Effects of Magnetic Braking and Tidal Friction on Hot Jupiters

TL;DR

This paper investigates how stellar magnetic braking influences the long-term orbital and spin evolution of Hot Jupiters, emphasizing the importance of coupled evolution models for accurate predictions and the implications for observed spin-orbit alignments.

Contribution

It introduces a combined model of tidal friction and magnetic braking effects, highlighting their impact on the evolution of Hot Jupiters and the importance of considering coupled orbital and stellar spin dynamics.

Findings

Magnetic braking significantly alters the orbital evolution of Hot Jupiters.

Coupled evolution of orbit and stellar spin is essential for accurate modeling.

Spin-orbit alignment timescales are comparable to inspiral times, affecting observational interpretations.

Abstract

Tidal friction is thought to be important in determining the long-term spin-orbit evolution of short-period extrasolar planetary systems. Using a simple model of the orbit-averaged effects of tidal friction Eggleton, Kiseleva & Hut (1998), we analyse the effects of the inclusion of stellar magnetic braking on the evolution of such systems. A phase-plane analysis of a simplified system of equations, including only the stellar tide together with a model of the braking torque proposed by Verbunt & Zwaan (1981), is presented. The inclusion of stellar magnetic braking is found to be extremely important in determining the secular evolution of such systems, and its neglect results in a very different orbital history. We then show the results of numerical integrations of the full tidal evolution equations, using the misaligned spin and orbit of the XO-3 system as an example, to study the accuracy of simple timescale estimates of tidal evolution. We find that it is essential to consider coupled evolution of the orbit and the stellar spin in order to model the behaviour accurately. In addition, we find that for typical Hot Jupiters the stellar spin-orbit alignment timescale is of the same order as the inspiral time, which tells us that if a planet is observed to be aligned, then it probably formed coplanar. This reinforces the importance of Rossiter-McLaughlin effect observations in determining the degree of spin-orbit alignment in transiting systems.