The Gaia -ESO Survey is an European Southern Observatory (ESO) public spectroscopic survey that targeted 10 5 stars in the Milky Way covering the major populations of the disk, bulge and halo. The ...observations were made using FLAMES on the VLT obtaining both UVES high ( R ~ 47 000) and GIRAFFE medium ( R ~ 20 000) resolution spectra. The analysis of the Gaia -ESO spectra was the work of multiple analysis teams (nodes) within five working groups (WG). The homogenisation of the stellar parameters within WG11 (high resolution observations of FGK stars) and the homogenisation of the stellar parameters within WG10 (medium resolution observations of FGK stars) is described here. In both cases, the homogenisation was carried out using a Bayesian Inference method developed specifically for the Gaia -ESO Survey by WG11. The method was also used for the chemical abundance homogenisation within WG11, however, the WG10 chemical abundance data set was too sparsely populated so basic corrections for each node analysis were employed for the homogenisation instead. The WG10 homogenisation primarily used the cross-match of stars with WG11 as the reference set in both the stellar parameter and chemical abundance homogenisation. In this way the WG10 homogenised results have been placed directly onto the WG11 stellar parameter and chemical abundance scales. The reference set for the metal-poor end was sparse which limited the effectiveness of the homogenisation in that regime. For WG11, the total number of stars for which stellar parameters were derived was 6 231 with typical uncertainties for T eff , log g and Fe/H of 32 K, 0.05 and 0.05 respectively. One or more chemical abundances out of a possible 39 elements were derived for 6 188 of the stars. For WG10, the total number of stars for which stellar parameters were derived was 76 675 with typical uncertainties for T eff , log g and Fe/H of 64 K, 0.15 and 0.07 respectively. One or more chemical abundances out of a possible 30 elements were derived for 64177 of the stars.
The
Gaia
-ESO Survey is a public spectroscopic survey that targeted ≳10
5
stars covering all major components of the Milky Way from the end of 2011 to 2018, delivering its final public release in May ...2022. Unlike other spectroscopic surveys,
Gaia
-ESO is the only survey that observed stars across all spectral types with dedicated, specialised analyses: from O (
T
eff
~ 30 000–52 000 K) all the way to K-M (≳3500 K). The physics throughout these stellar regimes varies significantly, which has previously prohibited any detailed comparisons between stars of significantly different types. In the final data release (internal data release 6) of the
Gaia
-ESO Survey, we provide the final database containing a large number of products, such as radial velocities, stellar parameters and elemental abundances, rotational velocity, and also, for example, activity and accretion indicators in young stars and membership probability in star clusters for more than 114 000 stars. The spectral analysis is coordinated by a number of working groups (WGs) within the survey, each specialised in one or more of the various stellar samples. Common targets are analysed across WGs to allow for comparisons (and calibrations) amongst instrumental setups and spectral types. Here we describe the procedures employed to ensure all survey results are placed on a common scale in order to arrive at a single set of recommended results for use by all survey collaborators. We also present some general quality and consistency checks performed on the entirety of the survey results.
Context.
The Gaia-ESO Survey (GES) is a large public spectroscopic survey that was carried out using the multi-object FLAMES spectrograph at the Very Large Telescope. The survey provides accurate ...radial velocities, stellar parameters, and elemental abundances for ~115 000 stars in all Milky Way components.
Aims.
In this paper, we describe the method adopted in the final data release to derive lithium equivalent widths (EWs) and abundances.
Methods.
Lithium EWs were measured using two different approaches for FGK and M-type stars, to account for the intrinsic differences in the spectra. For FGK stars, we fitted the lithium line using Gaussian components, while direct integration over a predefined interval was adopted for M-type stars. Care was taken to ensure continuity between the two regimes. Abundances were derived using a new set of homogeneous curves of growth that were derived specifically for GES, and which were measured on a synthetic spectral grid consistently with the way the EWs were measured. The derived abundances were validated by comparison with those measured by other analysis groups using different methods.
Results.
Lithium EWs were measured for ~40 000 stars, and abundances could be derived for ~38 000 of them. The vast majority of the measures (80%) have been obtained for stars in open cluster fields. The remaining objects are stars in globular clusters, or field stars in the Milky Way disc, bulge, and halo.
Conclusions.
The GES dataset of homogeneous lithium abundances described here will be valuable for our understanding of several processes, from stellar evolution and internal mixing in stars at different evolutionary stages to Galactic evolution.
ABSTRACT
Since the advent of Gaia astrometry, it is possible to identify massive accreted systems within the Galaxy through their unique dynamical signatures. One such system, Gaia-Sausage-Enceladus ...(GSE), appears to be an early ‘building block’ given its virial mass $\gt 10^{10}\, \mathrm{M_\odot }$ at infall (z ∼ 1−3). In order to separate the progenitor population from the background stars, we investigate its chemical properties with up to 30 element abundances from the GALAH+ Survey Data Release 3 (DR3). To inform our choice of elements for purely chemically selecting accreted stars, we analyse 4164 stars with low-α abundances and halo kinematics. These are most different to the Milky Way stars for abundances of Mg, Si, Na, Al, Mn, Fe, Ni, and Cu. Based on the significance of abundance differences and detection rates, we apply Gaussian mixture models to various element abundance combinations. We find the most populated and least contaminated component, which we confirm to represent GSE, contains 1049 stars selected via Na/Fe versus Mg/Mn in GALAH+ DR3. We provide tables of our selections and report the chrono-chemodynamical properties (age, chemistry, and dynamics). Through a previously reported clean dynamical selection of GSE stars, including $30 \lt \sqrt{J_R / \, \mathrm{kpc\, km\, s^{-1}}} \lt 55$, we can characterize an unprecedented 24 abundances of this structure with GALAH+ DR3. With our chemical selection we characterize the dynamical properties of the GSE, for example mean $\sqrt{J_R / \, \mathrm{kpc\, km\, s^{-1}}} =$$26_{-14}^{+9}$. We find only $(29\pm 1){{\ \rm per\ cent}}$ of the GSE stars within the clean dynamical selection region. Our methodology will improve future studies of accreted structures and their importance for the formation of the Milky Way.
Data from the SDSS-IV/Apache Point Observatory Galactic Evolution Experiment (APOGEE-2) have been released as part of SDSS Data Releases 13 (DR13) and 14 (DR14). These include high-resolution H-band ...spectra, radial velocities, and derived stellar parameters and abundances. DR13, released in 2016 August, contained APOGEE data for roughly 150,000 stars, and DR14, released in 2017 August, added about 110,000 more. Stellar parameters and abundances have been derived with an automated pipeline, the APOGEE Stellar Parameter and Chemical Abundance Pipeline (ASPCAP). We evaluate the performance of this pipeline by comparing the derived stellar parameters and abundances to those inferred from optical spectra and analysis for several hundred stars. For most elements-C, Na, Mg, Al, Si, S, Ca, Cr, Mn, Ni-the DR14 ASPCAP analyses have systematic differences with the comparisons samples of less than 0.05 dex (median), and random differences of less than 0.15 dex (standard deviation). These differences are a combination of the uncertainties in both the comparison samples as well as the ASPCAP analysis. Compared to the references, magnesium is the most accurate alpha-element derived by ASPCAP, and shows a very clear thin/thick disk separation, while nickel is the most accurate iron-peak element (besides iron itself).
ABSTRACT
Stellar ages are a crucial component to studying the evolution of the Milky Way. Using Gaia DR2 distance estimates, it is now possible to estimate stellar ages for a larger volume of evolved ...stars through isochrone matching. This work presents M/H–age and α/M–age relations derived for different spatial locations in the Milky Way disc. These relations are derived by hierarchically modelling the star formation history of stars within a given chemical abundance bin. For the first time, we directly observe that significant variation is apparent in the M/H–age relation as a function of both Galactocentric radius and distance from the disc mid-plane. The M/H–age relations support claims that radial migration has a significant effect in the plane of the disc. Using the M/H bin with the youngest mean age at each radial zone in the plane of the disc, the present-day metallicity gradient is measured to be −0.059 ± 0.010 dex kpc−1, in agreement with Cepheids and young field stars. We find a vertically flared distribution of young stars in the outer disc, confirming predictions of models and previous observations. The mean age of the M/H–α/M distribution of the solar neighbourhood suggests that the high-M/H stars are not an evolutionary extension of the low-α sequence. Our observational results are important constraints to Galactic simulations and models of chemical evolution.
Abstract
In the age of high-resolution spectroscopic stellar surveys of the Milky Way, the number of stars with detailed abundances of multiple elements is rapidly increasing. These elemental ...abundances are directly influenced by the evolutionary history of the Galaxy, but this can be difficult to interpret without an absolute timeline of the abundance enrichment. We present age–abundance trends for M/H, α/M, and 17 individual elements using a sample of 721 solar neighbourhood Hipparcos red giant stars observed by Apache Point Observatory Galactic Evolution Experiment. These age trends are determined through a Bayesian hierarchical modelling method presented by Feuillet et al. We confirm that the α/M–age relation in the solar neighbourhood is steep and relatively narrow (0.20 dex age dispersion), as are the O/M–age and Mg/M–age relations. The age trend of C/N is steep and smooth, consistent with stellar evolution. The M/H–age relation has a mean age dispersion of 0.28 dex and a complex overall structure. The oldest stars in our sample are those with the lowest and highest metallicities, while the youngest stars are those with solar metallicity. These results provide strong constraints on theoretical models of Galactic chemical evolution (GCE). We compare them to the predictions of one-zone GCE models and multizone mixtures, both analytic and numerical. These comparisons support the hypothesis that the solar neighbourhood is composed of stars born at a range of Galactocentric radii, and that the most metal-rich stars likely migrated from a region with earlier and more rapid star formation such as the inner Galaxy.
The Gaia-ESO Survey is an European Southern Observatory (ESO) public spectroscopic survey that targeted \(10^5\) stars in the Milky Way covering the major populations of the disk, bulge and halo. The ...observations were made using FLAMES on the VLT obtaining both UVES high (\(R\sim47,000\)) and GIRAFFE medium (\(R\sim20,000\)) resolution spectra. The analysis of the Gaia-ESO spectra was the work of multiple analysis teams (nodes) within five working groups (WG). The homogenisation of the stellar parameters within WG11 (high resolution observations of FGK stars) and the homogenisation of the stellar parameters within WG10 (medium resolution observations of FGK stars) is described here. In both cases, the homogenisation was carried out using a bayesian Inference method developed specifically for the Gaia-ESO Survey by WG11. The WG10 homogenisation primarily used the cross-match of stars with WG11 as the reference set in both the stellar parameter and chemical abundance homogenisation. In this way the WG10 homogenised results have been placed directly onto the WG11 stellar parameter and chemical abundance scales. The reference set for the metal-poor end was sparse which limited the effectiveness of the homogenisation in that regime. For WG11, the total number of stars for which stellar parameters were derived was 6,231 with typical uncertainties for Teff, log g and Fe/H of 32~K, 0.05 and 0.05 respectively. One or more chemical abundances out of a possible 39 elements were derived for 6,188 of the stars. For WG10, the total number of stars for which stellar parameters were derived was 76,675 with typical uncertainties for Teff, log g and Fe/H of 64~K, 0.15 and 0.07 respectively. One or more chemical abundances out of a possible 30 elements were derived for 64,177 of the stars.
We present stellar age distributions of the Milky Way bulge region using ages for ∼6000 high-luminosity ( ), metal-rich (Fe/H ≥ −0.5) bulge stars observed by the Apache Point Observatory Galactic ...Evolution Experiment. Ages are derived using The Cannon label-transfer method, trained on a sample of nearby luminous giants with precise parallaxes for which we obtain ages using a Bayesian isochrone-matching technique. We find that the metal-rich bulge is predominantly composed of old stars (>8 Gyr). We find evidence that the planar region of the bulge ( kpc) is enriched in metallicity, Z, at a faster rate (dZ/dt ∼ 0.0034 Gyr−1) than regions farther from the plane (dZ/dt ∼ 0.0013 Gyr−1 at kpc). We identify a nonnegligible fraction of younger stars (age ∼2-5 Gyr) at metallicities of +0.2 < Fe/H < +0.4. These stars are preferentially found in the plane ( kpc) and at Rcy 2-3 kpc, with kinematics that are more consistent with rotation than are the kinematics of older stars at the same metallicities. We do not measure a significant age difference between stars found inside and outside the bar. These findings show that the bulge experienced an initial starburst that was more intense close to the plane than far from the plane. Then, star formation continued at supersolar metallicities in a thin disk at 2 kpc Rcy 3 kpc until ∼2 Gyr ago.
ABSTRACT
We present a new suite of cosmological zoom-in hydrodynamical ($\approx 20\, \mathrm{pc}$ spatial resolution) simulations of Milky-Way mass galaxies to study how a varying mass ratio for a ...Gaia-Sausage-Enceladus (GSE) progenitor impacts the z = 0 chemodynamics of halo stars. Using the genetic modification approach, we create five cosmological histories for a Milky-Way-mass dark matter halo ($M_{200}\approx 10^{12} \, \mbox{M}_\mathrm{\odot }$), incrementally increasing the stellar mass ratio of a z ≈ 2 merger from 1:25 to 1:2, while fixing the galaxy’s final dynamical, stellar mass, and large-scale environment. We find markedly different morphologies at z = 0 following this change in early history, with a growing merger resulting in increasingly compact and bulge-dominated galaxies. Despite this structural diversity, all galaxies show a radially biased population of inner halo stars like the Milky-Way’s GSE which, surprisingly, has a similar magnitude, age, $\rm Fe/H$, and $\rm \alpha /Fe$ distribution whether the z ≈ 2 merger is more minor or major. This arises because a smaller ex-situ population at z ≈ 2 is compensated by a larger population formed in an earlier merger-driven starburst whose contribution to the GES can grow dynamically over time, and with both populations strongly overlapping in the $\rm Fe/H-\rm \alpha /Fe$ plane. Our study demonstrates that multiple high-redshift histories can lead to similar z = 0 chemodynamical features in the halo, highlighting the need for additional constraints to distinguish them, and the importance of considering the full spectrum of progenitors when interpreting z = 0 data to reconstruct our Galaxy’s past.
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