Nonlinear tidal excitation of super-harmonic gravity waves in main-sequence stars in binary and exoplanetary systems

TL;DR

This paper investigates how nonlinear interactions between tidally-excited gravity waves in main-sequence stars can enhance tidal dissipation, especially in stars with convective envelopes, with implications for star-planet and star-binary systems.

Contribution

It develops a new theory and numerical tests for the nonlinear excitation of secondary gravity waves near the stellar radiative/convective interface, expanding understanding of tidal dissipation mechanisms.

Findings

Secondary waves can be excited to large amplitudes more easily than wave breaking criteria suggest.

Nonlinear effects are significant in stars with convective envelopes, even for planetary-mass companions.

The criteria for nonlinear excitation are more easily satisfied than traditional wave breaking thresholds.

Abstract

We study the role of nonlinear effects on tidally-excited internal gravity waves in stellar radiation zones in exoplanetary or binary systems. We are partly motivated to study tides due to massive short-period hot Jupiters, which preferentially orbit stars with convective cores, for which wave breaking near the stellar centre cannot operate. We develop a theory (and test it with numerical calculations) for the nonlinear excitation of super-harmonic "secondary" waves (with frequencies $2\omega_p$) by a "primary" tidal wave (with frequency $\omega_p$) near the interface between the radiation zone and convective envelope. These waves have the same horizontal phase speeds to leading order, and this nonlinear effect could contribute importantly to tidal dissipation if the secondary waves can efficiently damp the primary. We derive criteria involving the orbital and stellar parameters required to excite these secondary waves to large amplitudes using a local model of the radiative/convective interface, which we convert to apply to tides in a spherical star. We numerically evaluate the critical amplitudes required for this new nonlinear effect to become important using stellar models, comparing them to the "conventional" criteria for wave breaking in radiative cores and the application of WKBJ theory near convective cores. The criteria for this new effect are easier to satisfy than the conventional measures of nonlinearity in $1.4$ and $2M_\odot$ stars on the main-sequence. We predict nonlinear effects to be important even for planetary-mass companions around the latter, but this effect is probably less important in stars with radiative cores.