Abstract
Asteroseismic parameters allow us to measure the basic stellar properties of field giants observed far across the Galaxy. Most of such determinations are, up to now, based on simple scaling ...relations involving the large-frequency separation, Δν, and the frequency of maximum power, νmax. In this work, we implement Δν and the period spacing, ΔP, computed along detailed grids of stellar evolutionary tracks, into stellar isochrones and hence in a Bayesian method of parameter estimation. Tests with synthetic data reveal that masses and ages can be determined with typical precision of 5 and 19 per cent, respectively, provided precise seismic parameters are available. Adding independent on the stellar luminosity, these values can decrease down to 3 and 10 per cent, respectively. The application of these methods to NGC 6819 giants produces a mean age in agreement with those derived from isochrone fitting, and no evidence of systematic differences between RGB and RC stars. The age dispersion of NGC 6819 stars, however, is larger than expected, with at least part of the spread ascribable to stars that underwent mass-transfer events.
We introduce a new generation of PARSEC-COLIBRI stellar isochrones that includes a detailed treatment of the thermally pulsing asymptotic giant branch (TP-AGB) phase, covering a wide range of initial ...metallicities (0.0001 < Zi < 0.06). Compared to previous releases, the main novelties and improvements are use of new TP-AGB tracks and related atmosphere models and spectra for M and C-type stars; inclusion of the surface H+He+CNO abundances in the isochrone tables, accounting for the effects of diffusion, dredge-up episodes and hot-bottom burning; inclusion of complete thermal pulse cycles, with a complete description of the in-cycle changes in the stellar parameters; new pulsation models to describe the long-period variability in the fundamental and first-overtone modes; and new dust models that follow the growth of the grains during the AGB evolution, in combination with radiative transfer calculations for the reprocessing of the photospheric emission. Overall, these improvements are expected to lead to a more consistent and detailed description of properties of TP-AGB stars expected in resolved stellar populations, especially in regard to their mean photometric properties from optical to mid-infrared wavelengths. We illustrate the expected numbers of TP-AGB stars of different types in stellar populations covering a wide range of ages and initial metallicities, providing further details on the "C-star island" that appears at intermediate values of age and metallicity, and about the AGB-boosting effect that occurs at ages close to 1.6-Gyr for populations of all metallicities. The isochrones are available through a new dedicated web server.
Abstract
Double-line eclipsing binaries (DLEBs) have been recently used to constrain the amount of central mixing as a function of stellar mass, with contrasting results. In this work, we reanalyse ...the DLEB sample by Claret & Torres, using a Bayesian method and new parsec tracks that account for both convective core overshooting and rotational mixing. Using overshooting alone, we obtain that, for masses larger than about 1.9 M⊙, the distribution of the overshooting parameter, λov, has a wide dispersion between 0.3 and 0.8, with essentially no values below λov = 0.3 and 0.4. While the lower limit supports a mild convective overshooting efficiency, the large dispersion derived is difficult to explain in the framework of current models of that process, which leave little room for large randomness. We suggest that a simple interpretation of our results can be rotational mixing: Different initial rotational velocities, in addition to a fixed amount of overshooting, could reproduce the high dispersion derived for intermediate-mass stars. After a reanalysis of the data, we find good agreement with models computed with a fixed overshooting parameter, $\mbox{$\lambda _\mathrm{ov}$}=0.4$, and initial rotational rates, ω, uniformly distributed in a wide range between 0 and 0.8 times the break-up value, at varying initial mass. We also find that our best-fitting models for the components of α Aurigae and TZ Fornacis agree with their observed rotational velocities, thus providing independent support to our hypothesis. We conclude that a constant efficiency of overshooting in concurrence with a star-to-star variation in the rotational mixing might be crucial in the interpretation of such data.
The Second APOKASC Catalog: The Empirical Approach Pinsonneault, Marc H.; Elsworth, Yvonne P.; Tayar, Jamie ...
The Astrophysical journal. Supplement series,
12/2018, Letnik:
239, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We present a catalog of stellar properties for a large sample of 6676 evolved stars with Apache Point Observatory Galactic Evolution Experiment spectroscopic parameters and Kepler asteroseismic data ...analyzed using five independent techniques. Our data include evolutionary state, surface gravity, mean density, mass, radius, age, and the spectroscopic and asteroseismic measurements used to derive them. We employ a new empirical approach for combining asteroseismic measurements from different methods, calibrating the inferred stellar parameters, and estimating uncertainties. With high statistical significance, we find that asteroseismic parameters inferred from the different pipelines have systematic offsets that are not removed by accounting for differences in their solar reference values. We include theoretically motivated corrections to the large frequency spacing (Δ ) scaling relation, and we calibrate the zero-point of the frequency of the maximum power ( max) relation to be consistent with masses and radii for members of star clusters. For most targets, the parameters returned by different pipelines are in much better agreement than would be expected from the pipeline-predicted random errors, but 22% of them had at least one method not return a result and a much larger measurement dispersion. This supports the usage of multiple analysis techniques for asteroseismic stellar population studies. The measured dispersion in mass estimates for fundamental calibrators is consistent with our error model, which yields median random and systematic mass uncertainties for RGB stars of order 4%. Median random and systematic mass uncertainties are at the 9% and 8% level, respectively, for red clump stars.
We derive age constraints for 1639 red giants in the APOKASC sample for which seismic parameters from Kepler, as well as effective temperatures, metallicities and α/Fe values from APOGEE DR12 (Apache ...Point Observatory Galactic Evolution Experiment Data Release 12) are available. We investigate the relation between age and chemical abundances for these stars, using a simple and robust approach to obtain ages. We first derive stellar masses using standard seismic scaling relations, then determine the maximum possible age for each star as function of its mass and metallicity, independently of its evolutionary stage. While the overall trend between maximum age and chemical abundances is a declining fraction of young stars with increasing α/Fe, at least 14 out of 241 stars with α/Fe >0.13 are younger than 6 Gyr. Five stars with α/Fe ≥0.2 have ages below 4 Gyr. We examine the effect of modifications in the standard seismic scaling relations, as well as the effect of very low helium fractions, but these changes are not enough to make these stars as old as usually expected for α-rich stars (i.e. ages greater than 8–9 Gyr). Such unusual α-rich young stars have also been detected by other surveys, but defy simple explanations in a galaxy evolution context.
ABSTRACT
The NASA Transiting Exoplanet Survey Satellite (NASA-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 TESS southern continuous viewing zone (SCVZ). Using three independent pipelines, we detect νmax and Δν in 41 per cent of the 15 405 star parent sample (6388 stars), with consistency at a level of $\sim \! 2{{\ \rm per\ cent}}$ in νmax and $\sim \! 5{{\ \rm per\ cent}}$ in Δν. Based on this, we predict that seismology will be attainable for ∼3 × 105 giants across the whole sky and at least 104 giants with ≥1 yr of observations in the TESS-CVZs, subject to improvements in analysis and data reduction techniques. The best quality TESS-CVZ data, for 5574 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. Furthermore, we demonstrate that mixed ℓ = 1 modes and rotational splitting are cleanly observed in the 1-yr data set. By combining TESS-CVZ data with TESS-HERMES, SkyMapper, APOGEE, and Gaia, we demonstrate its strong potential for Galactic archaeology studies, providing good age precision and accuracy that reproduces well the age of high α/Fe stars and relationships between mass and kinematics from previous studies based on e.g. Kepler. Better quality astrometry and simpler target selection than the 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.
We present a first determination of distances and extinctions for individual stars in the first release of the APOKASC catalogue, built from the joint efforts of the Apache Point Observatory Galactic ...Evolution Experiment (APOGEE) and the Kepler Asteroseismic Science Consortium (KASC). Our method takes into account the spectroscopic constraints derived from the APOGEE Stellar Parameters and Chemical Abundances Pipeline, together with the asteroseismic parameters from KASC. These parameters are then employed to estimate intrinsic stellar properties, including absolute magnitudes, using the Bayesian tool param. We then find the distance and extinction that best fit the observed photometry in Sloan Digital Sky Survey (SDSS), 2MASS, and WISE passbands. The first 1989 giants targetted by APOKASC are found at typical distances between 0.5 and 5 kpc, with individual uncertainties of just ∼1.8 per cent. Our extinction estimates are systematically smaller than provided in the Kepler Input Catalogue and by the Schlegel et al. maps. Distances to individual stars in the NGC 6791 and NGC 6819 star clusters agree to within their credible intervals. Comparison with the APOGEE red clump and SAGA catalogues provide another useful check, exhibiting agreement with our measurements to within a few per cent. Overall, present methods seem to provide excellent distance and extinction determinations for the bulk of the APOKASC sample. Approximately one third of the stars present broad or multiple-peaked probability density functions and hence increased uncertainties. Uncertainties are expected to be reduced in future releases of the catalogue, when a larger fraction of the stars will have seismically determined evolutionary status classifications.
Multiple stellar populations in NGC 1866 Costa, Guglielmo; Girardi, Léo; Bressan, Alessandro ...
Astronomy and astrophysics (Berlin),
11/2019, Letnik:
631
Journal Article
Recenzirano
Odprti dostop
We performed a comprehensive study of the stellar populations in the young Large Magellanic Cloud cluster NGC 1866, combining the analysis of its best-studied Cepheids with that of a very accurate ...colour–magnitude diagram (CMD) obtained from the most recent Hubble Space Telescope photometry. We used a Bayesian method based on new PARSEC stellar evolutionary tracks with overshooting and rotation to obtain ages and initial rotation velocities of five well-studied Cepheids of the cluster. We find that four of the five Cepheids belong to an initially slowly rotating young population (of 176 ± 5 Myr), while the fifth is significantly older, either 288 ± 20 Myr for models with high initial rotational velocity (ωi ∼ 0.9), or 202 ± 5 Myr for slowly rotating models. The complementary analysis of the CMD rules out the latter solution while strongly supporting the presence of two distinct populations of ∼176 Myr and ∼288 Myr, respectively. Moreover, the observed multiple main sequences and the turn-offs indicate that the younger population is mainly made of slowly rotating stars, as is the case of the four younger Cepheids, while the older population is made mainly of initially fast rotating stars, as is the case of the fifth Cepheid. Our study reinforces the notion that some young clusters like NGC 1866 harbour multiple populations. This work also hints that the first population, i.e. the older, may inherit the angular momentum from the parent cloud while stars of the second population, i.e. the younger, do not.
Abstract
Doppler-based planet surveys point to an increasing occurrence rate of giant planets with stellar mass. Such surveys rely on evolved stars for a sample of intermediate-mass stars (so-called ...retired A stars), which are more amenable to Doppler observations than their main-sequence progenitors. However, it has been hypothesized that the masses of subgiant and low-luminosity red-giant stars targeted by these surveys – typically derived from a combination of spectroscopy and isochrone fitting – may be systematically overestimated. Here, we test this hypothesis for the particular case of the exoplanet-host star HD 212771 using K2 asteroseismology. The benchmark asteroseismic mass ($1.45^{+0.10}_{-0.09}\,\text{M}_{\odot }$) is significantly higher than the value reported in the discovery paper (1.15 ± 0.08 M⊙), which has been used to inform the stellar mass–planet occurrence relation. This result, therefore, does not lend support to the above hypothesis. Implications for the fates of planetary systems are sensitively dependent on stellar mass. Based on the derived asteroseismic mass, we predict the post-main-sequence evolution of the Jovian planet orbiting HD 212771 under the effects of tidal forces and stellar mass-loss.
Multiple stellar populations in NGC 1866 Costa, Guglielmo; Girardi, Léo; Bressan, Alessandro ...
Astronomy and astrophysics (Berlin),
11/2019, Letnik:
631
Journal Article
Recenzirano
Odprti dostop
We performed a comprehensive study of the stellar populations in the young Large Magellanic Cloud cluster NGC 1866, combining the analysis of its best-studied Cepheids with that of a very accurate ...colour–magnitude diagram (CMD) obtained from the most recent
Hubble
Space Telescope photometry. We used a Bayesian method based on new PARSEC stellar evolutionary tracks with overshooting and rotation to obtain ages and initial rotation velocities of five well-studied Cepheids of the cluster. We find that four of the five Cepheids belong to an initially slowly rotating young population (of 176 ± 5 Myr), while the fifth is significantly older, either 288 ± 20 Myr for models with high initial rotational velocity (
ω
i
∼ 0.9), or 202 ± 5 Myr for slowly rotating models. The complementary analysis of the CMD rules out the latter solution while strongly supporting the presence of two distinct populations of ∼176 Myr and ∼288 Myr, respectively. Moreover, the observed multiple main sequences and the turn-offs indicate that the younger population is mainly made of slowly rotating stars, as is the case of the four younger Cepheids, while the older population is made mainly of initially fast rotating stars, as is the case of the fifth Cepheid. Our study reinforces the notion that some young clusters like NGC 1866 harbour multiple populations. This work also hints that the first population, i.e. the older, may inherit the angular momentum from the parent cloud while stars of the second population, i.e. the younger, do not.
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