One-dimensional stellar evolution models have been successful at representing the structure and evolution of stars in diverse astrophysical contexts, but complications have been noted in the context ...of young, magnetically active stars, as well as close binary stars with significant tidal interactions. Numerous puzzles are associated with pre-main-sequence (pre-MS) and active main sequence (MS) stars, relating to their radii, their colors, certain elemental abundances, and the coevality of young clusters, among others. A promising explanation for these puzzles is the distorting effects of magnetic activity and starspots on the structure of active stars. To assist the community in evaluating this hypothesis, we present the Stellar Parameters of Tracks with Starspots (SPOTS) models, a grid of solar-metallicity stellar evolutionary tracks and isochrones that include a treatment of the structural effects of starspots. The models range from 0.1 to 1.3 M and from spotless to a surface covering fraction of 85%, and are evolved from the pre-MS to the red giant branch (or 15 Gyr). We also produce two-temperature synthetic colors for our models using empirically calibrated color tables. We describe the physical ingredients included in the SPOTS models and compare their predictions to other modern evolution codes. Finally, we apply these models to several open questions in the field of active stars, including the radii of young eclipsing binaries, the color scale of pre-MS stars, and the existence of sub-subgiants, demonstrating that our models can explain many peculiar features of active stars.
We present an independent confirmation of the zero-point offset of Gaia Data Release 2 parallaxes using asteroseismic data of evolved stars in the Kepler field. Using well-characterized red giant ...branch stars from the APOKASC-2 catalog, we identify a Gaia astrometric pseudocolor ( eff )- and Gaia G-band magnitude-dependent zero-point offset of ϖ seis − ϖ Gaia = 52.8 2.4 (rand.) 8.6 (syst.) − (150.7 22.7)( eff − 1.5) − (4.21 0.77)(G − 12.2) as, in the sense that Gaia parallaxes are too small. The offset is found in high- and low-extinction samples, as well as among both shell H-burning red giant stars and core He-burning red clump stars. We show that errors in the asteroseismic radius and temperature scales may be distinguished from errors in the Gaia parallax scale. We estimate systematic effects on the inferred global Gaia parallax offset, c, due to radius and temperature systematics, as well as choices in bolometric correction and the adopted form for Gaia parallax spatial correlations. Because of possible spatially correlated parallax errors, as discussed by the Gaia team, our Gaia parallax offset model is specific to the Kepler field, but broadly compatible with the magnitude- and color-dependent offset inferred by the Gaia team and several subsequent investigations using independent methods.
ABSTRACT We assess the impact of starspots on the evolution of late-type stars during the pre-main sequence (pre-MS) using a modified stellar evolution code. We find that heavily spotted models of ...mass 0.1-1.2 M are inflated by up to 10% during the pre-MS, and up to 4% and 9% for fully and partially convective stars at the zero-age MS, consistent with measurements from active eclipsing binary systems. Spots similarly decrease stellar luminosity and , causing isochrone-derived masses to be underestimated by up to a factor of , and ages to be underestimated by a factor of 2-10, at 3 Myr. Consequently, pre-MS clusters and their active stars are systematically older and more massive than often reported. Cluster ages derived with the lithium depletion boundary technique are erroneously young by ∼15% and 10% at 30 and 100 Myr, respectively, if stars with 50% spots are interpreted with unspotted models. Finally, lithium depletion is suppressed in spotted stars with radiative cores, leading to a fixed-temperature lithium dispersion on the MS if a range of spot properties are present on the pre-MS. Such dispersions are large enough to explain Li abundance spreads seen in young open clusters, and imply a range of radii at fixed mass and age during the pre-MS Li-burning epoch. By extension, this implies that mass, composition, and age do not uniquely specify the Hertzsprung-Russell diagram location of pre-MS stars.
Stellar surface rotation carries information about stellar parameters-particularly ages-and thus the large rotational data sets extracted from Kepler time series represent powerful probes of stellar ...populations. In this article, we address the challenge of interpreting such data sets with a forward-modeling exercise. We combine theoretical models of stellar rotation, a stellar population model for the galaxy, and prescriptions for observational bias to predict the rotation distribution in the Kepler field under standard "vanilla" assumptions. We arrive at two central conclusions: first, that standard braking models fail to reproduce the observed distribution at long periods, and second, that the interpretation of the period distribution is complicated by a mixture of evolutionary states and observational uncertainties. If we assume that the detectability of rotational signatures scales with the Rossby number, we can show that the observed period distribution contains an apparent "Rossby edge" at , above which long-period, high Rossby number stars are either absent or undetected. This threshold suggests either that weakened magnetic braking is in operation in the full Kepler population or that stars undergo a transition in spottedness and activity. We discuss the observations necessary to disentangle these competing scenarios. Regardless of the physical origin of the edge, it biases the inferred age distributions, affecting stars older than ∼9 Gyr at , older than at solar temperatures, and at 6500 K. Below , rotation periods should be viable age diagnostics in even the oldest stars in the population.
The rapidly rotating cool dwarfs of the Pleiades are rich in lithium relative to their slowly rotating counterparts. Motivated by observations of inflated radii in young, active stars, and by ...calculations showing that radius inflation inhibits pre-main-sequence (pre-MS) Li destruction, we test whether this pattern could arise from a connection between stellar rotation rate and radius inflation on the pre-MS. We demonstrate that pre-MS radius inflation can efficiently suppress lithium destruction by rotationally induced mixing in evolutionary models, and that the net effect of inflation and rotational mixing is a pattern where rotation correlates with lithium abundance for M
* < M⊙, and anticorrelates with lithium abundance for M
* > M⊙, similar to the empirical trend in the Pleiades. Next, we adopt different prescriptions for the dependence of inflation on rotation, and compare their predictions to the Pleiades lithium/rotation pattern. We find that if a connection between rotation and radius inflation exists, then the important qualitative features of this pattern naturally and generically emerge in our models. This is the first consistent physical model to date that explains the Li–rotation correlation in the Pleiades. We discuss plausible mechanisms for inducing this correlation and suggest an observational test using granulation.
Abstract
The period versus mass diagrams (i.e., rotational sequences) of open clusters provide crucial constraints for angular momentum evolution studies. However, their memberships are often heavily ...contaminated by field stars, which could potentially bias the interpretations. In this paper, we use data from Gaia DR2 to reassess the memberships of seven open clusters with ground- and space-based rotational data, and present an updated view of stellar rotation as a function of mass and age. We use the Gaia astrometry to identify the cluster members in phase space, and the photometry to derive revised ages and place the stars on a consistent mass scale. Applying our membership analysis to the rotational sequences reveals that: (1) the contamination in clusters observed from the ground can reach up to ∼35%; (2) the overall fraction of rotational outliers decreases substantially when the field contaminants are removed, but some outliers persist; (3) there is a sharp upper edge in the rotation periods at young ages; (4) at young ages, stars in the 1.0–0.6
M
⊙
range inhabit a global maximum of rotation periods, potentially providing an optimal window for habitable planets. Additionally, we see clear evidence for a strongly mass-dependent spin-down process. In the regime where rapid rotators are leaving the saturated domain, the rotational distributions broaden (in contradiction with popular models), which we interpret as evidence that the torque must be lower for rapid rotators than for intermediate ones. The cleaned rotational sequences from ground-based observations can be as constraining as those obtained from space.
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 investigate lithium depletion in standard stellar models (SSMs) and main sequence (MS) open clusters, and explore the origin of the Li dispersion in young, cool stars of equal mass, age, and ...composition. We first demonstrate that SSMs accurately predict the Li abundances of solar analogs at the zero-age main sequence (ZAMS) within theoretical uncertainties. We then measure the rate of MS Li depletion by removing the Fe/H-dependent ZAMS Li pattern from three well-studied clusters, and comparing the detrended data. MS depletion is found to be mass-dependent, in the sense of more depletion at low mass. A dispersion in Li abundance at fixed T sub(eff) is nearly universal, and sets in by ~200 Myr. We discuss mass and age dispersion trends, and the pattern is mixed. We argue that metallicity impacts the ZAMS Li pattern, in agreement with theoretical expectations but contrary to the findings of some previous studies, and suggest Li as a test of cluster metallicity. Finally, we argue that a radius dispersion in stars of fixed mass and age, during the epoch of pre-MS Li destruction, is responsible for the spread in Li abundances and the correlation between rotation and Li in young cool stars, most well known in the Pleiades. We calculate stellar models, inflated to match observed radius anomalies in magnetically active systems, and the resulting range of Li abundances reproduces the observed patterns of young clusters. We discuss ramifications for pre-MS evolutionary tracks and age measurements of young clusters, and suggest an observational test.
ABSTRACT The mass of a star is arguably its most fundamental parameter. For red giant stars, tracers luminous enough to be observed across the Galaxy, mass implies a stellar evolution age. It has ...proven to be extremely difficult to infer ages and masses directly from red giant spectra using existing methods. From the Kepler and apogee surveys, samples of several thousand stars exist with high-quality spectra and asteroseismic masses. Here we show that from these data we can build a data-driven spectral model using The Cannon, which can determine stellar masses to ∼0.07 dex from apogee dr12 spectra of red giants; these imply age estimates accurate to ∼0.2 dex (40%). We show that The Cannon constrains these ages foremost from spectral regions with CN absorption lines, elements whose surface abundances reflect mass-dependent dredge-up. We deliver an unprecedented catalog of 70,000 giants (including 20,000 red clump stars) with mass and age estimates, spanning the entire disk (from the Galactic center to kpc). We show that the age information in the spectra is not simply a corollary of the birth-material abundances and , and that, even within a monoabundance population of stars, there are age variations that vary sensibly with Galactic position. Such stellar age constraints across the Milky Way open up new avenues in Galactic archeology.
ABSTRACT
We measure star-spot filling fractions for 240 stars in the Pleiades and M67 open star clusters using APOGEE high-resolution H-band spectra. For this work, we developed a modified ...spectroscopic pipeline which solves for star-spot filling fraction and star-spot temperature contrast. We exclude binary stars, finding that the large majority of binaries in these clusters (80 per cent) can be identified from Gaia DR3 and APOGEE criteria – important for field star applications. Our data agree well with independent activity proxies, indicating that this technique recovers real star-spot signals. In the Pleiades, filling fractions saturate at a mean level of 0.248 ± 0.005 for active stars with a decline at slower rotation; we present fitting functions as a function of Rossby number. In M67, we recover low mean filling fractions of 0.030 ± 0.008 and 0.003 ± 0.002 for main sequence GK stars and evolved red giants, respectively, confirming that the technique does not produce spurious spot signals in inactive stars. Star-spots also modify the derived spectroscopic effective temperatures and convective overturn time-scales. Effective temperatures for active stars are offset from inactive ones by −109 ± 11 K, in agreement with the Pecaut & Mamajek empirical scale. Star-spot filling fractions at the level measured in active stars changes their inferred overturn time-scale, which biases the derived threshold for saturation. Finally, we identify a population of stars statistically discrepant from mean activity–Rossby relations and present evidence that these are genuine departures from a Rossby scaling. Our technique is applicable to the full APOGEE catalogue, with broad applications to stellar, galactic, and exoplanetary astrophysics.
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