Internal gravity waves in massive stars

Abstract

Intermediate-mass stars in the main-sequence have radiative envelopes and convective cores. This configuration allows internal gravity waves (IGWs), generated stochastically at the convective-radiative boundary, to propagate through the radiation zone and produce signatures, which can be observed through space-based photometry and groundbased spectroscopy. In this thesis, we present results from the investigation of IGWs in intermediate-mass stars through theoretical and numerical studies. The study of IGWs in intermediate-mass stars can be broken down into IGW propagation and IGW generation. In our work, we start with the study of IGW propagation in the linear regime. In this regime, IGW amplitudes are affected by three main features: radiative diffusion, density stratification and geometric effects. We study the implications of these three features on waves travelling within the radiative zones of non-rotating stars. As a simple measure of induced wave dynamics, we define a criterion to see if waves can become nonlinear and if so, under what conditions. We find that the IGW generation spectrum, convective velocities and the strength of density stratification all play major roles in whether waves become nonlinear. With increasing stellar mass, there is an increasing trend in nonlinear wave energies. The trends with different metallicities and ages depend on the generation spectrum. Next, we move onto the study of non-linear IGW propagation using two-dimensional fully non-linear hydrodynamical simulations with realistic stellar reference states up to the stellar surface. When a single wave is forced, we observe wave self-interaction. When a spectrum of waves is forced, we find that the surface IGW frequency spectrum follows a power law with a slope consistent with recent observations. This power law is similar to that predicted by linear theory for the wave propagation, with small deviations which can be an effect of nonlinearities. When the same generation spectrum is applied to 3 M models at different stellar rotation and ages, the surface IGW spectrum slope is very similar to the generation spectrum slope. Finally, we study the IGW generation frequency spectrum from non-linear simulations of core convection as functions of stellar mass and age for intermediate-mass stars. This is an ongoing project and the current results show that the generation frequency slopes lie between -0 and -1 at lower frequencies, and between -1.5 and -3 at higher frequencies for all the stellar models, with the high-frequency slope of the 3M ZAMS model being consistent with previous numerical work (Rogers et al., 2013).