antia

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Author name code: antia
ADS astronomy entries on 2022-09-14
author:"Antia, H.M."

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Title: Lessons learnt from AstroSat observations of cyclotron
resonance scattering features from accretion powered pulsars
Authors: Mukerjee, Kallol; Dey, Prithwitosh; Antia, H. M.
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.

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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
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.

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Title: Changes in the Near-surface Shear Layer of the Sun
Authors: Antia, H. M.; Basu, Sarbani
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.

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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.
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.

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Title: Studies of Cepheus X-4 during the 2018 Outburst Observed
with AstroSat
Authors: Mukerjee, Kallol; Antia, H. M.
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.

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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.
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.

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Title: AstroSat Detection of a soft X-ray source in HR6819 (QV Tel)
Authors: Manchanda, R. K.; Katoch, T.; Antia, H. M.
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 &amp;
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.

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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
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.

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Title: GRB 210116: AstroSat LAXPC detection
Authors: Antia, H. M.; Katoch, Tilak; Shah, Parag; Dedhia, Dhiraj
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.

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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.
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.

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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
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.

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Title: AstroSat Observations of GRO J2058+42 during the 2019 Outburst
Authors: Mukerjee, Kallol; Antia, H. M.; Katoch, Tilak
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.

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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
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.

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Title: GRB 200326B: AstroSat LAXPC detection
Authors: Antia, H. M.; Katoch, Tilak; Shah, Parag; Dedhia TIFR,
Dhiraj; Mumbai; India.
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.

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Title: Time-Distance Helioseismology of Deep Meridional Circulation
Authors: Rajaguru, S. P.; Antia, H. M.
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.

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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
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.

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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.
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 &gt;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.
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.
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.
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.
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.
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
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.
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
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.
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.
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
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.
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.
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.
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”
(<A href="https://doi.org/10.3847/1538-4357/835/2/172">2017, ApJ,
835, 172</A>)
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.
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
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.
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 &amp;
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 &amp; 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
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.
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.
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.
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.
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
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.
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
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.
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
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 &gt; 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
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.
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.
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
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.
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
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
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.
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
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.
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.
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.
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.
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.
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 <A
href="http://www.aanda.org/10.1051/0004-6361/201526610/olm">http://www.aanda.org</A>

---------------------------------------------------------
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.
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.
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.
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 &gt;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 <A
href="http://www.aanda.org/10.1051/0004-6361/201425115/olm">http://www.aanda.org</A>

---------------------------------------------------------
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.
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.
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.
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.
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.
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.
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.
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
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
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.
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.
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.
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.
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 <A
href="http://www.aanda.org/10.1051/0004-6361/201323317/olm">http://www.aanda.org</A>

---------------------------------------------------------
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.
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.
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 <A href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
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.
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
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.
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.
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.
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
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.
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.
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.
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.
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.
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 <A href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
Title: Seismic detection of acoustic sharp features in the CoRoT
target <ASTROBJ>HD 49933</ASTROBJ>
Authors: Mazumdar, A.; Michel, E.; Antia, H. M.; Deheuvels, S.
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,
<ASTROBJ>HD 49933</ASTROBJ>. We are able to determine the acoustic depth
of the second helium ionisation zone of <ASTROBJ>HD 49933</ASTROBJ> 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 <ASTROBJ>HD 49933</ASTROBJ>.

---------------------------------------------------------
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.
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.
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
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
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
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.
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.
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
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.
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.
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.
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 &gt;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 ⊙ &gt; 0:94.

---------------------------------------------------------
Title: Changing Magnetic Fields in the Solar Interior
Authors: Baldner, C. S.; Antia, H. M.; Basu, S.; Larson, T. P.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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 (&gt;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.
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 (&gt;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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
2006ESASP.617E..50B Altcode: 2006soho...17E..50B
No abstract at ADS

---------------------------------------------------------
Title: Determining Solar Abundances Using Helioseismology
Authors: Antia, H. M.; Basu, Sarbani
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.
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.
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
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.
2004ESASP.559..301A Altcode: 2004soho...14..301A
No abstract at ADS

---------------------------------------------------------
Title: Temporal Variations in Zonal Flows
Authors: Antia, H. M.; Basu, S.
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.
2004ESASP.559..289A Altcode: 2004soho...14..289A
No abstract at ADS

---------------------------------------------------------
Title: Helioseismic Constraints on Photospheric Abundances
Authors: Basu, S.; Antia, H. M.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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 &gt;~ 120) modes.

---------------------------------------------------------
Title: Temporal variations in the rotation rate in the solar interior
Authors: Basu, Sarbani; Antia, H. M.
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.
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.
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.
2003LNP...619.....A Altcode: 2003lsp..conf.....A
No abstract at ADS

---------------------------------------------------------
Title: Seismic Sun
Authors: Chitre, S. M.; Antia, H. M.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 νe-νactive
and νe-νsterile cases and present the allowed
parameter regions in the Δm2-tan2 θ plane for
νe-νactive 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.
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.
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.
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
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
2000BASI...28..101A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Seismic Sun
Authors: Chitre, S. M.; Antia, H. M.
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.
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.
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.
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
1999ApJ...519..400A Altcode: 1998astro.ph.11154A
We determine the frequencies of solar oscillations covering a wide
range of degree (100&lt;l&lt;4000) and frequency (1.5&lt;ν&lt;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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
1998BASI...26..149A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Helioseismology and the solar neutrino problem
Authors: Antia, H.; Chitre, S.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
1997IAUS..181..151A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Seismology of the solar core
Authors: Antia, H. M.; Chitre, S. M.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
1995ASPC...76..649B Altcode: 1995gong.conf..649B
No abstract at ADS

---------------------------------------------------------
Title: Central Temperature of the Sun
Authors: Antia, H. M.; Chitre, S. M.
1995ASPC...76..164A Altcode: 1995gong.conf..164A
No abstract at ADS

---------------------------------------------------------
Title: Nonasymptotic helioseismic inversion for solar structure.
Authors: Antia, H. M.; Basu, S.
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&lt;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.
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.
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
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.
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.
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.
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.
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.

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Title: Mesogranulation as a Solar Convective Eigenmode
Authors: Antia, H. M.; Chitre, S. M.
1993ASPC...42...69A Altcode: 1993gong.conf...69A
No abstract at ADS

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Title: Rational Function Approximations for Fermi-Dirac Integrals
Authors: Antia, H. M.
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.

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Title: Theoretical Spectrum of Solar Convection
Authors: Antia, H. M.; Chitre, S. M.
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.

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Title: A Model for Stellar Convection and Spectral Line Asymmetries
Authors: Antia, H. M.
1990IAUS..138..417A Altcode:
No abstract at ADS

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Title: Convective Nonovershooting in Stellar Cores
Authors: Narasimha, D.; Antia, H. M.
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.

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Title: A Model for Stellar Surface Convection and Photospheric
Line Asymmetries
Authors: Antia, H. M.; Pandey, S. K.
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.

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Title: On the Excitation of Solar 5-MINUTE Oscillations
Authors: Antia, H. M.; Chitre, S. M.; Gough, D. O.
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.

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Title: Tidal Effects on Stellar Evolution in Close Binaries Formed
in Globular Clusters
Authors: Antia, H. M.; Kembhavi, A. K.; Ray, A.
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.

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Title: Helioseismology
Authors: Antia, H. M.
1988KodOB...9....7A Altcode:
No abstract at ADS

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Title: Evolution of stellar binaries formed by tidal capture
Authors: Ray, A.; Kembhavi, A. K.; Antia, H. M.
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.

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Title: Rapidly rotating stars and the Be star phenomenon
Authors: Apparao, K. M. V.; Antia, H. M.; Chitre, S. M.
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.

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Title: New Limits to Bias and the Amount of Dark Matter in the
Universe
Authors: Saslaw, William C.; Antia, H. M.; Chitre, S. M.
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.

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Title: Solar five-minute oscillations of low, intermediate, and
high degree
Authors: Antia, H. M.; Chitre, S. M.; Narashima, D.
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.

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Title: An excitation mechanism for solar five-minute oscillations
of intermediate and high degree.
Authors: Antia, H. M.; Chitre, S. M.; Narasimha, D.
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.

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Title: Convection in the envelopes of red giants
Authors: Antia, H. M.; Chitre, S. M.; Narasimha, D.
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.

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Title: Overstable solar oscillations of intermediate and high degree
Authors: Antia, H. M.; Chitre, S. M.; Narasimha, D.
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.

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Title: Solar five-minutes oscillations
Authors: Antia, H. M.; Chitre, S. M.; Narasimha, D.
1984stp..conf..457A Altcode:
No abstract at ADS

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Title: Influence of turbulent pressure on solar convective modes.
Authors: Antia, H. M.; Chitre, S. M.; Narasimha, D.
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.

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Title: Consistency of the mixing length theory
Authors: Narasimha, D.; Antia, H. M.
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.

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Title: Overstability of acoustic modes and the solar five-minute
oscillations
Authors: Antia, H. M.; Chitre, S. M.; Narasimha, D.
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.

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Title: Granulation and Supergranulation as Convective Modes in the
Solar Envelope
Authors: Antia, H. M.; Chitre, S. M.; Pandey, S. K.
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.

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Title: Composition and equation of state of hot dense matter
Authors: Antia, H. M.; Banerjee, B.; Chitre, S. M.
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.

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Title: Stability of a Steady Vertical Flow in a Viscous Fluid
Authors: Antia, H. M.; Chitre, S. M.
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.

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Title: Thermodynamics of a system of leptons, photons and interacting
nucleons
Authors: Antia, H. M.; Banerjee, B.; Chitre, S. M.
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.

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Title: Stability of Magneto-Acoustic Waves in a Thermally Conducting
Compressible Fluid
Authors: Antia, H. M.; Chitre, S. M.
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.

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Title: Instabilities in a Polytropic Atmosphere
Authors: Antia, H. M.; Chitre, S. M.
1980LNP...114...15A Altcode: 1980sttu.coll...15A; 1980IAUCo..51...15A
No abstract at ADS

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Title: Waves in the sunspot umbra.
Authors: Antia, H. M.; Chitre, S. M.
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.

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Title: Instabilities in a Penetrative Atmosphere
Authors: Pandey, S. K.; Antia, H. M.; Chitre, S. M.
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.

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Title: Validity of the linearized theory for complete viscous
polytropes.
Authors: Antia, H. M.; Chitre, S. M.
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.

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Title: Waves in the sunspot penumbra.
Authors: Antia, H. M.; Chitre, S. M.; Gokhale, M. H.
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.

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Title: Invalidity of the linearized theory for a complete polytrope.
Authors: Antia, H. M.; Chitre, S. M.
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.

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Title: Overstabilization of acoustic modes in a polytropic atmosphere.
Authors: Antia, H. M.; Chitre, S. M.; Kale, D. M.
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.