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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.

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