Abstract

Important clues on the formation and evolution of planetary systems can be inferred from the stellar obliquity psi. We study the distribution of obliquities using the California-Kepler Survey and the TEPCat Catalog of Rossiter-MacLaughlin (RM) measurements, from which we extract, respectively, 275 and 118 targets. We infer a "best fit" obliquity distribution in psi with a single parameter kappa. Large values of kappa imply that psi is distributed narrowly around zero, while small values imply approximate isotropy. Our findings are: (1) The distribution of psi in Kepler systems is narrower than found by previous studies and consistent with kappa similar to 15 (mean similar to 19 degrees and spread psi similar to 10 degrees). (2) The value of kappa in Kepler systems does not depend, at a statistically significant level, on planet multiplicity, stellar multiplicity or stellar age; on the other hand, metal-rich hosts, small planet hosts and long-period planet hosts tend to be more oblique than the general sample (at a similar to 2.5-sigma significance level). (3) Hot Jupiter (HJ) systems with RM measurements are consistent with kappa similar to 2, more broadly distributed than the general Kepler population. (4) A separation of the RM sample into cooler (T-eff less than or similar to 6250 K) and hotter (T-eff greater than or similar to 6250 K) HJ hosts results in two distinct distributions, kappa(cooler)similar to 4 and kappa(hotter)similar to 1 (4-sigma significance), both more oblique than the Kepler sample. We hypothesize that the total mass in planets may be behind the increasing obliquity with metallicity and planet radius, and that the period dependence could be due to primordial disk alignment rather than realignment of stellar spin.