The nature of the radiative hydrodynamic instabilities in radiatively supported Thomson atmospheres

Atmospheres having a significant radiative support are shown to be intrinsi cally unstable at luminosities above a critical fraction Gamma (crit) appro ximate to 0.5-0.85 of the Eddington limit, with the exact value depending o n the boundary conditions. Two different types of absolute radiation-hydrod ynamic instabilities of acoustic waves are found to take place even in the electron scattering-dominated limit. Both instabilities grow over dynamical timescales and both operate on nonradial modes. One is stationary and aris es only after the effects of the boundary conditions are taken into account , while the second is a propagating wave and is insensitive to the boundary conditions. Although a significant wind can be generated by these instabil ities even below the classical Eddington luminosity limit, quasi-stable con figurations can exist beyond the Eddington limit due to the generally reduc ed effective opacity. The study is done using a rigorous numerical linear a nalysis of a gray plane-parallel atmosphere under the Eddington approximati on. We also present more simplified analytical explanations.