Thermal Stability and Vertical Structure of Radiation Dominated accretion Disks

Yan-Fei Jiang

November 15, 2012

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Abstract: In the innermost regions of black hole and neutron star accretion flows, the standard thin disk model implies that radiation pressure may exceed gas pressure, and that the resulting disks are thermally unstable. However, in radiation magnetohydrodynamic simulations of disks with radiation treated via the flux-limited diffusion approximation, recent work has found that accretion disks driven by the magnetorotational instability are stable even when radiation pressure dominates. We check this surprising result with our recently developed radiation transfer module for the Athena code. I will first describe our module and explain why its superior to commonly used prescriptions, such as flux limited diffusion. Then I will summarize some test calculations that demonstrate the code is working accurately. Finally, I will describe in detail our work on the vertical structure and thermal stability of accretion disks in both the gas pressure and radiation pressure dominated regimes. Although we confirm some conclusions of the previous work, our results indicate that radiation dominated disks are unstable after all.