Obliquities of Kepler Stars: Comparison of Single- and Multiple-Transit Systems

Abstract

The stellar obliquity of a transiting planetary system can be constrained by combining measurements of the star's rotation period, radius, and projected rotational velocity. Here we present a hierarchical Bayesian technique for recovering the obliquity distribution of a population of transiting planetary systems, and apply it to a sample of 70 Kepler Objects of Interest. With ~95% confidence we find that the obliquities of stars with only a single detected transiting planet are systematically larger than those with multiple detected transiting planets. This suggests that a substantial fraction of Kepler's single-transiting systems represent dynamically hotter, less orderly systems than the "pancake-flat" multiple-transiting systems.