Suchergebnisse

CONTEXT. We have entered an era of large spectroscopic surveys in which we can measure, through automated pipelines, the atmospheric parameters and chemical abundances for large numbers of stars. Calibrating these survey pipelines using a set of "benchmark stars" in order to evaluate the accuracy and precision of the provided parameters and abundances is of utmost importance. The recent proposed set of Gaia FGK benchmark stars has up to five metal-poor stars but no recommended stars within -2.0 < [Fe/H] < -1.0 dex. However, this metallicity regime is critical to calibrate properly. AIMS. In this paper, we aim to add candidate Gaia benchmark stars inside of this metal-poor gap.We began with a sample of 21 metal-poor stars which was reduced to 10 stars by requiring accurate photometry and parallaxes, and high-resolution archival spectra. METHODS. The procedure used to determine the stellar parameters was similar to the previous works in this series for consistency. The di erence was to homogeneously determine the angular diameter and e ective temperature (Te ff) of all of our stars using the Infrared Flux Method utilizing multi-band photometry. The surface gravity (log g) was determined through fitting stellar evolutionary tracks. The [Fe/H] was determined using four di erent spectroscopic methods fixing the Te and log g from the values determined independent of spectroscopy. RESULTS. We discuss, star-by-star, the quality of each parameter including how it compares to literature, how it compares to a spectroscopic run where all parameters are free, and whether Fe I ionisation-excitation balance is achieved. CONCLUSIONS. From the 10 stars, we recommend a sample of five new metal-poor benchmark candidate stars which have consistent Te , log g, and [Fe/H] determined through several means. These stars, which are within -1.3 < [Fe/H] < -1.0, can be used for calibration and validation purpose of stellar parameter and abundance pipelines and should be of highest priority for future interferometric studies. ; K.H. is funded by the British Marshall Scholarship program and the King's College, Cambridge Studentship. This work was partly supported by the European Union FP7 programme through ERC grant number 320360. U.H. acknowledges support from the Swedish National Space Board (Rymdstyrelsen).

The Gaia-ESO Survey (GES) observed many open clusters as part of its programme to spectroscopically characterize the various Milky Way populations. GES spectroscopy and Gaia astrometry from its second data release are used here to assign membership probabilities to targets towards 32 open clusters with ages from 1 to 3800 Myr, based on maximum likelihood modelling of the 3D kinematics of the cluster and field populations. From a parent catalogue of 14 398 individual targets, 5032 stars with uniformly determined 3D velocities, Teff, log g, and chemistry are assigned cluster membership with probability >0.9, and with an average probability of 0.991. The robustness of the membership probabilities is demonstrated using independent membership criteria (lithium and parallax) in two of the youngest clusters. The addition of radial velocities improves membership discrimination over proper motion selection alone, especially in more distant clusters. The kinematically selected nature of the membership lists, independent of photometry and chemistry, makes the catalogue a valuable resource for testing stellar evolutionary models and investigating the time evolution of various parameters. ; This work was partly supported by the European Union FP7 programme through ERC grant number 320360 and by the Leverhulme Trust through grant no. RPG-2012-541. We acknowledge the support from INAF and Ministero dell' Istruzione, dell' Università' e della Ricerca (MIUR) in the form of the grant 'Premiale VLT 2012'. The results presented here benefit from discussions held during the Gaia-ESO workshops and conferences supported by the ESF (European Science Foundation) through the GREAT Research Network Programme. ; Peer reviewed