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. <BR /> 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. <BR /> 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. <BR /> 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 <ASTROBJ>psi Phe</ASTROBJ> for the first time. Based on NARVAL and HARPS data obtained within the Gaia DPAC (Data Processing and Analysis Consortium) and coordinated by the GBOG (Ground-Based Observations for Gaia) working group and on data retrieved from the ESO-ADP database.Tables 6-76 are only available at the CDS via anonymous ftp to <A href="http //cdsarc.u-strasbg.fr">http //cdsarc.u-strasbg.fr</A> (ftp //130.79.128.5) or via <A href="http //cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/564/A133">http //cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/564/A133</A>