The coupling between internal waves and shear-induced turbulence in stellar radiation zones: the critical layer

TL;DR

This paper investigates how internal gravity waves interact with critical layers in stellar radiation zones, revealing two regimes that influence angular momentum transport and highlighting the importance of shear-induced turbulence effects.

Contribution

It provides a mathematical analysis of IGW propagation near critical layers and applies these results to stellar evolution models, uncovering new regimes of wave-critical layer interactions.

Findings

Two regimes of IGW-critical layer interaction identified: stable attenuation and unstable over-reflection.

Over-reflection can significantly alter angular momentum transport in stars.

Enhanced damping of waves in the stable regime affects stellar rotation profiles.

Abstract

Internal gravity waves (hereafter IGWs) are known as one of the candidates for explaining the angular velocity profile in the Sun and in solar-type main-sequence and evolved stars, due to their role in the transport of angular momentum. Our bringing concerns critical layers, a process poorly explored in stellar physics, defined as the location where the local relative frequency of a given wave to the rotational frequency of the fluid tends to zero (i.e that corresponds to co-rotation resonances). IGW propagate through stably-stratified radiative regions, where they extract or deposit angular momentum through two processes: radiative and viscous dampings and critical layers. Our goal is to obtain a complete picture of the effects of this latters. First, we expose a mathematical resolution of the equation of propagation for IGWs in adiabatic and non-adiabatic cases near critical layers. Then, the use of a dynamical stellar evolution code, which treats the secular transport of angular momentum, allows us to apply these results to the case of a solar-like star.The analysis reveals two cases depending on the value of the Richardson number at critical layers: a stable one, where IGWs are attenuated as they pass through a critical level, and an unstable turbulent case where they can be reflected/transmitted by the critical level with a coefficient larger than one. Such over-reflection/transmission can have strong implications on our vision of angular momentum transport in stellar interiors. This paper highlights the existence of two regimes defining the interaction between an IGW and a critical layer. An application exposes the effect of the first regime, showing a strengthening of the damping of the wave. Moreover, this work opens new ways concerning the coupling between IGWs and shear instabilities in stellar interiors.