Stellar activity of planetary host star HD 189 733^*

¹ Institut d'Astrophysique de Paris, CNRS (UMR 7095), Université Pierre & Marie Curie, 98bis bd. Arago, 75014 Paris, France e-mail: iboisse@iap.fr
² Laboratoire d'Astrophysique de Marseille, CNRS (UMR 6110), Université de Provence, Pôle de l'Étoile Site de Château-Gombert, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
³ School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
⁴ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
⁵ Centro de Astrofísica, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁶ Dept. of Astronomy & Astrophysics, Univ. Toronto, 50 George St., Toronto, ON M5S 3H4, Canada
⁷ Laboratoire d'Astrophysique de Grenoble, CNRS (UMR 5571), Université Joseph Fourier, BP 53, 38041 Grenoble Cedex 9, France
⁸ University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada

Received: 22 July 2008
Accepted: 20 November 2008

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 189 733 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 189 733 over nearly two months. We refined the HD 189 733b 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 activity indices measured in He I (5875.62 Å), Hα (6562.81 Å), and both of the Ca II H&K lines (3968.47 Å and 3933.66 Å, respectively) exhibit a periodicity close to the stellar-rotation period and the correlations between them are consistent with a spotted stellar surface in rotation. We used these correlations to correct for the radial-velocity jitter due to stellar activity. This results in achieving high precision in measuring the orbital parameters, with a semi-amplitude K = 200.56 ± 0.88 m s^-1 and a derived planet mass of M_P = 1.13 ± 0.03 M_Jup.