The AMBRE project: Parameterisation of FGK-type stars from the ESO:HARPS archived spectra

Context. The AMBRE project is a collaboration between the European Southern Observatory (ESO) and the Observatoire de la Cote d'Azur (OCA). It has been established to determine the stellar atmospheric parameters of the archived spectra of four ESO spectrographs. Aims. The analysis of the ESO: HARPS archived spectra for the determination of their atmospheric parameters (effective temperature, surface gravity, global metallicities, and abundance of alpha-elements over iron) is presented. The sample being analysed (AMBRE: HARPS) covers the period from 2003 to 2010 and is comprised of 126 688 scientific spectra corresponding to similar to 17 218 different stars. Methods. For the analysis of the AMBRE: HARPS spectral sample, the automated pipeline developed for the analysis of the AMBRE: FEROS archived spectra has been adapted to the characteristics of the HARPS spectra. Within the pipeline, the stellar parameters are determined by the MATISSE algorithm, which has been developed at OCA for the analysis of large samples of stellar spectra in the framework of galactic archaeology. In the present application, MATISSE uses the AMBRE grid of synthetic spectra, which covers FGKM-type stars for a range of gravities and metallicities. Results. We first determined the radial velocity and its associated error for the similar to 15% of the AMBRE: HARPS spectra, for which this velocity had not been derived by the ESO: HARPS reduction pipeline. The stellar atmospheric parameters and the associated chemical index [alpha/Fe] with their associated errors have then been estimated for all the spectra of the AMBRE: HARPS archived sample. Based on key quality criteria, we accepted and delivered the parameterisation of 93 116 (74% of the total sample) spectra to ESO. These spectra correspond to similar to 10 706 stars; each are observed between one and several hundred times. This automatic parameterisation of the AMBRE: HARPS spectra shows that the large majority of these stars are cool main-sequence dwarfs with metallicities greater than -0.5 dex (as expected, given that HARPS has been extensively used for planet searches around GK-stars).