Thermal stability of optically thick accretion discs

For a wide range of possible viscosity laws, thermally unstable accretion discs are also unstable to convection. A simple analytical discussion suggests that energy transport by convection, described by a simple mixing-length theory, would remove thermal instability if convection carried a sufficiently large fraction of the energy. A more detailed study of disc structure shows that radiation always carries most of the energy and it appears that accretion discs will be thermally unstable if radiation pressure is sufficiently important. Convection increases the critical value of radiation pressure for instability but does not remove the instability. (author)