arXiv subjects

G. Kordopatis

Publications and source records attributed to G. Kordopatis.

At least 37 records · Page 2Linked to original sources

arXiv2022

Gaia Data Release 3: Astrophysical parameters inference system (Apsis) I -- methods and content overview

Gaia Data Release 3 contains a wealth of new data products for the community. Astrophysical parameters are a major component of this release. They were produced by the Astrophysical parameters inference system (Apsis) within the Gaia Data Processing and Analysis Consortium. The aim of this paper is to describe the overall content of the astrophysical parameters in Gaia Data Release 3 and how they were produced. In Apsis we use the mean BP/RP and mean RVS spectra along with astrometry and photometry, and we derive the following parameters: source classification and probabilities for 1.6 billion objects, interstellar medium characterisation and distances for up to 470 million sources, including a 2D total Galactic extinction map, 6 million redshifts of quasar candidates and 1.4 million redshifts of galaxy candidates, along with an analysis of 50 million outlier sources through an unsupervised classification. The astrophysical parameters also include many stellar spectroscopic and evolutionary parameters for up to 470 million sources. These comprise Teff, logg, and m_h (470 million using BP/RP, 6 million using RVS), radius (470 million), mass (140 million), age (120 million), chemical abundances (up to 5 million), diffuse interstellar band analysis (0.5 million), activity indices (2 million), H-alpha equivalent widths (200 million), and further classification of spectral types (220 million) and emission-line stars (50 thousand). This catalogue is the most extensive homogeneous database of astrophysical parameters to date, and it is based uniquely on Gaia data.

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arXiv2022

Gaia Data Release 3: Apsis II -- Stellar Parameters

The third Gaia data release contains, beyond the astrometry and photometry, dispersed light for hundreds of millions of sources from the Gaia prism spectra (BP and RP) and the spectrograph (RVS). This data release opens a new window on the chemo-dynamical properties of stars in our Galaxy, essential knowledge for understanding the structure, formation, and evolution of the Milky Way. To provide insight into the physical properties of Milky Way stars, we used these data to produce a uniformly-derived, all-sky catalog of stellar astrophysical parameters (APs): Teff, logg, [M/H], [$α$/Fe], activity index, emission lines, rotation, 13 chemical abundance estimates, radius, age, mass, bolometric luminosity, distance, and dust extinction. We developed the Apsis pipeline to infer APs of Gaia objects by analyzing their astrometry, photometry, BP/RP, and RVS spectra. We validate our results against other literature works, including benchmark stars, interferometry, and asteroseismology. Here we assessed the stellar analysis performance from Apsis statistically. We describe the quantities we obtained, including our results' underlying assumptions and limitations. We provide guidance and identify regimes in which our parameters should and should not be used. Despite some limitations, this is the most extensive catalog of uniformly-inferred stellar parameters to date. These comprise Teff, logg, and [M/H] (470 million using BP/RP, 6 million using RVS), radius (470 million), mass (140 million), age (120 million), chemical abundances (5 million), diffuse interstellar band analysis (1/2 million), activity indices (2 million), H{$α$} equivalent widths (200 million), and further classification of spectral types (220 million) and emission-line stars (50 thousand). More precise and detailed astrophysical parameters based on epoch BP, RP, and RVS are planned for the next Gaia data release.

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arXiv2022

Gaia Data Release 3. Stellar chromospheric activity and mass accretion from Ca II IRT observed by the Radial Velocity Spectrometer

The Gaia Radial Velocity Spectrometer provides the unique opportunity of a spectroscopic analysis of millions of stars at medium-resolution in the near-infrared. This wavelength range includes the Ca II infrared triplet (IRT), which is a good diagnostics of magnetic activity in the chromosphere of late-type stars. Here we present the method devised for inferring the Gaia stellar activity index together with its scientific validation. A sample of well studied PMS stars is considered to identify the regime in which the Gaia stellar activity index may be affected by mass accretion. The position of these stars in the colour-magnitude diagram and the correlation with the amplitude of the photometric rotational modulation is also scrutinised. Three regimes of the chromospheric stellar activity are identified, confirming suggestions made by previous authors on much smaller $R'_{\rm HK}$ datasets. The highest stellar activity regime is associated with PMS stars and RS CVn systems, in which activity is enhanced by tidal interaction. Some evidence of a bimodal distribution in MS stars with $T_{\rm eff}\ge$ 5000 K is also found, which defines the two other regimes, without a clear gap in between. Stars with 3500 K$\le T_{\rm eff} \le$ 5000 K are found to be either very active PMS stars or active MS stars with a unimodal distribution in chromospheric activity. A dramatic change in the activity distribution is found for $T_{\rm eff}\le$3500 K, with a dominance of low activity stars close to the transition between partially- and fully-convective stars and a rise in activity down into the fully-convective regime.

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arXiv2022

Gaia Data Release 3: Exploring and mapping the diffuse interstellar band at 862 nm

Diffuse interstellar bands (DIBs) are common interstellar absorption features in spectroscopic observations but their origins remain unclear. DIBs play an important role in the life cycle of the interstellar medium (ISM) and can also be used to trace Galactic structure. Here, we demonstrate the capacity of the Gaia-Radial Velocity Spectrometer (RVS) in Gaia DR3 to reveal the spatial distribution of the unknown molecular species responsible for the most prominent DIB at 862 nm in the RVS passband, exploring the Galactic ISM within a few kiloparsecs from the Sun. The DIBs are measured within the GSP-Spec module using a Gaussian profile fit for cool stars and a Gaussian process for hot stars. In addition to the equivalent widths and their uncertainties, Gaia DR3 provides their characteristic central wavelength, width, and quality flags. We present an extensive sample of 476.117 individual DIB measurements obtained in a homogeneous way covering the entire sky. We compare spatial distributions of the DIB carrier with interstellar reddening and find evidence that DIB carriers are present in a local bubble around the Sun which contains nearly no dust. We characterised the DIB equivalent width with a local density of $0.19 \pm 0.04$ Angström/kpc and a scale height of $\rm 98.60_{-8.46}^{+11.10}$ pc. The latter is smaller than the dust scale height, indicating that DIBs are more concentrated towards the Galactic plane. We determine the rest-frame wavelength with unprecedented precision ($\rm λ_{0} = 8620.86\, \pm 0.019$ Angström in air) and reveal a remarkable correspondence between the DIB velocities and the CO gas velocities, suggesting that the 862 nm DIB carrier is related to macro-molecules.

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arXiv2022

Gaia Data Release 3: Chemical cartography of the Milky Way

Gaia DR3 opens a new era of all-sky spectral analysis of stellar populations thanks to the nearly 5.6 million stars observed by the RVS and parametrised by the GSP-spec module. The all-sky Gaia chemical cartography allows a powerful and precise chemo-dynamical view of the Milky Way with unprecedented spatial coverage and statistical robustness. First, it reveals the strong vertical symmetry of the Galaxy and the flared structure of the disc. Second, the observed kinematic disturbances of the disc -- seen as phase space correlations -- and kinematic or orbital substructures are associated with chemical patterns that favour stars with enhanced metallicities and lower [alpha/Fe] abundance ratios compared to the median values in the radial distributions. This is detected both for young objects that trace the spiral arms and older populations. Several alpha, iron-peak elements and at least one heavy element trace the thin and thick disc properties in the solar cylinder. Third, young disc stars show a recent chemical impoverishment in several elements. Fourth, the largest chemo-dynamical sample of open clusters analysed so far shows a steepening of the radial metallicity gradient with age, which is also observed in the young field population. Finally, the Gaia chemical data have the required coverage and precision to unveil galaxy accretion debris and heated disc stars on halo orbits through their [alpha/Fe] ratio, and to allow the study of the chemo-dynamical properties of globular clusters. Gaia DR3 chemo-dynamical diagnostics open new horizons before the era of ground-based wide-field spectroscopic surveys. They unveil a complex Milky Way that is the outcome of an eventful evolution, shaping it to the present day (abridged).

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arXiv2022

Gaia Data Release 3: Analysis of RVS spectra using the General Stellar Parametriser from spectroscopy

The chemo-physical parametrisation of stellar spectra is essential for understanding the nature and evolution of stars and of Galactic stellar populations. Gaia DR3 contains the parametrisation of RVS data performed by the General Stellar Parametriser-spectroscopy, module. Here we describe the parametrisation of the first 34 months of RVS observations. GSP-spec estimates the chemo-physical parameters from combined RVS spectra of single stars. The main analysis workflow described here, MatisseGauguin, is based on projection and optimisation methods and provides the stellar atmospheric parameters; the individual chemical abundances of N, Mg, Si, S, Ca, Ti, Cr, FeI, FeII, Ni, Zr, Ce and Nd; the differential equivalent width of a cyanogen line; and the parameters of a DIB feature. Another workflow, based on an artificial neural network, provides a second set of atmospheric parameters that are useful for classification control. We implement a detailed quality flag chain considering different error sources. With about 5.6 million stars, the Gaia DR3 GSP-spec all-sky catalogue is the largest compilation of stellar chemo-physical parameters ever published and the first one from space data. Internal and external biases have been studied taking into account the implemented flags. In some cases, simple calibrations with low degree polynomials are suggested. The homogeneity and quality of the estimated parameters enables chemo-dynamical studies of Galactic stellar populations, interstellar extinction studies from individual spectra, and clear constraints on stellar evolution models. We highly recommend that users adopt the provided quality flags for scientific exploitation . The Gaia DR3 GSP-spec catalogue is a major step in the scientific exploration of Milky Way stellar populations, confirming the Gaia promise of a new Galactic vision (abridged).

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arXiv2021

The Pristine survey -- XV. A CFHT ESPaDOnS view on the Milky Way halo and disc populations

We present a one-dimensional, local thermodynamic equilibrium (1D-LTE) homogeneous analysis of 132 stars observed at high-resolution with ESPaDOnS. This represents the largest sample observed at high resolution (R$\sim$40,000) from the Pristine survey. This sample is based on the first version of the Pristine catalog and covers the full range of metallicities from [Fe/H]$\sim -3$ to $\sim +0.25$, with nearly half of our sample (58 stars) composed of very metal-poor stars ([Fe/H] $\le$ $-$2). This wide range of metallicities provides the opportunity of a new detailed study of the Milky Way stellar population. Because it includes both dwarf and giant stars, it also enables the analysis of any potential bias induced by the Pristine selection process. Based on Gaia EDR3, the orbital analysis of this Pristine$-$Espadons sample shows that it is composed of 65 halo stars and 67 disc stars. After a general assessment of the sample chemical properties with the $α$-elements Mg and Ca, we focus on the abundance of carbon and the neutron capture elements Ba and Sr. While most of our very metal-poor subsample is carbon normal, we also find that 14 stars out of the 38 stars with [Fe/H] $\leq$ $-$2 and measured carbon abundances turn out to be carbon enhanced metal-poor (CEMP) stars. We show that these CEMP stars are nearly exclusively (i.e. 12 stars out of 14) in the regime of low luminosity, unevolved, dwarf stars, which we interpret as the consequence of bias of the Pristine filter against C-rich giants. Among the very metal-poor (VMP) stars, we identify 2 CEMP stars with no enhancement in neutron-capture process elements (CEMP-no) and another one enriched in s-process element (CEMP-s). Finally, one VMP star is found with a very low [Sr/Fe] abundance ratio for its metallicity, as expected if it had been accreted from an ultra-faint dwarf galaxy.

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arXiv2021

The Pristine survey XIV: chemical analysis of two ultra-metal-poor stars

Elemental abundances of the most metal-poor stars reflect the conditions in the early Galaxy and the properties of the first stars. We present a spectroscopic follow-up of two ultra metal-poor stars ([Fe/H]<-4.0) identified by the survey {\em Pristine}: Pristine 221.8781+9.7844 and Pristine 237.8588+12.5660 (hereafter Pr 221 and Pr 237, respectively). Combining data with earlier observations, we find a radial velocity of -149.25 $\pm$ 0.27 and -3.18 $\pm$ 0.19 km/s for Pr 221 and Pr 237, respectively, with no evidence of variability between 2018 and 2020. From a one-dimensional (1D) local thermodynamic equilibrium (LTE) analysis, we measure [Fe/H]$_{\rm LTE}$=-4.79 $\pm$ 0.14 for Pr 221 and [Fe/H]$_{\rm LTE}$=-4.22 $\pm$ 0.12 for Pr 237, in good agreement with previous studies. Abundances of Li, Na, Mg, Al, Si, Ca, Ti, Fe, and Sr were derived based on the non-LTE (NLTE) line formation calculations. When NLTE effects are included, we measure slightly higher metallicities: [Fe/H]$_{\rm NLTE}$=-4.40 $\pm$ 0.13 and [Fe/H]$_{\rm NLTE}$=-3.93 $\pm$ 0.12, for Pr 221 and Pr 237, respectively. Analysis of the G-band yields [C/Fe]$_{\rm 1D-LTE} \leq$ +2.3 and [C/Fe]$_{\rm 1D-LTE} \leq$ +2.0 for Pr 221 and Pr 237. Both stars belong to the low-carbon band. Upper limits on nitrogen abundances are also derived. Abundances for other elements exhibit good agreement with those of stars with similar parameters. Finally, to get insight into the properties of their progenitors, we compare NLTE abundances to theoretical yields of zero-metallicity supernovae. This suggests that the supernovae progenitors had masses ranging from 10.6 to 14.4 M$_{\odot}$ and low-energy explosions with 0.3-1.2 $\times$ 10$^{51}$ erg.

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arXiv2021

The diffuse interstellar band around 8620 Å II. Kinematics and distance of the DIB carrier

We aim to make use of the measurements from the Giraffe Inner Bulge Survey (GIBS) and the Gaia$-$ESO survey (GES) to study the kinematics and distance of the carrier of DIB$\,λ$8620, as well as other properties. We successfully detected and measured DIB$\,λ$8620 in 760 of 4117 GES spectra. Combined with the DIBs measured in GIBS spectra, we confirmed a tight relation between EW and $E(J-K_{\rm S})$ as well as $A_{\rm V}$, with similar fitting coefficients to those found by previous works. With a more accurate sample and the consideration of the solar motion, the rest-frame wavelength of DIB$\,λ$8620 was redetermined as 8620.83 Å, with a mean fit error of 0.36 Å. We studied the kinematics of the DIB carriers by tracing their median radial velocities in each field in the local standard of rest ($V_{\rm LSR}$) and into the galactocentric frame ($V_{\rm GC}$), respectively, as a function of the Galactic longitudes. Based on the median $V_{\rm LSR}$ and two Galactic rotation models, we obtained valid kinematic distances of the DIB carriers for nine GIBS and ten GES fields. We also found a linear relation between the DIB$\,λ$8620 measured in this work and the near-infrared DIB in APOGEE spectra at $1.5273\,μm$. We demonstrate that the DIB carriers can be located much closer to the observer than the background stars based on the following arguments: (i) qualitatively, the carriers occupy in the Galactic longitude$-$velocity diagram typical rotation velocities of stars in the local Galactic disk, while the background stars in the GIBS survey are mainly located in the Galactic bulge; (ii) quantitatively, all the derived kinematic distances of the DIB carriers are smaller than the median distances to background stars in each field.

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arXiv2021

Gaia Early Data Release 3: The Galactic anticentre

We aim to demonstrate the scientific potential of the Gaia Early Data Release 3 (EDR3) for the study of the Milky Way structure and evolution. We used astrometric positions, proper motions, parallaxes, and photometry from EDR3 to select different populations and components and to calculate the distances and velocities in the direction of the anticentre. We explore the disturbances of the current disc, the spatial and kinematical distributions of early accreted versus in-situ stars, the structures in the outer parts of the disc, and the orbits of open clusters Berkeley 29 and Saurer 1. We find that: i) the dynamics of the Galactic disc are very complex with vertical asymmetries, and new correlations, including a bimodality with disc stars with large angular momentum moving vertically upwards from below the plane, and disc stars with slightly lower angular momentum moving preferentially downwards; ii) we resolve the kinematic substructure (diagonal ridges) in the outer parts of the disc for the first time; iii) the red sequence that has been associated with the proto-Galactic disc that was present at the time of the merger with Gaia-Enceladus-Sausage is currently radially concentrated up to around 14 kpc, while the blue sequence that has been associated with debris of the satellite extends beyond that; iv) there are density structures in the outer disc, both above and below the plane, most probably related to Monoceros, the Anticentre Stream, and TriAnd, for which the Gaia data allow an exhaustive selection of candidate member stars and dynamical study; and v) the open clusters Berkeley~29 and Saurer~1, despite being located at large distances from the Galactic centre, are on nearly circular disc-like orbits. We demonstrate how, once again, the Gaia are crucial for our understanding of the different pieces of our Galaxy and their connection to its global structure and history.

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arXiv2021

The AMBRE Project: Origin and evolution of sulfur in the Milky Way

Sulfur is a volatile chemical element that plays an important role in tracing the chemical evolution of galaxies. However, its nucleosynthesis origin and abundance variations are still unclear. The goal of the present article is to accurately and precisely study the S-content of large number of stars located in the solar neighbourhood. We use the parametrisation of thousands of HR stellar spectra provided by the AMBRE Project, and combine it with the automated abundance determination GAUGUIN to derive LTE sulfur abundances for 1855 slow-rotating FGK-type stars. This is the largest and most precise catalogue of S-abundances published to date. It covers a metallicity domain as high as ~2.5dex starting at [M/H]~-2.0dex. We find that the [S/M] abundances ratio is compatible with a plateau-like distribution in the metal-poor regime, and then starts to decrease continuously at [M/H]~-1.0dex. This decrease continues towards negative values for supersolar metallicity stars as recently reported for Mg and as predicted by Galactic chemical evolution models. Moreover, sulfur-rich stars having [M/H] in the range [-1.0,-0.5] have very different kinematical and orbital properties with respect to more metal-rich and sulfur-poor ones. Two disc components, associated with the thin and thick discs, are thus seen independently in kinematics and sulfur abundances. The sulfur radial gradients in the Galactic discs have also been estimated. Finally, the enrichment in sulfur with respect to iron is nicely correlated with stellar ages: older metal-poor stars have higher [S/M] ratios than younger metal-rich ones. This work has confirmed that sulfur is an alfa-element that could be considered to explore the Galactic populations properties.

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arXiv2020

The diffuse interstellar band around 8620 Å I. Methods and application to the GIBS data set

We developed a set of procedures to automatically detect and measure the DIB around 8620 Å (the Gaia DIB) for a wide range of temperatures. The DIB profile is fit with a Gaussian function. Specifically, the DIB feature is extracted from the spectra of late-type stars by subtracting the corresponding synthetic spectra. For early-type stars we applied a specific model based on the Gaussian process that needs no prior knowledge of the stellar parameters. The method was tested on $\sim$5000 spectra from the Giraffe Inner Bulge Survey (GIBS). After validation, we obtained 4194 reasonable fitting results from the GIBS database. An EW versus $E(J\,{-}\,K_{\rm S})$ relation is derived as $E(J\,{-}\,K_{\rm S})\,{=}\,1.875\,({\pm}\,0.152)\,{\times}\,{\rm EW}\,{-}\,0.011\,({\pm}\,0.048)$, according to $E(B\,{-}\,V)/{\rm EW}\,{=}\,2.721$, which is highly consistent with previous results toward similar sightlines. After a correction based on the VVV database for both EW and reddening, the coefficient derived from individual GIBS fields, $E(J\,{-}\,K_{\rm S})/{\rm EW}\,{=}\,1.884\,{\pm}\,0.225$, is also in perfect agreement with literature values. Based on a subsample of 1015 stars toward the Galactic center within $-3^{\circ}\,{<}\,b\,{<}\,3^{\circ}$ and $-6^{\circ}\,{<}\,l\,{<}\,3^{\circ}$, we determined a rest-frame wavelength of the Gaia DIB as 8620.55 Å. A Gaussian profile is proved to be a proper and stable assumption for the Gaia DIB as no intrinsic asymmetry is found.

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arXiv2020

The RAdial Velocity Experiment (RAVE): Parameterisation of RAVE spectra based on convolutional neural networks

In the context of large spectroscopic surveys of stars, data-driven methods are key in deducing physical parameters for millions of spectra in a short time. Convolutional neural networks (CNNs) enable us to connect observables (e.g. spectra, stellar magnitudes) to physical properties (atmospheric parameters, chemical abundances, or labels in general). We trained a CNN, adopting stellar atmospheric parameters and chemical abundances from APOGEE DR16 (resolution R=22500) data as training set labels. As input, we used parts of the intermediate-resolution RAVE DR6 spectra (R~7500) overlapping with the APOGEE DR16 data as well as broad-band ALL_WISE and 2MASS photometry, together with Gaia DR2 photometry and parallaxes. We derived precise atmospheric parameters Teff, log(g), and [M/H] along with the chemical abundances of [Fe/H], [alpha/M], [Mg/Fe], [Si/Fe], [Al/Fe], and [Ni/Fe] for 420165 RAVE spectra. The precision typically amounts to 60K in Teff, 0.06 in log(g) and 0.02-0.04 dex for individual chemical abundances. Incorporating photometry and astrometry as additional constraints substantially improves the results in terms of the accuracy and precision of the derived labels. We provide a catalogue of CNN-trained atmospheric parameters and abundances along with their uncertainties for 420165 stars in the RAVE survey. CNN-based methods provide a powerful way to combine spectroscopic, photometric, and astrometric data without the need to apply any priors in the form of stellar evolutionary models. The developed procedure can extend the scientific output of RAVE spectra beyond DR6 to ongoing and planned surveys such as Gaia RVS, 4MOST, and WEAVE. We call on the community to place a particular collective emphasis and on efforts to create unbiased training samples for such future spectroscopic surveys.

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arXiv2020

An optimised tiling pattern for multi-object spectroscopic surveys: application to the 4MOST survey

Large multi-object spectroscopic surveys require automated algorithms to optimise their observing strategy. One of the most ambitious upcoming spectroscopic surveys is the 4MOST survey. The 4MOST survey facility is a fibre-fed spectroscopic instrument on the VISTA telescope with a large enough field of view to survey a large fraction of the southern sky within a few years. Several Galactic and extragalactic surveys will be carried out simultaneously, so the combined target density will strongly vary. In this paper, we describe a new tiling algorithm that can naturally deal with the large target density variations on the sky and which automatically handles the different exposure times of targets. The tiling pattern is modelled as a marked point process, which is characterised by a probability density that integrates the requirements imposed by the 4MOST survey. The optimal tilling pattern with respect to the defined model is estimated by the tiles configuration that maximises the proposed probability density. In order to achieve this maximisation a simulated annealing algorithm is implemented. The algorithm automatically finds an optimal tiling pattern and assigns a tentative sky brightness condition and exposure time for each tile, while minimising the total execution time that is needed to observe the list of targets in the combined input catalogue of all surveys. Hence, the algorithm maximises the long-term observing efficiency and provides an optimal tiling solution for the survey. While designed for the 4MOST survey, the algorithm is flexible and can with simple modifications be applied to any other multi-object spectroscopic survey.

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arXiv2020

Probabilistic fibre-to-target assignment algorithm for multi-object spectroscopic surveys

Context. Several new multi-object spectrographs are currently planned or under construction that are capable of observing thousands of Galactic and extragalactic objects simultaneously. Aims. In this paper we present a probabilistic fibre-to-target assignment algorithm that takes spectrograph targeting constraints into account and is capable of dealing with multiple concurrent surveys. We present this algorithm using the 4-metre Multi-Object Spectroscopic Telescope (4MOST) as an example. Methods. The key idea of the proposed algorithm is to assign probabilities to fibre-target pairs. The assignment of probabilities takes the fibre positioner's capabilities and constraints into account. Additionally, these probabilities include requirements from surveys and take the required exposure time, number density variation, and angular clustering of targets across each survey into account. The main advantage of a probabilistic approach is that it allows for accurate and easy computation of the target selection function for the different surveys, which involves determining the probability of observing a target, given an input catalogue. Results. The probabilistic fibre-to-target assignment allows us to achieve maximally uniform completeness within a single field of view. The proposed algorithm maximises the fraction of successfully observed targets whilst minimising the selection bias as a function of exposure time. In the case of several concurrent surveys, the algorithm maximally satisfies the scientific requirements of each survey and no specific survey is penalised or prioritised. Conclusions. The algorithm presented is a proposed solution for the 4MOST project that allows for an unbiased targeting of many simultaneous surveys. With some modifications, the algorithm may also be applied to other multi-object spectroscopic surveys.

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arXiv2020

The Pristine survey VIII: The metallicity distribution function of the Milky Way halo down to the extremely metal-poor regime

The Pristine survey uses narrow-band photometry to derive precise metallicities down to the extremely metal-poor regime ([Fe/H] < -3), and currently consists of over 4 million FGK-type stars over a sky area of $\sim 2~500\, \mathrm{deg}^2$. We focus our analysis on a subsample of $\sim 80~000$ main sequence turnoff stars with heliocentric distances between 6 and 20 kpc, which we take to be a representative sample of the inner halo. The resulting metallicity distribution function (MDF) has a peak at [Fe/H] = -1.6, and a slope of $Δ$(LogN)/$Δ[Fe/H] = 1.0 \pm 0.1$ in the metallicity range of -3.4 < [Fe/H] < -2.5. This agrees well with a simple closed-box chemical enrichment model in this range, but is shallower than previous spectroscopic MDFs presented in the literature, suggesting that there may be a larger proportion of metal-poor stars in the inner halo than previously reported. We identify the Monoceros/TriAnd/ACS/EBS/A13 structure in metallicity space in a low latitude field in the anticenter direction, and also discuss the possibility that the inner halo is dominated by a single, large merger event, but cannot strongly support or refute this idea with the current data. Finally, based on the MDF of field stars, we estimate the number of expected metal-poor globular clusters in the Milky Way halo to be 5.4 for [Fe/H] < -2.5 and 1.5 for [Fe/H] < -3, suggesting that the lack of low metallicity globular clusters in the Milky Way is not due simply to statistical undersampling.

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arXiv2019

Properties of carbon stars in the Solar neighbourhood based on Gaia DR2 astrometry

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. 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. 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 larger than ~ 500 pc from the Galactic Plane, and show a significant drift with respect to the local standard of rest. 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_o 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 one would imply that stars with mass up to ~5 Mo may become carbon star on the AGB. J-type stars not only differ significantly 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 dvifferent type whose origin is still unknown. The derived luminosities of R-hot stars make these stars unlikely to be in the red-clump as previously claimed. On the other hand, the derived spatial distribution and kinematic properties, together with their metallicity, 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.

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arXiv2019

The Gaia-ESO survey: Calibrating a relationship between Age and the [C/N] abundance ratio with open clusters

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.

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