Spectroscopic survey of the Galaxy with Gaia- I. Design and performance of the Radial Velocity Spectrometer

The definition and optimization studies for the Gaia satellite spectrograph, the 'radial velocity spectrometer' (RVS), converged in late 2002 with the adoption of the instrument baseline. This paper reviews the characteristics of the selected configuration and presents its expected performance. The RVS is a 2.0 1.6 degree integral field spectrograph, dispersing the light of all sources entering its field of view with a resolving power R = / = 11 500 over the wavelength range [848, 874] nm. The RVS will continuously and repeatedly scan the sky during the 5-yr Gaia mission. On average, each source will be observed 102 times over this period. The RVS will collect the spectra of about 100-150 million stars up to magnitude V 17-18. At the end of the mission, the RVS will provide radial velocities with precisions of 2 km s -1 at V = 15 and 15-20 km s -1 at V = 17, for a solar-metallicity G5 dwarf. The RVS will also provide rotational velocities, with precisions (at the end of the mission) for late-type stars of v sin i 5 km s -1 at V 15 as well as atmospheric parameters up to V 14-15. The individual abundances of elements such as silicon and magnesium, vital for the understanding of Galactic evolution, will be obtained up to V 12-13. Finally, the presence of the 862.