Spin and Obliquity Evolution of Hot Jupiter Hosts from Resonance Locks

TL;DR

This paper explores how resonance locking with non-axisymmetric stellar gravity modes influences the obliquity and spin evolution of hot Jupiter host stars, providing explanations for observed misalignments and spin rates across different stellar types.

Contribution

It extends previous models by including non-axisymmetric modes in resonance locking, revealing their effects on stellar obliquity damping and spin evolution, and compares predictions with observations.

Findings

Resonance locking affects obliquity damping for all modes ($-2 \, ext{to} \, 2$).

Stars spin up or down depending on the mode azimuthal number ($m$).

Model reproduces observed trends in obliquity and stellar rotation rates.

Abstract

When a hot Jupiter orbits a star whose effective temperature exceeds $\sim$6100 K, its orbit normal tends to be misaligned with the stellar spin axis. Cooler stars typically have smaller obliquities, which may have been damped by hot Jupiters in resonance lock with axisymmetric stellar gravity modes (azimuthal number $m=0$). Here we allow for resonance locks with non-axisymmetric modes, which affect both stellar obliquity and spin frequency. Obliquities damp for all modes ($-2 \leq m \leq 2$). Stars spin up for $m > 0$, and spin down for $m < 0$. We carry out a population synthesis that assumes hot Jupiters form misaligned around both cool and hot stars, and subsequently lock onto modes whose $m$-values yield the highest mode energies for given starting obliquities. Core hydrogen burning enables hot Jupiters to torque low-mass stars, but not high-mass stars, into spin-orbit alignment. Resonance locking plus stellar spin-down from magnetic braking largely reproduces observed obliquities and stellar rotation rates and how they trend with stellar effective temperature and orbital separation. The possible suppression of resonance locking by non-linear dissipation of gravity waves remains an outstanding issue.