WASP-23b: a transiting hot Jupiter around a K dwarf and its Rossiter-McLaughlin effect^{⋆,⋆⋆,⋆⋆⋆}

¹ Observatoire Astronomique de l’Université de Genève, Chemin des Maillettes 51, 1290 Sauverny, Switzerland
e-mail: Amaury.Triaud@unige.ch
² Astrophysics Group, Keele University, Staffordshire, ST55BG, UK
³ Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août, 17, Bât. B5C, Liège 1, Belgium
⁴ Department of Physics and Astronomy, Vanderbilt University, Nashville, TN37235, USA
⁵ SUPA, School of Physics & Astronomy, University of St Andrews, North Haugh, KY16 9SS, St Andrews, Fife, Scotland, UK
⁶ Centro de Astrofísica, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁷ Institut d’Astrophysique de Paris, CNRS (UMR 7095), Université Pierre & Marie Curie, 98bis bd. Arago, 75014 Paris, France
⁸ Observatoire de Haute-Provence, CNRS/OAMP, 04870 St Michel l’ Observatoire, France
⁹ Las Cumbres Observatory, 6740 Cortona Dr. Suite 102, Santa Barbara, CA 93117, USA
¹⁰ Astrophysics Research Centre, School of Mathematics & Physics, Queens University, University Road, Belfast, BT71 NN, UK
¹¹ Department of Physics and Astronomy, Universityof Leicester, Leicester, LE17 RH, UK

Received: 20 December 2010
Accepted: 13 March 2011

Abstract

We report the discovery of a new transiting planet in the southern hemisphere. It was found by the WASP-south transit survey and confirmed photometrically and spectroscopically by the 1.2 m Swiss Euler telescope, LCOGT 2m Faulkes South Telescope, the 60 cm TRAPPIST telescope, and the ESO 3.6 m telescope. The orbital period of the planet is 2.94 days. We find that it is a gas giant with a mass of 0.88 ± 0.10   M_J and an estimated radius of 0.96 ± 0.05   R_J. We obtained spectra during transit with the HARPS spectrograph and detect the Rossiter-McLaughlin effect despite its small amplitude. Because of the low signal-to-noise ratio of the effect and a small impact parameter, we cannot place a strong constraint on the projected spin-orbit angle. We find two conflicting values for the stellar rotation. We find, via spectral line broadening, that v   sin   I = 2.2 ± 0.3 km s^-1, while applying another method, based on the activity level using the index , gives an equatorial rotation velocity of only v = 1.35 ± 0.20 km s^-1. Using these as priors in our analysis, the planet might be either misaligned or aligned. This result raises doubts about the use of such priors. There is evidence of neither eccentricity nor any radial velocity drift with time.