Understanding Mixing Processes in Stars Using Hydrodynamic Simulations Internal Gravity Waves in Stellar Interiors

Abstract

Waves that propagate in stellar interiors are essential to stellar physics for two reasons. First, the interiors of stars are studied by detection of global modes of oscillations resulting from wave interference. Secondly, waves are involved in various transport phenomena. In stars, there are two main types of waves: acoustic and gravity. This duality of waves as observational tools and physical processes impacting stellar structure makes them a crucial field of study in astrophysics.

In this thesis, we focus on internal gravity waves (IGWs), which are well known for transporting angular momentum, energy and chemical elements in stably stratified media. Despite observations of very high precision, detection of IGWs is still challenging and their properties in stellar interiors remain poorly understood and/or constrained. This is mostly because IGWs are inherently 3D, non-linear and anisotropic phenomena. Consequently, multidimensional modelling is a great tool to study these waves. However, stellar hydrodynamics faces important challenges such as numerical stability and thermal relaxation. To face them, an artificial increase of the stellar luminosity and of the thermal diffusivity by several orders of magnitudes is a commonly used tactic. Using two-dimensional simulations of a solar-like model, we quantify the impact of such a technique on IGWs. Our results suggest that this technique affect the excitation of IGWs, because of an impact on convective motions and overshooting, but also their damping.

Main-sequence intermediate-mass stars, with M ≳ 2M_⊙, possess a convective core and a radiative envelope. It remains unclear if waves generated at the edge of the convective core should be able to propagate up to the stellar surface. In this context, we have carried out an analysis of IGWs in simulations of 5 M_⊙star model. Our results show that low frequency waves excited by core convection are strongly impacted by radiative effects as they propagate. In the upper layers of the simulation domain, we observe an increase of the temperature, likely due to heat added in these layers by IGWs damped by radiative diffusion. We show that nonlinear effects linked to large amplitude IGWs may be relevant just above the convective core. Both these effects are intensified by the artificial enhancement of the luminosity and radiative diffusivity. Our results also highlight that direct comparison between numerical simulations with enhanced luminosity and observations must be made with caution.