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Session 48 - Circumstellar Envelopes.
Display session, Tuesday, January 14
Metropolitan Ballroom,
To study the effect of multiple scattering on polarization, we model the transport of polarized radiation in a dusty medium of spherical geometry with spherical dust grains of silicate and amorphous carbon. The equation of radiative transport is solved for the Stokes parameters for linear polarization (I and Q) by an iterative method. We iterate between the solution of the nonlinear moment equation using a finite difference method, which determines the source function due to scattering and thermal emission, and the solution of the linear ray equations using a short characteristic method, which determines the angular distribution of the radiation field.
Initial work has assumed Rayleigh scattering and has been extended to include larger grains using Mie scattering. The scattering matrix for Mie grains is calculated using a sum over Fourier components and general spherical functions. Polarization as a function of impact parameter is calculated for a resolved source, from which polarization maps are constructed. In addition to optical depth effects, the effect of grain size distribution, grain composition, and spectral features in the dielectric function on the polarization in the ultraviolet, visible, and near infrared is studied. Results are presented for models with power law size distributions of silicate and amorphous carbon grains and applied to the study of AGB type stars.
