On the accuracy of GAIA radial velocities

Abstract

We have obtained 782 real spectra and used them as inputs for 6700 automatic cross-correlation runs to investigate the 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/s), of solar metallicity ( = -0.07) and not binary. The results are accurately described by the simple law: log σ = 0.6 (log S/N)2 - 2.4 log S/N + 1.75 log D + 3, where σ is the cross-correlation standard error (in km/s) 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/s accuracy of tangential motions, provided the observations are performed at a dispersion not less than 0.5 Å/pix.