The Distribution of Grain Sizes in Circumstellar Dust Shells

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Session 40 -- Computational Astrophysics I
Display presentation, Wednesday, 1, 1994, 9:20-6:30

[40.15] The Distribution of Grain Sizes in Circumstellar Dust Shells

M. P. Egan (Phillips Laboratory - GPOB)

I have used a maximum entropy technique to determine the distribution of grain sizes throughout a model circumstellar envelope (CSE). The CSE has been constructed using a time-dependent model which solves the hydrodynamics of the shell, modified to include grain formation. Grain nucleation is calculated using kinetic equations, while the growth of larger particles is determined through the use of equations describing the evolution of moments of the grain size distribution function. For particles larger than the cutoff size in the kinetic equations, only average grain properties, as determined by the moments are known. Maximum entropy reconstruction, constrained by the moments of the distribution, gives a good approximation to the true grain size distribution. Comparison of distributions at various points and times in the outflow allows for investigation of grain formation and destruction mechanisms within the flow. The average grain sizes determined by the moments suggest that very large grains form close to the star. These grains accelerate into low density regions where some evaporation takes place. Smaller grains then form in the low density region. The maximum entropy derived grain size distributions are consistent with this picture. They show that the distribution is initially dominated by a narrow (in size range) large grain component in the inner (high density) regions of the dust shell. In the outer (lower density) part of the dust shell, the grain size distribution generally consists of a diminished large grain component plus a broad small grain component.

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