Eccentricity in planetary systems and the role of binarity: sample definition, initial results, and the system of HD 211847

TL;DR

This study investigates the relationship between stellar binarity and planetary eccentricity, revealing that high-eccentricity exoplanet systems are more often associated with stellar companions, though most high-eccentricity planets are in single-star systems.

Contribution

It provides new high-resolution imaging data showing the correlation between stellar binarity and planetary eccentricity, and clarifies the nature of previously detected companions in these systems.

Findings

High-eccentricity systems have a higher binary fraction than circular systems.

Binaries with separations less than 50 au are less common among exoplanet hosts than in field stars.

Most high-eccentricity planets are not in multiple stellar systems.

Abstract

We explore the multiplicity of exoplanet host stars with high-resolution images obtained with VLT/SPHERE. Two different samples of systems were observed: one containing low-eccentricity outer planets, and the other containing high-eccentricity outer planets. We find that 10 out of 34 stars in the high-eccentricity systems are members of a binary, while the proportion is 3 out of 27 for circular systems. Eccentric-exoplanet hosts are, therefore, significantly more likely to have a stellar companion than circular-exoplanet hosts. The median magnitude contrast over the 68 data sets is 11.26 and 9.25, in H and K, respectively, at 0.30 arcsec. The derived detection limits reveal that binaries with separations of less than 50au are rarer for exoplanet hosts than for field stars. Our results also imply that the majority of high-eccentricity planets are not embedded in multiple stellar systems (24 out of 34), since our detection limits exclude the presence of a stellar companion. We detect the low-mass stellar companions of HD 7449 and HD 211847, both members of our high-eccentricity sample. HD 7449B was already detected by Rodigas et al (2016) and our independent observation is in agreement with this earlier work. HD 211847's substellar companion, previously detected by the radial velocity method, is actually a low-mass star seen face-on. The role of stellar multiplicity in shaping planetary systems is confirmed by this work, although it does not appear as the only source of dynamical excitation.