SOPHIE velocimetry of Kepler transit candidates

I. Detection of the low-mass white dwarf KOI 74b^⋆

¹ Laboratoire d’Astrophysique de Grenoble, Université Joseph Fourier, CNRS (UMR 5571), BP 53, 38041 Grenoble Cedex 9, France
e-mail: david.ehrenreich@obs.ujf-grenoble.fr
² Institut d’Astrophysique de Paris, Université Pierre et Marie Curie, CNRS (UMR 7095), 98 bis boulevard Arago, 75014 Paris, France
³ Observatoire de Haute-Provence, CNRS/OAMP, 04870 Saint-Michel-l’Observatoire, France
⁴ Observatoire de Genève, Université de Genève, 51, chemin des Maillettes, 1290 Sauverny, Switzerland
⁵ Laboratoire d’Astrophysique de Marseille, Université de Provence, CNRS (UMR 6110), 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
⁶ School of Physics, University of Exeter, Exeter EX4 4QL, UK
⁷ Centro de Astrofísica, Universidade do Porto, rua das Estrelas, 4150-762 Porto, Portugal
⁸ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Portugal

Received: 9 September 2010
Accepted: 10 October 2010

Abstract

The Kepler mission has detected transits and occultations of a hot compact object around an early-type star, the Kepler Object of Interest KOI 74. The mass of this transiting object was photometrically assessed in a previous study using the presence of the relativistic beaming effect (so-called “Doppler boosting”) in the light curve. Our aim was to provide a spectroscopic validation of this pioneering approach. We measured the radial velocity variations of the A1V star KOI 74 with the SOPHIE spectrograph at the 1.93-m telescope of the Observatoire de Haute-Provence (France). Radial velocity measurements of this star are challenging because of the high level of stellar pulsations and the few available spectral lines. Using a technique dedicated to early-type main-sequence stars, we measured radial velocity variations compatible with a companion of mass 0.252 ± 0.025 M_⊙, in good agreement with the value derived from the Kepler light curve. This work strengthens the scenario suggesting that KOI 74 is a blue straggler orbited by a stellar core despoiled of its envelope, the low-mass white dwarf KOI 74b.