The Doppler shadow of WASP-3b

A tomographic analysis of Rossiter-McLaughlin observations

¹ School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK
e-mail: gm228@st-andrews.ac.uk
² Astrophysics Research Centre, School of Mathematics & Physics, Queen’s University, University Road, Belfast BT7 1NN, UK
³ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁴ Observatoire de l’Université de Genève, Chemin des Maillettes 51, 1290 Sauverny, Switzerland
⁵ Institut d’Astrophysique de Paris, UMR7095 CNRS, Université Pierre & Marie Curie, 98bis Bd Arago, 75014 Paris, France
⁶ Laboratoire d’Astrophysique de Marseille, BP 8, 13376 Marseille, Cedex 12, France
⁷ Isaac Newton Group of Telescopes, Apartado de Correos 321, 38700 Santa Cruz de la Palma, Tenerife, Spain

Received: 26 May 2010
Accepted: 16 August 2010

Abstract

Context. Hot-Jupiter planets must form at large separations from their host stars where the temperatures are cool enough for their cores to condense. They then migrate inwards to their current observed orbital separations. Different theories of how this migration occurs lead to varying distributions of orbital eccentricity and the alignment between the rotation axis of the star and the orbital axis of the planet.

Aims. The spin-orbit alignment of a transiting system is revealed via the Rossiter-McLaughlin effect, which is the anomaly present in the radial velocity measurements of the rotating star during transit due to the planet blocking some of the starlight. In this paper we aim to measure the spin-orbit alignment of the WASP-3 system via a new way of analysing the Rossiter-McLaughlin observations.

Methods. We apply a new tomographic method for analysing the time variable asymmetry of stellar line profiles caused by the Rossiter-McLaughlin effect. This new method eliminates the systematic error inherent in previous methods used to analyse the effect.

Results. We find a value for the projected stellar spin rate of v sin i = 13.9  ±  0.03 km s^-1 which is in agreement with previous measurements but has a much higher precision. The system is found to be well aligned, with ° which favours an evolutionary history for WASP-3b involving migration through tidal interactions with a protoplanetary disc. From comparison with isochrones we put an upper limit on the age of the star of 2 Gyr.