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 x 1.6 degree integral field spectrograph, dispersing the light of all sources entering its field of view with a resolving power R = lambda/Deltalambda = 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 similar or equal to 17-18. At the end of the mission, the RVS will provide radial velocities with precisions of similar to2 km s(-1) at V = 15 and similar to15-20 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 sigma(upsilonsin) similar or equal to (i) similar or equal to 5 km s(-1) at V similar or equal to 15 as well as atmospheric parameters up to V similar or equal to 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 similar or equal to 12-13. Finally, the presence of the 862.0-nm diffuse interstellar band (DIB) in the RVS wavelength range will make it possible to derive the three-dimensional structure of the interstellar reddening