The frequency, \(\nu_{\rm max}\), at which the envelope of pulsation power peaks for solar-like oscillators is an important quantity in asteroseismology. We measure \(\nu_{\rm max}\) for the Sun ...using 25 years of Sun-as-a-Star Doppler velocity observations with the Birmingham Solar-Oscillations Network (BiSON), by fitting a simple model to binned power spectra of the data. We also apply the fit to Sun-as-a-Star Doppler velocity data from GONG and GOLF, and photometry data from VIRGO/SPM on the ESA/NASA SOHO spacecraft. We discover a weak but nevertheless significant positive correlation of the solar \(\nu_{\rm max}\) with solar activity. The uncovered shift between low and high activity, of \(\simeq 25\,\rm \mu Hz\), translates to an uncertainty of 0.8 per cent in radius and 2.4 per cent in mass, based on direct use of asteroseismic scaling relations calibrated to the Sun. The mean \(\nu_{\rm max}\) in the different datasets is also clearly offset in frequency. Our results flag the need for caution when using \(\nu_{\rm max}\) in asteroseismology.
The Sch\"onberg-Chandrasekhar (SC) limit is a well-established result in the
understanding of stellar evolution. It provides an estimate of the point at
which an evolved isothermal core embedded in ...an extended envelope begins to
contract. We investigate contours of constant fractional mass in terms of
homology invariant variables U and V and find that the SC limit exists because
the isothermal core solution does not intersect all the contours for an
envelope with polytropic index 3. We find that this analysis also applies to
similar limits in the literature including the inner mass limit for polytropic
models of quasi-stars. Consequently, any core solution that does not intersect
all the fractional mass contours exhibits an associated limit and we identify
several relevant cases where this is so. We show that a composite polytrope is
at a fractional core mass limit when its core solution touches but does not
cross the contour of the corresponding fractional core mass. We apply this test
to realistic models of helium stars and find that stars typically expand when
their cores are near a mass limit. Furthermore, it appears that stars that
evolve into giants have always first exceeded an SC-like limit.
During the first half of main-sequence lifetimes, the evolution of rotation and magnetic activity in solar-type stars appears to be strongly coupled. Recent observations suggest that rotation rates ...evolve much more slowly beyond middle-age, while stellar activity continues to decline. We aim to characterize this mid-life transition by combining archival stellar activity data from the Mount Wilson Observatory with asteroseismology from the Transiting Exoplanet Survey Satellite (TESS). For two stars on opposite sides of the transition (88 Leo and \(\rho\) CrB), we independently assess the mean activity levels and rotation periods previously reported in the literature. For the less active star (\(\rho\) CrB), we detect solar-like oscillations from TESS photometry, and we obtain precise stellar properties from asteroseismic modeling. We derive updated X-ray luminosities for both stars to estimate their mass-loss rates, and we use previously published constraints on magnetic morphology to model the evolutionary change in magnetic braking torque. We then attempt to match the observations with rotational evolution models, assuming either standard spin-down or weakened magnetic braking. We conclude that the asteroseismic age of \(\rho\) CrB is consistent with the expected evolution of its mean activity level, and that weakened braking models can more readily explain its relatively fast rotation rate. Future spectropolarimetric observations across a range of spectral types promise to further characterize the shift in magnetic morphology that apparently drives this mid-life transition in solar-type stars.
(Abridged) We present the first APOKASC catalog of spectroscopic and asteroseismic data for 415 dwarfs and subgiants. Asteroseismic data have been obtained by Kepler in short cadence. The ...spectroscopic parameters are based on spectra taken as part of APOGEE and correspond to DR13 of SDSS. We analyze our data using two Teff scales, the spectroscopic values from DR13 and those derived from SDSS griz photometry. We use the differences in our results arising from these choices as a test of systematic Teff, and find that they can lead to significant differences in the derived stellar properties. Determinations of surface gravity (\(\log{g}\)), mean density (\(\rho\)), radius (\(R\)), mass (\(M\)), and age (\(\tau\)) for the whole sample have been carried out with stellar grid-based modeling. We have assessed random and systematic error sources in the spectroscopic and seismic data, as well as in the grid-based modeling determination of the stellar quantities in the catalog. We provide stellar properties for both Teff scales. The median combined (random and systematic) uncertainties are 2% (0.01 dex; \(\log{g}\)), 3.4% (\(\rho\)), 2.6% (\(R\)), 5.1% (\(M\)), and 19% (\(\tau\)) for the photometric Teff scale and 2% (\(\log{g}\)), 3.5% (\(\rho\)), 2.7% (\(R\)), 6.3% (\(M\)), and 23% (\(\tau\)) for the spectroscopic scale. Comparisons with stellar quantities in the catalog by Chaplin et al.(2014) highlight the importance of metallicity measurements for determining stellar parameters accurately. We compare our results with those from other sources, including stellar radii determined from TGAS parallaxes and asteroseismic analyses based on individual frequencies. We find a very good agreement in all cases. Comparisons give strong support to the determination of stellar quantities based on global seismology, a relevant result for future missions such as TESS and PLATO. Table 5 corrected (wrongly listed SDSS Teff before).
We present a convariant formulation for radiative transfer in curved space time and demonstrate some applications in the black-hole systems. We calculate the emission from semi-transparent accretion ...tori around black holes, for opacity provided by the Fe K lines and for opacity dominated by electron scattering. We also calculate the emission from radiative inefficient accretion flow in black holes with opacity provided by electron-positron annihilation lines. Finally we show shadows cast by accreting black holes with different spins and with different distribution of warm material around them.
Asteroseismology of bright stars has become increasingly important as a method to determine fundamental properties (in particular ages) of stars. The Kepler Space Telescope initiated a revolution by ...detecting oscillations in more than 500 main-sequence and subgiant stars. However, most Kepler stars are faint, and therefore have limited constraints from independent methods such as long-baseline interferometry. Here, we present the discovery of solar-like oscillations in \(\alpha\) Men A, a naked-eye (V=5.1) G7 dwarf in TESS's Southern Continuous Viewing Zone. Using a combination of astrometry, spectroscopy, and asteroseismology, we precisely characterize the solar analog alpha Men A (Teff = 5569 +/- 62 K, R = 0.960 +/- 0.016 Rsun, M = 0.964 +/- 0.045 Msun). To characterize the fully convective M dwarf companion, we derive empirical relations to estimate mass, radius, and temperature given the absolute Gaia magnitude and metallicity, yielding M = 0.169 +/- 0.006, R = 0.19 +/- 0.01 and Teff = 3054 +/- 44 K. Our asteroseismic age of 6.2 +/- 1.4 (stat) +/- 0.6 (sys) Gyr for the primary places \(\alpha\) Men B within a small population of M dwarfs with precisely measured ages. We combined multiple ground-based spectroscopy surveys to reveal an activity cycle of 13.1 +/- 1.1 years, a period similar to that observed in the Sun. We used different gyrochronology models with the asteroseismic age to estimate a rotation period of ~30 days for the primary. Alpha Men A is now the closest (d=10pc) solar analog with a precise asteroseismic age from space-based photometry, making it a prime target for next-generation direct imaging missions searching for true Earth analogs.
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 $\Delta\nu$ and
$\delta\nu_{02}$; the behavior of line widths with frequency and line widths at
$\nu_{\rm max}$ with $T_{\rm eff}$, 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.
The NASA-\(\it{TESS}\) mission presents a treasure trove for understanding the stars it observes and the Milky Way, in which they reside. We present a first look at the prospects for Galactic and ...stellar astrophysics by performing initial asteroseismic analyses of bright (\(G < 11\)) red giant stars in the \(\it{TESS}\) Southern Continuous Viewing Zone (SCVZ). Using three independent pipelines, we detect \(\nu_{\mathrm{max}}\) and \(\Delta\nu\) in 41% of the 15,405 star parent sample (6,388 stars), with consistency at a level of \(\sim 2\%\) in \(\nu_{\mathrm{max}}\) and \(\sim 5\%\) in \(\Delta\nu\). Based on this, we predict that seismology will be attainable for \(\sim 3\times10^{5}\) giants across the whole sky, subject to improvements in analysis and data reduction techniques. The best quality \(\it{TESS}\)-CVZ data, for 5,574 stars where pipelines returned consistent results, provide high quality power spectra across a number of stellar evolutionary states. This makes possible studies of, for example, the Asymptotic Giant Branch bump (AGBb). We demonstrate that mixed \(\ell=1\) modes and rotational splitting are cleanly observed in the 1-year data set. By combining \(\it{TESS}\)-CVZ data with \(\it{TESS}\)-HERMES, \(\it{SkyMapper}\), APOGEE and \(\it{Gaia}\) we demonstrate the potential for Galactic archaeology studies using the data, which provides good age precision and accuracy that reproduces the age of high \(\mathrm{\alpha/Fe}\) stars and relationships between mass and kinematics from studies based on \(\it{Kepler}\). Better quality astrometry and simpler target selection than the \(\it{Kepler}\) sample makes this data ideal for studies of the local star formation history and evolution of the Galactic disc. These results provide a strong case for detailed spectroscopic follow-up in the CVZs to complement that which has been (or will be) collected by current surveys. Abridged
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 modelling 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 \(\sim\)2\% in radius, \(\sim\)4\% in mass, and \(\sim\)10\% in age, making this the best-characterised sample of main-sequence stars available to date. Our predicted initial abundances and mixing-length parameters are checked against inferences from chemical enrichment laws \(\Delta Y / \Delta Z\) 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.
