Issue |
A&A
Volume 385, Number 1, April I 2002
|
|
---|---|---|
Page(s) | 365 - 376 | |
Section | Section $secnum inconnue | |
DOI | https://doi.org/10.1051/0004-6361:20020158 | |
Published online | 15 April 2002 |
Multiple scattering of polarized radiation by non-spherical grains: First results
1
Thuringian State Observatory Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
2
Sobolev Astronomical Institute, St. Petersburg University, Universitetskii Prosp. 28, 198504 Stary Peterhof-St. Petersburg, Russia and Isaac Newton Institute of Chile, St. Petersburg Branch, Russia
3
Astrophysical Institute and University Observatory (AIU), Friedrich Schiller University, Schillergäßchen 3, 07745 Jena, Germany
Corresponding author: N. V. Voshchinnikov, nvv@astro.spbu.ru
Received:
29
December
2000
Accepted:
21
January
2002
We present the first numerical radiative transfer simulation of
multiple light scattering in dust configurations
containing aligned non-spherical (spheroidal) dust grains.
Such models are especially important if one wants
to explain the circular polarization of light, observed in a variety of astronomical
objects.
The optical properties of the spheroidal grains are calculated
using the method of separation of variables developed by Voshchinnikov
& Farafonov ([CITE]).
The radiative transfer problem is solved on the basis of the Monte Carlo
method. Test simulations, confirming the correct numerical
implementation of the scattering mechanism, are presented.
As a first application, we investigate the
linear and circular polarization of light coming from
a spherical circumstellar shell. This shell contains perfectly aligned
prolate or oblate spheroidal grains.
We investigate the dependence of the results on the grain parameters
(equivolume radius, aspect ratio) and
the shell parameters (inner/outer radius, optical thickness).
The most remarkable features 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 origin of null points lies in the
physics of light scattering by non-spherical particles
and is not related to the cancellation of polarization
as was discussed in previous models.
The maps of circular polarization have a sector-like
structure with maxima at the ends of lines inclined
to the grain alignment axis by .
Key words: polarization / radiative transfer / scattering / methods: numerical / (stars:) circumstellar matter / ISM: clouds
© ESO, 2002
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