The CORALIE survey for Southern extra-solar planets. IV. Intrinsic stellar limitations to planet searches with radial-velocity techniques

Activity related phenomena can induce radial-velocity variations, which can be very important when dealing with extra-solar planet search programmes requiring high-precision radial-velocity measurements. In this paper we present a new chromospheric activity index, S_COR, based on the Ca Ii H line central reemission, and constructed using CORALIE spectra. After one year of measurements, values of S_COR are available for a sub-sample of stars of the Geneva extra-solar planet search programme. After transforming the S_COR values into the Mount-Wilson ``S'' scale we obtained values of the Ca Ii H and K flux corrected from photospheric emission (R'_HK) for the stars. The first results are presented, and in particular we focus on the study of the relation between the observed radial-velocity scatter and the chromospheric activity index R'_HK, for F, G and K dwarfs. Based on observations collected at the La Silla Observatory, ESO (Chile), with the echelle spectrograph CORALIE at the 1.2-m Euler Swiss telescope