Gaia FGK Benchmark Stars - Metallicity
  To calibrate automatic pipelines that determine atmospheric parameters of
stars, one needs a sample of stars -- ``benchmark stars'' -- with well defined
parameters to be used as a reference We provide a detailed documentation of the
determination of the iron abundance of the 34 FGK-type benchmark stars selected
to be the pillars for calibration of the one billion Gaia stars. They cover a
wide range of temperatures, surface gravities and metallicities. Up to seven
different methods were used to analyze an observed spectral library of high
resolution and high signal-to-noise ratio. The metallicity was determined
assuming a value of effective temperature and surface gravity obtained from
fundamental relations, i.e. these parameters were known a priori independently
from the spectra. 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 a detailed documentation of the associated uncertainties. Finally,
we report for the first time a value of the metallicity of the cool giant psi
Phe.
