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Multiple scattering of polarized radiation by non-spherical
grains: first results

S. Wolf
Thuringian State Observatory Tautenburg

N.V. Voshchinnikov
Astronomy Department and Sobolev Astronomical Institute,
St.Petersburg University

Th. Henning
Astrophysical Institute and University Observatory,
Friedrich Schiller University, Jena


Abstract

We present the first solution to the radiative transfer problem in
dust configuration containing aligned non-spherical (spheroidal)
dust grains. The optical properties of spheroidal grains are
calculated using the method of separation of variables developed by
Voshchinnikov & Farafonov (1993). The radiative transfer problem is
solved on the basis of a Monte-Carlo method for three-dimensional
self-consistent simulations (Wolf et al. 1999). The test simulations
confirming the correct numerical implementation of the scattering
mechanism are presented. As a first application, we investigate the
linear and circular polarization from a spherical circumstellar
shell containing perfectly aligned prolate or oblate spheroidal
grains. The dependence of the results on the grain parameters
(equivolume radius, aspect ratio) and the shell parameters
(inner/outer radius, optical thickness) is considered. The most
remarkable feature of the simulated linear polarization maps are
so-called polarization null points where the reversal of
polarization occurs. They appear in the case when the grain
alignment axis is perpendicular to the line of sight. The position
of these points may be used for the estimation of grain shape and
geometrical structure of the shell. The maps of circular
polarization has the sector-like structure with maxima at the ends
of line inclined to the grain alignment axis by +-45 deg.