Local Radiative Hydrodynamic and Magnetohydrodynamic Instabilities in Optically Thick Media

Abstract

We examine the local conditions for radiative damping and driving of short wavelength, propagating hydrodynamic and magnetohydrodynamic (MHD) waves in static, optically thick, stratified equilibria. We show that so-called strange modes in stellar oscillation theory and magnetic photon bubbles are intimately related and are both fundamentally driven by the background radiation flux acting on compressible waves. We identify the necessary criteria for unstable driving of these waves, and show that this driving can exist in both gas and radiation pressure dominated media, as well as pure Thomson scattering media in the MHD case. The equilibrium flux acting on opacity fluctuations can drive both hydrodynamic acoustic waves and magnetosonic waves unstable. In addition, magnetosonic waves can be driven unstable by a combination of the equilibrium flux acting on density fluctuations and changes in the background radiation pressure along fluid displacements. We briefly describe the conditions under which these instabilities might be manifested in both main sequence stellar envelopes and accretion disks.

Nonlinear Evolution of the Radiation-Driven Magneto-Acoustic Instability

“…We have confirmed the predictions of Blaes & Socrates (2003). In particular, the RMI instability operates when gas pressure dominates over radiation pressure and when the magnetic field is weak.…”

General Overview of Black Hole Accretion Theory

“…The most well-explored of such instabilities are magnetically-mediated "photon bubble" instabilities (Arons 1992;Gammie 1998). On short length scales where photons diffuse rapidly, such instabilities amount to radiatively amplified magnetosonic modes (Blaes & Socrates 2003) that develop into highly inhomogeneous trains of shocks (Begelman 2001;Turner et al 2005, see Figure 7). In principle such inhomogeneities could allow locally super-Eddington fluxes to escape from the disk atmosphere without driving an outflow (Begelman 2002).…”

Instabilities in the Envelopes and Winds of Very Massive Stars

“…The response of this acceleration to local perturbations, e.g. in the gas density ρ, gives rise to the so-called strange-mode instability (Blaes & Socrates, 2003;Glatzel & Kiriakidis, 1993;Glatzel, 1994Glatzel, , 2005. The most general form requires a global analysis of the envelope structure; but in the limit of wavelengths shorter than the local gravitational scale height, we can apply the so-called "WKB approximation" that ignores background gradients and analyzes the effect of localized, small-amplitude perturbations δ ρ, δ v, etc.…”

“…A generalization (Blaes & Socrates, 2003) that takes proper account of associated perturbations in the radiation field shows that the instability competes against the damping from a "radiation drag" effect, yielding now a net growth rate (cf. eqn.…”