SOPHIE velocimetry of Kepler transit candidates VI. An additional companion in the KOI-13 system

TL;DR

This study reports the discovery of a new stellar companion in the KOI-13 system, revealing a hierarchical triple system with implications for the transiting planet's characterization.

Contribution

The paper presents the detection of an additional stellar companion in KOI-13 using SOPHIE radial velocities, and clarifies the system's hierarchical structure and its impact on planet mass limits.

Findings

Discovered a stellar companion with 0.4-1 Msun orbiting KOI-13B.

Placed upper limits on the transiting planet's mass at 14.8 and 9.4 Mjup.

Confirmed the transiting planet orbits KOI-13A in a hierarchical triple system.

Abstract

We report the discovery of a new stellar companion in the KOI-13 system. KOI-13 is composed by two fast-rotating A-type stars of similar magnitude. One of these two stars hosts a transiting planet discovered by Kepler. We obtained new radial velocity measurements using the SOPHIE spectrograph at the Observatoire de Haute-Provence that revealed an additional companion in this system. This companion has a mass between 0.4 and 1 Msun and orbits one of the two main stars with a period of 65.831 \pm 0.029 days and an eccentricity of 0.52 \pm 0.02. The radial velocities of the two stars were derived using a model of two fast-rotating line profiles. From the residuals, we found a hint of the stellar variations seen in the Kepler light curve with an amplitude of about 1.41 km/s and a period close to the rotational period. This signal appears to be about three order of magnitude larger than expected for stellar activity. From the analysis of the residuals, we also put a 3-sigma upper-limit on the mass of the transiting planet KOI-13.01 of 14.8 Mjup and 9.4 Mjup, depending on which star hosts the transit. We found that this new companion has no significant impact on the photometric determination of the mass of KOI-13.01 but is expected to affect precise infrared photometry. Finally, using dynamical simulations, we infer that the new companion is orbiting around KOI-13B while the transiting planet candidate is expected to orbit KOI-13A. Thus, the transiting planet candidate KOI-13.01 is orbiting the main component of a hierarchical triple system.