Nonlinear Evolution of the Radiation-Driven Magneto-Acoustic Instability (RMI)

TL;DR

This study investigates the nonlinear behavior of the radiation-driven magneto-acoustic instability (RMI) through numerical simulations, revealing how it saturates and impacts astrophysical environments like stellar envelopes.

Contribution

It provides the first detailed numerical analysis of RMI saturation in various energy regimes, confirming theoretical predictions and exploring its astrophysical implications.

Findings

RMI operates even in weakly magnetized, gas pressure-dominated environments.

Saturation amplitude increases with the ratio of radiation to gas pressure.

Maximum saturation occurs when magnetic pressure is comparable to radiation pressure.

Abstract

We examine the nonlinear development of unstable magnetosonic waves driven by a background radiative flux -- the Radiation-Driven Magneto-Acoustic Instability (RMI, a.k.a. the "photon bubble" instability). The RMI may serve as a persistent source of density, radiative flux, and magnetic field fluctuations in stably-stratified, optically-thick media. The conditions for instability are present in a variety of astrophysical environments, and do not require the radiation pressure to dominate or the magnetic field to be strong. Here we numerically study the saturation properties of the RMI, covering three orders of magnitude in the relative strength of radiation, magnetic field, and gas energies. Two-dimensional, time-dependent radiation-MHD simulations of local, stably-stratified domains are conducted with Zeus-MP in the optically-thick, highly-conducting limit. Our results confirm the theoretical expectations of Blaes and Socrates (2003) in that the RMI operates even in gas pressure-dominated environments that are weakly magnetized. The saturation amplitude is a monotonically increasing function of the ratio of radiation to gas pressure. Keeping this ratio constant, we find that the saturation amplitude peaks when the magnetic pressure is comparable to the radiation pressure. We discuss the implications of our results for the dynamics of magnetized stellar envelopes, where the RMI should act as a source of sub-photospheric perturbations.