002IAUCo.188...71A; 2002astro.ph..8339A; 2002solm.conf...71A Using even-order frequency splitting coefficients of global p-modes it is possible to infer the magnetic field in the solar interior as a function of radial distance and latitude. Results obtained using GONG and MDI data are discussed. While there is some signal of a possible magnetic field in the convection zone, there is little evidence for any temporal variation of the magnetic field in the solar interior. Limits on possible magnetic field in the solar core are also discussed. It is generally believed that the solar dynamo is located in the tachocline region. Seismic studies do not show any significant temporal variation in the tachocline region, though a significant latitudinal variation in the properties of the tachocline are found. There is some evidence to suggest that the latitudinal variation is not continuous and the tachocline may consist of two parts. Title: Seismic tests for solar models with tachocline mixing Authors: Brun, A. S.; Antia, H. M.; Chitre, S. M.; Zahn, J. -P. Bibcode: 2002A&A...391..725B Altcode: 2002astro.ph..6180B We have computed accurate 1-D solar models including both a macroscopic mixing process in the solar tachocline as well as up-to-date microscopic physical ingredients. Using sound speed and density profiles inferred through primary inversion of the solar oscillation frequencies coupled with the equation of thermal equilibrium, we have extracted the temperature and hydrogen abundance profiles. These inferred quantities place strong constraints on our theoretical models in terms of the extent and strength of our macroscopic mixing, on the photospheric heavy elements abundance, on the nuclear reaction rates such as S11 and S34 and on the efficiency of the microscopic diffusion. We find a good overall agreement between the seismic Sun and our models if we introduce a macroscopic mixing in the tachocline and allow for variation within their uncertainties of the main physical ingredients. From our study we deduce that the solar hydrogen abundance at the solar age is Xinv=0.732+/- 0.001 and that based on the 9Be photospheric depletion, the maximum extent of mixing in the tachocline is 5% of the solar radius. The nuclear reaction rate for the fundamental pp reaction is found to be S11(0)=4.06+/- 0.07 10-25 MeV barns, i.e., 1.5% higher than the present theoretical determination. The predicted solar neutrino fluxes are discussed in the light of the new SNO/SuperKamiokande results. Title: Changes in high-degree mode characteristics with magnetic activity Authors: Bogart, Richard S.; Basu, Sarbani; Antia, H. M. Bibcode: 2002ESASP.508..145B Altcode: 2002soho...11..145B We compare mode frequencies and other characteristics as determined from ring-diagram analysis of selected small regions of the Sun exhibiting strong variations in magnetic activity. These regions were observed with the Michelson Doppler Imager (MDI) on SOHO in its high-resolution mode during several years from solar minimum to maximum. To better understand the systematic uncertainties in fitting the ridges to the high-resolution data, we compare our results with those for the same regions concurrently observed in the MDI full-disc mode. We find that the properties of high degree p-modes are different in active and quiet regions and that the magnitude of the changes depend on the activity level. Title: The solar rotation rate from solar minimum to maximum Authors: Basu, Sarbani; Antia, H. M. Bibcode: 2002ESASP.508...59B Altcode: 2002soho...11...59B Frequency splittings obtained from GONG and MDI observations over the last 6 years are used to study how the rotation rate in the solar convection zone has evolved with time. The pole rotation rate is found to have had a minimum in 1999, distinctly before the maximum solar activity. The bands of faster and slower than average rotation rate are found to move towards the equator at low latitudes and towards the poles at high latitudes. Title: Temporal variations of the solar meridional flows from ring diagram analysis Authors: Basu, Sarbani; Antia, H. M. Bibcode: 2002ESASP.508..151B Altcode: 2002soho...11..151B We use MDI data obtained over the past six years to determine changes in solar meridional flows with time. We have used ring diagram analysis to study the flows. We also study the North-South antisymmetric component of solar rotation. We find distinct solar activity related changes in the meridional flows, in particular, the anti-symmetric component of the meridional flow shows a decrease in speed with time. Title: Latitudinal and temporal variations of the tachocline Authors: Basu, Sarbani; Antia, H. M. Bibcode: 2002ESASP.508...63B Altcode: 2002soho...11...63B Frequency splittings from GONG and MDI data over the last 6 years are used to study the temporal and latitudinal variations in the properties of the tachocline. In particular, we study changes in the position and the width of the tachocline. We find good evidence for latitudinal variations, but only marginal evidence for any temporal variation. The position of the tachocline at high latitudes may be varying slightly with time. Title: Seismic View of the Solar Interior Authors: Antia, H. M. Bibcode: 2002JApA...23....3A Altcode: The interior of the Sun is not directly observable to us. Nevertheless, it is possible to infer the physical conditions prevailing in the solar interior with the help of theoretical models coupled with observational input providedby measured frequencies of solar oscillations. The frequencies of these solar oscillations depend on the internal structure and dynamics of the Sun and from the knowledge of these frequencies it is possible to infer the internal structure as well as the large scale flows inside the Sun, in the same way as the observations of seismic waves on the surface of Earth help us in the study of its interior. With the accumulation of seismic data over the last six years it has also become possible to study temporal variations in the solar interior. Some of these seismic inferences would be described. Title: Effect of flares on solar oscillations characteristics Authors: Ambastha, Ashok; Basu, Sarbani; Antia, H. M. Bibcode: 2002ESASP.508...43A Altcode: 2002soho...11...43A We use ring diagram analysis to study the effect of solar flares on oscillation mode characteristics, using data from GONG+ and MDI. The data taken around the flares of June 6-7, 2000; March 29, 2001 and April 10-11, 2001 are studied. We find that during some flares, the power in acoustic modes increases beyond the normal values expected from the influence of magnetic field. Title: Global oscillation analysis of solar neutrino data with helioseismically constrained fluxes Authors: Choubey, Sandhya; Goswami, Srubabati; Kar, Kamales; Antia, H. M.; Chitre, S. M. Bibcode: 2001PhRvD..64k3001C Altcode: 2001hep.ph....6168C A seismic model for the Sun calculated using the accurate helioseismic data predicts a lower 8B neutrino flux as compared to the standard solar model (SSM). However, there persists a discrepancy between the predicted and measured neutrino fluxes and it seems necessary to invoke neutrino oscillations to explain the measurements. In this work, we have performed a global, unified oscillation analysis of the latest solar neutrino data (including the results of SNO charged current rate) using the seismic model fluxes as theoretical predictions. We determine the best-fit values of the neutrino oscillation parameters and the χ2min for both νeactive and νesterile cases and present the allowed parameter regions in the Δm2-tan2 θ plane for νeactive transition. The results are compared with those obtained using the latest SSM by Bahcall and co-workers. Title: Ring Diagram Analysis of the Characteristics of Solar Oscillation Modes in Active Regions Authors: Rajaguru, S. P.; Basu, Sarbani; Antia, H. M. Bibcode: 2001ApJ...563..410R Altcode: 2001astro.ph..8227R The presence of intense magnetic fields in and around sunspots is expected to modify solar structure and oscillation frequencies. Applying the ring diagram technique to data from the Michelson Doppler Imager on board the Solar and Heliospheric Observatory, we analyze the characteristics of high-degree f- and p-modes near active regions and compare them with the characteristics of the modes in quiet regions. As expected from earlier results, the f- and p-mode frequencies of high-degree modes are found to be significantly larger in magnetically active regions. In addition, we find that the power in both f- and p-modes is lower in active regions while the widths of the peaks are larger, indicating smaller lifetimes. We also find that the oscillation modes are more asymmetric in active regions than those in quiet regions, indicating that modes in active regions are excited closer to the surface. While the increase in mode frequency is monotonic in frequency, all other characteristics show more complex frequency dependences. Title: Solar-cycle variation of the sound-speed asphericity from GONG and MDI data 1995-2000 Authors: Antia, H. M.; Basu, S.; Hill, F.; Howe, R.; Komm, R. W.; Schou, J. Bibcode: 2001MNRAS.327.1029A Altcode: 2001astro.ph..9326A We study the variation of the frequency splitting coefficients describing the solar asphericity in both GONG and MDI data, and use these data to investigate temporal sound-speed variations as a function of both depth and latitude during the period 1995-2000 and a little beyond. The temporal variations in even splitting coefficients are found to be correlated to the corresponding component of magnetic flux at the solar surface. We confirm that the sound-speed variations associated with the surface magnetic field are superficial. Temporally averaged results show a significant excess in sound speed around r=0.92Rsolar and latitude of 60°. Title: Seismic study of stellar convective cores Authors: Mazumdar, A.; Antia, H. M. Bibcode: 2001A&A...377..192M Altcode: 2001astro.ph..7576M It has been shown that a discontinuity in the derivatives of the sound speed at the edge of the convective regions inside a star gives rise to a characteristic oscillatory signal in the frequencies of stellar oscillations. This oscillatory signal has been suggested as a means to study the base of the outer convection zone in low mass stars and possibly the outer edge of the convective core in high mass stars. Using stellar models we show that because of a phenomenon similar to aliasing in a Fourier transform, it may not be possible to use this signal to detect the convective core. Nevertheless, it may be possible to determine the size of convective cores using the frequency separation nu n+1,l-nu n,l. Title: Temporal Variations of the Solar Rotation Rate at High Latitudes Authors: Antia, H. M.; Basu, Sarbani Bibcode: 2001ApJ...559L..67A Altcode: 2001astro.ph..8226A Frequency splitting coefficients from Global Oscillation Network Group and Michelson Doppler Imager observations covering the period 1995-2001 are used to study temporal variations in the solar rotation rate at high latitudes. The torsional oscillation pattern in the Sun is known to penetrate to a depth of about 0.1 Rsolar with alternate bands of faster and slower rotating plasma. At lower latitudes, the bands move toward the equator with time. At higher latitudes, however, the bands appear to move toward the poles. This is similar to the observed poleward movement of large-scale magnetic fields at high latitudes. This also supports theoretical results of poleward-moving bands at high latitudes in some mean field dynamo models. The polar rotation rate is found to decrease between 1995 and 1999, after which it has started increasing. Title: A study of possible temporal and latitudinal variations in the properties of the solar tachocline Authors: Basu, Sarbani; Antia, H. M. Bibcode: 2001MNRAS.324..498B Altcode: 2001astro.ph..1314B Temporal variations of the structure and the rotation rate of the solar tachocline region are studied using helioseismic data from the Global Oscillation Network Group (GONG) and the Michelson Doppler Imager (MDI) obtained during the period 1995-2000. We do not find any significant temporal variation in the depth of the convection zone, the position of the tachocline or the extent of overshoot below the convection zone. No systematic variation in any other properties of the tachocline, like width, etc., is found either. The possibility of periodic variations in these properties is also investigated. Time-averaged results show that the tachocline is prolate with a variation of about 0.02Rsolar in its position. Neither the depth of the convection zone nor the extent of overshoot shows any significant variation with latitude. Title: High Resolution Analysis of the Upper Convection Zone Authors: Basu, S.; Bogart, R. S.; Schou, J.; Antia, H. M. Bibcode: 2001AGUSM..SP31A02B Altcode: Plane-wave (ring-diagram) analysis of high-degree modes in data from the SOI/MDI instrument on SOHO have enabled us us to determine the structure and dynamics of about 0.1 solar radius below the photosphere, and study how these quantities change with time. The bulk of the work done so far has utilized full-disc data for which the analysis has the ability to resolve structures with a scale of about 180 Mm. Additional data with three times the spatial resolution over a small portion of the disc are available for shorter durations at various times. Here we report on details of various trends which not resolved by full-disc data. Title: Seismic detection of stellar tachoclines Authors: Mazumdar, A.; Antia, H. M. Bibcode: 2001A&A...368L...8M Altcode: 2001astro.ph..2008M Helioseismic inversions for the rotation rate have established the presence of a tachocline near the base of the solar convection zone. We show that the tachocline produces a characteristic oscillatory signature in the splitting coefficients of low degree modes, which could be observed on distant stars. Using this signature it may be possible to determine the characteristics of the tachocline using only low degree modes. The limitations of this technique in terms of observational uncertainties are discussed, to assess the possibility of detecting tachoclines on distant stars. Title: How correlated are f-mode frequencies with solar activity? Authors: Antia, H. M.; Basu, Sarbani; Pintar, J.; Schou, Jesper Bibcode: 2001ESASP.464...27A Altcode: 2001soho...10...27A Temporal variations of solar f-mode frequencies are studied using data from the Global Oscillation Network Group (GONG) and the Michelson Doppler Imager (MDI) covering the period from 1995 to 2000. The frequencies show an increase with activity. There appears to be one component in the time varying part of the frequencies which is correlated with the solar activity indices. Superposed on this is an oscillatory variation with a period of 1 year, whose origin is not clear. The amplitudes of both the oscillatory and non-oscillatory component increases with the degree (and hence frequency) of the mode. Title: Studying asphericity in the solar sound speed from MDI and GONG data Authors: Antia, H. M.; Basu, S.; Hill, F.; Howe, R.; Komm, R. W.; Schou, J. Bibcode: 2001ESASP.464...45A Altcode: 2001soho...10...45A We study the variation of the frequency splitting coefficients describing the solar asphericity in both GONG and MDI data, and use these data to investigate temporal sound-speed variations as a function of both depth and latitude during the period 1995-2000. The temporal variations in even splitting coefficients are found to be correlated with the corresponding component of magnetic flux at the solar surface. The sound-speed variations associated with the surface magnetic field appear to be superficial. Temporally averaged results show a significant excess in sound speed around r = 0.92 Rsolar and latitude of 60°. Title: Spatially-resolved Analysis of the the Upper Covnection Zone Authors: Bogart, R. S.; Schou, J.; Basu, S.; Haber, D. A.; Hill, F.; Antia, H. M. Bibcode: 2001IAUS..203..183B Altcode: Plane-wave (ring-diagram) analysis of high-degree modes in data from the SOI/MDI instrument on SOHO permits us to determine spatial and temporal variations of the structure and dynamics of the upper convection zone, to a depth of about 0.1 solar radius below the photosphere. The spatial resolution achieved with full-disc data is at least 15 heliographic degrees (180 Mm), and the temporal resolution is of order 1 day. Data useful for such analysis cover at least two full Carrington rotations in each year since 1996. Additional data with three times the spatial resolution over a small portion of the disc are available for shorter durations at various times. Analyses of the full-disc data from the earlier years have already revealed systematic patterns in the global meridional flow and flows associated with active regions during the early phase of the solar cycle. Here we report on variations and trends seen in the flows as the activity level of the cycle approaches maximum. Title: Seismic constraints on neutrino oscillation parameters Authors: Goswami, Srubabati; Kar, Kamales; Antia, H. M.; Chitre, S. M. Bibcode: 2001ESASP.464..519G Altcode: 2001soho...10..519G The neutrino fluxes calculated using a seismically inferred solar model are compared with measured fluxes from the three solar neutrino experiments. Treating the neutrino fluxes from seismic model as theoretical predictions, the latest solar neutrino data is analyzed assuming vacuum oscillation of neutrinos. The best-fit values of the neutrino mixing angle and mass squared difference are found and the allowed regions are determined. Title: Seismic investigation of changes in the rotation rate in the solar interior Authors: Basu, Sarbani; Antia, H. M. Bibcode: 2001ESASP.464..179B Altcode: 2001soho...10..179B Frequency splitting coefficients from Global Oscillation Network Group (GONG) and Michelson Doppler Imager (MDI) data obtained during the period 1995-2000 are used to study temporal variations in the solar rotation rate. The torsional oscillation pattern in the Sun is known to penetrate to depths greater than the subsurface shear layer seen in rotation inversions. We study temporal and latitudinal variations in the properties of this shear layer. We also investigate the reported periodic variations of the rotation rate in the tachocline region in an attempt to test the results independently. Title: A study of temporal variations of the tachocline Authors: Basu, Sarbani; Antia, H. M. Bibcode: 2001ESASP.464..297B Altcode: 2001soho...10..297B Temporal variations of the structure and rotation rate in the solar tachocline region are studied using helioseismic data from the Global Oscillation Network Group (GONG) and the Michelson Doppler Imager (MDI) obtained during the period 1995-2000. We do not find any significant temporal variation in the depth of the convection zone, the position or thickness of the tachocline or the extent of overshoot below the convection zone. We find evidence to suggest that the mean position of tachocline becomes shallower with increase in latitude, strengthening earlier results. The convection zone depth is found to be essentially independent of latitude. Title: Characteristics of high degree p-modes using ring diagram analyses Authors: Basu, Sarbani; Antia, H. M.; Bogart, R. S. Bibcode: 2001ESASP.464..183B Altcode: 2001soho...10..183B We study the properties of high-degree p-modes using ring diagram analyses. Ring diagrams produced from full-disc Doppler velocity, continuum and line-depth images of the Sun obtained by the Michelson Doppler Imager (MDI) are studied to check how mode characteristics such as asymmetry, line-width etc. vary with the type of observable used for producing the spectra. We have selected data from a low solar activity period to ensure that the activity-related effects do not influence our conclusions. Title: Temporal Variations of the Rotation Rate in the Solar Interior Authors: Antia, H. M.; Basu, Sarbani Bibcode: 2000ApJ...541..442A Altcode: 2000astro.ph..4335A The temporal variations of the rotation rate in the solar interior are studied using frequency splittings from Global Oscillations Network Group (GONG) data obtained during the period 1995-1999. We find alternating latitudinal bands of faster and slower rotation that appear to move toward the equator with time-similar to the torsional oscillations seen at the solar surface. This flow pattern appears to persist to a depth of about 0.1 Rsolar, and in this region its magnitude is well correlated with solar activity indices. We do not find any periodic or systematic changes in the rotation rate near the base of the convection zone. Title: The Sun's acoustic asphericity and magnetic fields in the solar convection zone Authors: Antia, H. M.; Chitre, S. M.; Thompson, M. J. Bibcode: 2000A&A...360..335A Altcode: 2000astro.ph..5587A The observed splittings of solar oscillation frequencies can be employed to separate the effects of internal solar rotation and to estimate the contribution from a large-scale magnetic field or any latitude- dependent thermal perturbation inside the Sun. The surface distortion estimated from the rotation rate in the solar interior is found to be in good agreement with the observed oblateness at solar surface. After subtracting out the estimated contribution from rotation, there is some residual signal in the even splitting coefficients, which may be explained by a magnetic field of approximately 20 kG strength located at a depth of 30000 km below the surface or an equivalent aspherical thermal perturbation. An upper limit of 300 kG is derived for a toroidal field near the base of the convection zone. Title: Helioseismic Search for Magnetic Field in the Solar Interior Authors: Antia, H. M.; Chitre, S. M.; Thompson, M. J. Bibcode: 2000JApA...21..343A Altcode: No abstract at ADS Title: Temporal Variation of Large Scale Flows in the Solar Interior Authors: Basu, S.; Antia, H. M. Bibcode: 2000JApA...21..353B Altcode: No abstract at ADS Title: Studying Asphericity in the Solar Sound Speed from MDI and GONG Data 1995-1999 Authors: Schou, J.; Antia, H. M.; Basu, S.; Howe, R.; Hill, F.; Komm, R. W. Bibcode: 2000SPD....31.0111S Altcode: 2000BAAS...32..803S We study the variation of the frequency splitting coefficients describing the solar asphericity in both GONG and MDI data, and use these data to investigate temporal sound-speed variations as a function of both depth and latitude during the period 1995--99. We confirm that the sound-speed variations associated with the surface magnetic field are superficial. Title: Helioseismic constraints on the proton-proton reaction cross-section Authors: Antia, H. M.; Chitre, S. M. Bibcode: 2000BASI...28..105A Altcode: No abstract at ADS Title: Effect of Asymmetry in Peak Profiles on Solar Oscillation Frequencies Authors: Basu, Sarbani; Antia, H. M. Bibcode: 2000ApJ...531.1088B Altcode: 1999astro.ph.11007B Most helioseismic analyses are based on solar oscillation frequencies obtained by fitting symmetric peak profiles to the power spectra. However, it has now been demonstrated that the peaks are not symmetric. In this work we study the effects of the asymmetry of the peak profiles on the solar oscillation frequencies of p-modes for low and intermediate degrees. We also investigate how the resulting shift in frequencies affects helioseismic inferences. Title: Possible solar cycle variations in the convection zone Authors: Basu, Sarbani; Antia, H. M. Bibcode: 2000SoPh..192..449B Altcode: 2000astro.ph..1444B Using data from the Global Oscillations Network Group (GONG) that covers the period from 1995 to 1998 we study the change in frequencies of solar oscillations with solar activity. From these frequencies we attempt to determine any possible variation in solar structure with solar activity. We do not find any evidence of a change in the convection zone depth or extent of overshoot below the convection zone during the solar cycle. Title: Effect of asymmetry in peak profile on f-mode frequencies Authors: Antia, H. M.; Basu, Sarbani Bibcode: 2000BASI...28...97A Altcode: No abstract at ADS Title: Solar Cycle Variation in Solar f-Mode Frequencies and Radius Authors: Antia, H. M.; Basu, Sarbani; Pintar, J.; Pohl, B. Bibcode: 2000SoPh..192..459A Altcode: 2000astro.ph..1293A Using data from the Global Oscillation Network Group (GONG) covering the period from 1995 to 1998, we study the change with solar activity in solar f-mode frequencies. The results are compared with similar changes detected from the Michelson Doppler Imager (MDI) data. We find variations in f-mode frequencies which are correlated with solar activity indices. If these changes are due to variation in solar radius then the implications are that the solar radius decreases by about 5 km from minimum to maximum activity. Title: Solar Cycle Variations of Large-Scale Flows in the sun Authors: Basu, Sarbani; Antia, H. M. Bibcode: 2000SoPh..192..469B Altcode: 2000astro.ph..1294B Using data from the Michelson Doppler Imager (MDI) instrument on board the Solar and Heliospheric Observatory (SOHO), we study the large-scale velocity fields in the outer part of the solar convection zone using the ring diagram technique. We use observations from four different times to study possible temporal variations in flow velocity. We find definite changes in both the zonal and meridional components of the flows. The amplitude of the zonal flow appears to increase with solar activity and the flow pattern also shifts towards lower latitude with time. Title: Rotation rate and flows in the solar interior Authors: Antia, H. M. Bibcode: 2000BASI...28...75A Altcode: The rotation rate in the solar interior can be inferred from measured splittings of solar oscillation frequencies using various inversion techniques. While other large scale flows can be studied using local techniques, like ring diagrams or time-distance helioseismology. These techniques can be used to study variation in flow velocities in the three spatial dimensions as well as the temporal variations. Title: Effect of a magnetic field on solar oscillation frequencies Authors: Antia, H. M.; Chitre, S. M. Bibcode: 2000BASI...28..101A Altcode: No abstract at ADS Title: Seismic Sun Authors: Chitre, S. M.; Antia, H. M. Bibcode: 1999CSci...77.1454C Altcode: No abstract at ADS Title: Large-Scale Flows in the Solar Interior: Effect of Asymmetry in Peak Profiles Authors: Basu, Sarbani; Antia, H. M. Bibcode: 1999ApJ...525..517B Altcode: 1999astro.ph..6252B Ring diagram analysis can be used to study large-scale velocity fields in the outer part of the solar convection zone. All previous works assume that the peak profiles in the solar oscillation power spectrum are symmetrical. However, it has now been demonstrated that the peaks are not symmetrical. In this work we study how the explicit use of asymmetrical peak profiles in ring diagram analysis influences the estimated velocity fields. We find that the use of asymmetrical profiles leads to significant improvement in the fits, but the estimated velocity fields are not substantially different from those obtained using a symmetrical profile to fit the peaks. The resulting velocity fields are compared with those obtained by other investigators. Title: Can Naked Singularities Yield Gamma-ray Bursts? Authors: Antia, H. M. Bibcode: 1999GReGr..31.1675A Altcode: 1998gr.qc.....7019A Gamma-ray bursts are believed to be the most luminous objects in the Universe. There has been some suggestion that these arise from quantum processes around naked singularities. The main problem with this suggestion is that all known examples of naked singularities are massless and hence there is effectively no source of energy. It is argued that a globally naked singularity coupled with quantum processes operating within a distance of the order of Planck length of the singularity will probably yield energy burst of the order of M_pc^2\approx2\times 10^{16} ergs, where M_p is the Planck mass. Title: Limits on the proton-proton reaction cross-section from helioseismology Authors: Antia, H. M.; Chitre, S. M. Bibcode: 1999A&A...347.1000A Altcode: 1999astro.ph..6007A Primary inversions of solar oscillation frequencies coupled with the equations of thermal equilibrium and other input physics, enable us to infer the temperature and hydrogen abundance profiles inside the Sun. These profiles also help in setting constraints on the input physics that is consistent with the accurately measured oscillation frequencies data. Helioseismic limits on the cross-section of proton-proton nuclear reaction as a function of heavy element abundance in the solar core are derived. We demonstrate that it is not possible to infer the heavy element abundance profile, in addition to temperature and hydrogen abundance profiles, with the helioseismic constraints. Title: High-Frequency and High-Wavenumber Solar Oscillations Authors: Antia, H. M.; Basu, Sarbani Bibcode: 1999ApJ...519..400A Altcode: 1998astro.ph.11154A We determine the frequencies of solar oscillations covering a wide range of degree (100<l<4000) and frequency (1.5<ν<10 mHz) using the ring diagram technique applied to power spectra obtained from Michelson Doppler Imager (MDI) data. The f-mode ridge extends up to l~3000, where the line width becomes very large, which implies a damping time that is comparable to the time period. The frequencies of high-degree f-modes are significantly different from those given by the simple dispersion relation ω2=gk. The f-mode peaks in power spectra are distinctly asymmetric, and use of an asymmetric profile increases the fitted frequency, bringing it closer to the frequencies computed for a solar model. Title: Influence of surface layers on the seismic estimate of the solar radius Authors: Tripathy, S. C.; Antia, H. M. Bibcode: 1999SoPh..186....1T Altcode: Possible systematic errors in determining the solar radius from the f-mode frequencies are studied to find that the input physics governing the structure of outermost layers of the Sun has significant influence on the estimated radius. It is investigated how treatment of convection and low temperature opacity may influence the determination of radius and it is found that it may be changed by as much as 100 km. The best estimate of the solar radius is found to be 695.77±0.1 Mm, where the error bars represent estimate of systematic errors, while the statistical errors are very small (≈1 km). Title: Ring Diagram Analysis of Near-Surface Flows in the Sun Authors: Basu, Sarbani; Antia, H. M.; Tripathy, S. C. Bibcode: 1999ApJ...512..458B Altcode: 1998astro.ph..9309B Ring diagram analysis of solar oscillation power spectra obtained from Michelson Doppler Imager data is carried out to study the velocity fields in the outer part of the solar convection zone. The three-dimensional power spectra are fitted to a model that has a Lorentzian profile in frequency and includes the advection of the wave front by horizontal flows in order to obtain the two components of the subsurface flows as a function of the horizontal wave number and radial order of the oscillation modes. This information is then inverted using the optimally localized averages method and regularized least squares method to infer the variation in horizontal flow velocity with depth. The average rotation velocity at different latitudes obtained by this technique agrees reasonably with helioseismic estimates made using frequency-splitting data. The shear layer just below the solar surface appears to consist of two parts, with the outer part measuring up to a depth of 4 Mm where the velocity gradient does not show any reversal up to a latitude of 60°. In the deeper part the velocity gradient shows reversal in sign around a latitude of 55°. The zonal flow velocities inferred in the outermost layers appear to be similar to those obtained by other measurements. A meridional flow from equator poleward is found. It has a maximum amplitude of about 30 m s-1 near the surface, and the amplitude is nearly constant in the outer shear layer. Title: Possible Solar Cycle Variation in the Convection Zone Authors: Basu, Sarbani; Antia, H. M. Bibcode: 1999soho....9E..38B Altcode: Using GONG data covering the period from 1995 to 1998 we study the change in frequencies of solar oscillations with solar activity. From these frequencies we find the depth of the solar convection zone as well as the extent of overshoot below the solar convection zone and attempt to determine any possible variation with solar activity. Title: Effect of Asymmetry in Peak Profiles on Ring Diagram Analysis of Large Scale Flows in the Solar Interior Authors: Basu, Sarbani; Antia, H. M. Bibcode: 1999soho....9E..37B Altcode: Ring diagram analysis has been extensively used in studying large scale flows in outer part of the solar convection zone. Most of these studies assume that the peaks in the solar oscillation power spectra are symmetric. Since it has been demonstrated that the peaks are not symmetric, we study how the explicit use of asymmetric peak profiles in ring-diagram analysis influences the estimated velocity fields. Title: Can 3He redistribution solve the solar neutrino problem ? Authors: Antia, H. M.; Chitre, S. M. Bibcode: 1999BASI...27...69A Altcode: No abstract at ADS Title: Solar Cycle Variation in the F-Mode Frequencies and Solar Radius Authors: Antia, H. M.; Basu, Sarbani; Pintar, J.; Pohl, B. Bibcode: 1999soho....9E..33A Altcode: Using GONG data covering the period from 1995 to 1998 we study the change in frequencies of f-modes with solar cycle. The results are compared with similar changes detected in MDI data. Considering the estimated errors in the measured frequencies, it should in principle, be possible to determine changes in solar radius by a few kilometers over the solar cycle. We examine the possibility of these frequency changes arising from likely changes in solar radius. Title: Determination of temperature and chemical composition profiles in the solar interior from seismic models Authors: Antia, H. M.; Chitre, S. M. Bibcode: 1998A&A...339..239A Altcode: 1997astro.ph.10159A The primary inversion of the solar oscillation frequencies coupled with the equations of thermal equilibrium and input physics enable us to infer the temperature and hydrogen abundance profiles inside the Sun. The inferred hydrogen abundance profile is smoother than that in a solar model with conventional treatment of diffusion, in the region just beneath the solar convection zone (rga0 .68R_sun). Such a mixing process could account for the observed low lithium abundance in the solar envelope. It is also possible to constrain the nuclear reaction rates using the inferred temperature and hydrogen abundance profiles. The helioseismically estimated cross-section for pp nuclear reaction turns out to be (4.15+/-0.25)*E(-25) MeV barns, where the error estimates include those from opacities arising from up to 50% uncertainty in heavy element abundance Z. Title: Helioseismic Studies of Differential Rotation in the Solar Envelope by the Solar Oscillations Investigation Using the Michelson Doppler Imager Authors: Schou, J.; Antia, H. M.; Basu, S.; Bogart, R. S.; Bush, R. I.; Chitre, S. M.; Christensen-Dalsgaard, J.; Di Mauro, M. P.; Dziembowski, W. A.; Eff-Darwich, A.; Gough, D. O.; Haber, D. A.; Hoeksema, J. T.; Howe, R.; Korzennik, S. G.; Kosovichev, A. G.; Larsen, R. M.; Pijpers, F. P.; Scherrer, P. H.; Sekii, T.; Tarbell, T. D.; Title, A. M.; Thompson, M. J.; Toomre, J. Bibcode: 1998ApJ...505..390S Altcode: The splitting of the frequencies of the global resonant acoustic modes of the Sun by large-scale flows and rotation permits study of the variation of angular velocity Ω with both radius and latitude within the turbulent convection zone and the deeper radiative interior. The nearly uninterrupted Doppler imaging observations, provided by the Solar Oscillations Investigation (SOI) using the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO) spacecraft positioned at the L1 Lagrangian point in continuous sunlight, yield oscillation power spectra with very high signal-to-noise ratios that allow frequency splittings to be determined with exceptional accuracy. This paper reports on joint helioseismic analyses of solar rotation in the convection zone and in the outer part of the radiative core. Inversions have been obtained for a medium-l mode set (involving modes of angular degree l extending to about 250) obtained from the first 144 day interval of SOI-MDI observations in 1996. Drawing inferences about the solar internal rotation from the splitting data is a subtle process. By applying more than one inversion technique to the data, we get some indication of what are the more robust and less robust features of our inversion solutions. Here we have used seven different inversion methods. To test the reliability and sensitivity of these methods, we have performed a set of controlled experiments utilizing artificial data. This gives us some confidence in the inferences we can draw from the real solar data. The inversions of SOI-MDI data have confirmed that the decrease of Ω with latitude seen at the surface extends with little radial variation through much of the convection zone, at the base of which is an adjustment layer, called the tachocline, leading to nearly uniform rotation deeper in the radiative interior. A prominent rotational shearing layer in which Ω increases just below the surface is discernible at low to mid latitudes. Using the new data, we have also been able to study the solar rotation closer to the poles than has been achieved in previous investigations. The data have revealed that the angular velocity is distinctly lower at high latitudes than the values previously extrapolated from measurements at lower latitudes based on surface Doppler observations and helioseismology. Furthermore, we have found some evidence near latitudes of 75° of a submerged polar jet which is rotating more rapidly than its immediate surroundings. Superposed on the relatively smooth latitudinal variation in Ω are alternating zonal bands of slightly faster and slower rotation, each extending some 10° to 15° in latitude. These relatively weak banded flows have been followed by inversion to a depth of about 5% of the solar radius and appear to coincide with the evolving pattern of ``torsional oscillations'' reported from earlier surface Doppler studies. Title: Solar internal rotation rate and the latitudinal variation of the tachocline Authors: Antia, H. M.; Basu, Sarbani; Chitre, S. M. Bibcode: 1998MNRAS.298..543A Altcode: 1997astro.ph..9083A A new set of accurately measured frequencies of solar oscillations is used to infer the rotation rate inside the Sun, as a function of radial distance as well as latitude. We have adopted a regularized least-squares technique with iterative refinement for both 1.5D inversion, using the splitting coefficients, and 2D inversion using individual m splittings. The inferred rotation rate agrees well with earlier estimates showing a shear layer just below the surface and another one around the base of the convection zone. The tachocline or the transition layer where the rotation rate changes from differential rotation in the convection zone to an almost latitudinally independent rotation rate in the radiative interior is studied in detail. No compelling evidence for any latitudinal variation in the position and width of the tachocline is found, although it appears that the tachocline probably shifts to a slightly larger radial distance at higher latitudes and possibly also becomes thicker. However, these variations are within the estimated errors and more accurate data would be needed to make a definitive statement about latitudinal variations. Title: Estimate of solar radius from f-mode frequencies Authors: Antia, H. M. Bibcode: 1998A&A...330..336A Altcode: 1997astro.ph..7226A Frequency and rotational splittings of the solar f-modes are estimated from the GONG data. Contrary to earlier observations the frequencies of f-modes are found to be close to the theoretically computed values for a standard solar model. The f-mode being essentially a surface mode is a valuable diagnostic probe of the properties of the solar surface, and also provides an independent measure of solar radius. The estimated solar radius is found to be about 0.03% less than what is traditionally used in construction of standard solar models. If this decrease in solar radius is confirmed then the current solar models as well as inversion results will need to be revised. The rotational splittings of the f-modes yield an independent measure of the rotation rate near the solar surface, which is compared with other measurements. Title: Latitudinal Variations in the Properties of the Tachocline Authors: Basu, Sarbani; Antia, H. M. Bibcode: 1998ESASP.418..711B Altcode: 1998soho....6..711B The tachocline or the transition layer where the rotation rate changes from differential rotation in the convection zone to almost latitudinally independent rotation rate in the radiative interior is studied using recent data on splitting of solar p-modes from MDI. Attempt is made to study the latitudinal variation in position and thickness of tachocline using forward modelling techniques. Title: Ring Diagram Analysis of Velocity Fields within the Solar Convection Zone Authors: Basu, Sarbani; Antia, H. M.; Tripathy, S. C. Bibcode: 1998ESASP.418..705B Altcode: 1998soho....6..705B; 1998astro.ph..6308B Ring diagram analysis of solar oscillation power spectra obtained from GONG and MDI data is performed to study the velocity fields within the solar convection zone. The three dimensional power spectra are fitted to a model with Lorentzian profile in frequency and includes the advection of the wave front by horizontal flows. We obtain the two horizontal components of flows as a function of the horizontal wave number and radial order of the oscillation modes. This information is then inverted using the OLA and RLS techniques to infer the variation in flow velocity with depth. Title: Helioseismic inferences from the GONG data Authors: Antia, H. M. Bibcode: 1998BASI...26..149A Altcode: No abstract at ADS Title: Helioseismology and the solar neutrino problem Authors: Antia, H.; Chitre, S. Bibcode: 1998IAUS..185...41A Altcode: The accurately measured frequencies of solar oscillations can be inverted to determine the profiles of sound speed and density through a large part of the Sun's interior. This acoustic structure can be used to obtain the temperature and chemical composition profiles inside the Sun and also to calculate the expected neutrino fluxes. In the framework of standard neutrino physics, but with the allowance of arbitrary variations in the input opacities and even relaxation of the thermal equilibrium condition, it turns out to be difficult to produce a seismic model that is simultaneously consistent with any two of the existing solar neutrino experiments. It is therefore tempting to suggest that the low observed fluxes of solar neutrinos should be attributed to nonstandard neutrino physics. Title: What do solar f-mode frequencies tell us? Authors: Antia, H. M.; Chitre, S. M. Bibcode: 1998IAUS..185..165A Altcode: Frequency and rotational splittings of the solar f-modes are estimated from the GONG data. The f-mode being essentially a surface mode should provide a valuable diagnostic probe of the properties of the solar surface. Contrary to earlier observations the frequencies of f-modes are found to be close to the theoretically computed values for a standard solar model. It is therefore desirable to investigate the systematic differences between various observations. The rotational splittings of the f-modes yield an independent measure of the rotation rate near the solar surface, which can be compared with other measurements. The inclusion of the f-mode splittings in rotation inversions thus improves the inverted rotation profiles in the near surface region. Title: Probes of the solar interior Authors: Chitre, S. M.; Antia, H. M. Bibcode: 1998BASI...26..143C Altcode: No abstract at ADS Title: The SOI-MDI High-Latitude Jet: the Evidence For and Against Authors: Howe, R.; Antia, H.; Basu, S.; Christensen-Dalsgaard, J.; Korzennik, S. G.; Schou, J.; Thompson, M. J. Bibcode: 1998ESASP.418..803H Altcode: 1998soho....6..803H The apparent detection of a prograde jet at latitude 75-circ and at a radius of about 0.95Rodot in some inversions of rotation data from SOI--MDI (Schou et al., 1998) has excited considerable interest, but whether the jet really exists in the solar interior is certainly not yet firmly established. The detection of the feature is sensitive both to the inversion techniques used and to the methods of mode parameter estimation used to generate the input data. In particular, the feature is much more apparent in Regularized Least-Squares inversions than in inversions using an Optimally Localized Average approach, and is not detected at all in the present GONG data when analysed with the GONG peakfinding algorithm, or indeed in SOI data when analysed with the GONG algorithm. Therefore in this poster we examine critically the current evidence for the source and existence of this jet in the light of forward and inverse analyses. Title: Effect of turbulent pressure on solar oscillation frequencies Authors: Antia, H. M.; Basu, S. Bibcode: 1997ASSL..225...51A Altcode: 1997scor.proc...51A We use observed frequencies of solar $p$-modes to test different formulations for calculating the convective flux. In particular, models using the usual mixing length theory and the formulation of Canuto and Mazzitelli for calculating the convective flux are compared to find that the latter yields frequencies that are closer to observed values. Inclusion of turbulent pressure is also found to improve the agreement with observed frequencies, but the magnitude of the difference is much smaller. Title: Rotation and Zonal Flows in the Solar Envelope from the SOHO/MDI Observations Authors: Scherrer, P. H.; Schou, J.; Bogart, R. S.; Bush, R. I.; Hoeksema, J. T.; Kosovichev, A. G.; Antia, H. M.; Chitre, S. M.; Christensen-Dalsgaard, J.; Larsen, R. M.; Pijpers, F. P.; Eff-Darwich, A.; Korzennik, S. G.; Gough, D. O.; Sekii, T.; Howe, R.; Tarbell, T.; Title, A. M.; Thompson, M. J.; Toomre, J. Bibcode: 1997AAS...191.7310S Altcode: 1997BAAS...29.1322S We report on the latest inferences concerning solar differential rotation that have been drawn from the helioseismic data that are now available from the Solar Oscillations Investigation (SOI) using the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO). That spacecraft is positioned in a halo orbit near the Sun-Earth Lagrangian point L_1, in order to obtain continuous Doppler-imaged observations of the sun with high spatial fidelity. Doppler velocity, intensity and magnetic field images are recorded, based on modulations of the 676.8 nm Ni I solar absorption line. The high spatial resolution of MDI thereby permits the study of many millions of global resonant modes of solar oscillation. Determination and subsequent inversion of the frequencies of these modes, including the degeneracy-splitting by the rotation of the sun, enables us to infer how the sun's angular velocity varies throughout much of the interior. The current MDI data are providing substantial refinements to the helioseismic deductions that can be made about differential rotation both within the convection zone and in its transition to the radiative interior. The shearing layer evident in the angular velocity Omega just below the solar surface is becoming better defined, as is the adjustment layer or tachocline near the base of the convection zone. The MDI data are also revealing a prominent decrease in Omega at high latitudes from the rotation rate expressed by a simple three-term expansion in latitude that was originally deduced from surface Doppler measurements. Further, there are indications that a submerged polar vortex involving somewhat faster Omega than its surroundings exists at about 75(deg) in latitudes. Title: Helioseismic models and solar neutrino fluxes Authors: Antia, H. M.; Chitre, S. M. Bibcode: 1997MNRAS.289L...1A Altcode: An inversion of the accurately measured frequencies of solar oscillations leads to a determination of the profiles of sound speed and density through most of the solar interior. This seismically inferred structure can be used to obtain the temperature and chemical abundance profiles inside the Sun and also to calculate expected neutrino fluxes. Even allowing for arbitrary variations in the input opacities and relaxing the requirement of thermal equilibrium, but assuming standard neutrino properties, it turns out to be difficult to construct a seismic model that is simultaneously consistent with any two of the three existing solar neutrino experiments. This conclusion is in agreement with similar results derived from general considerations, independently of any solar model. It therefore seems that non-standard neutrino physics is very likely to be responsible for low observed fluxes of solar neutrinos. Title: Seismic measurement of the depth of the solar convection zone Authors: Basu, Sarbani; Antia, H. M. Bibcode: 1997MNRAS.287..189B Altcode: The observed frequencies of solar oscillations have been used to determine the depth of the convection zone. The effect of diffusion of helium and heavy elements on this measurement is studied and it is found that the discontinuity in the composition gradient at the base of the CZ due to diffusion gives rise to some systematic errors in this measurement. Taking into account these systematic errors the base of the CZ is estimated to be at a radial distance of (0.713+/-0.001)R_solar. Further, the estimated opacity at the base of the CZ is found to be consistent with that calculated from the OPAL opacity tables using the current value of Z/X. Assuming that the OPAL tables correctly represent the opacity of solar material the surface Z/X is estimated to be 0.0245+/-0.0008. Title: Seismic investigation of the solar structure using GONG frequencies Authors: Tripathy, S. C.; Antia, H. M.; Hill, F.; Ambastha, A. Bibcode: 1997astro.ph..3179T Altcode: Using the recently obtained GONG frequencies, we investigate the properties of the solar interior by constructing solar models with various input physics like opacities, equation of state, nuclear reaction rates etc. The differential asymptotic inversion technique is then used to infer the relative difference in sound speed between the Sun and solar models. Here we apply these results to test equation of state and different formulation for calculating the convective flux. Title: Admissibility of initial data in spherical collapse Authors: Antia, H. M. Bibcode: 1997gr.qc.....2029A Altcode: Gravitational collapse of a spherically symmetric cloud has been extensively studied to investigate the nature of resulting singularity. However, there has been considerable debate about the admissibility of certain initial density distributions. Using the Newtonian limit of the equations governing collapse of a fluid with an equation of state p=p(\rho) it is shown that the density distribution has to be even function of r in a spherically symmetric situation provided dp/d\rho \ne 0, even in comoving coordinates. We show that recent claim by Singh that the discrepancy pointed out earlier is due to their use of comoving coordinates is totally incorrect. It is surprising that he expects the use of comoving coordinates to make any difference in this matter. It is also argued that strong curvature naked singularities in gravitational collapse of spherically symmetric dust do not violate the cosmic censorship hypothesis. Title: Naked singularities and admissibility of initial conditions Authors: Antia, H. M. Bibcode: 1997gr.qc.....1023A Altcode: Gravitational collapse of a spherically symmetric cloud has been extensively studied to investigate the nature of resulting singularity. However, there has been considerable debate about the admissibility of certain initial density distributions. Using the Newtonian limit of the equations governing collapse of a fluid with an equation of state $p=p(\rho)$ it is shown that the density distribution has to be even function of r in a spherically symmetric situation provided $dp/d\rho\ne0$. Implications of this result on formation of strong naked singularities are examined. Title: The seismic structure of the Sun from GONG Authors: Anderson, E.; Antia, H. M.; Basu, S.; Chaboyer, B.; Chitre, S. M.; Christensen-Dalsgaard, J.; Eff-Darwich, A.; Elliott, J. R.; Giles, P. M.; Gough, D. O.; Guzik, J. A.; Harvey, J. W.; Hill, F.; Leibacher, J. W.; Kosovichev, A. G.; Monteiro, M. J. P. F. G.; Richard, O.; Sekii, T.; Shibahashi, H.; Takata, M.; Thompson, M. J.; Toomre, J.; Vauclair, S.; Vorontsov, S. V. Bibcode: 1997IAUS..181..151A Altcode: No abstract at ADS Title: Seismology of the solar core Authors: Antia, H. M.; Chitre, S. M. Bibcode: 1996BASI...24..321A Altcode: No abstract at ADS Title: Bulletin of the Astronomical Society of India Authors: Krishan, Vinod; Bhatt, H. C.; Antia, H. M.; Chitre, S. M. Bibcode: 1996STIN...9635971K Altcode: This issue of the bulletin of the Astronomical Society of India contains papers presented at the International Conference on 'Windows on the Sun's Interior'. Although the internal layers of the Sun are not directly observable, it is possible to deduce the structural properties and dynamic processes operating in the solar interior. Two diagnostic tools are (1) neutrinos generated in the nuclear reaction network operating in the deep solar core which yields the measured neutrino counting rate, and (2) helio-seismology. Also, data from the Global Oscillation Network Group (GONG) and the Solar and Heliospheric Observatory (SOHO) should help clarify a number of outstanding problems in solar physics. Title: Windows on the Sun's interior. Proceedings. International Conference on Windows on the Sun's Interior, Bombay (India), 19 - 21 Oct 1995. Authors: Antia, H. M.; Chitre, S. M. Bibcode: 1996BASI...24...87A Altcode: The following topics were dealt with: solar structure; helioseismology, solar neutrinos. Title: The Current State of Solar Modeling Authors: Christensen-Dalsgaard, J.; Dappen, W.; Ajukov, S. V.; Anderson, E. R.; Antia, H. M.; Basu, S.; Baturin, V. A.; Berthomieu, G.; Chaboyer, B.; Chitre, S. M.; Cox, A. N.; Demarque, P.; Donatowicz, J.; Dziembowski, W. A.; Gabriel, M.; Gough, D. O.; Guenther, D. B.; Guzik, J. A.; Harvey, J. W.; Hill, F.; Houdek, G.; Iglesias, C. A.; Kosovichev, A. G.; Leibacher, J. W.; Morel, P.; Proffitt, C. R.; Provost, J.; Reiter, J.; Rhodes, E. J., Jr.; Rogers, F. J.; Roxburgh, I. W.; Thompson, M. J.; Ulrich, R. K. Bibcode: 1996Sci...272.1286C Altcode: Data from the Global Oscillation Network Group (GONG) project and other helioseismic experiments provide a test for models of stellar interiors and for the thermodynamic and radiative properties, on which the models depend, of matter under the extreme conditions found in the sun. Current models are in agreement with the helioseismic inferences, which suggests, for example, that the disagreement between the predicted and observed fluxes of neutrinos from the sun is not caused by errors in the models. However, the GONG data reveal subtle errors in the models, such as an excess in sound speed just beneath the convection zone. These discrepancies indicate effects that have so far not been correctly accounted for; for example, it is plausible that the sound-speed differences reflect weak mixing in stellar interiors, of potential importance to the overall evolution of stars and ultimately to estimates of the age of the galaxy based on stellar evolution calculations. Title: The Seismic Structure of the Sun Authors: Gough, D. O.; Kosovichev, A. G.; Toomre, J.; Anderson, E.; Antia, H. M.; Basu, S.; Chaboyer, B.; Chitre, S. M.; Christensen-Dalsgaard, J.; Dziembowski, W. A.; Eff-Darwich, A.; Elliott, J. R.; Giles, P. M.; Goode, P. R.; Guzik, J. A.; Harvey, J. W.; Hill, F.; Leibacher, J. W.; Monteiro, M. J. P. F. G.; Richard, O.; Sekii, T.; Shibahashi, H.; Takata, M.; Thompson, M. J.; Vauclair, S.; Vorontsov, S. V. Bibcode: 1996Sci...272.1296G Altcode: Global Oscillation Network Group data reveal that the internal structure of the sun can be well represented by a calibrated standard model. However, immediately beneath the convection zone and at the edge of the energy-generating core, the sound-speed variation is somewhat smoother in the sun than it is in the model. This could be a consequence of chemical inhomogeneity that is too severe in the model, perhaps owing to inaccurate modeling of gravitational settling or to neglected macroscopic motion that may be present in the sun. Accurate knowledge of the sun's structure enables inferences to be made about the physics that controls the sun; for example, through the opacity, the equation of state, or wave motion. Those inferences can then be used elsewhere in astrophysics. Title: Differential Rotation and Dynamics of the Solar Interior Authors: Thompson, M. J.; Toomre, J.; Anderson, E. R.; Antia, H. M.; Berthomieu, G.; Burtonclay, D.; Chitre, S. M.; Christensen-Dalsgaard, J.; Corbard, T.; De Rosa, M.; Genovese, C. R.; Gough, D. O.; Haber, D. A.; Harvey, J. W.; Hill, F.; Howe, R.; Korzennik, S. G.; Kosovichev, A. G.; Leibacher, J. W.; Pijpers, F. P.; Provost, J.; Rhodes, E. J., Jr.; Schou, J.; Sekii, T.; Stark, P. B.; Wilson, P. R. Bibcode: 1996Sci...272.1300T Altcode: Splitting of the sun's global oscillation frequencies by large-scale flows can be used to investigate how rotation varies with radius and latitude within the solar interior. The nearly uninterrupted observations by the Global Oscillation Network Group (GONG) yield oscillation power spectra with high duty cycles and high signal-to-noise ratios. Frequency splittings derived from GONG observations confirm that the variation of rotation rate with latitude seen at the surface carries through much of the convection zone, at the base of which is an adjustment layer leading to latitudinally independent rotation at greater depths. A distinctive shear layer just below the surface is discernible at low to mid-latitudes. Title: The Solar Acoustic Spectrum and Eigenmode Parameters Authors: Hill, F.; Stark, P. B.; Stebbins, R. T.; Anderson, E. R.; Antia, H. M.; Brown, T. M.; Duvall, T. L., Jr.; Haber, D. A.; Harvey, J. W.; Hathaway, D. H.; Howe, R.; Hubbard, R. P.; Jones, H. P.; Kennedy, J. R.; Korzennik, S. G.; Kosovichev, A. G.; Leibacher, J. W.; Libbrecht, K. G.; Pintar, J. A.; Rhodes, E. J., Jr.; Schou, J.; Thompson, M. J.; Tomczyk, S.; Toner, C. G.; Toussaint, R.; Williams, W. E. Bibcode: 1996Sci...272.1292H Altcode: The Global Oscillation Network Group (GONG) project estimates the frequencies, amplitudes, and linewidths of more than 250,000 acoustic resonances of the sun from data sets lasting 36 days. The frequency resolution of a single data set is 0.321 microhertz. For frequencies averaged over the azimuthal order m, the median formal error is 0.044 microhertz, and the associated median fractional error is 1.6 x 10-5. For a 3-year data set, the fractional error is expected to be 3 x 10-6. The GONG m-averaged frequency measurements differ from other helioseismic data sets by 0.03 to 0.08 microhertz. The differences arise from a combination of systematic errors, random errors, and possible changes in solar structure. Title: The Sun's rotation rate in its equatorial plane. Authors: Antia, H. M.; Chitre, S. M.; Thompson, M. J. Bibcode: 1996A&A...308..656A Altcode: We investigate the Sun's rotation rate in its equatorial plane, as a function of depth, using Big Bear data for the years 1986, 1988, 1989 and 1990. One novel feature of this investigation is that we have used an iterative inversion technique, based upon the regularized least-squares method. Such methods generally have one or more adjustable parameters, and the details of the inferred rotation profile depend upon the values chosen for those parameters. We find that the iterative technique produces results that are much less sensitive to the values of the parameters, which may be seen as one advantage of our procedure. Our results exhibit various features, including a secular decrease in the rotation rate beneath the convection zone, a locally enhanced rotation rate near 0.9Rsun_ (Rsun_ being the photospheric radius), and a local minimum near 0.6Rsun_. Not only are these features apparently fairly robust from year to year, they have all variously been seen in inversions of other datasets. However, we demonstrate that some caution should be exercised in interpreting such features in the Sun's equatorial rotation rate. Title: Nonasymptotic helioseismic inversion: iterated seismic solar model. Authors: Antia, H. M. Bibcode: 1996A&A...307..609A Altcode: A nonasymptotic technique based on the variational formulation of the equations of stellar oscillations is used for helioseismic inversion. The resulting nonlinear integral equations are solved iteratively to obtain a seismic model of the Sun. Tests using pairs of known solar models show that this technique is capable of determining the sound speed to an accuracy of better than 0.2% and the density to an accuracy of better than 1% in most of the solar interior. It is demonstrated that the resulting seismic solar model is not particularly sensitive to the choice of regularization parameter. Influence of systematic differences in the observed frequencies is studied to find that it is probably the dominant source of uncertainties in helioseismic inversions. The seismic solar model is found to be close to a standard solar model including the diffusion of helium and heavy elements. Further, just below the HeII ionization zone OPAL equation of state is found to be in better agreement with that of solar material as compared to the MHD equation of state. Title: Stability of naked singularities in spherically symmetric dust collapse Authors: Antia, H. M. Bibcode: 1996PhRvD..53.3472A Altcode: The stability of certain counterexamples leading to a strong curvature naked singularity in the collapse of a spherically symmetric dust cloud is examined to find that these solutions are unstable to small perturbations in the initial conditions. Hence, the cosmic censorship hypothesis may be modified to exclude unstable solutions. Title: Helioseismic Determination of Sound Speed in the Sun Authors: Basu, S.; Antia, H. M. Bibcode: 1995JApAS..16..392B Altcode: No abstract at ADS Title: Seismology of the Solar f-Mode. I. Basic Signatures of Shearing Velocity Fields Authors: Ghosh, Pranab; Antia, H. M.; Chitre, S. M. Bibcode: 1995ApJ...451..851G Altcode: The observed frequencies of the solar f-mode show systematic shifts from the (parabolic) dispersion relation characteristic of a pure surface mode, owing to the perturbing effects of such phenomena in the outer layers of the Sun as velocity fields, magnetic fields, and temperature gradients/discontinuities. These frequency shifts thus provide a good diagnostic probe of these phenomena. In this paper, we wish to focus our attention on the possible influence of shearing velocity fields on the f-mode frequencies. We show here that velocity fields of a wide class leave a signature on the frequency-shift profile which is consistent with observations, including the characteristic change of sign (crossover) in δω at a spherical harmonic degree of approximately 800. We demonstrate that quantitative fits to the observed frequency shifts are possible with parameters which are characteristic of the underlying motions of the solar convection zone. However, the simultaneous effects of other phenomena, e.g., chromospheric magnetic fields, should also be taken into account. Title: Helium abundance in the solar envelope Authors: Basu, Sarbani; Antia, H. M. Bibcode: 1995MNRAS.276.1402B Altcode: The abundance of helium in the solar envelope can be determined using the variation of the adiabatic index of the stellar material in the second helium ionization zone. All techniques for inferring helium abundance from the observed frequencies of solar p modes are known to be sensitive to the equation of state used in the reference models. The sensitivity of inferred helium abundance to the equation of state is studied by using different reference models with MHD and OPAL equations of state. Recent observations of high-degree solar p-mode frequencies yield a helium abundance Y=0.246 when determined using reference models with the MHD equation of state and Y=0.249 using the OPAL equation of state. Further, the models constructed with the OPAL equation of state are found to be in better agreement with the inferred sound speed below the HeII ionization zone. Title: Helium Abundance in the Solar Envelope Authors: Basu, S.; Antia, H. M. Bibcode: 1995ESASP.376b..35B Altcode: 1995soho....2...35B; 1995help.confP..35B The variations in the adiabatic index in the second helium ionization zone of the Sun can be used to infer the helium abundance in the solar envelope using the observed solar oscillation frequencies. These variations leave their signature on the sound-speed in this region, hence, techniques based on solar sound speed inversion can be used to determine the abundance of helium. These techniques are known to be sensitive to the equation of state used in the reference models. Sensitivity of the helium abundance measurements to the equation of state is studied using models constructed with MHD or OPAL equations of state. Observations of high degree solar p-modes yield helium abundance Y = 0.246 and 0.249, respectively, using reference models with MHD and OPAL equations of state. Further, the models constructed using the OPAL equation of state are found to be in better agreement with the inferred sound speed in the Sun, particularly below the second helium ionization zone. Title: Effects of surface layers on helioseismic inversion Authors: Antia, H. M. Bibcode: 1995MNRAS.274..499A Altcode: In helioseismic inversions it is traditionally assumed that uncertainties in the treatment of surface layers of the Sun introduce frequency shifts which, after suitable scaling, can be expressed as a function of frequency alone. By considering explicit examples it is demonstrated that, although this assumption is correct to first order, the actual frequency changes are not a function of frequency alone to the accuracy of measured frequencies. This is true even for intermediate degree modes which are most useful for inversion. The errors introduced in inversion as a result of such simplifying assumptions are studied. Title: Helioseismic bounds in the central temperature of the Sun Authors: Antia, H. M.; Chitre, S. M. Bibcode: 1995ApJ...442..434A Altcode: The information concerning the variation of sound speed and density inside the Sun as inferred from helioseismic inversion is employed to constrain the temperature and composition profiles in the solar core. The usual thermal transport and energy generation equations governing stellar structure are adopted to demonstrate that in order to reduce the chlorine neutrino flux to match the observed values, the opacity needs to be reduced by a factor upwards of 1.5. Further, assuming that the uncertainties in the tabulated OPAL opacity values are no larger than 20% in the core, the central temperature is bounded by 15.2 x 106 Tc 16.1 x 106 K. In order to get a temperature profile which is consistent with the observed solar luminosity and the helioseismic data, it appears that the nuclear energy generation rates will need to be revised upward by a few percent, although the estimate of the central temperature is not particularly affected by these uncertainties. With the current OPAL opacities, the central temperature is found to be (15.6 + 0.40) x 106 K. Title: Helioseismic Test of Stellar Convection Theories Authors: Basu, S.; Antia, H. M. Bibcode: 1995ASPC...76..649B Altcode: 1995gong.conf..649B No abstract at ADS Title: Central Temperature of the Sun Authors: Antia, H. M.; Chitre, S. M. Bibcode: 1995ASPC...76..164A Altcode: 1995gong.conf..164A No abstract at ADS Title: Nonasymptotic helioseismic inversion for solar structure. Authors: Antia, H. M.; Basu, S. Bibcode: 1994A&AS..107..421A Altcode: A nonasymptotic technique based on the variational formulation of the equations of stellar oscillations is used for helioseismic inversion. This method can give the difference in sound speed and density between two solar models or between a solar model and the Sun using the corresponding frequency differences. Tests using pairs of known solar models show that this technique is capable of reproducing the sound speed to an accuracy of better than 0.1% for r<0.97Rsun_ , while the density can be determined to an accuracy of better than 0.5% in most of the radiative interior. Using the observed frequencies of solar oscillations it is found that the sound speed and density in a solar model with gravitational settling of helium and heavy elements is very close to that in the Sun, the maximum difference being 0.5% for the sound speed and 1.5% for density. It is possible to use this technique to obtain the adiabatic index {GAMMA}_1_ inside the Sun, which can be employed as a probe for the equation of state in stellar interiors. The relative sound speed difference between the reference solar model and the Sun just below the He ii ionization zone indicates that the adiabatic index of the solar material in this region is smaller than that in the model. The inverted density profile is found to be stable against convection in the solar core, while the depth of the surface convection zone is found to be approximately 0.29Rsun_. Title: Effects of Diffusion on the Extent of Overshoot Below the Solar Convection Zone Authors: Basu, S.; Antia, H. M. Bibcode: 1994MNRAS.269.1137B Altcode: Discontinuities in the derivatives of the sound speed at the base of the overshoot layer below the solar convection zone introduce a characteristic oscillatory component in the frequencies of solar p-modes as a function of the radial order n. The amplitude of this oscillatory part may be used to measure the extent of overshoot. However, sharp changes in the mean molecular weight due to gravitational settling of helium, as well as sharp changes in the opacity due to diffusion of metals, can also give rise to an oscillatory component in the frequencies of solar p-modes. Thus the estimate of overshoot will be affected by the diffusion of helium and metals. It is found that helium diffusion tends to increase the amplitude of the oscillatory component in the frequencies of p-modes, while metal diffusion tends to decrease this amplitude, but the net effect is to increase the amplitude. Apart from diffusion, the amplitude also depends on the depth of the convection zone, and to a lesser extent on the hydrogen and metal abundance in the solar envelope. In the absence of diffusion, or when the composition gradient near the base of the convection zone is smooth, the amplitude of the oscillatory component in a solar model with an overshoot of 0.05Hp is found to be consistent with that in the observed frequencies. Further, models with diffusion that have a sharp change in their composition profiles at the base of the solar convection zone do not seem to be favoured by observations. Key words: convection - diffusion - radiative transfer - Sun: interior - Sun: oscillations. Title: Seismology of the solar convection zone Authors: Basu, Sarbani; Antia, H. M. Bibcode: 1994JApA...15..143B Altcode: An attempt is made to infer the structure of the solar convection zone from observed p-mode frequencies of solar oscillations. The differential asymptotic inversion technique is used to fine the sound speed in the solar envelope. It is found that envelope models which use the Canuto-Mazzitelli (CM) formulation for calculating the convective flux give significantly better agreement with observations than models constructed using the mixing length formalism. This inference can be drawn from both the scaled frequency differences and the sound speed difference. The sound speed in the CM envelope model is within 0.2% of that in the sun except in the region with r greater than 0.99 solar radius. The envelope models are extended below the convection zone, to find some evidence for the gravitational settling of helium beneath the base of the convection zone. It turns out that for models with a steep composition gradient below the convection zone, the convection zone depth has to be increased by about 6 Mm in order to get agreement with helioseismic observartions. Title: Measuring the Helium Abundance in the Solar Envelope: The Role of the Equation of State Authors: Antia, H. M.; Basu, Sarbani Bibcode: 1994ApJ...426..801A Altcode: Variations in the adiabatic index of stellar material in the second helium ionization zone enable one to infer the helium abundance in the solar envelope, using the observed frequencies of solar oscillations. Three techniques based on the differential asymptotic method for sound speed inversion are considered. With the help of the signature of helium abundance on various tracers of ionization, it is possible to estimate the helium abundance. Using several test models, the systematic errors in these techniques are estimated. All these techniques are found to be sensitive to the equation of state. The magnetohydrodynamic (MHD) equation of state is found to be close to that of solar material. Using reference models employing MHD equation of state we find the solar helium abundance Y = 0.252 +/- 0.003. Title: Helioseismic measurement of the extent of overshoot below the solar convection zone Authors: Basu, Sarbani; Antia, H. M.; Narasimha, D. Bibcode: 1994MNRAS.267..209B Altcode: The discontinuity in the derivatives of the sound speed at the base of the overshoot layer below the solar convection zone introduces a characteristic oscillatory component in the frequencies of solar p-modes as a function of the radial order n. The amplitude of this oscillatory part is calibrated as a function of the extent of overshoot using a sequence of solar models constructed with varying extent of overshoot. Using this calibration, an attempt is made to measure the extent of overshoot below the solar convection zone using the available frequencies of the p-modes. It is found that the observed frequencies are consistent with a solar model without overshoot. Further, taking account of the errors in observations, it is possible to put a 2-sigma upper limit of 0.1 H(p) on the extent of overshoot. Title: Helioseismic detection of overshoot below the solar convection zone. Authors: Basu, Sarbani; Antia, H. M.; Narasimha, D. Bibcode: 1993BASI...21..669B Altcode: It is generally accepted that there is no adequate theory to describe astrophysical convection. In particular, there is no agreement among different theories about the extent of overshoot from stellar convection zones. The solar photosphere seems to show, substantial overshoot. On the basis of this result it is sometimes concluded that there is substantial overshoot below the solar convection zone also. Since this layer is not directly observable, we attempt to verify this conjecture using the available helioseismic data (Libbrecht et al. 1990). Title: Penetration at the Base of Solar Convection Zone Authors: Antia, H. M.; Chitre, S. M. Bibcode: 1993ApJ...413..778A Altcode: The extent of overshoot from stellar convection zones into the adjoining stable layers has been recognized to have a nonnegligible influence on evolutionary tracks of stars. Recently, Stothers and Chin (1992) have carried out a detailed model-independent analysis of substantial body of observational data to conclude that the maximum permissible overshoot is 0.2 times the local pressure scale height. In the present work a realistic solar convection zone model is constructed by employing a nonlocal equation for the velocity of convective elements and by including dissipative effects in the calculations. The convection model approach and the analysis of linear eigenmodes are combined to estimate the penetration depths below the base of the convection zone and into the overlying solar atmosphere. It is demonstrated that for an arbitrary extent of overshoot into the underlying stable region, it may not be possible to find a combination of linear modes capable of reproducing the model convective flux profile over the overshoot layers. The acceptable overshoot distance below the base of the convection zone turns out to be 0.2H(p) or less with a probable value of 0.1H(p), which appears to be consistent with helioseismological data. Title: Discrete Cellular Scales of Solar Convection Authors: Antia, H. M.; Chitre, S. M. Bibcode: 1993SoPh..145..227A Altcode: The theoretical power spectrum of velocity fields and flux fluctuations at the solar photosphere is calculated using a quasi-nonlinear framework of superposition of unstable convective eigenmodes excited in the solar convection zone. It is demonstrated that this power spectrum exhibits at least three distinct peaks corresponding to granulation, mesogranulation and supergranulation. The vertical velocity and the brightness fluctuation at the solar surface are found to be correlated. The theoretical framework can be adopted for application to other types of stars in order to predict the dominant length scales in the power spectrum of convection in these stars. Title: Mesogranulation as a Solar Convective Eigenmode Authors: Antia, H. M.; Chitre, S. M. Bibcode: 1993ASPC...42...69A Altcode: 1993gong.conf...69A No abstract at ADS Title: Rational Function Approximations for Fermi-Dirac Integrals Authors: Antia, H. M. Bibcode: 1993ApJS...84..101A Altcode: Rational function minimax approximations are given for the complete Fermi-Dirac integrals of orders -½, ½, 3/2, and 5/2. In each case, three sets of approximations are provided with maximum relative error ≍ l0-4, 10-8, and 10-12, respectively. These approximations can be used to compute the Fermi-Dirac integrals efficiently over the entire range. Approximations to the corresponding inverse functions with an accuracy of ≍l0-4 and 10-8 are also obtained. Title: Theoretical Spectrum of Solar Convection Authors: Antia, H. M.; Chitre, S. M. Bibcode: 1991LNP...388..157A Altcode: 1991ctsm.conf..157A The theoretical power spectrum of solar velocity field and flux variation is obtained with amplitudes of the unstable convective eigenmodes determined by linearly superposing these modes to yield the model convective flux profile. The numerical results are compared with the observed spectrum of solar convection and the technique is applied to other stars. Title: A Model for Stellar Convection and Spectral Line Asymmetries Authors: Antia, H. M. Bibcode: 1990IAUS..138..417A Altcode: No abstract at ADS Title: Convective Nonovershooting in Stellar Cores Authors: Narasimha, D.; Antia, H. M. Bibcode: 1990LNP...367...45N Altcode: 1990psss.conf...45N Mixing-length approximation appears to be consistent with the normal mode analysis of the transport of heat flux due to turbulent convection, in the stellar core as well as in the envelope of stars of a range of spectral types and luminosity classes that we have investigated. However, in spite of demonstrating the self-consistency of the mixing length approximation we do not see any justification for accepting any constant multiple of the mixing-length as a measure of the scale length for convective overshooting into the radiative zones. The convective velocity field in the interior of model of a star of ZAMS mass 10M has been examined at three representative epochs during the main sequence phase, using the linear convective modes. The extent of overshooting is found to be less than O.1H pin all the cases while the mixing-length within the convection zone is typically around 1/3H p. Title: A Model for Stellar Surface Convection and Photospheric Line Asymmetries Authors: Antia, H. M.; Pandey, S. K. Bibcode: 1989ApJ...341.1097A Altcode: A model for stellar convection zones based on linear convective modes using a nonlocal mixing length theory is developed to study the spectral line asymmetries resulting from convective motions in the stellar photospheric region. The amplitudes of these linear convective modes is estimated by demanding that the convective flux due to a linear superposition of such modes should reprodeuce the convective flux required by the mixing length model. The mode with the largest amplitude in the photospheric line formation region is chosen to represent the stellar surface structure. Synthetic spectral line profiles are obtained by summing locally symmetric profiles over the stellar disk according to the local Doppler velocity and intensity fluctuations. Four stars, i.e., the sun, Alpha Cen A, Arcturus, and Procyon, which have characteristically different observed line bisector shapes are chosen for the study. It is found that the simple model considered here can explain the gross features of the observed bisector shapes for these stars of different spectral types. Title: On the Excitation of Solar 5-MINUTE Oscillations Authors: Antia, H. M.; Chitre, S. M.; Gough, D. O. Bibcode: 1988IAUS..123..371A Altcode: A simple prescription for the dynamics of convection perturbed by stellar pulsation is used in an estimation of the growth rates of solar five-minute modes. Convection appears to enhance the excitation of the modes, and the maximum in the growth rate versus frequency found previously when oscillatory convective perturbations were ignored is still present. Title: Tidal Effects on Stellar Evolution in Close Binaries Formed in Globular Clusters Authors: Antia, H. M.; Kembhavi, A. K.; Ray, A. Bibcode: 1988IAUS..126..671A Altcode: A mechanism of forming X-ray binaries by close collision of a neutron star and a normal star in a globular cluster core (GC) was proposed by Fabian, Pringle and Rees (1975). Press and Teukolsky (1977) (PT) made detailed computations of tidal energy deposition in the non-radial modes of a main sequence (MS) star (approximated by a n = 3 polytrope) and two-body tidal capture cross-section. Here, the authors correct numerical errors in PT for the n = 3 polytrope; extend the calculation to the n = 3/2 case and discuss the effects of tidal energy dissipation on the evolution of the MS star and the binary orbit. Title: Helioseismology Authors: Antia, H. M. Bibcode: 1988KodOB...9....7A Altcode: No abstract at ADS Title: Evolution of stellar binaries formed by tidal capture Authors: Ray, A.; Kembhavi, A. K.; Antia, H. M. Bibcode: 1987A&A...184..164R Altcode: Two-body tidal capture, as proposed by Fabian et al. (1975), is the favored mechanism for the formation of X-ray binaries in globular clusters. The tidal capture formation and subsequent evolution of a system consisting of a neutron star and a low mass main sequence star is considered, yielding the amount of tidal energy deposited during the first and later close passages, and the radial distribution of this energy. Going further, the effects of the viscous dissipation of the tidal energy on the structure of the low-mass star and on the binary system are examined. The tidal energy is thermalized on a timescale of 10,000 yr. The consequent high tidal luminosity causes the star to expand and overflow its Roche lobe, resulting in the formation of a common envelope. This makes the stellar core and the neutron star spiral towards each other because of the frictional drag. The state reached by the system after the dissipation of the tidal energy depends on the relative values of the various timescales relevant to the system. Depending on these values the system may evolve into any of the following configurations: an X-ray binary, a detached binary, a neutron star surrounded by a massive accretion disk and a cloud of matter, or a Thorne-Zytkow object. Title: Rapidly rotating stars and the Be star phenomenon Authors: Apparao, K. M. V.; Antia, H. M.; Chitre, S. M. Bibcode: 1987A&A...177..198A Altcode: The authors have outlined a scenario in which a Be star ejects a ring of matter in a quasi-periodic fashion. With an assumed rapidly rotating core, it is found that the transport of angular momentum to the outer regions of the star can destabilize the already near-critical outer layers. These outer regions are confined from flowing out by a magnetic field and with increasing angular momentum a Rayleigh-Taylor type instability can develop leading to the episodic ejection of gas in the equatorial regions. Title: New Limits to Bias and the Amount of Dark Matter in the Universe Authors: Saslaw, William C.; Antia, H. M.; Chitre, S. M. Bibcode: 1987ApJ...315L...1S Altcode: The observed thermodynamic distribution function f(N) for galaxies places significant constraints on the amount of structured dark matter in the universe. The simplest models of cold dark matter require the cosmological density parameter Ω0 ⪉ 0.4. Biased galaxy formation in more complicated models must have specific forms which depend on the amount and structure of dark matter in the model. Title: Solar five-minute oscillations of low, intermediate, and high degree Authors: Antia, H. M.; Chitre, S. M.; Narashima, D. Bibcode: 1986Ap&SS.118..169A Altcode: The overstability of acoustic modes trapped inside the Sun is studied with mechanical and thermal effects of turbulence included in an approximate manner through the eddy diffusivities. Many of the acoustic modes are found to be overstable with the most rapidly growing modes occupying a region centred around 3.3 mHz and spread over a wide range of length-scales. The numerical results turn out to be in reasonable accord with the observed power-spectrum of the five-minute oscillations of arbitrary degree. It is demonstrated that these oscillations are most likely to be driven by a simultaneous operation of the κ-mechanism and the convective Cowling mechanism, the dominant contribution to the generation of self-excited acoustic waves arising from the turbulent diffusion. Title: An excitation mechanism for solar five-minute oscillations of intermediate and high degree. Authors: Antia, H. M.; Chitre, S. M.; Narasimha, D. Bibcode: 1984sses.nasa..345A Altcode: 1984sss..conf..345A The overstability of acoustic modes trapped in the solar convection zone is studied with mechanical and thermal effects of turbulence included, in an approximate manner, through the eddy transport coefficients. Many of these acoustic modes are found to be overstable with the most rapidly growing modes occupying a region centered around 3.2 mHz and spread over a wide range of length-scales. The numerical results are in reasonable accord with the observed power-spectrum of the five-minute oscillations of intermediate and high degree. The oscillations are probably driven by a simultaneous operation of the kappa-mechanism and the turbulent conduction (convective Cowling) mechanism, the dominant contribution to the generation of self-excited acoustic waves arising from the convective Cowling mechanism. Title: Convection in the envelopes of red giants Authors: Antia, H. M.; Chitre, S. M.; Narasimha, D. Bibcode: 1984ApJ...282..574A Altcode: The nature of convection in the envelopes of red giants is investigated, and the linear convective modes are computed to demonstrate the consistency of the mixing length theory. The mixing length at a given depth is tentatively identified with an equivalent width of the luminosity profile of the convective eigenmodes. The dominant convective element turns out to be comparable in size to the radius of the star, and this could account for the observed irregular variations in red giants. Title: Overstable solar oscillations of intermediate and high degree Authors: Antia, H. M.; Chitre, S. M.; Narasimha, D. Bibcode: 1984MmSAI..55..175A Altcode: The stability of linear acoustic modes trapped in the solar envelope is investigated with the thermal and mechanical effects of turbulence on the mean flow incorporated through turbulent conductivity, viscosity and turbulent pressure. A number of these modes turn out to be overstable, and the most rapidly growing acoustic modes are found to occupy a region centred around 3.2 mHz and spread over a wide range of length scales. These numerical results are in reasonable accord with the observed power-spectrum of the five-minute oscillations. Title: Solar five-minutes oscillations Authors: Antia, H. M.; Chitre, S. M.; Narasimha, D. Bibcode: 1984stp..conf..457A Altcode: No abstract at ADS Title: Influence of turbulent pressure on solar convective modes. Authors: Antia, H. M.; Chitre, S. M.; Narasimha, D. Bibcode: 1983MNRAS.204..865A Altcode: The stability of linear convective modes in the solar envelope model is investigated by incorporating in a very crude manner the effects of turbulent pressure and eddy transport coefficients calculated in the mixing length approximation. It is demonstrated that for a reasonable choice of the parameters there occur two peaks in the growth rate versus horizontal wavenumber plot which correspond to the observed features associated with granulation and supergranulation. Title: Consistency of the mixing length theory Authors: Narasimha, D.; Antia, H. M. Bibcode: 1982ApJ...262..358N Altcode: It is found that the structure of the solar convection zone calculated according to the mixing length theory is consistent with the transport of convective flux by a linear superposition of statistically independent unstable convective modes, provided the effects of turbulent conductivity and viscosity are taken into account. The resultant vertical velocity agrees reasonably well with the observed granular velocity in the atmosphere. The horizontal velocity, however, turns out to be too large for low l modes. Title: Overstability of acoustic modes and the solar five-minute oscillations Authors: Antia, H. M.; Chitre, S. M.; Narasimha, D. Bibcode: 1982SoPh...77..303A Altcode: The stability of linear convective and acoustic modes in solar envelope models is investigated by incorporating the thermal and mechanical effects of turbulence through the eddy transport coefficients. With a reasonable value of the turbulent Prandtl number it is possible to obtain the scales of motion corresponding to granulation, supergranulation and the five-minute oscillations. Several of the acoustic modes trapped in the solar convection zone are found to be overstable and the most unstable modes, spread over a region centred predominantly around a period of 300 s with a wide range of horizontal length scales, are in reasonable accord with the observed power-spectrum of the five-minute oscillations. It is demonstrated that these oscillations are driven by a simultaneous action of the κ-mechanism and the radiative and turbulent conduction mechanisms operating in the strongly superadiabatic region in the hydrogen ionization zone, the turbulent transport being the dominant process in overstabilizing the acoustic modes. Title: Granulation and Supergranulation as Convective Modes in the Solar Envelope Authors: Antia, H. M.; Chitre, S. M.; Pandey, S. K. Bibcode: 1981SoPh...70...67A Altcode: The stability of linear convective modes in the solar convection zone is investigated by incorporating the mechanical and thermal effects of turbulence through the eddy transport coefficients. The inclusion of turbulent thermal conductivity and viscosity, calculated in the framework of the mixing length approximation, is demonstrated to have a profound influence on the convective growth rates. The solar envelope model of Spruit (1977) is used to show that that most rapidly growing fundamental mode and the first harmonic are in reasonable accord with the observed features of granulation and supergranulation, respectively. Title: Composition and equation of state of hot dense matter Authors: Antia, H. M.; Banerjee, B.; Chitre, S. M. Bibcode: 1981PhLB...98..319A Altcode: The composition and equation of state of an equilibrium mixture of non-interacting baryons, pions and leptons is computed in the density range 1014-1015.5 g/cm3 for two values of the entropy per baryon, S=1 and 2. These parameters are chosen because of their possible importance in the supernova explosion problem. The threshold densities for the appearance of hyperons are found to be lowered compared to the zero temperature case. Title: Stability of a Steady Vertical Flow in a Viscous Fluid Authors: Antia, H. M.; Chitre, S. M. Bibcode: 1980SoPh...66...71A Altcode: The one-dimensional non-linear equations for a viscous fluid with finite thermal conductivity are solved to get an exact solution for a steady vertical flow. The stability of such a steady flow is examined to find that the viscosity has a very pronounced stabilizing influence on convective and acoustic modes. Title: Thermodynamics of a system of leptons, photons and interacting nucleons Authors: Antia, H. M.; Banerjee, B.; Chitre, S. M. Bibcode: 1980Ap&SS..69..471A Altcode: The thermodynamics of matter composed of interacting nucleons with an admixutre of leptons and photons is investigated in the density-temperature region relevant to the problem of supernova explosion. A quantum mechanical calculation is carried out using Skyrme effective interaction between nucleons. The equation of state is shown to soften below the nuclear density and suddenly stiffen in the transnuclear density region, a condition which is suitable for producing a strong bounce shock that is needed for effective supernova explosion and leaving a remnant neutron star behind. Title: Stability of Magneto-Acoustic Waves in a Thermally Conducting Compressible Fluid Authors: Antia, H. M.; Chitre, S. M. Bibcode: 1980Ap&SS..68..183A Altcode: The stability of magneto-acoustic waves in an inviscid, perfectly conducting isothermal fluid, stratified under constant gravity and subjected to a horizontal magnetic field is investigated in the presence of thermal dissipation. Title: Instabilities in a Polytropic Atmosphere Authors: Antia, H. M.; Chitre, S. M. Bibcode: 1980LNP...114...15A Altcode: 1980sttu.coll...15A; 1980IAUCo..51...15A No abstract at ADS Title: Waves in the sunspot umbra. Authors: Antia, H. M.; Chitre, S. M. Bibcode: 1979SoPh...63...67A Altcode: The magnetoacoustic modes excited in a thermally conducting polytropic fluid layer in the presence of a vertical magnetic field are examined with a view to classify them with the help of phase diagrams. The possibility of identifying the umbral flashes with overstable magnetoacoustic modes is explored. Title: Instabilities in a Penetrative Atmosphere Authors: Pandey, S. K.; Antia, H. M.; Chitre, S. M. Bibcode: 1979Ap&SS..63..103P Altcode: The destabilization of convective, gravity and acoustic modes in a compressible atmosphere consisting of a stable layer overlying an unstable layer is investigated in the optically thin approximation. It is shown that penetration into the stable layer promotes instability under suitable conditions. Title: Validity of the linearized theory for complete viscous polytropes. Authors: Antia, H. M.; Chitre, S. M. Bibcode: 1979MNRAS.186..491A Altcode: A previous evaluation of the validity of the linearized approximation in the case of an inviscid thermally conducting polytropic fluid is extended to complete polytropes, where both viscosity and thermal conductivity are taken into account. It is shown that the linearized theory is self-consistent in the case of a viscous polytrope even when the temperature vanishes at one of the boundaries. An analysis is performed which demonstrates that the linearized theory is self-consistent for both Eulerian and Lagrangian perturations and, in the case of the latter, for the optically thick as well as the optically thin case. Title: Waves in the sunspot penumbra. Authors: Antia, H. M.; Chitre, S. M.; Gokhale, M. H. Bibcode: 1978SoPh...60...31A Altcode: The stability of a plane-parallel polytropic fluid layer in the presence of a uniform horizontal magnetic field is investigated to explore the possibility of identifying the running penumbral waves and the penumbral filaments with different types of instabilities. Title: Invalidity of the linearized theory for a complete polytrope. Authors: Antia, H. M.; Chitre, S. M. Bibcode: 1978MNRAS.184..211A Altcode: The reported investigation has the objective to demonstrate that the linearized approximation, which assumes that perturbations in the steady-state values of the various physical quantities are small compared to the corresponding steady-state values themselves, breaks down when the temperature vanishes at the top boundary. It is shown that for optically thin disturbances in a polytropic atmosphere when the top temperature tends to be zero, the linear theory breaks down for growing convective and acoustic modes. On the other hand for optically thick disturbances the linear theory breaks down for all modes, for all values of four parameters, and for the considered three sets of boundary conditions. For a proper stability analysis it is essential to solve the equations using the nonlinear theory. Title: Overstabilization of acoustic modes in a polytropic atmosphere. Authors: Antia, H. M.; Chitre, S. M.; Kale, D. M. Bibcode: 1978SoPh...56..275A Altcode: The overstability of sound waves in a polytropic atmosphere is examined for disturbances of arbitrary optical thickness. It is concluded that the Cowling-Spiegel mechanism can operate in the solar convective zone, although the κ-mechanism is predominantly responsible for the observed five-minute oscillations.