On the accuracy of GAIA radial velocities

We have obtained 782 real spectra and used them as inputs for 6700 automatic crosscorrelation runs to investigate GAIA potential in terms of radial velocity accuracy. We have explored the dispersions 0.25, 0.5, 1 and 2 Å/pix over the 8490–8740 Å GAIA range. We have investigated late-F to early-M stars (constituting the vast majority of GAIA targets), slowly rotating (< Vrot sin i > = 4 km sec ), of solar metallicity (< [Fe/H]> = −0.07) and not binary. The results are accurately described by the simple law: lg σ = 0.6× (lg S N ) − 2.4× lg S N + 1.75× lgD + 3, where σ is the cross-correlation standard error (in km sec) and D is the spectral dispersion (in Å/pix). The spectral dispersion has turned out to be the dominant factor, with S/N being less important and the spectral mis-match being a weak player at the lowest S/N. Our results show that mission-averaged radial velocities of faint GAIA targets (V∼15 mag) can match the ∼0.5 km sec accuracy of tangential motions, provided the observations are performed at a dispersion not less than 0.5 Å/pix.