Misaligned spin-orbit in the XO-3 planetary system?^*

¹ Institut d'Astrophysique de Paris, UMR7095 CNRS, Université Pierre & Marie Curie, 98bis boulevard Arago, 75014 Paris, France e-mail: hebrard@iap.fr
² Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
³ Laboratoire d'Astrophysique de Marseille, Université de Provence, CNRS (UMR 6110), BP 8, 13376 Marseille Cedex 12, France
⁴ Instituto de Astrofísica de Canarias, La Laguna, Tenerife, Spain
⁵ Centro de Astrofísica, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁶ Laboratoire d'Astrophysique de Grenoble, CNRS (UMR 5571), Université J. Fourier, BP53, 38041 Grenoble, France
⁷ Observatoire de Genève, Université de Genève, 51 Chemin des Maillettes, 1290 Sauverny, Switzerland

Received: 25 April 2008
Accepted: 4 June 2008

Abstract

The transiting extrasolar planet XO-3b is remarkable, with a high mass and eccentric orbit. These unusual characteristics make it interesting to test whether its orbital plane is parallel to the equator of its host star, as it is observed for other transiting planets. We performed radial velocity measurements of XO-3 with the SOPHIE spectrograph at the 1.93 m telescope of Haute-Provence Observatory during a planetary transit and at other orbital phases. This allowed us to observe the Rossiter-McLaughlin effect and, together with a new analysis of the transit light curve, to refine the parameters of the planet. The unusual shape of the radial velocity anomaly during the transit provides a hint of a nearly transverse Rossiter-McLaughlin effect. The sky-projected angle between the planetary orbital axis and the stellar rotation axis should be λ = 70° ± 15° to be compatible with our observations. This suggests that some close-in planets might result from gravitational interaction between planets and/or stars rather than migration due to interaction with the accretion disk. This surprising result requires confirmation by additional observations, especially at lower airmass, to fully exclude the possibility that the signal is due to systematic effects.