Abstract
Aims. Extra-solar planet search programs require high-precision velocity measurements. They need to determine how to differentiate between radial-velocity variations due to Doppler motion and the noise induced by stellar activity. Methods. We monitored the active K2V star HD 189733 and its transiting planetary companion, which has a 2.2-day orbital period. We used the high-resolution spectograph SOPHIE mounted on the 1.93-m telescope at the Observatoire de Haute-Provence to obtain 55 spectra of HD 189733 over nearly two months. We refined the HD 189733b orbit parameters and placed limits on both the eccentricity and long-term velocity gradient. After subtracting the orbital motion of the planet, we compared the variability in spectroscopic activity indices with the evolution in the radial-velocity residuals and the shape of spectral lines. Results. The radial velocity, the spectral-line profile, and the acti vity indices measured in He I (5875.62 A), Hα (6562.81 A), and both of the Ca II H&K lines (3968.47 A and 3933.66 A, respectively) exhibit a periodicity close to the stellar-rotation pe riod and the correlations between them are consistent with a spotted stellar surface in rotation. We used these correlations to corr ect for the radialvelocity jitter due to stellar activity. This results in ach ieving high precision in measuring the orbital parameters, with a semi-amplitude ◦ .
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