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.
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.
ABSTRACT 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.
Abstract
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.
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.
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 (Δ ) 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.
Abstract
We present timing and spectral results for the 2018 outburst of Cepheus X-4, observed twice by AstroSat at luminosities of 2.04 × 10
37
erg s
−1
and 1.02 × 10
37
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.
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 and the behavior of line widths with frequency and line widths at with , 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.
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