MULTIPLE-SCATTERING OF POLARIZED RADIATION IN CIRCUMSTELLAR DUST SHELLS

On the basis of recently developed modification of the Monte Carlo app roach (the method of symmetrized trajectories), a numerical code for t he transfer of the polarized radiation in circumstellar shells has bee n created. This code allows to study light scattering by dust grains i n prolate or oblate spheroidal shells of a moderate optical thickness (tau less-than-or-similar-to 5 - 8) seen at various angles. The descri ption of the Monte Carlo approach and various tests are presented. The extensive calculations were performed for the cases of Rayleigh scatt ering (grain albedo LAMBDA = 0.1, 0.5, and 1.0) and Mie scattering (si licate-graphite mixtures, wavelength range 1000 angstrom - 1 mum). The dependencies of the intensity and polarization on the parameters of d ust grains and homogeneous circumstellar shells are examined. It was f ound that the light scattered by circumstellar dust is a noticeable fr action of the observed stellar radiation in the visual and ultraviolet spectral regions. The fraction of scattered radiation grows with incr easing LAMBDA and decreasing the shell semiaxis ratio. The ratio of th e scattered radiation intensity to the direct stellar one has a maximu m at about 2000 - 3000 angstrom and can reach I(sca)/I almost-equal-t o 0.3 - 0.4 for the uniform oblate spheroidal shell. In some cases, th e summary influence of the scattered radiation and the circumstellar ( and interstellar) extinction can produce the wavelength independent ex tinction curves. Such curves cannot be analyzed by the colour-excess m ethod. The degree of linear polarization usually does not exceed 1 - 2 % and has a weak wavelength dependence in the visual and red regions o f the spectrum. At the ultraviolet wavelengths, the polarization has a maximum in the region of the bump (lambda almost-equal-to 2200 angstr om) and a minimum at lambda almost-equal-to 1800 - 2000 angstrom. We h ave also considered the case of the screening of stellar radiation by dense circumstellar clouds which occurs for some young stars (e.g., He rbig Ae/Be stars). If only the scattered radiation is observed, the up per limit on the stellar brightness variations is of 1m - 2m in the ul traviolet and 3m - 4m in the red wavelength region. The maximum degree of linear polarization grows with increasing wavelength and can reach 8 - 10%.