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We aim to prepare the machine-learning ground for the next generation of spectroscopic surveys, such as 4MOST and WEAVE. Our goal is to show that convolutional neural networks can predict accurate stellar labels from relevant spectral features in a physically meaningful way. We built a neural network and trained it on GIRAFFE spectra with associated stellar labels from the sixth internal Gaia-ESO data release. Our neural network predicts the atmospheric parameters Teff and log(g) as well as the chemical abundances [Mg/Fe], [Al/Fe], and [Fe/H] for 30115 stellar spectra. The scatter of predictions from eight slightly different network models shows a high internal precision of the network results: 24 K for Teff, 0.03 for log(g), 0.02 dex for [Mg/Fe], 0.03 dex for [Al/Fe], and 0.02 dex for [Fe/H]. The network gradients reveal that the network is inferring the labels in a physically meaningful way from spectral features. Validation with benchmark stars and several scientific applications confirm that our network predictions are accurate for individual stars and recover the properties of different stellar populations in the Milky Way galaxy. Such a study provides very good insights into the application of machine-learning for the spectral analysis of large-scale spectroscopic surveys, such as WEAVE and 4MIDABLE-LR and -HR (4MOST Milky Way disk and bulge low- and high-resolution). The community will have to put a substantial effort into building proactive training sets for machine-learning methods to minimize the possible systematics., Comment: Submitted to A&A

The Gaia-ESO Public Spectroscopic Survey is an ambitious project designed to obtain astrophysical parameters and elemental abundances for 100,000 stars, including large representative samples of the stellar populations in the Galaxy, and a well-defined sample of 60 (plus 20 archive) open clusters. We provide internally consistent results calibrated on benchmark stars and star clusters, extending across a very wide range of abundances and ages. This provides a legacy data set of intrinsic value, and equally a large wide-ranging dataset that is of value for homogenisation of other and future stellar surveys and Gaia's astrophysical parameters. This article provides an overview of the survey methodology, the scientific aims, and the implementation, including a description of the data processing for the GIRAFFE spectra. A companion paper (arXiv:2206.02901) introduces the survey results. Gaia-ESO aspires to quantify both random and systematic contributions to measurement uncertainties. Thus all available spectroscopic analysis techniques are utilised, each spectrum being analysed by up to several different analysis pipelines, with considerable effort being made to homogenise and calibrate the resulting parameters. We describe here the sequence of activities up to delivery of processed data products to the ESO Science Archive Facility for open use. The Gaia-ESO Survey obtained 202,000 spectra of 115,000 stars using 340 allocated VLT nights between December 2011 and January 2018 from GIRAFFE and UVES. The full consistently reduced final data set of spectra was released through the ESO Science Archive Facility in late 2020, with the full astrophysical parameters sets following in 2022., Comment: 38 pages. A&A in press

In the last 15 years different ground-based spectroscopic surveys have been started (and completed) with the general aim of delivering stellar parameters and elemental abundances for large samples of Galactic stars, complementing Gaia astrometry. Among those surveys, the Gaia-ESO Public Spectroscopic Survey (GES), the only one performed on a 8m class telescope, was designed to target 100,000 stars using FLAMES on the ESO VLT (both Giraffe and UVES spectrographs), covering all the Milky Way populations, with a special focus on open star clusters. This article provides an overview of the survey implementation (observations, data quality, analysis and its success, data products, and releases), of the open cluster survey, of the science results and potential, and of the survey legacy. A companion article (Gilmore et al.) reviews the overall survey motivation, strategy, Giraffe pipeline data reduction, organisation, and workflow. The GES has determined homogeneous good-quality radial velocities and stellar parameters for a large fraction of its more than 110,000 unique target stars. Elemental abundances were derived for up to 31 elements for targets observed with UVES. Lithium abundances are delivered for about 1/3 of the sample. The analysis and homogenisation strategies have proven to be successful; several science topics have been addressed by the Gaia-ESO consortium and the community, with many highlight results achieved. The final catalogue has been released through the ESO archive at the end of May 2022, including the complete set of advanced data products. In addition to these results, the Gaia-ESO Survey will leave a very important legacy, for several aspects and for many years to come., Comment: Accepted for publication in Astronomy and Astrophysics. 30 pages, 30 figures, 4 tables

In fulfilling the aims of the planetary and asteroseismic research missions, such as that of the NASA Transiting Exoplanet Survey Satellite (TESS) space telescope, accurate stellar atmospheric parameters and a detailed chemical composition are required as input. We have observed high-resolution spectra for all 848 bright (V<8 mag) stars that are cooler than F5 spectral class in the area up to 12 deg surrounding the northern TESS continuous viewing zone and uniformly determined the main atmospheric parameters, ages, orbital parameters, velocity components, and precise abundances of up to 24 chemical species (C(C2), N(CN), [O I], Na I, Mg I, Al I, Si I, Si I, Ca I, Ca II, Sc I, Sc II, Ti I, Ti II, V I, Cr I, Cr II, Mn I, Fe I, Fe II, Co I, Ni I, Cu I, and Zn I) for 740 slowly rotating stars. The analysis of 25 planet-hosting stars in our sample drove us to the following conclusions: the dwarf stars hosting high-mass planets are more metal rich than those with low-mass planets. We find slightly negative C/O and Mg/Si slopes toward the stars with high-mass planets. All the low-mass planet hosts in our sample show positive $\Delta$[El/Fe] versus condensation temperature slopes, in particular, the star with the large number of various planets. The high-mass planet hosts have a diversity of slopes, but in more metal rich, older, and cooler stars, the positive elemental abundance slopes are more common., Comment: 17 pages, 8 figures. arXiv admin note: text overlap with arXiv:2005.07526

The last decade has seen a revolution in our knowledge of the Galaxy thanks to the Gaia and asteroseismic space missions and the ground-based spectroscopic surveys. To complete this picture, it is necessary to map the ages of its stellar populations. During recent years, the dependence on time of abundance ratios involving slow (s) neutron-capture and $\alpha$ elements (called chemical-clocks) has been used to provide estimates of stellar ages, usually in a limited volume close to the Sun. We aim to analyse the relations of chemical clocks in the Galactic disc extending the range to R$_{\rm GC}\sim$6-20~kpc. Using the sixth internal data release of the Gaia-ESO survey, we calibrated several relations between stellar ages and abundance ratios [s/$\alpha$] using a sample of open clusters, the largest one so far used with this aim. Thanks to their wide galactocentric coverage, we investigated the radial variations of the shape of these relations, confirming their non-universality. We estimated our accuracy and precision in recovering the global ages of open clusters, and the ages of their individual members. We applied the multi-variate relations with the highest correlation coefficients to the field star population. We confirm that there is no single age-chemical clock relationship valid for the whole disc, but that there is a dependence on the galactocentric position, which is related to the radial variation of the star formation history combined with the non-monotonic dependence on metallicity of the yields of the s-process elements from low- and intermediate-mass stars. Finally, the abundance ratios [Ba/$\alpha$] are more sensitive to age than those with [Y/$\alpha$] for young disc stars, and their slopes vary less with galactocentric distance., Comment: 14 pages, 10 figures + Appendix (3 tables and 2 figures)

Context. NGC 1851 is one of several globular clusters for which multiple stellar populations of the subgiant branch have been clearly identified and a difference in metallicity detected. A crucial piece of information on the formation history of this cluster can be provided by the sum of A(C+N+O) abundances. However, these values have lacked a general consensus thus far. The separation of the subgiant branch can be based on age and/or A(C+N+O) abundance differences. Aims. Our main aim was to determine carbon, nitrogen, and oxygen abundances for evolved giants in the globular cluster NGC1851 in order to check whether or not the double populations of stars are coeval. Methods. High-resolution spectra, observed with the FLAMES-UVES spectrograph on the ESO VLT telescope, were analysed using a differential model atmosphere method. Results. We provide abundances of up to 29 chemical elements for a sample of 45 giants in NGC 1851. The investigated stars can be separated into two populations with a difference of 0.07 dex in the mean metallicity, 0.3 dex in the mean C/N, and 0.35 dex in the mean s-process dominated element-to-iron abundance ratios [s/Fe]. No significant difference was determined in the mean values of A(C+N+O) as well as in abundance to iron ratios of carbon, alpha- and iron-peak-elements, and of europium. Conclusions. As the averaged A(C+N+O) values between the two populations do not differ, additional evidence is given that NGC 1851 is composed of two clusters, the metal-rich cluster being by about 0.6 Gyr older than the metal-poor one. A global overview of NGC 1851 properties and the detailed abundances of chemical elements favour its formation in a dwarf spheroidal galaxy that was accreted by the Milky Way., Comment: 13 pages, 17 figures, accepted to Astronomy and Astrophysics

Eclipsing binary stars allow derivation of accurate and precise masses and radii. When they reside in star clusters, properties of even higher precision, along with additional information, can be extracted. Asteroseismology of solar-like oscillations offers similar possibilities for single stars. We improve the previously established properties of the Hyades eclipsing binary HD27130 and re-assess the asteroseismic properties of the giant star $\epsilon$ Tau. The physical properties of these members of the Hyades are then used to constrain the helium content and age of the cluster. New multi-colour light curves were combined with multi-epoch radial velocities to yield masses and radii of HD27130. $T_{\rm eff}$ was derived from spectroscopy and photometry, and verified using the Gaia parallax. We estimate the cluster age from re-evaluated asteroseismic properties of $\epsilon$ Tau while using HD27130 to constrain the helium content. The masses and radii, and $T_{\rm eff}$ of HD 27130 were found to be $M=1.0245\pm0.0024 M_{\odot}$, $R=0.9226\pm0.015 R_{\odot}$, $T_{\rm eff}=5650\pm50$ K for the primary, and $M=0.7426\pm0.0016 M_{\odot}$, $R=0.7388\pm0.026 R_{\odot}$, $T_{\rm eff}=4300\pm100$ K for the secondary component. Our re-evaluation of $\epsilon$ Tau suggests that the previous literature estimates are trustworthy, and that the Hipparcos parallax is more reliable than the Gaia DR2 parallax. The helium content of HD27130 and thus of the Hyades is found to be $Y=0.27$ but with significant model dependence. Correlations with the adopted metallicity results in a robust helium enrichment law with $\frac{\Delta Y}{\Delta Z}$ close to 1.2. We estimate the age of the Hyades to be 0.9 $\pm$ 0.1 (stat) $\pm$ 0.1 (sys) Gyr in slight tension with recent age estimates based on the cluster white dwarfs. (abridged), Comment: 14 pages, 8 figures, accepted for publication in Astronomy & Astrophysics

Accurate atmospheric parameters and chemical composition of stars play a vital role in characterizing physical parameters of exoplanetary systems and understanding of their formation. A full asteroseismic characterization of a star is also possible if its main atmospheric parameters are known. The NASA Transiting Exoplanet Survey Satellite (TESS) space telescope will play a very important role in searching of exoplanets around bright stars and stellar asteroseismic variability research. We have observed all 302 bright (V < 8 mag) and cooler than F5 spectral class stars in the northern TESS continuous viewing zone with a 1.65 m telescope at the Moletai Astronomical Observatory of Vilnius University and the high-resolution Vilnius University Echelle Spectrograph. We uniformly determined the main atmospheric parameters, ages, orbital parameters, velocity components, and precise abundances of 24 chemical species ( C(C2), N(CN), [O I], Na I, Mg I, Al I, Si I, Si II, Ca I, Ca II, Sc I, Sc II, Ti I, Ti II, V I, Cr I, Cr II, Mn I, Fe I, Fe II, Co I, Ni I, Cu I, and Zn I) for 277 slowly rotating single stars in the field. About 83 % of the sample stars exhibit the Mg/Si ratios greater than 1.0 and may potentially harbor rocky planets in their systems., Comment: 15 pages, 12 figures, published in ApJS, 2020 May 12

In the era of large high-resolution spectroscopic surveys, high-quality spectra can contribute to our understanding of the Galactic chemical evolution, providing chemical abundances belonging to the different nucleosynthesis channels, and also providing constraints to stellar age. Some abundance ratios have been proven to be excellent indicators of stellar ages. We aim at providing an empirical relationship between stellar ages and [C/N] using, as calibrators, open star clusters observed by both the Gaia-ESO and APOGEE surveys. We use stellar parameters and abundances from the Gaia-ESO and APOGEE of the Galactic field and open cluster stars. Ages of star clusters are retrieved from the literature sources and validated using a common set of isochrones. We use the same isochrones to determine, for each age and metallicity, the surface gravity at which the first dredge-up and red giant branch bump occur. We study the effect of extra-mixing processes in our sample of giant stars, and we derive the mean [C/N] in evolved stars, including only stars without evidence of extra-mixing. Combining the Gaia-ESO and APOGEE samples of open clusters, we derive a linear relationship between [C/N] and logarithmic cluster ages. We apply our relationship to selected giant field stars in both Gaia-ESO and APOGEE. We find an age separation between thin and thick disc stars and age trends within their populations, with an increasing age towards lower metallicity populations. With such empirical relationship, we are able to provide an age estimate for giant stars in which C and N abundances are measured. Isochrone fitting is less sensitive for giant than dwarf stars at the turn off. The present method can be thus considered as an additional tool to give an independent estimate of the age of giant stars, with uncertainties in their ages comparable to those obtained using isochrone fitting for dwarf stars., Comment: 28 pages, 24 figures. Accepted by A&A

As extensive ground-based observations and characterisation of different variable stars are of the utmost importance in preparing optimal input catalogues for space missions, our aim was to search for new variable stars in selected fields of the northern sky. We obtained 24470 CCD images and analysed photometric time series of stars using the DAOPHOT based package Muniwin as the first step, and the Period04 package was used to further analyse the suspected new variable stars. The light curves and other observational results are presented for 3598 stars online. We found 81 new variable stars, among them is an eclipsing binary with a variable component and possibly eccentric orbits TYC4038-693-1 which we also observed spectroscopically, four {\delta} Scuti candidates, six other variable stars with periods falling into the interval of 35 minutes to 20 days. Furthermore, we identified 70 slowly varying stars with so far undefined periodicity. Additional photometric and spectral observations were carried out for TYC 2764-1997-1, and its previous candidacy for eclipsing binaries was approved., Comment: 17 pages, 13 figures. Accepted to Astrophysics and Space Science. Manuscript number ASTR-D-18-00575R2

The abundance ratio N/O is a useful tool to study the interplay of galactic processes, e.g. star formation efficiency, time-scale of infall and outflow loading factor We aim to trace log(N/O) versus [Fe/H] in the Milky Way and to compare it with a set of chemical evolution models to understand the role of infall, outflow and star formation efficiency in the building-up of the Galactic disc. We use the abundances from idr2-3, idr4, idr5 data releases of the Gaia-ESO Survey both for Galactic field and open cluster stars.We determine membership and average composition of open clusters and we separate thin and thick disc field stars.We consider the effect of mixing in the abundance of N in giant stars. We compute a grid of chemical evolution models, suited to reproduce the main features of our Galaxy, exploring the effects of the star formation efficiency, the infall time-scale and the differential outflow. With our samples, we map the metallicity range -0.6<[Fe/H]<0.3 with a corresponding -1.2

The GES survey using FLAMES at the VLT has obtained high-resolution UVES spectra for a large number of giant stars, allowing a determination of the abundances of the key chemical elements C and N at their surface. The surface abundances of these chemical species are well-known to change in stars during their evolution on the red giant branch after the first dredge-up episod, as a result of extra-mixing phenomena. We investigate the effects of thermohaline mixing on C and N abundances using the first comparison between the GES [C/N] determinations with simulations of the observed fields using a model of stellar population synthesis. We explore the effects of thermohaline mixing on the chemical properties of giants through stellar evolutionary models computed with the stellar evolution code STAREVOL. We include these stellar evolution models in the Besan\c{c}on Galaxy model to simulate the [C/N] distributions determined from the UVES spectra of the GES and compare them with the observations. Theoretical predictions including the effect of thermohaline mixing are in good agreement with the observations. However, the field stars in the GES with C and N-abundance measurements have a metallicity close to solar, where the efficiency of thermohaline mixing is not very large. The C and N abundances derived by the GES in open and globular clusters clearly show the impact of thermohaline mixing at low-metallicity, allowing to explain the [C/N] ratio observed in lower-mass and older giant stars. Using independent observations of carbon isotopic ratio in clump field stars and open clusters, we also confirm that thermohaline mixing should be taken into account to explain the behavior of 12C/13C ratio as a function of stellar age. Overall the current model including thermohaline mixing is able to reproduce very well the C- and N-abundances over the whole metallicity range investigated by the GES data., Comment: Accepted for publication in A&A ; 12 pages, 12 figures

During its four years of photometric observations, the Kepler space telescope detected thousands of exoplanets and exoplanet candidates. One of Kepler's greatest heritages has been the confirmation and characterization of hundreds of multi-planet systems via Transit Timing Variations (TTVs). However, there are many interesting candidate systems displaying TTVs on such long time scales that the existing Kepler observations are of insufficient length to confirm and characterize them by means of this technique. To continue with Kepler's unique work we have organized the "Kepler Object of Interest Network" (KOINet). The goals of KOINet are, among others, to complete the TTV curves of systems where Kepler did not cover the interaction timescales well. KOINet has been operational since March, 2014. Here we show some promising first results obtained from analyzing seven primary transits of KOI-0410.01, KOI-0525.01, KOI-0760.01, and KOI-0902.01 in addition to Kepler data, acquired during the first and second observing seasons of KOINet. While carefully choosing the targets we set demanding constraints about timing precision (at least 1 minute) and photometric precision (as good as 1 part per thousand) that were achieved by means of our observing strategies and data analysis techniques. For KOI-0410.01, new transit data revealed a turn-over of its TTVs. We carried out an in-depth study of the system, that is identified in the NASA's Data Validation Report as false positive. Among others, we investigated a gravitationally-bound hierarchical triple star system, and a planet-star system. While the simultaneous transit fitting of ground and space-based data allowed for a planet solution, we could not fully reject the three-star scenario. New data, already scheduled in the upcoming 2018 observing season, will set tighter constraints on the nature of the system., Comment: 22 figures

We analyze 494 main sequence turnoff and subgiant stars from the AMBRE:HARPS survey. These stars have accurate astrometric information from \textit{Gaia}/DR1, providing reliable age estimates with relative uncertainties of $\pm1-2$ 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 a high-[Mg/Fe] sequence, which are often associated with the thick disk. 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. 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 both sequences, the high-[Mg/Fe] sequence has lower velocity dispersion than the low-[Mg/Fe] sequence for stars of similar age. 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-and low-[Mg/Fe] sequences are most likely a reflection of the chemical enrichment history of the inner and outer disk populations; 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., Comment: 5 pages, 5 figures

We describe the methodologies that, taking advantage of Gaia-DR1 and the Gaia-ESO Survey data, enable the comparison of observed open star cluster sequences with stellar evolutionary models. The final, long-term goal is the exploitation of open clusters as age calibrators. We perform a homogeneous analysis of eight open clusters using the Gaia-DR1 TGAS catalogue for bright members, and information from the Gaia-ESO Survey for fainter stars. Cluster membership probabilities for the Gaia-ESO Survey targets are derived based on several spectroscopic tracers. The Gaia-ESO Survey also provides the cluster chemical composition. We obtain cluster parallaxes using two methods. The first one relies on the astrometric selection of a sample of bona fide members, while the other one fits the parallax distribution of a larger sample of TGAS sources. Ages and reddening values are recovered through a Bayesian analysis using the 2MASS magnitudes and three sets of standard models. Lithium depletion boundary (LDB) ages are also determined using literature observations and the same models employed for the Bayesian analysis. For all but one cluster, parallaxes derived by us agree with those presented in Gaia Collaboration et al. (2017), while a discrepancy is found for NGC 2516; we provide evidence supporting our own determination. Inferred cluster ages are robust against models and are generally consistent with literature values. The systematic parallax errors inherent in the Gaia DR1 data presently limit the precision of our results. Nevertheless, we have been able to place these eight clusters onto the same age scale for the first time, with good agreement between isochronal and LDB ages where there is overlap. Our approach appears promising and demonstrates the potential of combining Gaia and ground-based spectroscopic datasets., Comment: Accepted for publication by A&A. 22 pages main text plus 8 pages appendices, 12 figures main text, 7 figures appendices

We attempt to determine the relative fraction of stars that have undergone significant radial migration by studying the orbital properties of metal-rich ([Fe/H]$>0.1$) stars within 2 kpc of the Sun using a sample of more than 3,000 stars selected from iDR4 of the Gaia-ESO Survey. We investigate the kinematic properties, such as velocity dispersion and orbital parameters, of stellar populations near the sun as a function of [Mg/Fe] and [Fe/H], which could show evidence of a major merger in the past history of the Milky Way. This was done using the stellar parameters from the Gaia-ESO Survey along with proper motions from PPMXL to determine distances, kinematics, and orbital properties for these stars to analyze the chemodynamic properties of stellar populations near the Sun. Analyzing the kinematics of the most metal-rich stars ([Fe/H]$>0.1$), we find that more than half have small eccentricities ($e<0.2$) or are on nearly circular orbits. Slightly more than 20\% of the metal-rich stars have perigalacticons $R_p>7$ kpc. We find that the highest [Mg/Fe], metal-poor populations have lower vertical and radial velocity dispersions compared to lower [Mg/Fe] populations of similar metallicity by $\sim10$ km s$^{-1}$. The median eccentricity increases linearly with [Mg/Fe] across all metallicities, while the perigalacticon decreases with increasing [Mg/Fe] for all metallicities. Finally, the most [Mg/Fe]-rich stars are found to have significant asymmetric drift and rotate more than 40 km s$^{-1}$ slower than stars with lower [Mg/Fe] ratios. While our results cannot constrain how far stars have migrated, we propose that migration processes are likely to have played an important role in the evolution of the Milky Way, with metal-rich stars migrating from the inner disk toward to solar neighborhood and past mergers potentially driving enhanced migration of older stellar populations in the disk., Comment: 7 pages, 7 Figures, accepted A&A

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 AMBRE Project. The data suggest that the stars in the thick and thin discs form two distinct sequences with the thick disc stars showing higher [{\alpha}/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 timescale; 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] vs. [Fe/H] and the metallicity distribution functions in the two Galactic components, we conclude that the parallel approach can account for a group of {\alpha}-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 timescale of accretion of 0.1 Gyr, whereas the thin disc formed on a timescale 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., Comment: 12 pages, 10 figures. Accepted for publication in MNRAS

Abridged: We used the fourth internal data release of the Gaia-ESO survey to characterize the bulge chemistry, spatial distribution, kinematics, and to compare it chemically with the thin and thick disks. The sample consist on ~2500 red clump stars in 11 bulge fields ($-10^\circ\leq l\leq+8^\circ$ and $-10^\circ\leq b\leq-4^\circ$), and a set of ~6300 disk stars selected for comparison. The bulge MDF is confirmed to be bimodal across the whole sampled area, 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. Metal-poor bulge stars have a more isotropic hot kinematics and do not participate in the X-shape bulge. With similar Mg-enhancement levels, the position of the metal-poor bulge sequence "knee" is observed at [Fe/H]$_{knee}=-0.37\pm0.09$, being 0.06 dex higher than that of the thick disk. It suggests a higher SFR for the bulge than for the thick disk. 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 at early epochs. We associate metal-rich stars with the B/P bulge formed from the 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 $\alpha$-abundances has been performed in a large area on the basis of a homogeneous, fully spectroscopic analysis of high-resolution, high S/N data., Comment: Accepted in A&A 30 March 2017

We take advantage of the Gaia-ESO Survey iDR4 bulge data to search for abundance anomalies that could shed light on the composite nature of the Milky Way bulge. The alpha-elements (Mg, Si, and whenever available, Ca) abundances, and their trends with Fe abundances have been analysed for a total of 776 bulge stars. In addition, the aluminum abundances and their ratio to Fe and Mg have also been examined. Our analysis reveals the existence of low-alpha element abundance stars with respect to the standard bulge sequence in the [alpha/Fe] vs. [Fe/H] plane. 18 objects present deviations in [alpha/Fe] ranging from 2.1 to 5.3 sigma with respect to the median standard value. Those stars do not show Mg-Al anti-correlation patterns. Incidentally, this sign of the existence of multiple stellar populations is reported firmly for the first time for the bulge globular cluster NGC 6522. The identified low-alpha abundance stars have chemical patterns compatible with those of the thin disc. Their link with massive dwarf galaxies accretion seems unlikely, as larger deviations in alpha abundance and Al would be expected. The vision of a bulge composite nature and a complex formation process is reinforced by our results. The used approach, a multi-method and model-driven analysis of high resolution data seems crucial to reveal this complexity., Comment: A&A Letter, accepted

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. We automatically derived iron peak (Mn, Fe, Ni, Cu, and Zn) and alpha element (Mg) chemical abundances for 4666 stars. We used the bimodal distribution of [Mg/Fe] to chemically classify sample stars into different Galactic substructures: thin disc, metal-poor and high-alpha metal rich, high-alpha and low-alpha metal-poor populations. High-alpha and low-alpha metal-poor populations are fully distinct in Mg, Cu, and Zn. Thin disc trends of [Ni/Fe] and [Cu/Fe] are very similar and show a small increase at supersolar metallicities. Thin and thick disc trends of Ni and Cu are very similar and indistinguishable. Mn looks 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. [Zn/Fe] trends look very similar to those of [alpha/Fe] trends. The dispersion of results in both discs is low (approx 0.05 dex for [Mg, Mn, and Cu/Fe]) and is even much lower for [Ni/Fe] (approx 0.035 dex). Zn is an alpha-like element and could be used to separate thin and thick disc stars. [Mn/Mg] ratio could also be a very good tool for tagging Galactic substructures. Some models can partially reproduce the observed Mg, Zn, and, Cu behaviours. 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. Very low scatter (approx 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., Comment: 21 pages, 20 figures

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. 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. 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. 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 alpha-rich, metal-rich stars has a peak at A(Li)~3 dex. 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., Comment: 15 pages, 12 figures, 2 tables

We analyse the kinematics of $\sim 2000$ giant stars in the direction of the Galactic bulge, extracted from the Gaia-ESO survey in the region $-10^\circ \lesssim \ell \lesssim 10^\circ$ and $-11^\circ \lesssim b \lesssim -3^\circ$. We find distinct kinematic trends in the metal rich ($\mathrm{[M/H]}>0$) and metal poor ($\mathrm{[M/H]}<0$) stars in the data. The velocity dispersion of the metal-rich stars drops steeply with latitude, compared to a flat profile in the metal-poor stars, as has been seen previously. We argue that the metal-rich stars in this region are mostly on orbits that support the boxy-peanut shape of the bulge, which naturally explains the drop in their velocity dispersion profile with latitude. The metal rich stars also exhibit peaky features in their line-of-sight velocity histograms, particularly along the minor axis of the bulge. We propose that these features are due to stars on resonant orbits supporting the boxy-peanut bulge. This conjecture is strengthened through the comparison of the minor axis data with the velocity histograms of resonant orbits generated in simulations of buckled bars. The 'banana' or 2:1:2 orbits provide strongly bimodal histograms with narrow velocity peaks that resemble the Gaia-ESO metal-rich data., Comment: 6 pages, 8 figures, accepted to ApJ Letters

(Abridged) Recent spectroscopic surveys have begun to explore the Galactic disk system outside the solar neighborhood on the basis of large data samples. In this way, they provide valuable information for testing spatial and temporal variations of disk structure kinematics and chemical evolution. We used a Gaussian mixture model algorithm, as a rigurous mathematical approach, to separate in the [Mg/Fe] vs. [Fe/H] plane a clean disk star subsample from the Gaia-ESO survey internal data release 2. We find that the sample is separated into five groups associated with major Galactic components; the metal-rich end of the halo, the thick disk, and three subgroups for the thin disk sequence. This is confirmed with a sample of red clump stars from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. The two metal-intermediate and metal-rich groups of the thin disk decomposition ([Fe/H]>-0.25 dex) highlight a change in the slope at solar metallicity. This holds true at different radial regions. The distribution of Galactocentric radial distances of the metal-poor part of the thin disk ([Fe/H]<-0.25 dex) is shifted to larger distances than those of the more metal-rich parts. Moreover, the metal-poor part of the thin disk presents indications of a scale height intermediate between those of the thick and the rest of the thin disk, and it displays higher azimuthal velocities than the latter. These stars might have formed and evolved in parallel and/or dissociated from the inside-out formation taking place in the internal thin disk. Their enhancement levels might be due to their origin from gas pre-enriched by outflows from the thick disk or the inner halo. The smooth trends of their properties (their spatial distribution with respect to the plane, in particular) with [Fe/H] and [Mg/Fe] suggested by the data indicates a quiet dynamical evolution, with no relevant merger events.

Understanding the history and the evolution of the Milky Way disc is one of the main goals of modern astrophysics. We study the velocity dispersion behaviour of Galactic disc stars as a function of the [Mg/Fe] ratio, which can be used as a proxy of relative age. This key relation is essential to constrain the formation mechanisms of the disc stellar populations as well as the cooling processes. We used the recommended parameters and chemical abundances of 7800 FGK Milky Way field stars from the second internal data release of the Gaia-ESO Survey. These stars were observed with the GIRAFFE spectrograph, and cover a large spatial volume (6+0.2dex. First, the velocity dispersion increases with [Mg/Fe] at all [Fe/H] ratios for the thin-disc stars, and then it decreases for the thick-disc at the highest [Mg/Fe] abundances. Similar trends are observed within the errors for the azimuthal velocity dispersion, while a continuous increase with [Mg/Fe] is observed for the vertical velocity dispersion. The velocity dispersion decrease agrees with previous measurements of the RAVE survey, although it is observed here for a greater metallicity interval and a larger spatial volume. We confirm the existence of [Mg/Fe]-rich thick-disc stars with cool kinematics in the generally turbulent context of the primitive Galactic disc. This is discussed in the framework of the different disc formation scenarios., Comment: 14 pages, 13 figures, A&A accepted

We investigate, using the Gaia-ESO Survey internal Data-Release 2, the properties of the double sequence of the Milky Way discs (defined chemically as the high-alpha and low-alpha populations), and discuss their compatibility with discs defined by other means such as metallicity, kinematics or positions. This investigation uses two different approaches: in velocity space for stars located in the extended Solar neighbourhood, and in chemical space for stars at different ranges of Galactocentric radii and heights from the plane. The separation we find in velocity space allows us to investigate, in a novel manner, the extent in metallicity of each of the two sequences, identifying them with the two discs, without making any assumption about the shape of their metallicity distribution functions. Then, using the separation in chemical space, we characterise the spatial variation of the slopes of the [alpha/Fe] - [Fe/H] sequences for the thick and thin discs and the way in which the relative proportions of the two discs change across the Galaxy. We find that the thick disc (high-alpha sequence), extends up to [Fe/H]~ +0.2 and the thin disc (low-alpha sequence), at least down to [Fe/H]~ -0.8. Radial and vertical gradients in alpha-abundances are found for the thin disc, with mild spatial variations in its [alpha/Fe] - [Fe/H] paths, whereas for the thick disc we do not detect any such spatial variations. The small variations in the spatial [alpha/Fe] - [Fe/H] paths of the thin disc do not allow us to distinguish between formation models of this structure. On the other hand, the lack of radial gradients and [alpha/Fe] - [Fe/H] variations for the thick disc indicate that the mechanism responsible for the mixing of the metals in the young Galaxy (e.g. radial stellar migration or turbulent gaseous disc) was more efficient before the (present) thin disc started forming., Comment: 21 pages, 18 figures, accepted for publication in Astronomy & Astrophysics

In the current era of large spectroscopic surveys of the Milky Way, reference stars for calibrating astrophysical parameters and chemical abundances are of paramount importance. We determine elemental abundances of Mg, Si, Ca, Sc, Ti, V, Cr, Mn, Co and Ni for our predefined set of Gaia FGK benchmark stars. By analysing high-resolution and high-signal to noise spectra taken from several archive datasets, we combined results of eight different methods to determine abundances on a line-by-line basis. We perform a detailed homogeneous analysis of the systematic uncertainties, such as differential versus absolute abundance analysis, as well as we assess errors due to NLTE and the stellar parameters in our final abundances. Our results are provided by listing final abundances and the different sources of uncertainties, as well as line-by-line and method-by-method abundances. The Gaia FGK benchmark stars atmospheric parameters are already being widely used for calibration of several pipelines applied to different surveys. With the added reference abundances of 10 elements this set is very suitable to calibrate the chemical abundances obtained by these pipelines., Comment: Accepted for publication in A&A, Printer format: 65 pages, 18 figures, 25 tables

High-precision spectroscopy of large stellar samples plays a crucial role for several topical issues in astrophysics. Examples include studying the chemical structure and evolution of the Milky Way galaxy, tracing the origin of chemical elements, and characterizing planetary host stars. Data are accumulating from instruments that obtain high-quality spectra of stars in the ultraviolet, optical and infrared wavelength regions on a routine basis. These instruments are located at ground-based 2- to 10-m class telescopes around the world, in addition to the spectrographs with unique capabilities available at the Hubble Space Telescope. The interpretation of these spectra requires high-quality transition data for numerous species, in particular neutral and singly ionized atoms, and di- or triatomic molecules. We rely heavily on the continuous efforts of laboratory astrophysics groups that produce and improve the relevant experimental and theoretical atomic and molecular data. The compilation of the best available data is facilitated by databases and electronic infrastructures such as the NIST Atomic Spectra Database, the VALD database, or the Virtual Atomic and Molecular Data Centre (VAMDC). We illustrate the current status of atomic data for optical stellar spectra with the example of the Gaia-ESO Public Spectroscopic Survey. Data sources for 35 chemical elements were reviewed in an effort to construct a line list for a homogeneous abundance analysis of up to 100000 stars., Comment: Published 30 April 2015 in Physica Scripta

Photometry alone is not sufficient to unambiguously distinguish between ultra-faint star clusters and dwarf galaxies because of their overlap in morphological properties. Here we report on VLT/GIRAFFE spectra of candidate member stars in two recently discovered ultra-faint satellites Reticulum 2 and Horologium 1, obtained as part of the ongoing Gaia-ESO Survey. We identify 18 members in Reticulum 2 and 5 in Horologium 1. We find Reticulum 2 to have a velocity dispersion of ~3.22 km/s, implying a M/L ratio of ~ 500. We have inferred stellar parameters for all candidates and we find Reticulum 2 to have a mean metallicity of [Fe/H] = -2.46+/-0.1, with an intrinsic dispersion of ~ 0.29, and is alpha-enhanced to the level of [alpha/Fe]~0.4. 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 [Fe/H] ~ -2 and V_hel ~ 220 km/s, far from the systemic velocity of Reticulum 2. For Horologium 1 we infer a velocity dispersion of 4.9^{+2.8}_{-0.9} km/s and a consequent 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 [Fe/H] = -2.76 +/- 0.1 and is similarly alpha-enhanced: [alpha/Fe] ~ 0.3. Despite a large error-bar, we also measure a significant spread of metallicities of 0.17 dex which strengthen the evidence that Horologium 1 is indeed a dwarf galaxy. The line-of-sight velocity of Reticulum 2 is offset by some 100 km/s from the prediction of the orbital velocity of the LMC, thus making its association with the Cloud uncertain. However, at the location of Horologium 1, both the backward integrated LMC's orbit and the LMC's halo are predicted to have radial velocities similar to that of the dwarf. Therefore, it is likely that Horologium 1 is or once was a member of the Magellanic Family., Comment: submitted to ApJ, abstract abridged

Aim of this work is to determine C, N, and O abundances in stars of Galactic open clusters of the Gaia-ESO survey and to compare the observed abundances with those predicted by current stellar and Galactic evolution models. In this pilot paper, we investigate the first three intermediate-age open clusters. High-resolution spectra, observed with the FLAMES-UVES spectrograph on the ESO VLT telescope, were analysed using a differential model atmosphere method. Abundances of carbon were derived using the C2 band heads at 5135 and 5635.5 {\AA}. The wavelength interval 6470- 6490 {\AA}, with CN features, was analysed to determine nitrogen abundances. Oxygen abundances were determined from the [O i] line at 6300 {\AA}. The mean values of the elemental abundances in Trumpler 20 as determined from 42 stars are: [Fe/H] = 0.10 +- 0.08 (s.d.), [C/H] = -0.10 +- 0.07, [N/H] = 0.50 +- 0.07, and consequently C/N = 0.98 +- 0.12. We measure from five giants in NGC 4815: [Fe/H] = -0.01 +- 0.04, [C/H] = -0.17 +- 0.08, [N/H] = 0.53 +- 0.07, [O/H] = 0.12 +- 0.09, and C/N = 0.79 +- 0.08. We obtain from 27 giants in NGC 6705: [Fe/H] = 0.0 +- 0.05, [C/H] = -0.08 +- 0.06, [N/H] = 0.61 +- 0.07, [O/H] = 0.13 +- 0.05, and C/N = 0.83 +- 0.19. The C/N ratios of stars in the investigated open clusters were compared with the ratios predicted by stellar evolutionary models. For the corresponding stellar turn-off masses from 1.9 to 3.3 solar masses, the observed C/N ratio values are very close to the predictions of standard first dredge-up models as well as to models of thermohaline extra-mixing. The average [O/H] abundance ratios of NGC 4815 and NGC 6705 are compared with the predictions of two Galactic chemical evolution models. The data are consistent with the evolution at the solar radius within the errors., Comment: 13 pages, 11 figures, accepted for publication in Astronomy & Astrophysics, 2014

We performed a detailed study of the ratio of low-{\alpha} to high-{\alpha} stars in the Galactic halo as observed by the Gaia-ESO Survey. Using a sample of 381 metal-poor stars from the second internal data release, we found that the value of this ratio did not show evidence of systematic trends as a function of metallicity, surface gravity, Galactic latitude, Galactic longitude, height above the Galactic plane, and Galactocentric radius. We conclude that the {\alpha}-poor/{\alpha}-rich value of 0.28 $\pm$ 0.08 suggests that in the inner halo, the larger portion of stars were formed in a high star formation rate environment, and about 15% of the metal-poor stars originated from much lower star formation rate environments.

The Gaia-ESO Survey is obtaining high-quality spectroscopic data for about 10^5 stars using FLAMES at the VLT. UVES high-resolution spectra are being collected for about 5000 FGK-type stars. These UVES spectra are analyzed in parallel by several state-of-the-art methodologies. Our aim is to present how these analyses were implemented, to discuss their results, and to describe how a final recommended parameter scale is defined. We also discuss the precision (method-to-method dispersion) and accuracy (biases with respect to the reference values) of the final parameters. These results are part of the Gaia-ESO 2nd internal release and will be part of its 1st public release of advanced data products. The final parameter scale is tied to the one defined by the Gaia benchmark stars, a set of stars with fundamental atmospheric parameters. A set of open and globular clusters is used to evaluate the physical soundness of the results. Each methodology is judged against the benchmark stars to define weights in three different regions of the parameter space. The final recommended results are the weighted-medians of those from the individual methods. The recommended results successfully reproduce the benchmark stars atmospheric parameters and the expected Teff-log g relation of the calibrating clusters. Atmospheric parameters and abundances have been determined for 1301 FGK-type stars observed with UVES. The median of the method-to-method dispersion of the atmospheric parameters is 55 K for Teff, 0.13 dex for log g, and 0.07 dex for [Fe/H]. Systematic biases are estimated to be between 50-100 K for Teff, 0.10-0.25 dex for log g, and 0.05-0.10 dex for [Fe/H]. Abundances for 24 elements were derived: C, N, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Mo, Ba, Nd, and Eu. The typical method-to-method dispersion of the abundances varies between 0.10 and 0.20 dex., Comment: 39 pages, A&A accepted

Most high-resolution spectroscopic studies of the Galactic 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 both discs, using the first internal data release of the Gaia-ESO survey. We derive and discuss the abundances of eight elements (Mg, Al, Si, Ca, Ti, Fe, Cr, Ni, and Y). The trends of these elemental abundances with iron are very similar to those in the solar neighbourhood. We find a natural division between alpha-rich and alpha-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. 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 Recio-Blanco et al. (2014b), 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 and vertical gradients in metallicity, iron, four alpha-element abundances, and Al for the two populations, taking into account the identified correlation between R_GC and |Z|. Radial metallicity gradient is found in the thin disc. The positive radial individual [alpha/M] gradients found are at variance from the gradients observed in the RAVE survey. The thin disc also hosts a negative vertical metallicity gradient, accompanied by positive individual [alpha/M] and [Al/M] gradients. The thick-disc, presents no radial metallicity gradient, a shallower vertical metallicity gradient than the thin-disc, an alpha-elements-to-iron radial gradient in the opposite sense than that of the thin disc, and positive vertical individual [alpha/M] and [Al/M] gradients., Comment: 24 pages, 10 figures

Context. The homogenization of the stellar parameters is an important goal for large observational spectroscopic surveys, but it is very difficult to achieve it because of the diversity of the spectroscopic analysis methods used within a survey, such as spectrum synthesis and the equivalent width method. To solve this problem, constraints to the spectroscopic analysis can be set, such as the use of a common line-list. Aims. We present a procedure for selecting the best spectral lines from a given input line-list, which then allows us to derive accurate stellar parameters with the equivalent width method. Methods. To select the lines, we used four very well known benchmark stars, for which we have high-quality spectra. From an initial line-list, the equivalent width of each individual line was automatically measured for each benchmark star using ARES, then we performed a local thermodynamic equilibrium analysis with MOOG to compute individual abundances. The results allowed us to choose the best lines which give consistent abundance values for all the benchmark stars from which we then created a final line-list. Results. To verify its consistency, the compiled final line-list was tested for a small sample of stars. These stars were selected to cover different ranges in the parameter space for FGK stars. We show that the obtained parameters agree well with previously determined values., Comment: 9 pages. 5 figures, accepted for publication in A&A