SOPHIE velocimetry of Kepler transit candidates. VIII. KOI-205 b: a brown-dwarf companion to a K-type dwarf

TL;DR

This paper reports the discovery of a transiting brown dwarf orbiting a K-type star, combining Kepler photometry and SOPHIE radial velocity data, providing new insights into brown dwarf companions around such stars.

Contribution

First detection of a bona fide brown dwarf companion around a K-type star using combined photometric and spectroscopic methods.

Findings

Brown dwarf mass: 39.9 ± 1.0 MJup

Brown dwarf radius: 0.81 ± 0.02 RJup

First such detection around a K0 dwarf

Abstract

We report the discovery of a transiting brown dwarf companion to KOI-205, a K0 main-sequence star, in a 11.720125-day period orbit. The transits were detected by the Kepler space telescope, and the reflex motion of the star was measured using radial velocity observations obtained with the SOPHIE spectrograph. The atmospheric parameters of the host stars were determined from the analysis of high-resolution, high signal-to-noise ratio ESPaDOns spectra obtained for this purpose. Together with spectrophotometric measurements recovered from the literature, these spectra indicate that the star is a mildly metallic K0 dwarf with Teff 5237 $\pm$ 60 K. The mass of the companion is 39.9 $\pm$ 1.0 MJup and its radius is 0.81 $\pm$ 0.02 RJup, in agreement with current theoretical predictions. This is the first time a bona fide brown dwarf companion is detected in orbit around a star of this type. The formation and orbital evolution of brown dwarf companions is briefly discussed in the light of this new discovery.