Most Sub-Arcsecond Companions of Kepler Exoplanet Candidate Host Stars are Gravitationally Bound

TL;DR

This study uses high-resolution imaging to estimate that most close companions to Kepler exoplanet host stars are gravitationally bound binaries, with a binary fraction similar to that of field stars.

Contribution

It provides the first statistical estimate of the binary fraction among Kepler exoplanet host stars using speckle imaging data and binary star models.

Findings

Most sub-arcsecond companions are gravitationally bound binaries.

The binary fraction of Kepler host stars is approximately 40-50%.

Detected companions are mostly long-period binary stars.

Abstract

Using the known detection limits for high-resolution imaging observations and the statistical properties of true binary and line-of-sight companions, we estimate the binary fraction of {\it Kepler} exoplanet host stars. Our speckle imaging programs at the WIYN 3.5-m and Gemini North 8.1-m telescopes have observed over 600 {\it Kepler} objects of interest (KOIs) and detected 49 stellar companions within $\sim$1 arcsecond. Assuming binary stars follow a log-normal period distribution for an effective temperature range of 3,000 to 10,000 K, then the model predicts that the vast majority of detected sub-arcsecond companions are long period ($P>50$ years), gravitationally bound companions. In comparing the model predictions to the number of real detections in both observational programs, we conclude that the overall binary fraction of host stars is similar to the 40-50\% rate observed for field stars.