Stellar parametrization from Gaia RVS spectra
  Among the myriad of data collected by the ESA Gaia satellite, about 150
million spectra will be delivered by the Radial Velocity Spectrometer (RVS) for
stars as faint as G_RVS~16. A specific stellar parametrization will be
performed for most of these RVS spectra. Some individual chemical abundances
will also be estimated for the brightest targets. We describe the different
parametrization codes that have been specifically developed or adapted for RVS
spectra within the GSP-spec working group of the analysis consortium. The
tested codes are based on optimization (FERRE and GAUGUIN), projection
(MATISSE) or pattern recognition methods (Artificial Neural Networks). We
present and discuss their expected performances in the recovered stellar
atmospheric parameters (Teff, log(g), [M/H]) for B- to K- type stars. The
performances for the determinations of [alpha/Fe] ratios are also presented for
cool stars. For all the considered stellar types, stars brighter than
G_RVS~12.5 will be very efficiently parametrized by the GSP-spec pipeline,
including solid estimations of [alpha/Fe]. Typical internal errors for FGK
metal-rich and metal-intermediate stars are around 40K in Teff , 0.1dex in
log(g), 0.04dex in [M/H], and 0.03dex in [alpha/Fe] at G_RVS=10.3. Similar
accuracies in Teff and [M/H] are found for A-type stars, while the log(g)
derivation is more accurate. For the faintest stars, with G_RVS>13-14, a
spectrophotometric Teff input will allow the improvement of the final GSP-spec
parametrization. The reported results show that the contribution of the RVS
based stellar parameters will be unique in the brighter part of the Gaia survey
allowing crucial age estimations, and accurate chemical abundances. This will
constitute a unique and precious sample for which many pieces of the Milky Way
history puzzle will be available, with unprecedented precision and statistical
relevance.
