P. de Laverny

s-Process nucleosynthesis in carbon stars

We present the first detailed and homogeneous analysis of the s-element content in Galactic carbon stars of N type. Abundances of Sr, Y, Zr (low-mass s-elements, or ls), Ba, La, Nd, Sm, and Ce (high-mass s-elements, or hs) are derived using the spectral synthesis technique from high-resolution spectra. The N stars analyzed are of nearly solar metallicity and show moderate s-element enhancements, similar to those found in S stars, but smaller than those found in the only previous similar study (Utsumi 1985), and also smaller than those found in supergiant post-asymptotic giant branch (post-AGB) stars. This is in agreement with the present understanding of the envelope s-element enrichment in giant stars, which is increasing along the spectral sequence M → MS → S → SC → C during the AGB phase. We compare the observational data with recent s-process nucleosynthesis models for different metallicities and stellar masses. Good agreement is obtained between low-mass AGB star models (M 3 M☉) and s-element observations. In low-mass AGB stars, the 13C(α, n)16O reaction is the main source of neutrons for the s-process; a moderate spread, however, must exist in the abundance of 13C that is burnt in different stars. By combining information deriving from the detection of Tc, the infrared colors, and the theoretical relations between stellar mass, metallicity, and the final C/O ratio, we conclude that most (or maybe all) of the N stars studied in this work are intrinsic, thermally pulsing AGB stars; their abundances are the consequence of the operation of third dredge-up and are not to be ascribed to mass transfer in binary systems.

Gaia FGK benchmark stars: Metallicity

Context. To calibrate automatic pipelines that determine atmospheric parameters of stars, one needs a sample of stars, or “benchmark stars”, with well-defined parameters to be used as a reference. Aims. We provide detailed documentation of the iron abundance determination of the 34 FGK-type benchmark stars that are selected to be the pillars for calibration of the one billion Gaia stars. They cover a wide range of temperatures, surface gravities, and metallicities. Methods. Up to seven different methods were used to analyze an observed spectral library of high resolutions and high signal-to-noise ratios. The metallicity was determined by assuming a value of effective temperature and surface gravity obtained from fundamental relations; that is, these parameters were known a priori and independently from the spectra. Results. We present a set of metallicity values obtained in a homogeneous way for our sample of benchmark stars. In addition to this value, we provide detailed documentation of the associated uncertainties. Finally, we report a value of the metallicity of the cool giant ψ Phe for the first time.

Automated derivation of stellar atmospheric parameters and chemical abundances: the MATISSE algorithm

We present an automated procedure for the derivation of atmospheric parameters (T eff , log g, [M/H]) and individual chemical abundances from stellar spectra. The MATrix Inversion for Spectral SynthEsis (MATISSE) algorithm determines a basis, B θ (λ), allowing the derivation of a particular stellar parameter θ by projection of an observed spectrum. The Be(>.) function is determined from an optimal linear combination of theoretical spectra and it relates, in a quantitative way, the variations in the spectrum flux with variations in 6. An application of this method to the Gaia Radial Velocity Spectrograph spectral range is described, together with its performances for different types of stars of various metallicities. Blind tests with synthetic spectra of randomly selected parameters and observed input spectra are also presented. The method gives rapid, accurate and stable results and it can be efficiently applied to the study of stellar populations through the analysis of large spectral data sets, including moderate to low signal-to-noise ratio spectra.

The Gaia astrophysical parameters inference system (Apsis) - Pre-launch description

The Gaia satellite will survey the entire celestial sphere down to 20th magnitude, obtaining astrometry, photometry, and low resolution spectrophotometry on one billion astronomical sources, plus radial velocities for over one hundred million stars. Its main objective is to take a census of the stellar content of our Galaxy, with the goal of revealing its formation and evolution. Gaias unique feature is the measurement of parallaxes and proper motions with hitherto unparalleled accuracy for many objects. As a survey, the physical properties of most of these objects are unknown. Here we describe the data analysis system put together by the Gaia consortium to classify these objects and to infer their astrophysical properties using the satellites data. This system covers single stars, (unresolved) binary stars, quasars, and galaxies, all covering a wide parameter space. Multiple methods are used for many types of stars, producing multiple results for the end user according to different models and assumptions. Prior to its application to real Gaia data the accuracy of these methods cannot be assessed definitively. But as an example of the current performance, we can attain internal accuracies (RMS residuals) on F,G,K,M dwarfs and giants at G=15 (V=15-17) for a wide range of metallicites and interstellar extinctions of around 100K in effective temperature (Teff), 0.1mag in extinction (A0), 0.2dex in metallicity ([Fe/H]), and 0.25dex in surface gravity (logg). The accuracy is a strong function of the parameters themselves, varying by a factor of more than two up or down over this parameter range. After its launch in November 2013, Gaia will nominally observe for five years, during which the system we describe will continue to evolve in light of experience with the real data.

The Gaia-ESO Survey: Kinematic structure in the Gamma Velorum cluster

This work was partially supported by the Gaia Research for European Astronomy Training (GREAT-ITN) Marie Curie network, funded through the European Union Seventh Framework Programme [FP7/2007-2013] under grant agreement 264895 and supported by the European Union FP7 programme through ERC grant number 320360 and by the Leverhulme Trust through grant RPG-2012-541. We acknowledge the support from INAF and Ministero dell’ Istruzione, dell’ Universita e della Ricerca (MIUR) in the form of the grant “Premiale VLT 2012”. RJJ acknowledges financial support from the UK Science and Technology Facilities Council.

A spectroscopic survey of thick disc stars outside the solar neighbourhood

Context. In the era of large spectroscopic surveys, Galactic archaeology aims to understand the formation and evolution of the Milky Way by means of large datasets. In particular, the kinematic and chemical study of the thick disc can give valuable information on the merging history of the Milky Way. Aims. Our aim is to detect and characterise the Galactic thick disc chemically and dynamically by analysing F, G, and K stars, whose atmospheres reflect their initial chemical composition. Methods. We performed a spectroscopic survey of nearly 700 stars probing the Galactic thick disc far from the solar neighbourhood towards the Galactic coordinates (l ∼ 277 ◦ , b ∼ 47 ◦ ). The derived effective temperatures, surface gravities and overall metallicities were then combined with stellar evolution isochrones, radial velocities and proper motions to derive the distances, kinematics and orbital parameters of the sample stars. The targets belonging to each Galactic component (thin disc, thick disc, halo) were selected either on their kinematics or according to their position above the Galactic plane, and the vertical gradients were also estimated. Results. We present here atmospheric parameters, distances and kinematics for this sample and a comparison of our kinematic and metallicity distributions with the Besancon model of the Milky Way. The thick disc far from the solar neighbourhood is found to differ only slightly from the thick disc properties as derived in the solar vicinity. For regions where the thick disc dominates (1 Z 4 kpc), we measured vertical velocity and metallicity trends of ∂Vφ/∂Z = 19 ± 8k m s −1 kpc −1 and ∂[M/H]/∂Z = −0.14 ± 0.05 dex kpc −1 , respectively. These trends can be explained as a smooth transition between the different Galactic components, although intrinsic gradients could not be excluded. In addition, a correlation ∂Vφ/∂[M/H] = −45 ± 12 km s −1 dex −1 between the orbital velocity and the metallicity of the thick disc is detected. This gradient is inconsistent with the SDSS photometric survey analysis, which did not detect any such trend, and challenges radial migration models of thick disc formation. Estimations of the scale heights and scale lengths for different metallicity bins of the thick disc result in consistent values, with hR ∼ 3.4 ± 0.7 kpc, and hZ ∼ 694 ± 45 pc, showing no evidence of relics of destroyed massive satellites.

Spectroscopic survey of the Galaxy with Gaia– II. The expected science yield from the Radial Velocity Spectrometer

The Gaia mission is designed as a Galaxy explorer, and will measure simultaneously, in a survey mode, the five or six phase-space parameters of all stars brighter than 20th magnitude, as well as providing a description of their astrophysical characteristics. These measurements are obtained by combining an astrometric instrument with micro-arcsecond capabilities, a photometric system giving the magnitudes and colours in 15 bands and a medium-resolution spectrograph named the Radial Velocity Spectrometer (RVS). The latter instrument will produce spectra in the 848- to 874-nm wavelength range, with a resolving power R = 11500, from which radial velocities, rotational velocities, atmospheric parameters and abundances can be derived. A companion paper has presented the characteristics of the RVS and its performance. The present paper details the outstanding scientific impact of this important part of the Gaia satellite on some key open questions in present-day astrophysics. The unbiased and simultaneous acquisition of multi-epoch radial velocities and individual abundances of key elements in parallel with the astrometric parameters is essential for the determination of the dynamical state and formation history of our Galaxy. Moreover, for stars brighter than V similar or equal to 15, the resolving power of the RVS will give information about most of the effects that influence the position of a star in the Hertzsprung-Russell diagram, placing unprecedented constraints on the age, internal structure and evolution of stars of all types. Finally, the RVS multi-epoch observations are ideally suited to the identification, classification and characterization of the many types of double, multiple and variable stars.

The Gaia-ESO Survey: metallicity and kinematic trends in the Milky Way bulge

Aims. Observational studies of the Milky Way bulge are providing increasing evidence of its complex chemo-dynamical patterns and morphology. Our intent is to use the iDR1 Gaia-ESO Survey (GES) data set to provide new constraints on the metallicity and kinematic trends of the Galactic bulge, exploring the viability of the currently proposed formation scenarios. Methods. We analyzed the stellar parameters and radial velocities of similar to 1200 stars in five bulge fields wich are located in the region -10 degrees < / < 7 degrees and -10 degrees < b < -4 degrees. We use VISTA Variables in the Via Lactea (VVV) photometry to verify the internal consistency of the atmospheric parameters recommended by the consortium. As a by-product, we obtained reddening values using a semi-empirical Tdf -color calibration. We constructed the metallicity distribution functions and combined them with photometric and radial velocity data to analyze the properties of the stellar populations in the observed fields. Results. From a Gaussian decomposition of the metallicity distribution functions, we unveil a clear bimodality in all fields, with the relative size of components depending of the specific position on the sky. In agreement with some previous studies, we find a mild gradient along the minor axis (-0.05 dex/deg between b = -6 degrees and b = -10 degrees) that arises from the varying proportion of metal-rich and metal-poor components. The number of metal-rich stars fades in favor of the metal-poor stars with increasing b. The K-magnitude distribution of the metal-rich population splits into two peaks for two of the analyzed fields that intersects the near and far branches of the X-shaped bulge structure. In addition, two lateral fields at (l,b) = (7, -9) and (l, b) = (-10, 8) present contrasting characteristics. In the former, the metallicity distribution is dominated by metal-rich stars, while in the latter it presents a mix of a metal-poor population and and a metal-intermediate one, of nearly equal sizes. Finally, we find systematic differences in the velocity dispersion between the metal-rich and the metal-poor components of each field. Conclusions. The iDR I bulge data show chemo-dynamical distributions that are consistent with varying proportions of stars belonging to (i) a metal-rich boxy/peanut X-shaped component, with bar-like kinematics; and (ii) a metal-poor more extended rotating structure with a higher velocity dispersion that dominates far from the Galactic plane. These first GES data already allow studying the detailed spatial dependence of the Galactic bulge populations, thanks to the analysis of individual fields with relatively high statistics. (Less)

Automatic stellar spectra parameterisation in the IR Ca ii triplet region

Context. Galactic archaeology aims to determine the evolution of the Galaxy from the chemical and kinematical properties of its individual stars. This requires the analysis of data from la rge spectroscopic surveys, with sample sizes in tens of thou sands at present, with millions of stars being reached in the near future. Such large samples require automated analysis techniques and cl ssification algorithms to obtain robust estimates of the stellar parame ter values. Several on-going and planned spectroscopic sur veys have selected their wavelength region to contain the IR Ca ii triplet (∼ λλ 8500 Å) and the work presented in this paper focuses on the aut omatic analysis of such spectra. Aims. We aim to develop and test an automatic method by which one can obtain estimates of values of the stellar atmospheric parameters (e ffective temperature, surface gravity, overall metallicity ) from a stellar spectrum. We also explore the degeneracies i n parameter space, estimate the uncertainties in the derived parameter values and investigate the consequences of these limitations for achieving the goals of galactic archaeology. Methods. We investigated two algorithms, both of which compare the ob served spectrum to a grid of synthetic spectra, but each uses a different mathematical approach for finding the optimum match an d he ce the best values of the stellar parameters. Our invest igation of these algorithms’ robustness can be widely applied becau se it amplifies the main problems that the other methods can en counter. The first algorithm, MATISSE, derives the values of each stel lar parameter through a local fit to the spectrum such that eac h pixel in wavelength space is treated separately. The sensitivity of he flux at each wavelength to the value of a given stellar pa rameter is determined from the synthetic spectra. The observed spectr um is then projected using these sensitivity vectors to give an stimated value of the stellar parameters. This value depends on findin g the true minimum in the fit and the algorithm must avoid being trapped in false local minima. The second algorithm, DEGAS, uses a pa ttern-recognition approach and consequently has a more glo bal vision of the parameter space. The best-fit synthetic spectrum is de rive through a series of comparisons between the observed a nd synthetic spectra, summed over wavelength pixels, with additional re finements in the set of synthetic spectra after each stage, i. e. a decision tree. Results. We identified physical degeneracies in di fferent regions of the H–R diagram: hot dwarf and giant stars sh are t e same spectral signatures. Furthermore, it is very di fficult to determine an accurate value for the surface gravity o f co ler dwarfs. These e ffects are intensified when the lack of information increases, which ha ppens for low-metallicity stars or spectra with low signalto-noise ratios (SNRs). Our results demonstrate that the local projection m ethod is preferred for spectra with high SNR, whereas the dec ision-tree method is preferred for spectra of lower SNR. We therefore pr opose a hybrid approach, combining these methods, and demon strate that sufficiently accurate results for the purposes of galactic archa eology studies are retrieved down to SNR ∼20 for typical parameter values of stars belonging to the local thin or thick disc, and for SNR down to∼50 for the more metal-poor giant stars of the halo. Conclusions. If unappreciated, degeneracies in stellar parameters can i ntroduce biases and systematic errors in derived quantitie s for target stars such as distances and full space motions. These can be minimised using the knowledge gained by thorough test ing of the proposed stellar classification algorithm, which in tur n lead to robust automated methods for the coming extensive s pectroscopic surveys of stars in the Local Group.

Kinematics and chemistry of recently discovered reticulum 2 and horologium 1 dwarf galaxies

We report on VLT/GIRAFFE spectra of stars in two recently discovered ultra-faint satellites, Reticulum 2 and Horologium 1, obtained as part of the Gaia-ESO Survey. We identify 18 members in Reticulum 2 and five in Horologium 1. We find Reticulum 2 to have a velocity dispersion of 3.22(-0.49)(+1.64) km s(-1) , implying a mass-to-light ratio (M/L) of similar to 500. The mean metallicity of Reticulum 2 is [Fe/H] = -2.46, with an intrinsic dispersion of similar to 0.3 dex and alpha-enhancement of similar to 0.4 dex. We conclude that Reticulum 2 is a dwarf galaxy. We also report on the serendipitous discovery of four stars in a previously unknown stellar substructure near Reticulum 2 with [Fe/H] similar to -2 and V-hel similar to 220 km s(-1), far from the systemic velocity of Reticulum 2. For Horologium 1 we infer a velocity dispersion of sigma (V) = 4.9(-0.9)(+2.8) km s(-1) and a M/L ratio of similar to 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) and is similarly alpha-enhanced: [alpha/Fe] similar to 0.3 dex with a significant spread of metallicities of 0.17 dex. The line-of-sight velocity of Reticulum 2 is offset by 100 km s(-1) from the prediction of the orbital velocity of the Large Magellanic Cloud (LMC), thus making its association with the Cloud uncertain. However, at the location of Horologium 1, both the backward-integrated orbit of the LMC and its halo are predicted to have radial velocities similar to that of the dwarf. Therefore, it is possible that Horologium 1 is or once was a member of the Magellanic family.