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J. E. Bjorkman (U. Toledo), B. A. Whitney (Space Science Institute), K. Wood (U. St. Andrews)
One of the principal limitations of Monte Carlo radiation transfer simulations is the very large number of interactions that occur when the optical depth becomes very large. In practice, the upper limit for most spherically symmetric codes is an optical depth of around 100. For circumstellar disks, the lower optical depth in the polar direction provides an escape channel that mitigates the problem. Nonetheless, when the vertical optical depth becomes very large, photons can become trapped within the disk. Such high optical depths occur in practice for protostellar disks, where the radial optical depth can easily exceed a million.
To deal with this problem, we present a method that combines Monte Carlo simulation in a ``disk atmosphere'' with a diffusion approximation in the disk interior. By appropriately matching the boundary conditions between the diffusion region and the Monte Carlo atmosphere, we can reduce the optical depth of the Monte Carlo layer to only a few mean free paths (typically between 3 and 10), which greatly decreases the execution time. We also show how to include the diffusion region when performing radiative equilibrium calculations, and how to incorporate energy generation (e.g., accretion luminosity) in both the disk interior (diffusion region) and disk atmosphere (Monte Carlo region).
This work is supported by NSF Grant AST-9819928 and NASA Grant NAG5-8794.
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.