Three-dimensional simulations of internal wave breaking and the fate of planets around solar-type stars

TL;DR

This paper uses 3D simulations to explore how internal gravity waves in solar-type stars dissipate energy and angular momentum, affecting the fate of close-in planets and star spin-up.

Contribution

It extends previous 2D work to 3D, providing new insights into wave reflection, breaking, and tidal dissipation mechanisms in stars with planetary companions.

Findings

Wave reflection is nearly perfect for moderate amplitudes.

Breaking waves deposit angular momentum, spinning up the star.

The mechanism explains survival of short-period planets around FGK stars.

Abstract

We study the fate of internal gravity waves approaching the centre of a non-rotating solar-type star, by performing 3D numerical simulations using a Boussinesq-type model. These waves are excited at the top of the radiation zone by the tidal forcing of a short-period planet on a circular, coplanar orbit. This extends previous work done in 2D by Barker & Ogilvie. We first derive a linear wave solution, which is not exact in 3D; however, the reflection of ingoing waves from the centre is close to perfect for moderate amplitude waves. Waves with sufficient amplitude to cause isentropic overturning break, and deposit their angular momentum near the centre. This forms a critical layer, at which the angular velocity of the flow matches the orbital angular frequency of the planet. This efficiently absorbs ingoing waves, and spins up the star from the inside out, while the planet spirals into the star. We also perform numerical integrations to determine the linearised adiabatic tidal response throughout the star, for solar-type stellar models with masses in the range 0.5 < m_star/M_sun < 1.1, throughout their main sequence lifetimes. The aim is to study the launching region for these waves at the top of the radiation zone in more detail, and to determine the accuracy of a semi-analytic approximation for the tidal torque on the star, that was derived under the assumption that all ingoing wave angular momentum is absorbed in a critical layer. The main conclusions are that this nonlinear mechanism of tidal dissipation could provide an explanation for the survival of all short-period extrasolar planets observed around FGK stars, while it predicts the destruction of more massive planets. This work provides further support for the model outlined in a previous paper by Barker & Ogilvie, and makes predictions that will be tested by ongoing observational studies, such as WASP and Kepler.