WASP-50 b: a hot Jupiter transiting a moderately active solar-type star^⋆

¹ Université de Liège, Allée du 6 août 17, Sart Tilman, Liège 1, Belgium
e-mail: michael.gillon@ulg.ac.be
² Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK
³ Observatoire de Genève, Université de Genève, 51 chemin des Maillettes, 1290 Sauverny, Switzerland
⁴ Astrophysics Research Centre, School of Mathematics & Physics, Queen’s University, University Road, Belfast, BT7 1NN, UK
⁵ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁶ School of Physics and Astronomy, University of St. Andrews, North Haugh, Fife, KY16 9SS, UK
⁷ Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK

Received: 6 May 2011
Accepted: 1 August 2011

Abstract

We report the discovery by the WASP transit survey of a giant planet in a close orbit (0.0295  ±  0.0009 AU) around a moderately bright (V = 11.6, K = 10) G9 dwarf (0.89  ±  0.08   M_⊙, 0.84  ±  0.03   R_⊙) in the Southern constellation Eridanus. Thanks to high-precision follow-up photometry and spectroscopy obtained by the telescopes TRAPPIST and Euler, the mass and size of this planet, WASP-50 b, are well constrained to 1.47  ±  0.09   M_Jup and 1.15  ±  0.05   R_Jup, respectively. The transit ephemeris is 2   455   558.6120 (±0.0002) + N  ×  1.955096 (±0.000005) HJD_UTC. The size of the planet is consistent with basic models of irradiated giant planets. The chromospheric activity () and rotational period (P_rot = 16.3  ±  0.5 days) of the host star suggest an age of 0.8  ±  0.4 Gy that is discrepant with a stellar-evolution estimate based on the measured stellar parameters (ρ_∗ = 1.48  ±  0.10 ρ_⊙, T_eff = 5400  ±  100 K, [Fe/H] = −0.12  ±  0.08) which favors an age of 7  ±  3.5 Gy. This discrepancy could be explained by the tidal and magnetic influence of the planet on the star, in good agreement with the observations that stars hosting hot Jupiters tend to show faster rotation and magnetic activity. We measure a stellar inclination of  deg, disfavoring a high stellar obliquity. Thanks to its large irradiation and the relatively small size of its host star, WASP-50 b is a good target for occultation spectrophotometry, making it able to constrain the relationship between hot Jupiters’ atmospheric thermal profiles and the chromospheric activity of their host stars.