An investigation into the radial velocity variations of CoRoT-7^*

A. P. Hatzes¹, R. Dvorak², G. Wuchterl¹, P. Guterman³, M. Hartmann¹, M. Fridlund⁴, D. Gandolfi^1,4, E. Guenther¹ and M. Pätzold⁵

¹ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany e-mail: artie@tls-tautenburg.de
² Institute for Astronomy, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
³ Laboratoire d'Astrophysique de Marseille, UMR 6110, Technopole de Marseille-Étoile, 13388 Marseille Cedex 13, France
⁴ Research and Scientific Support Department, European Space Agency, ESTEC, 2200 Noordwijk, The Netherlands
⁵ Rheinisches Institut für Umweltforschung, Universität zu Köln, Abt. Planetenforschung, Aachener Str. 209, 50931 Köln, Germany

Received: 15 April 2010
Accepted: 22 June 2010

Abstract

Context. CoRoT-7b, the first transiting “superearth” exoplanet, has a radius of 1.7  and a mass of 4.8 . The HARPS radial velocity (RV) measurements used for deriving this mass also detected an additional companion with a period of 3.7 days and a mass of 8.4 . The mass of CoRoT-7b is a crucial parameter for planet structure models, but is difficult to determine because CoRoT-7 is a modestly active star and there is at least one additional companion.

Aims. The aims of this paper are to assess the statistical significance of the RV variations of CoRoT-7b and CoRoT-7c, to obtain a better measurement of the planet mass for CoRoT-7b, and to search for additional companions in the RV data.

Methods. A Fourier analysis is performed on the HARPS spectral data of CoRoT-7. These data include RV measurements, spectral line bisectors, the full width at half maximum of the cross-correlation function, and Ca II emission. The latter 3 quantities vary due to stellar activity and were used to assess the nature of the observed RV variations. An analysis of a sub-set of the RV measurements where multiple observations were made per night was also used to estimate the RV amplitude from CoRoT-7b that was less sensitive to activity variations.

Results. Our analysis indicates that the 0.85-d and 3.7-d RV signals of CoRoT-7b and CoRoT-7c are present in the spectral data with a high degree of statistical significance. We also find evidence for another significant RV signal at 9 days. An analysis of the activity indicator data reveals that this 9-d signal most likely does not arise from activity, but possibly from an additional companion. If due to a planetary companion the mass is m = 19.5 , assuming co-planarity with CoRoT-7b. A dynamical study of the three planet system shows that it is stable over several hundred millions of years. Our analysis yields a RV amplitude of 5.04 ± 1.09 m s^-1 for CoRoT-7b which corresponds to a planet mass of m = 6.9 ± 1.4 . This increased mass would make the planet CoRoT-7b more Earth-like in its internal structure.

Conclusions. CoRoT-7 is confirmed to be a planet system with at least 2 and possibly 3 exoplanets having masses in the range 7–20 . If the third companion can be confirmed then CoRoT-7 may represent a case of an ultra-compact planetary system.