Variability of stellar granulation and convective blueshift with spectral type and magnetic activity

In solar type stars, the attenuation of convective blueshift by stellar magnetic activity dominates the radial velocity variations over small mass planets. Models of stars different from the Sun request a good knowledge of these properties to allow a realistic extrapolation. It is therefore crucial to precisely determine not only the amplitude of the convective blueshift for different types of stars, but also the variability of this convective blueshift with magnetic activity, as they are key factors in models producing the radial velocity variations. We study a large sample of G and K stars and focus on their temporally averaged properties, i.e. the activity level and a criterion allowing to characterize the amplitude of the convective blueshift using the variation of the velocity versus the intensity at the bottom of spectral lines. We find this criteria to depend on spectral type, on wavelength and on the activity level. We derive a convective blueshift decreasing towards lower temperature, with a good agreement with models. Smaller differences remain to be examined in details. We quantify the variability of granulation properties in stars other than the Sun due to magnetic field. The relative variation (with respect to the average convective blueshift for a given stellar type) seems to be constant over the considered range of spectral types. We finally compare the observed radial velocity  variation amplitudes with those derived from our convective blueshift estimations using a simple law and find a general agreement on the amplitude, and show that inclination plays a major role.