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Centro de Astrofísica da Universidade do Porto

Disentangling between stellar activity and planetary signals

I. Boisse, F. Bouchy, G. Hébrard, X. Bonfils, N. C. Santos, S. Vauclair

Abstract
Photospheric stellar activity (i.e. dark spots or bright plages) might be an important source of noise and confusion in the radial-velocity (RV) measurements. Radial-velocimetry planet search surveys as well as follow-up of photometric transit surveys require a deeper understanding and characterization of the effects of stellar activities to disentangle it from planetary signals.We simulate dark spots on a rotating stellar photosphere. The variations of the photometry, RV and spectral line shapes are characterized and analyzed according to the stellar inclination, the latitude and the number of spots. We show that the anti-correlation between RV and bisector span, known to be a signature of activity, requires a good sampling to be resolved when there are several spots on the photosphere. The Lomb-Scargle periodograms of the RV variations induced by activity present power at the rotational period Prot of the star and its two-first harmonics Prot/2 and Prot/3. Three adjusted sinusoids fixed at the fundamental period and its two-first harmonics allow to remove about 90% of the RV jitter amplitude.We apply and validate our approach on four known active planet-host stars: HD189733, GJ 674, CoRoT-7 and ?Hor. We succeed in fitting simultaneously activity and planetary signals on GJ674 and CoRoT-7. This simultaneous modeling of the activity and planetary parameters leads to slightly larger masses of CoRoT-7b and c: respectively, 5.7 ± 2.5 MEarth and 13.1 ± 4.1 MEarth. The larger uncertainties take into account properly for the stellar active jitter. We excluded short-period lowmass exoplanets around ?Hor. For data with realistic time-sampling and white Gaussian noise, we use simulations to show that our approach is efficient to disentangle reflex-motion due to a planetary companion and stellar-activity induced-RV variations provided that 1) the planetary orbital period is not close to that of the stellar rotation or one of its two-first harmonics, 2) the semi-amplitude of the planet exceeds ∼30% of the semi-amplitude of the active signal, 3) the rotational period of the star is accurately known, 4) the data cover more than one stellar rotational period.

Keywords
techniques: radial velocities - stars: activity - stars: individual: ι Hor - stars: individual: HD189733 - stars: individual: GJ 674 - stars: individual: CoRoT-7

Astronomy and Astrophysics
Volume 528, Page A4_1
April 2011

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