SOPHIE velocimetry of Kepler transit candidates. I. Detection of the low-mass white dwarf KOI-74b

TL;DR

This study spectroscopically confirms the low-mass white dwarf KOI-74b orbiting KOI 74, validating the use of Doppler boosting in Kepler data for detecting compact objects around early-type stars.

Contribution

It provides the first spectroscopic validation of Doppler boosting for detecting a low-mass white dwarf companion to an early-type star.

Findings

Radial velocity variations consistent with a 0.252 Msun companion.

Agreement between spectroscopic and photometric mass estimates.

Strengthens the scenario of KOI 74 being a blue straggler with a white dwarf companion.

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 Msun, 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.