Parallel Acquisition of Radial Velocities and Metallicities for the Gaia Mission

We discuss possible options for the acquisition of radial velocities in parallel with the astrometric data for the global astrometry mission GAIA, currently under study in the framework of the scienti c programme of the European Space Agency. The scienti c rationale of a parallel radial velocity programme, in the light of the recent evolution of the GAIA concept toward signi cantly fainter magnitudes, is discussed, and a possible instrument concept is presented. The approach currently being investigated could yield radial velocities accurate to ' 10 km s 1 for targets as faint as V ' 17 mag, and as accurate as ' 1 kms 1 for relatively bright stars (V ' 10{12 mag). These performance gures, derived for a realistic instrumental con guration, compatible with the GAIA con guration currently being studied, would greatly enhance the scienti c return from GAIA itself, and produce signi cant science of its own.