Constraints on the Obliquities of Kepler Planet-Hosting Stars

TL;DR

This study investigates the obliquities of Kepler planet-hosting stars, finding most have low obliquities except for hot stars with hot Jupiters, which often show high obliquity.

Contribution

It provides the first comprehensive analysis of obliquities across a wide range of planet sizes, especially focusing on stars smaller than Neptune.

Findings

Most stars with small planets have low obliquities.

Hot stars with hot Jupiters often exhibit high obliquities.

The vsini distribution supports low obliquity in most planet hosts.

Abstract

Stars with hot Jupiters have obliquities ranging from 0-180 degrees, but relatively little is known about the obliquities of stars with smaller planets. Using data from the California-Kepler Survey, we investigate the obliquities of stars with planets spanning a wide range of sizes, most of which are smaller than Neptune. First, we identify 156 planet hosts for which measurements of the projected rotation velocity (vsini) and rotation period are both available. By combining estimates of v and vsini, we find nearly all the stars to be compatible with high inclination, and hence, low obliquity (less than about 20 degrees). Second, we focus on a sample of 159 hot stars (> 6000K) for which vsini is available but not necessarily the rotation period. We find 6 stars for which vsini is anomalously low, an indicator of high obliquity. Half of these have hot Jupiters, even though only 3% of the stars that were searched have hot Jupiters. We also compare the vsini distribution of the hot stars with planets to that of 83 control stars selected without prior knowledge of planets. The mean vsini of the control stars is lower than that of the planet hosts by a factor of approximately pi/4, as one would expect if the planet hosts have low obliquities. All these findings suggest that the Kepler planet-hosting stars generally have low obliquities, with the exception of hot stars with hot Jupiters.