Companions to Kepler giant stars: A long-period eccentric substellar companion to KIC 3526061 and a stellar companion to HD 187878

TL;DR

This study reports the discovery and characterization of a long-period eccentric sub-stellar companion to a giant star and a stellar companion to another giant, expanding understanding of planetary and stellar companions around evolved intermediate-mass stars.

Contribution

It presents the first detailed orbital and mass characterization of companions around two intermediate-mass giant stars, including a brown dwarf with a highly eccentric orbit.

Findings

KIC 3526061 b is a brown dwarf with 18.15 Jupiter masses, orbiting in 3552 days with eccentricity 0.85.

HD 187878 B is a stellar companion with a mass of 0.51 solar masses, determined through combined spectroscopic and astrometric data.

The system KIC 3526061 is the most evolved with a sub-stellar companion on a highly eccentric, wide orbit.

Abstract

Context. Our knowledge of populations and occurrence of planets orbiting evolved intermediate-mass stars is still incomplete. In 2010 we started a planet-search program among 95 giant stars observed by the Kepler mission to increase the sample of giant stars with planets and with reliable estimates of stellar masses and radii. Aims. We present the two systems KIC 3526061 and HD 187878 from our planet-search program for which we could characterise their companions. Methods. We used precise stellar radial velocity measurements taken with four different echelle spectrographs to derive an orbital solution. We used Gaia astrometric measurements to obtain the inclination of the HD 187878 system and Kepler photometric observations to estimate the stellar mass and radius. Results. We report the discovery of a sub-stellar and a stellar companion around two intermediate-mass red giant branch stars. KIC 3526061 b is most likely a brown dwarf with a minimum mass of 18.15 Jupiter masses in a long-period eccentric orbit, with the orbital period 3552 d and orbital eccentricity 0.85. It is the most evolved system found having a sub-stellar companion with such a large eccentricity and wide separation. HD 187878 B has a minimum mass of 78.4 Jupiter masses. Combining the spectroscopic orbital parameters with the astrometric proper motion anomaly we derived an orbital inclination 9.8 deg, which corresponds to the companion's mass in the stellar regime of 0.51 Sun mass. Conclusions. A sub-stellar companion of KIC 3526061 extends the sample of known red giant branch stars with sub-stellar companions on very eccentric wide orbits and might provide a probe of the dynamical evolution of such systems over time.