The Impact of Hot Jupiters on the Spin-down of their Host Stars

TL;DR

This study uses numerical MHD simulations to show that close-in giant planets significantly reduce stellar wind-driven angular momentum loss, affecting star spin-down, with the effect depending on planetary magnetic field strength and orientation.

Contribution

It provides the first detailed numerical analysis of how close-in giant planets alter stellar wind structure and angular momentum loss, highlighting the role of magnetic interactions.

Findings

Angular momentum loss rate decreases by up to 4 times with close-in planets.

The reduction depends mainly on planetary magnetic field strength.

Anti-aligned magnetic fields cause interactions at greater distances.

Abstract

We present a numerical Magnetohydrodynamic (MHD) study of the dependence of stellar mass and angular momentum- loss rates on the orbital distance to close-in giant planets. We find that the mass loss rate drops by a factor of $\approx$1.5-2, while the angular momentum loss rate drops by a factor of $\approx$4 as the distance decreases past the Alfv\'en surface. This reduction in angular momentum loss is due to the interaction between the stellar and planetary Alfv\'en surfaces, which modifies the global structure of the stellar corona and stellar wind on the hemisphere facing the planet, as well as the opposite hemisphere. The simulation also shows that the magnitude of change in angular momentum loss rate depends mostly on the strength of the planetary magnetic field and not on its polarity. The interaction however, begins at greater separation if the overall field topology of the star and the planet are of anti-aligned. Our results are consistent with evidence for excess angular momentum in stars harboring close-in giant planets, and show that the reduction in wind-driven angular momentum loss can compete with, and perhaps dominate, spin-up due to tidal interaction.