Abstract
We study the chemical evolution of the thick and thin discs of the Galaxy by comparing detailed chemical evolution models with recent data from the Archéologie avec Matisse Basée sur les ...aRchives de l'ESO project. The data suggest that the stars in the thick and thin discs form two distinct sequences with the thick disc stars showing higher α/Fe ratios. We adopt two different approaches to model the evolution of thick and thin discs. In particular, we adopt (i) a two-infall approach where the thick disc forms fast and before the thin disc and by means of a fast gas accretion episode, whereas the thin disc forms by means of a second accretion episode on a longer time-scale; (ii) a parallel approach, where the two discs form in parallel but at different rates. By comparing our model results with the observed Mg/Fe versus Fe/H and the metallicity distribution functions in the two Galactic components, we conclude that the parallel approach can account for a group of α-enhanced metal-rich stars present in the data, whereas the two-infall approach cannot explain these stars unless they are the result of stellar migration. In both approaches, the thick disc has formed on a time-scale of accretion of 0.1 Gyr, whereas the thin disc formed on a time-scale of 7 Gyr in the solar region. In the two-infall approach, a gap in star formation between the thick and thin disc formation of several hundreds of Myr should be present, at variance with the parallel approach where no gap is present.
Context. The chemical evolution of lithium in the Milky Way represents a major problem in modern astrophysics. Indeed, lithium is, on the one hand, easily destroyed in stellar interiors, and, on the ...other hand, produced at some specific stellar evolutionary stages that are still not well constrained. Aims. The goal of this paper is to investigate the lithium stellar content of Milky Way stars in order to put constraints on the lithium chemical enrichment in our Galaxy, in particular in both the thin and thick discs. Methods. Thanks to high-resolution spectra from the ESO archive and high quality atmospheric parameters, we were able to build a massive and homogeneous catalogue of lithium abundances for 7300 stars derived with an automatic method coupling, a synthetic spectra grid, and a Gauss-Newton algorithm. We validated these lithium abundances with literature values, including those of the Gaia benchmark stars. Results. In terms of lithium galactic evolution, we show that the interstellar lithium abundance increases with metallicity by 1 dex from M/H = −1 dex to + 0.0 dex. Moreover, we find that this lithium ISM abundance decreases by about 0.5 dex at super-solar metalllicity. Based on a chemical separation, we also observed that the stellar lithium content in the thick disc increases rather slightly with metallicity, while the thin disc shows a steeper increase. The lithium abundance distribution of α-rich, metal-rich stars has a peak at ALi ~ 3 dex. Conclusions. We conclude that the thick disc stars suffered of a low lithium chemical enrichment, showing lithium abundances rather close to the Spite plateau while the thin disc stars clearly show an increasing lithium chemical enrichment with the metallicity, probably thanks to the contribution of low-mass stars.
Atomic data for the Gaia -ESO Survey Heiter, U.; Lind, K.; Bergemann, M. ...
Astronomy and astrophysics (Berlin),
2021, Letnik:
645
Journal Article
Recenzirano
Odprti dostop
Context.
We describe the atomic and molecular data that were used for the abundance analyses of FGK-type stars carried out within the
Gaia
-ESO Public Spectroscopic Survey in the years 2012 to 2019. ...The
Gaia
-ESO Survey is one among several current and future stellar spectroscopic surveys producing abundances for Milky-Way stars on an industrial scale.
Aims.
We present an unprecedented effort to create a homogeneous common line list, which was used by several abundance analysis groups using different radiative transfer codes to calculate synthetic spectra and equivalent widths. The atomic data are accompanied by quality indicators and detailed references to the sources. The atomic and molecular data are made publicly available at the CDS.
Methods.
In general, experimental transition probabilities were preferred but theoretical values were also used. Astrophysical
gf
-values were avoided due to the model-dependence of such a procedure. For elements whose lines are significantly affected by a hyperfine structure or isotopic splitting, a concerted effort has been made to collate the necessary data for the individual line components. Synthetic stellar spectra calculated for the Sun and Arcturus were used to assess the blending properties of the lines. We also performed adetailed investigation of available data for line broadening due to collisions with neutral hydrogen atoms.
Results.
Among a subset of over 1300 lines of 35 elements in the wavelength ranges from 475 to 685 nm and from 850 to 895 nm, we identified about 200 lines of 24 species which have accurate
gf
-values and are free of blends in the spectra of the Sun and Arcturus. For the broadening due to collisions with neutral hydrogen, we recommend data based on Anstee-Barklem-O’Mara theory, where possible. We recommend avoiding lines of neutral species for which these are not available. Theoretical broadening data by R.L. Kurucz should be used for Sc
II
, Ti
II
, and Y
II
lines; additionally, for ionised rare-earth species, the Unsöld approximation with an enhancement factor of 1.5 for the line width can be used.
Conclusions.
The line list has proven to be a useful tool for abundance determinations based on the spectra obtained within the
Gaia
-ESO Survey, as well as other spectroscopic projects. Accuracies below 0.2 dex are regularly achieved, where part of the uncertainties are due to differences in the employed analysis methods. Desirable improvements in atomic data were identified for a number of species, most importantly Al
I
, S
I
, and Cr
II
, but also Na
I
, Si
I
, Ca
II
, and Ni
I
.
Context. The pattern of chemical abundance ratios in stellar populations of the Milky Way is a fingerprint of the Galactic chemical history. In order to interpret such chemical fossils of Galactic ...archaeology, chemical evolution models have to be developed. However, despite the complex physics included in the most recent models, significant discrepancies between models and observations are widely encountered. Aims. The aim of this paper is to characterise the abundance patterns of five iron-peak elements (Mn, Fe, Ni, Cu, and Zn) for which the stellar origin and chemical evolution are still debated. Methods. We automatically derived iron peak (Mn, Fe, Ni, Cu, and Zn) and α element (Mg) chemical abundances for 4666 stars, adopting classical LTE spectral synthesis and 1D atmospheric models. Our observational data collection is composed of high-resolution, high signal-to-noise ratios HARPS and FEROS spectra, which were previously parametrised by the AMBRE project. Results. We used the bimodal distribution of the magnesium-to-iron abundance ratios to chemically classify our sample stars into different Galactic substructures: thin disc, metal-poor and high-α metal rich, high-α, and low-α metal-poor populations. Both high-α and low-α metal-poor populations are fully distinct in Mg, Cu, and Zn, but these substructures are statistically indistinguishable in Mn and Ni. Thin disc trends of Ni/Fe and Cu/Fe are very similar and show a small increase at supersolar metallicities. Also, both thin and thick disc trends of Ni and Cu are very similar and indistinguishable. Yet, Mn looks very different from Ni and Cu. Mn/Fe trends of thin and thick discs actually have noticeable differences: the thin disc is slightly Mn richer than the thick disc. The Zn/Fe trends look very similar to those of α/Fe trends. The typical dispersion of results in both discs is low (≈0.05 dex for Mg, Mn, and Cu/Fe) and is even much lower for Ni/Fe (≈0.035 dex). Conclusions. It is clearly demonstrated that Zn is an α-like element and could be used to separate thin and thick disc stars. Moreover, we show that the Mn/Mg ratio could also be a very good tool for tagging Galactic substructures. From the comparison with Galactic chemical evolutionary models, we conclude that some recent models can partially reproduce the observed Mg, Zn, and, Cu behaviours in thin and thick discs and metal-poor sequences. Models mostly fail to reproduce Mn and Ni in all metallicity domains, however, models adopting yields normalised from solar chemical properties reproduce Mn and Ni better, suggesting that there is still a lack of realistic theoretical yields of some iron-peak elements. The very low scatter (≈0.05 dex) in thin and thick disc sequences could provide an observational constrain for Galactic evolutionary models that study the efficiency of stellar radial migration.
ABSTRACT We report on VLT/GIRAFFE spectra of stars in two recently discovered ultra-faint satellites, Reticulum 2 and Horologium 1, obtained as part of the Gaia-ESO Survey. We identify 18 members in ...Reticulum 2 and five in Horologium 1. We find Reticulum 2 to have a velocity dispersion of , implying a mass-to-light ratio (M/L) of ∼500. The mean metallicity of Reticulum 2 is , with an intrinsic dispersion of ∼0.3 dex and -enhancement of ∼0.4 dex. We conclude that Reticulum 2 is a dwarf galaxy. We also report on the serendipitous discovery of four stars in a previously unknown stellar substructure near Reticulum 2 with and , far from the systemic velocity of Reticulum 2. For Horologium 1 we infer a velocity dispersion of and a M/L ratio of ∼600, leading us to conclude that Horologium 1 is also a dwarf galaxy. Horologium 1 is slightly more metal-poor than Reticulum 2 ( ) and is similarly -enhanced: with a significant spread of metallicities of 0.17 dex. The line-of-sight velocity of Reticulum 2 is offset by 100 km s−1 from the prediction of the orbital velocity of the Large Magellanic Cloud (LMC), thus making its association with the Cloud uncertain. However, at the location of Horologium 1, both the backward-integrated orbit of the LMC and its halo are predicted to have radial velocities similar to that of the dwarf. Therefore, it is possible that Horologium 1 is or once was a member of the Magellanic family.
Context. As observational evidence steadily accumulates, the nature of the Galactic bulge has proven to be rather complex: the structural, kinematic, and chemical analyses often lead to contradictory ...conclusions. The nature of the metal-rich bulge – and especially of the metal-poor bulge – and their relation with other Galactic components, still need to be firmly defined on the basis of statistically significant high-quality data samples. Aims. We used the fourth internal data release of the Gaia-ESO survey to characterize the bulge metallicity distribution function (MDF), magnesium abundance, spatial distribution, and correlation of these properties with kinematics. Moreover, the homogeneous sampling of the different Galactic populations provided by the Gaia-ESO survey allowed us to perform a comparison between the bulge, thin disk, and thick disk sequences in the Mg/Fe vs. Fe/H plane in order to constrain the extent of their eventual chemical similarities. Methods. We obtained spectroscopic data for ~2500 red clump stars in 11 bulge fields, sampling the area −10° ≤ l ≤ + 8° and −10° ≤ b ≤ −4° from the fourth internal data release of the Gaia-ESO survey. A sample of ~6300 disk stars was also selected for comparison. Spectrophotometric distances computed via isochrone fitting allowed us to define a sample of stars likely located in the bulge region. Results. From a Gaussian mixture models (GMM) analysis, the bulge MDF is confirmed to be bimodal across the whole sampled area. The relative ratio between the two modes of the MDF changes as a function of b, with metal-poor stars dominating at high latitudes. The metal-rich stars exhibit bar-like kinematics and display a bimodality in their magnitude distribution, a feature which is tightly associated with the X-shape bulge. They overlap with the metal-rich end of the thin disk sequence in the Mg/Fe vs. Fe/H plane. On the other hand, metal-poor bulge stars have a more isotropic hot kinematics and do not participate in the X-shape bulge. Their Mg enhancement level and general shape in the Mg/Fe vs. Fe/H plane is comparable to that of the thick disk sequence. The position at which Mg/Fe starts to decrease with Fe/H, called the “knee”, is observed in the metal-poor bulge at Fe/H knee = −0.37 ± 0.09, being 0.06 dex higher than that of the thick disk. Although this difference is inside the error bars, it suggest a higher star formation rate (SFR) for the bulge than for the thick disk. We estimate an upper limit for this difference of Δ Fe/H knee = 0.24 dex. Finally, we present a chemical evolution model that suitably fits the whole bulge sequence by assuming a fast (<1 Gyr) intense burst of stellar formation that takes place at early epochs. Conclusions. We associate metal-rich stars with the bar boxy/peanut bulge formed as the product of secular evolution of the early thin disk. On the other hand, the metal-poor subpopulation might be the product of an early prompt dissipative collapse dominated by massive stars. Nevertheless, our results do not allow us to firmly rule out the possibility that these stars come from the secular evolution of the early thick disk. This is the first time that an analysis of the bulge MDF and α-abundances has been performed in a large area on the basis of a homogeneous, fully spectroscopic analysis of high-resolution, high S/N data.
Aims. Until recently, most high-resolution spectroscopic studies of the Galactic thin and thick discs were mostly confined to objects in the solar vicinity. Here we aim at enlarging the volume in ...which individual chemical abundances are used to characterise the thin and thick discs, using the first internal data release of the Gaia-ESO survey (GES iDR1). Methods. We used the spectra of around 2000 FGK dwarfs and giants from the GES iDR1, obtained at resolutions of up to R ~ 20 000 with the FLAMES/GIRAFFE spectrograph. We derive and discuss the abundances of eight elements (Mg, Al, Si, Ca, Ti, Fe, Cr, Ni, and Y). Results. We show that the trends of these elemental abundances with iron are very similar to those in the solar neighbourhood. We find a natural division between α-rich and α-poor stars, best seen in the bimodality of the Mg/M distributions in bins of metallicity, which we attribute to thick- and thin-disc sequences, respectively. This separation is visible for most α-elements and for aluminium. With the possible exception of Al, the observed dispersion around the trends is well described by the expected errors, leaving little room for astrophysical dispersion. Using previously derived distances from the first paper from this series for our sample, we further find that the thick-disc is more extended vertically and is more centrally concentrated towards the inner Galaxy than the thin-disc, which indicates a shorter scale-length. We derive the radial (4 to 12 kpc) and vertical (0 to 3.5 kpc) gradients in metallicity, iron, four α-element abundances, and aluminium for the two populations, taking into account the identified correlation between RGC and | Z |. Similarly to other works, a radial metallicity gradient is found in the thin disc. The positive radial individual α/M gradients found are at variance from the gradients observed in the RAVE survey. The thin disc also hosts a negative vertical metallicity gradient in the solar cylinder, accompanied by positive individual α/M and Al/M gradients. The thick-disc, on the other hand, presents no radial metallicity gradient, a shallower vertical metallicity gradient than the thin-disc, an α-elements-to-iron radial gradient in the opposite sense than that of the thin disc, and positive vertical individual α/M and Al/M gradients. We examine several thick-disc formation scenarii in the light of these radial and vertical trends.
We analyze 494 main sequence turnoff and subgiant stars from the AMBRE:HARPS survey. These stars have accurate astrometric information from Gaia DR1, providing reliable age estimates with relative ...uncertainties of ±1 or 2 Gyr and allowing precise orbital determinations. The sample is split based on chemistry into a low-Mg/Fe sequence, which are often identified as thin disk stellar populations, and high-Mg/Fe sequence, which are often associated with thick disk stellar populations. We find that the high-Mg/Fe chemical sequence has extended star formation for several Gyr and is coeval with the oldest stars of the low-Mg/Fe chemical sequence: both the low- and high-Mg/Fe sequences were forming stars at the same time. We find that the high-Mg/Fe stellar populations are only vertically extended for the oldest, most-metal poor and highest Mg/Fe stars. When comparing vertical velocity dispersion for the low- and high-Mg/Fe sequences, the high-Mg/Fe sequence has lower vertical velocity dispersion than the low-Mg/Fe sequence for stars of similar age. This means that identifying either group as thin or thick disk based on chemistry is misleading. The stars belonging to the high-Mg/Fe sequence have perigalacticons that originate in the inner disk, while the perigalacticons of stars on the low-Mg/Fe sequence are generally around the solar neighborhood. From the orbital properties of the stars, the high-Mg/Fe and low-Mg/Fe sequences are most likely a reflection of the chemical enrichment history of the inner and outer disk populations, respectively; radial mixing causes both populations to be observed in situ at the solar position. Based on these results, we emphasize that it is important to be clear in defining what populations are being referenced when using the terms thin and thick disk, and that ideally the term thick disk should be reserved for purely geometric definitions to avoid confusion and be consistent with definitions in external galaxies.
Context.
In June 2022,
Gaia
DR3 provided the astronomy community with about one million spectra from the Radial Velocity Spectrometer (RVS) covering the CaII triplet region. In the next
Gaia
data ...releases, we anticipate the number of RVS spectra to successively increase from several 10 million spectra to eventually more than 200 million spectra. Thus, stellar spectra are projected to be produced on an ‘industrial scale’, with numbers well above those for current and anticipated ground-based surveys. However, one-third of the published spectra have 15 ≤
S /N
≤ 25 per pixel such that they pose problems for classical spectral analysis pipelines, and therefore, alternative ways to tap into these large datasets need to be devised.
Aims.
We aim to leverage the versatility and capabilities of machine learning techniques for supercharged stellar parametrisation by combining
Gaia
-RVS spectra with the full set of
Gaia
products and high-resolution, high-quality ground-based spectroscopic reference datasets.
Methods.
We developed a hybrid convolutional neural network (CNN) that combines the
Gaia
DR3 RVS spectra, photometry (G, G_BP, G_RP), parallaxes, and XP coefficients to derive atmospheric parameters (
T
eff
, log(g) as well as overall M/H) and chemical abundances (Fe/H and
α
/M). We trained the CNN with a high-quality training sample based on APOGEE DR17 labels.
Results.
With this CNN, we derived homogeneous atmospheric parameters and abundances for 886 080 RVS stars that show remarkable precision and accuracy compared to external datasets (such as GALAH and asteroseismology). The CNN is robust against noise in the RVS data, and we derive very precise labels down to S/N =15. We managed to characterise the
α
/M - M/H bimodality from the inner regions to the outer parts of the Milky Way, which has never been done using RVS spectra or similar datasets.
Conclusions.
This work is the first to combine machine learning with such diverse datasets and paves the way for large-scale machine learning analysis of
Gaia
-RVS spectra from future data releases. Large, high-quality datasets can be optimally combined thanks to the CNN, thereby realising the full power of spectroscopy, astrometry, and photometry.
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