Local Radiative Hydrodynamic and Magnetohydrodynamic Instabilities in Optically Thick Media

We examine the local conditions for radiative damping and driving of shortwavelength, propagating hydrodynamic and magnetohydrodynamic (MHD) waves instatic, optically thick, stratified equilibria. We show that so-called strangemodes in stellar oscillation theory and magnetic photon bubbles are intimatelyrelated and are both fundamentally driven by the background radiation fluxacting on compressible waves. We identify the necessary criteria for unstabledriving of these waves, and show that this driving can exist in both gas andradiation pressure dominated media, as well as pure Thomson scattering media inthe MHD case. The equilibrium flux acting on opacity fluctuations can driveboth hydrodynamic acoustic waves and magnetosonic waves unstable. In addition,magnetosonic waves can be driven unstable by a combination of the equilibriumflux acting on density fluctuations and changes in the background radiationpressure along fluid displacements. We briefly describe the conditions underwhich these instabilities might be manifested in both main sequence stellarenvelopes and accretion disks.Comment: 55 pages, revised version accepted for publication by ApJ. New appendix added justifying WKB analysi