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

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.

accepted by ApJ with minor changes

References in corpus (5)

• A Magnetic Confinement vs. Rotation Classification of Massive-Star Magnetospheres
• Clumping in the inner winds of hot, massive stars from hydrodynamical line-driven instability simulations
• NGC 1624-2: A slowly rotating, X-ray luminous Of?cp star with an extraordinarily strong magnetic field
• Modeling Ultraviolet Wind Line Variability in Massive Hot Stars
• Coulomb Bubbles: Over-stable Driving of Magnetoacoustic Waves Due to the Rapid and Anisotropic Diffusion of Energy

Cited by in corpus (4)

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• Radiative MHD Simulations of Photon Bubbles in Radiation-Supported Magnetized Atmospheres of Neutron Stars with Isotropic Thomson Scattering
• Photon Feedback: Screening and the Eddington Limit
• Testing the Reliability of X-rays as a Tool for Constraining Mass-loss Rates of Hot Stars