The planet search program at the ESO Coude Echelle spectrometer I. Data modeling technique and radial velocity precision tests

We present the modeling technique we apply to analyse data of the ESO Coude Echelle Spectrometer (CES) planet search program, in order to obtain high precision stellar radial velocity (RV measurements. The detection of the Do ppler reflex motion of a star due to an orbiting planet has so far been the most successful method to discover planetary companions to solar-type star s. The first extrasolar planet discovered by the CES program is the compani on to the young (ZAMS) G0V star iota Hor (Kurster et al. 2000). To achieve a high long-term RV measurement stability the CES spectra are self-calibrat ed by a superimposed iodine vapor (I-2) reference spectrum. In the modeling process a multi-parameter chi (2)-optimization is employed to generate an accurate description of the observation. We largely follow the general mode ling approach for I-2-calibrated data outlined by Butler et al. (1996) and Valenti et al. (1995). The sharp I-2 reference lines allow the reconstructi on of the spectrograph instrumental profile (IP) and the precise determinat ion of the Doppler shift of the stellar absorption lines. We show examples of the application of the modeling technique to CES data obtained over a ti me span of five and a half years at ESO La Silla. The small spectral bandwi dth of only 4.85 nm for CES spectra makes this data challenging for high pr ecision radial velocities. We demonstrate for the known RV-constant star ta u Get, for two stars with known extrasolar planets, 51 Peg and 70 Vir; and for a Men and GJ 570A, two stars from our survey that the achievable RV pre cision for CES data is 8-15 ms-(1).