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

Activity related phenomena can induce radialvelocity variations, which can be very important when dealing with extra-solar planet search programmes requiring highprecision radial-velocity measurements. In this paper we present a new chromospheric activity index, SCOR, based on the Ca ii H line central reemission, and constructed using CORALIE spectra. After one year of measurements, values of SCOR are available for a sub-sample of stars of the Geneva extra-solar planet search programme. After transforming the SCOR 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.