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Register a new userDeciphering the evolution of the Milky Way discs: Gaia APOGEE Kepler giant stars and the Besanc{c}on Galaxy Model
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We investigate the properties of the double sequences of the Milky Way discs visible in the [$alpha$/Fe] vs [Fe/H] diagram. In the framework of Galactic formation and evolution, we discuss the complex relationships between age, metallicity, [$alpha$/Fe], and the velocity components. We study stars with measured chemical, seismic and astrometric properties from the APOGEE survey, the Kepler and Gaia satellites, respectively. We separate the [$alpha$/Fe]-[Fe/H] diagram into 3 stellar populations: the thin disc, the high-$alpha$ metal-poor thick disc and the high-$alpha$ metal-rich thick disc and characterise each of these in the age-chemo-kinematics parameter space. We compare results obtained from different APOGEE data releases and using two recent age determinations. We use the Besanc{c}on Galaxy model (BGM) to highlight selection biases and mechanisms not included in the model. The thin disc exhibits a flat age-metallicity relation while [$alpha$/Fe] increases with stellar age. We confirm no correlation between radial and vertical velocities with [Fe/H], [$alpha$/Fe] and age for each stellar population. Considering both samples, V$_varphi$ decreases with age for the thin disc, while it increases with age for the h$alpha$mp thick disc. Although the age distribution of the h$alpha$mr thick disc is very close to that of the h$alpha$mp thick disc between 7 and 14 Gyr, its kinematics seems to follow that of the thin disc. This feature, not predicted by the hypotheses included in the BGM, suggests a different origin and history for this population. Finally, we show that there is a maximum dispersion of the vertical velocity, $sigma_Z$, with age for the h$alpha$mp thick disc around 8 Gyr. The comparisons with the BGM simulations suggest a more complex chemo-dynamical scheme to explain this feature, most likely including mergers and radial migration effects
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We construct a dynamical model of the Milky Way disk from a data set, which combines Gaia EDR3 and APOGEE data throughout Galactocentric radii between $5.0leq Rleq19.5$ kpc. We make use of the spherically-aligned Jeans Anisotropic Method to model the stellar velocities and their velocity dispersions. Building upon our previous work, our model now is fitted to kinematic maps that have been extended to larger Galactocentric radii due to the expansion of our data set, probing the outer regions of the Galactic disk. Our best-fitting dynamical model suggests a logarithmic density slope of $alpha_{rm DM}=-1.602pm0.079_{rm syst}$ for the dark matter halo and a dark matter density of $rho_{rm DM}(R_{odot})=(8.92pm0.56_{rm syst})times 10^{-3}$ M$_{odot}$ pc$^{-3}$ ($0.339pm0.022_{rm syst}$ GeV cm$^{3}$). We estimate a circular velocity at the solar radius of $v_{rm circ}=(234.7pm1.7_{rm syst})$ km s$^{-1}$ with a decline towards larger radii. The total mass density is $rho_{rm tot}(R_{odot})$=$(0.0672pm0.0015_{rm syst})$ M$_{odot}$ pc$^{-3}$ with a slope of $alpha_{rm tot}$=$-2.367pm0.047_{rm syst}$ for $5leq Rleq19.5$ kpc and the total surface density is $Sigma(R{_odot}, |z|leq$ 1.1 kpc)=$(55.5pm1.7_{rm syst})$ M$_{odot}$ pc$^{-2}$. While the statistical errors are small, the error budget of the derived quantities is dominated by the 3 to 7 times larger systematic uncertainties. These values are consistent with our previous determination, but systematic uncertainties are reduced due to the extended data set covering a larger spatial extent of the Milky Way disk. Furthermore, we test the influence of non-axisymmetric features on our resulting model and analyze how a flaring disk model would change our findings.
[Abridged] Ensemble studies of red-giant stars with exquisite asteroseismic, spectroscopic, and astrometric constraints offer a novel opportunity to recast and address long-standing questions concerning the evolution of stars and of the Galaxy. Here, we infer masses and ages for nearly 5400 giants with available Kepler light curves and APOGEE spectra, and discuss some of the systematics that may affect the accuracy of the inferred stellar properties. First, we look at age-chemical-abundances relations. We find a dearth of young, metal-rich stars, and the existence of a significant population of old (8-9 Gyr), low-[$alpha$/Fe], super-solar metallicity stars, reminiscent of the age and metallicity of the well-studied open cluster NGC6791. The age-chemo-kinematic properties of these stars indicate that efficient radial migration happens in the thin disk. We find that ages and masses of the nearly 400 $alpha$-element-rich red-giant-branch (RGB) stars in our sample are compatible with those of an old (~11 Gyr), nearly coeval, chemical-thick disk population. Using a statistical model, we show that 95% of the population was born within ~1.5 Gyr. Moreover, we find a difference in the vertical velocity dispersion between low- and high-[$alpha$/Fe] populations, confirming their different chemo-dynamical histories. We then exploit the almost coeval $alpha$-rich population to gain insight into processes that may have altered the mass of a star along its evolution, which are key to improve the mapping of the observed stellar mass to age. We find evidence for a mean integrated RGB mass loss <$Delta$M>= 0.10 $pm$ 0.02 Msun and that the occurrence of massive (M $gtrsim$ 1.1 Msun) $alpha$-rich stars is of the order of 5% on the RGB, and significantly higher in the RC, supporting the scenario in which most of these stars had undergone interaction with a companion.
We use models of stellar angular momentum evolution to determine ages for $sim500$ stars in the APOGEE-textit{Kepler} Cool Dwarfs sample. We focus on lower main-sequence stars, where other age-dating tools become ineffective. Our age distributions are compared to those derived from asteroseismic and giant samples and solar analogs. We are able to recover gyrochronological ages for old, lower-main-sequence stars, a remarkable improvement over prior work in hotter stars. Under our model assumptions, our ages have a median relative uncertainty of $14%$, comparable to the age precision inferred for more massive stars using traditional methods. We investigate trends of galactic $alpha$-enhancement with age, finding evidence of a detection threshold between the age of the oldest $alpha$-poor stars and that of the bulk $alpha$-rich population. We argue that gyrochronology is an effective tool reaching ages of 10--12 Gyr in K- and early M-dwarfs. Finally, we present the first effort to quantify the impact of detailed abundance patterns on rotational evolution. We estimate a $sim15%$ bias in age for cool, $alpha$-enhanced (+ 0.4 dex) stars when standard solar-abundance-pattern rotational models are used for age inference, rather than models that appropriately account for $alpha$-enrichment.
The history of the Milky Way is encoded in the spatial distributions, kinematics, and chemical enrichment patterns of its resolved stellar populations. SEGUE-2 and APOGEE, two of the four surveys that comprise SDSS-III (the Sloan Digital Sky Survey III), will map these distributions and enrichment patterns at optical and infrared wavelengths, respectively. Using the existing SDSS spectrographs, SEGUE-2 will obtain spectra of 140,000 stars in selected high-latitude fields to a magnitude limit r ~ 19.5, more than doubling the sample of distant halo stars observed in the SDSS-II survey SEGUE (the Sloan Extension for Galactic Understanding and Exploration). With spectral resolution R ~ 2000 and typical S/N per pixel of 20-25, SEGUE and SEGUE-2 measure radial velocities with typical precision of 5-10 km/s and metallicities ([Fe/H]) with a typical external error of 0.25 dex. APOGEE (the Apache Point Observatory Galactic Evolution Experiment) will use a new, 300-fiber H-band spectrograph (1.5-1.7 micron) to obtain high-resolution (R ~ 24,000), high signal-to-noise ratio (S/N ~ 100 per pixel) spectra of 100,000 red giant stars to a magnitude limit H ~ 12.5. Infrared spectroscopy penetrates the dust that obscures the inner Galaxy from our view, allowing APOGEE to carry out the first large, homogeneous spectroscopic survey of all Galactic stellar populations. APOGEE spectra will allow radial velocity measurements with < 0.5 km/s precision and abundance determinations (with ~ 0.1 dex precision) of 15 chemical elements for each program star, which can be used to reconstruct the history of star formation that produced these elements. (abridged)
We summarise recent results from analysis of APOGEE/Gaia data for stellar populations in the Galactic halo, disk, and bulge, leading to constraints on the contribution of dwarf galaxies and globular clusters to the stellar content of the Milky Way halo. Interpretation of the extant data in light of cosmological numerical simulations suggests that the Milky Way has been subject to an unusually intense accretion history at z >~ 1.5.
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