Multiple scattering of polarized radiation in circumstellar dust shells

Details Multiple scattering of polarized radiation in circumstellar dust shells Multiple scattering of polarized radiation in circumstellar dust shells

Aug 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...288..883v&link_type=abstract

Astronomy and Astrophysics 288, 883-896 (1994)

Astronomy and Astrophysics

Astrophysics

26

Circumstellar Matter, Radiative Transfer, Polarization, Stars: Late-Type, Stars: Pre-Main-Sequence

Scientific paper

On the basis of recently developed modification of the Monte Carlo approach (the method of symmetrized trajectories), a numerical code for the transfer of the polarized radiation in circumstellar shells has been created. This code allows to study light scattering by dust grains in prolate or oblate spheroidal shells of a moderate optical thickness (τ<~5-8) seen at various angles. The description of the Monte Carlo approach and various tests are presented. The extensive calculations were performed for the cases of Rayleigh scattering (grain albedo {LAMBDA}=0.1, 0.5, and 1.0) and Mie scattering (silicate-graphite mixtures, wavelength range 1000A-1μm). The dependencies of the intensity and polarization on the parameters of dust grains and homogeneous circumstellar shells are examined. It was found that the light scattered by circumstellar dust is a noticeable fraction of the observed stellar radiation in the visual and ultraviolet spectral regions. The fraction of scattered radiation grows with increasing {LAMBDA} and decreasing the shell semiaxis ratio. The ratio of the scattered radiation intensity to the direct stellar one has a maximum at about 2000-3000A and can reach Isca/I*=~0.3-0.4 for the uniform oblate spheroidal shell. In some cases, the summary influence of the scattered radiation and the circumstellar (and interstellar) extinction can produce the wavelength independent extinction curves. Such curves cannot be analyzed by the colour-excess method. The degree of linear polarization usually does not exceed 1-2 % and has a weak wavelength dependence in the visual and red regions of the spectrum. At the ultraviolet wavelengths, the polarization has a maximum in the region of the bump (λ=~2200A) and a minimum at λ=~1800-2000A. We have also considered the case of the screening of stellar radiation by dense circumstellar clouds which occurs for some young stars (e.g., Herbig Ae/Be stars). If only the scattered radiation is observed, the upper limit on the stellar brightness variations is of 1-2mag in the ultraviolet and 3-4mag in the red wavelength region. The maximum degree of linear polarization grows with increasing wavelength and can reach 8-10 %.

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