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. Revised spectroscopic parameters for the HF molecule and a new CN line list in the 2.3 μm region have recently become available, facilitating a revision of the F content in asymptotic giant ...branch (AGB) stars. Aims. AGB carbon stars are the only observationally confirmed sources of fluorine. Currently, there is no consensus on the relevance of AGB stars in its Galactic chemical evolution. The aim of this article is to better constrain the contribution of these stars with a more accurate estimate of their fluorine abundances. Methods. Using new spectroscopic tools and local thermodynamical equilibrium spectral synthesis, we redetermine fluorine abundances from several HF lines in the K-band in a sample of Galactic and extragalactic AGB carbon stars of spectral types N, J, and SC, spanning a wide range of metallicities. Results. On average, the new derived fluorine abundances are systematically lower by 0.33 dex with respect to previous determinations. This may derive from a combination of the lower excitation energies of the HF lines and the larger macroturbulence parameters used here as well as from the new adopted CN line list. Yet, theoretical nucleosynthesis models in AGB stars agree with the new fluorine determinations at solar metallicities. At low metallicities, an agreement between theory and observations can be found by handling the radiative/convective interface at the base of the convective envelope in a different way. Conclusions. New fluorine spectroscopic measurements agree with theoretical models at low and at solar metallicity. Despite this, complementary sources are needed to explain its observed abundance in the solar neighbourhood.
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.
The discovery of lithium-rich giants contradicts expectations from canonical stellar evolution. Here we report on the serendipitous discovery of 20 Li-rich giants observed during the Gaia-ESO Survey, ...which includes the first nine Li-rich giant stars known towards the CoRoT fields. Most of our Li-rich giants have near-solar metallicities and stellar parameters consistent with being before the luminosity bump. This is difficult to reconcile with deep mixing models proposed to explain lithium enrichment, because these models can only operate at later evolutionary stages: at or past the luminosity bump. In an effort to shed light on the Li-rich phenomenon, we highlight recent evidence of the tidal destruction of close-in hot Jupiters at the sub-giant phase. We note that when coupled with models of planet accretion, the observed destruction of hot Jupiters actually predicts the existence of Li-rich giant stars, and suggests that Li-rich stars should be found early on the giant branch and occur more frequently with increasing metallicity. A comprehensive review of all known Li-rich giant stars reveals that this scenario is consistent with the data. However, more evolved or metal-poor stars are less likely to host close-in giant planets, implying that their Li-rich origin requires an alternative explanation, likely related to mixing scenarios rather than external phenomena.
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.
Aims. We use kinematical and chemical properties of 754 CoRoT stars to characterise the stellar populations of the Milky Way disc in three beams close the Galactic plane. Methods. From the ...atmospheric parameters derived in Gazzano et al. (2010, A&A, 523, A91) with the MATISSE algorithm, we derived stellar distances using isochrones. Combining these data with proper motions, we provide the complete kinematical description of stars in three CoRoT fields. Finally, we used kinematical criteria to identify the Galactic populations in our sample and study their characteristics, particularly their chemistry. Results. Comparing our kinematics with the Besançon Galactic model, we show that, within 3σ, simulated and observed kinematical distributions are in good agreement. We study the characteristics of the thin disc, finding a correlation that is significant at a value of 2-σ between the V-velocity component and the metallicity for two different radial distance bins (8−9 kpc and 9−10 kpc; but not for the most inner bin 7−8 kpc, probably because of the uncertainties in the abundances), which could be interpreted as radial migration evidence. We also measured a radial metallicity gradient value of −0.097 ± 0.015 dex kpc-1 with giant stars, and −0.053 ± 0.015 dex kpc-1 with dwarfs. Finally, we identified metal-rich stars with peculiar, high α/Fe values in the directions pointing to the inner part of the Galaxy Applying the same methodology to the planet-hosting stars detected by CoRoT shows that they mainly belong to the thin disc population with normal chemical and kinematical properties.
Context. The chemical evolution of neutron capture elements in the Milky Way disc is still a matter of debate. There is a lack of statistically significant catalogues of such element abundances, ...especially those of the r-process. Aims. We aim to understand the chemical evolution of r-process elements in Milky Way disc. We focus on three pure r-process elements Eu, Gd, and Dy. We also consider a pure s-process element, Ba, in order to disentangle the different nucleosynthesis processes. Methods. We take advantage of high-resolution FEROS, HARPS, and UVES spectra from the ESO archive in order to perform a homogeneous analysis on 6500 FGK Milky Way stars. The chemical analysis is performed thanks to the automatic optimization pipeline GAUGUIN. We present abundances of Ba (5057 stars), Eu (6268 stars), Gd (5431 stars), and Dy (5479 stars). Based on the α/Fe ratio determined previously by the AMBRE Project, we chemically characterize the thin and the thick discs, and a metal-rich α-rich population. Results. First, we find that the Eu/Fe ratio follows a continuous sequence from the thin disc to the thick disc as a function of the metallicity. Second, in thick disc stars, the Eu/Ba ratio is found to be constant, while the Gd/Ba and Dy/Ba ratios decrease as a function of the metallicity. These observations clearly indicate a different nucleosynthesis history in the thick disc between Eu and Gd–Dy. The r/Fe ratio in the thin disc is roughly around +0.1 dex at solar metallicity, which is not the case for Ba. We also find that the α-rich metal-rich stars are also enriched in r-process elements (like thick disc stars), but their Ba/Fe is very different from thick disc stars. Finally, we find that the r/α ratio tends to decrease with metallicity, indicating that supernovae of different properties probably contribute differently to the synthesis of r-process elements and α-elements. Conclusions. We provide average abundance trends for Ba/Fe and Eu/Fe with rather small dispersions, and for the first time for Gd/Fe and Dy/Fe. This data may help to constrain chemical evolution models of Milky Way r- and s-process elements and the yields of massive stars. We emphasize that including yields of neutron-star or black hole mergers is now crucial if we want to quantitatively compare observations to Galactic chemical evolution models.
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.
Stars evolving along the asymptotic giant branch (AGB) can become carbon rich in the final part of their evolution. The detailed description of their spectra has led to the definition of ...several spectral types: N, SC, J, and R. To date, differences among them have been partially established only on the basis of their chemical properties.
Aims.
An accurate determination of the luminosity function (LF) and kinematics together with their chemical properties is extremely important for testing the reliability of theoretical models and establishing on a solid basis the stellar population membership of the different carbon star types.
Methods.
Using
Gaia
Data Release 2 (
Gaia
DR2) astrometry, we determine the LF and kinematic properties of a sample of 210 carbon stars with different spectral types in the solar neighbourhood with measured parallaxes better than 20%. Their spatial distribution and velocity components are also derived. Furthermore, the use of the infrared Wesenheit function allows us to identify the different spectral types in a
Gaia
-2MASS diagram.
Results.
We find that the combined LF of N- and SC-type stars are consistent with a Gaussian distribution peaking at
M
bol
∼ −5.2 mag. The resulting LF, however, shows two tails at lower and higher luminosities more extended than those previously found, indicating that AGB carbon stars with solar metallicity may reach
M
bol
∼ −6.0 mag. This contrasts with the narrower LF derived in Galactic carbon Miras from previous studies. We find that J-type stars are about half a magnitude fainter on average than N- and SC-type stars, while R-hot stars are half a magnitude brighter than previously found, although fainter in any case by several magnitudes than other carbon types. Part of these differences are due to systematically lower parallaxes measured by
Gaia
DR2 with respect to H
IPPARCOS
values, in particular for sources with parallax
ϖ
< 1 mas. The Galactic spatial distribution and velocity components of the N-, SC-, and J-type stars are very similar, while about 30% of the R-hot stars in the sample are located at distances greater than ∼500 pc from the Galactic plane, and show a significant drift with respect to the local standard of rest.
Conclusions.
The LF derived for N- and SC-type in the solar neighbourhood fully agrees with the expected luminosity of stars of 1.5−3
M
⊙
on the AGB. On a theoretical basis, the existence of an extended low-luminosity tail would require a contribution of extrinsic low-mass carbon stars, while the high-luminosity tail would imply that stars with mass values up to ∼5
M
⊙
may become carbon stars on the AGB. J-type stars differ significantly not only in their chemical composition with respect to the N- and SC-types, but also in their LF, which reinforces the idea that these carbon stars belong to a different type whose origin is still unknown. The derived luminosities of R-hot stars means that it is unlikely that these stars are in the red-clump, as previously claimed. On the other hand, the derived spatial distribution and kinematic properties, together with their metallicity values, indicate that most of the N-, SC-, and J-type stars belong to the thin disc population, while a significant fraction of R-hot stars show characteristics compatible with the thick disc.
PDF
