We present an in-depth study of metal-poor stars, based high resolution spectra combined with newly released astrometric data from Gaia, with special attention to observational uncertainties. The results are compared to those of other studies, including Gaia benchmark stars. Chemical evolution models are discussed, highlighting few puzzles that are still affecting our understanding of stellar nucleosynthesis and of the evolution of our Galaxy.

Various Galactic stellar spectroscopic surveys are currently underway, and each is expected to achieve high internal accuracy in terms of stellar parameters and abundances. A number of questions related to the formation and evolution of the Galaxy may be addressed based on samples of stars observed within each survey. In addition, complementary samples of stars may be constructed by combining data from different surveys. The Gaia FGK Benchmark Stars provide the necessary link to bring the quantities measured from different spectra with different methods onto the same scale. We selected 28 FGK stars and 5 M giants with available angular diameter θ, bolometric flux Fbol, and distance. We used the fundamental relation Teff ∝ F/θ0.5 to determine a reference effective temperature. We used the fundamental relation g ∝ M/R2, where M is the mass and R the radius, to determine a reference surface gravity. A homogeneous analysis of a high resolution and high signal-to-noise spectral library provides a reference metallicity (Blanco-Cuaresma et al. 2014A&A...566A..98B, Jofré et al. 2014A&A...564A.133J). These stars serve as a reference for testing and homogenizing large stellar surveys such as the Gaia mission and the Gaia-ESO public spectroscopic survey and for improving models of FGK-star atmospheres. A detailed discussion of the fundamental Teff and logg values will be presented in Heiter et al. (to be submitted).

This paper introduces the The Milky Way Unravelled by Gaia conference proceedings.

This paper gives a brief overview of the Gaia Research for European Astronomy Training (GREAT) network, including a description of the GREAT-ESF Research Network Programme and the GREAT Initial Training Network (GREAT-ITN). Scientific highlights from the GREAT-ITN are noted.

Solar twins are stars that replicate the solar astrophysical properties. They represent natural candidates for harboring planetary systems similar to ours. Solar siblings are stars which were born in the same cluster than the Sun. They may differ from the Sun by their mass and radius, but they have the same age, the same chemical composition, and similar spatial velocities. Solar twins and siblings may have formed in the same conditions as the Sun, and thus they may give some clues on where and how the Sun formed in our Galaxy. In this study, we search the closest solar twins among thousands of solar type stars observed at high spectral resolution over the full optical range. The spectra are directly compared to solar ones observed by reflection on asteroids or the Moon with the same instrument. This purely differential method, based on the reduced χ2, is independant of any model. We applied it to a selection of ∼17700 spectra of ∼3500 different stars from the ELODIE archive. The star HD 146233 (18 Sco) keeps its status of the closest solar twin. The second closest star is the known solar twin HD 138573. Some other stars among the top twenty best stars have never been studied before, like HD 168009, HD 056124, and HD 029150. Atmospheric parameters of the top twenty solar twins were determined with the iSpec code. Their effective temperatures and gravities are found to be within 100 K and 0.1 dex from those of the Sun, respectively. We find the metallicity of the twins to be higher by 0.05 dex on average than that of the Sun. We derived the chemical abundances for the 200 closest solar twins in a line by line differential basis relative to the Sun. The absorption lines used in this study come from a rigorous analysis of 670 lines measured in our 14 solar spectra. We find several stars with all abundances within 0.01 dex from those of the Sun. Several of them fall on the Yonsei-Yale theoritical isochrone of solar age and metallicity. Thus they could have been formed in the same cluster as the Sun.

Past studies have already shown that stars in open clusters are chemically homogeneous (e.g. De Silva et al. 2006, 2007 and 2009). These results support the idea that stars born from the same giant molecular cloud should have the same chemical composition. In this context, the chemical tagging technique was proposed by Freeman et al. (2002). The principle is to recover disrupted stellar clusters by looking only to the stellar chemical composition. In order to evaluate the feasibility of this approach, it is necessary to test if we can distinguish between stars born from different molecular clouds. For this purpose, we studied the chemical composition of stars in 32 old and intermediate-age open clusters, and we applied machine learning algorithms to recover the original cluster by only considering the chemical signatures.

We have applied the chemical tagging technique to study three old Open Clusters (NGC2632/Praesepe, NGC752 and NGC1817) by using our own spectral analysis code.

We have chosen the name of GYES, one of the mythological giants with one hundred arms,offspring of Gaia and Uranus, for our instrument study of a multifibre spectrograph forthe prime focus of the Canada-France-Hawaii Telescope. Such an instrument could provide anexcellent ground-based complement for the Gaia mission and a northern complement to theHERMES project on the AAT. The CFHT is well known for providing a stable prime focusenvironment, with a large field of view, which has hosted several imaging instruments, buthas never hosted a multifibre spectrograph. Building upon the experience gained at GÉPIwith FLAMES-Giraffe and X-Shooter, we are investigating the feasibility of a highmultiplex spectrograph (about 500 fibres) over a field of view one degree in diameter. Weare investigating an instrument with resolution in the range 15 000 to 30 000, whichshould provide accurate chemical abundances for stars down to 16th magnitude and radialvelocities, accurate to 1 km s-1 for fainter stars. The study is led byGÉPI-Observatoire de Paris with a contribution from Oxford for the study of thepositioner. The financing for the study comes from INSU CSAA and Observatoire de Paris.The conceptual study will be delivered to CFHT for review by October 1st 2010.

During the five years of the mission, the Gaia spectrograph, the Radial Velocity Spectrometer (RVS) will repeatedly survey the celestial sphere down to magnitude V ~ 17–18. This talk presents: (i) the system which is currently developed within the Gaia Data Processing and Analysis Consortium (DPAC) to reduce and calibrate the spectra and to derive the radial and rotational velocities, (ii) the RVS expected performances and (iii) scientific returns.

The calibration of the Radial Velocity Zero-Point (RVZP) of the Radial VelocitySpectrometer (RVS) will be performed with the help of Radial Velocity (RV) standard starsand asteroids. A full-sky list of RV standard stars candidates has been built for thispurpose within the Development Unit (DU) 640 of the Gaia DPAC Consortium. A ground-basedcampaign of RV observations has been initiated to eliminate unsuitable candidates.Simulations have also been performed in order to estimate in advance the number of RVstandard stars and asteroids crossing the RVS field of view. These standard stars andasteroids will also be very helpful for controlling any harmful effect (especially the CCDradiation damage) on calibration and RVZP during the lifetime of the Gaia project.Kinematic RV are expected to be published at the end of the project for the brighteststars, taking into account gravitational redshift and convective shifts for the linespresent in the RVS spectral wavelength range.

Using the method of spectral synthesis we derived the abundances of Li in the atmospheres of 100 stars in the range of metallicities −3${<}$[Fe/H]${<}$0.2. The investigated spectra are part of the library collected at the Haute Provence Observatory, they were obtained with the 193 cm telescope equipped with ELODIE spectrometer (R=42000). For the metal-poor dwarfs, in the “Spite plateau” region ([Fe/H]${<}$−1.7, $T_{\rm {eff}}{>}$5700 K) we obtain logA(Li)${=}$2.30${\pm}$0.05, which is in a good agreement with the results of other authors. Our values of Li abundances do not indicate any trends either with $T_{\rm {eff}}$ or with [Fe/H]. The “plateau” is also traced in the metallicity range −0.7${<}$[Fe/H]${<}$−0.3. The behavior of the lithium abundance for stars with [Fe/H]${>}$−1.7, $T_{\rm {eff}}{<}$5700 K shows a depleting mechanism in these stars, the growth of its efficiency with an increase of the metallicity. The “lithium plateau” was also found for thick disk dwarfs with $T_{\rm {eff}}{>}$5800 K.

A sample of nearly 400 stars, selected from the Tycho-2 catalog, has beenobserved towards the North Galactic Pole with the high resolution echellespectrograph Elodie. Combining those data with a local sample from the Hipparcos catalog, adetailed study of the disk components is done. A dynamical analysis is alsoperformed, enabling us to put new constraints on the local mass density.

A simple model for the evolution of an isothermal disk has been developed.It combines both the chemical evolution (through the Age-MetallicityRelation) and dynamical evolution (through the age-dispersion relation). Applying this model to a sample of Red Clump Stars from the Tycho-2 catalog,observed with the high resolution echelle spectrograph Elodie at theObservatoire de Haute Provence, we derive the underlying Star FormationHistory and compare it with local determinations using chromosphericactivity survey by Rocha-Pinto et al. (2000b).

The ‘1996 Edition’ of the Catalogue of [Fe/H] determinations by Cayrel de Strobel et al. (1997, A&A S 124,1) and two recent papers by Castro et al. (1997, AJ Vol.114, N.1) and by Feltzing and Gustafsson (A&A in press) have made possible to increase in the theoretical HR diagram (log Teff, Mbol the number of SMR stars.

The SMR Stars of this new enlarged sample had to have reliable absolute magnitudes, coming all from Hipparcos parallaxes, precise bolometric corrections, effective temperatures and metal abundances from high resolution detailed spectroscopic analyses.

With the help of an appropriate grid of isochrones computed by Lebreton (1997, Perryman et al. A&A, in press), ‘turn-off ages’could then be attributed to the slightly evolved stars (subgiants) of the sample. The (log Teff, Mbol) diagram constituted by the new sample of SMR stars, shows that the conclusions in a former paper by Cayrel de Strobel (1987, A&AJ 8,141) remain valid: the SMR stars areold stars in spite of their higher than solar metallicity. The result, that metal-rich stars were in the mean old stars, was interpreted in the 1987 paper as due to a more chemical uniformity of the nowadays interstellar medium of the Galaxy with respect of the older much more active interstellar medium.

The aim of our stellar population study is to investigate the kinematical and chemical characteristics of the thin disk, thick disk and halo of the Galaxy. We have selected 51 stars in 2 astrometric and photometric surveys at l = 42°, b = +79° (Soubiran 1992) and l = 167°, b = +47° (Ojha et al. 1994), on the basis of the Reduced Proper Motion Diagram. They were observed with the 193cm telescope at Observatoire de Haute-Provence, with the CARELEC spectrograph (dispersion of 66Å/mm, FWHM of 3.0Å, range λλ4600 – 5500Å), together with 43 comparison stars with known fundamental parameters. The derivation of Teff, logg and [Fe/H] was done differentially using a grid of synthetic spectra and the comparison stars, as described in Cayrel et al. (1991). Twenty of the target stars were found to be more deficient than −0.5. In the (V, [Fe/H]) distribution, the halo stars are clearly separated from the other stars with a mean of (V, [Fe/H]) ≃ (−210km/s, – 1.4dex). Because of the small size of the sample, it was not possible to discriminate the thick disk from the thin disk. We have taken 200 more spectra, and with these new observations, we hope to be able to deconvolve the 3 populations in the (U, V, W, [Fe/H]) space as we did previously with the (U, V) velocity (Soubiran 1993).

A sample including 2370 stars with (U, V) velocities has been analyzed up to z = 2.5 kpc. It is shown that the observed vertical gradient in the velocity distribution can be explained by the sum of 3 discrete populations with constant kinematics. The observations are well fitted by exponential density laws for the thin disk and the thick disk with scale lengths of 280 pc and 700 pc respectively, and with local densities of 6% and 0.15% for the thick disk and halo respectively.