SOPHIE velocimetry of Kepler transit candidates III. KOI-423b: an 18 Mjup transiting companion around an F7IV star

TL;DR

This paper reports on the radial velocity follow-up of four Kepler transiting candidates, identifying KOI-423b as a massive planet/brown dwarf hybrid around an F7IV star, and discusses the characteristics of these companions.

Contribution

It presents the discovery and characterization of KOI-423b, a transiting object in the mass range between massive planets and brown dwarfs, using SOPHIE spectrograph data.

Findings

KOI-423b has a mass of 18 Mjup and radius of 1.22 Rjup.

At least three candidates have Jupiter-sized transiting companions.

Massive companions are more common around F-type stars than G-type stars.

Abstract

We report the strategy and results of our radial velocity follow-up campaign with the SOPHIE spectrograph (1.93-m OHP) of four transiting planetary candidates discovered by the Kepler space mission. We discuss the selection of the candidates KOI-428, KOI-410, KOI-552, and KOI-423. KOI-428 was established as a hot Jupiter transiting the largest and the most evolved star discovered so far and is described by Santerne et al. (2011a). KOI-410 does not present radial velocity change greater than 120 m/s, which allows us to exclude at 3 sigma a transiting companion heavier than 3.4 Mjup. KOI-552b appears to be a transiting low-mass star with a mass ratio of 0.15. KOI-423b is a new transiting companion in the overlapping region between massive planets and brown dwarfs. With a radius of 1.22 +- 0.11 Rjup and a mass of 18.0 +- 0.92 Mjup, KOI-423b is orbiting an F7IV star with a period of 21.0874 +- 0.0002 days and an eccentricity of 0.12 +- 0.02. From the four selected Kepler candidates, at least three of them have a Jupiter-size transiting companion, but two of them are not in the mass domain of Jupiter-like planets. KOI-423b and KOI-522b are members of a growing population of known massive companions orbiting close to an F-type star. This population currently appears to be absent around G-type stars, possibly due to their rapid braking and the engulfment of their companions by tidal decay.