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Session 12 - Stellar Evolution - Theory.
Display session, Wednesday, January 07
Exhibit Hall,

[12.04] Radiative Equilibrium and Temperature Correction Procedures for use in Monte Carlo Radiation Transfer

J. E. Bjorkman (U. Toledo), K. Wood (Harvard SAO)

We present a Monte Carlo technique for calculating the radiative equilibrium temperature and emergent spectral energy distribution from circumstellar envelopes. Since the Monte Carlo simulation is inherently a 3-D method, our radiative equilibrium code is well-suited for modeling complex circumstellar environments. Our simulation tracks individual equal-energy monochromatic photon packets emitted by the star. As they traverse the envelope, they are scattered and absorbed at random locations determined by the envelope opacity (dust in these simulations). Whenever a photon is absorbed, we increment the number absorbed by that grid cell and calculate the temperature required to reradiate the cumulative absorbed energy. We then reemit the photon with a frequency distribution that corrects that of any previous emission, and we continue tracking it until it finally escapes the envelope, whereupon we bin it into frequency and direction-of-observation bins. Since the number of photons absorbed by a cell equals the number emitted, our procedure automatically enforces radiative equilibrium. Note that the reemission converts stellar photons into envelope photons with a frequency shift determined by the envelope temperature; this is how the envelope redistributes the input spectral energy distribution. Once a stellar photon is converted to an envelope photon, it may still be absorbed in other grid cells; thus, the diffuse radiation field is also automatically included. We continue emitting stellar photons, correcting the envelope temperature, and reemitting absorbed photons until they all exit the system. At this point, we have determined the emergent spectral energy distribution and equilibrium temperature structure throughout the entire envelope. On current workstations, our code yields accurate results for spherically symmetric and two-dimensional cases in about one to ten minutes, respectively. We show the results of our method compared to other spherically symmetric methods, and we also present the results of simulations for two-dimensional axisymmetric density structures.

Program listing for Wednesday