High circular polarization of near-infrared light induced by micron-sized dust grains

Abstract

ABSTRACT We explore the induction of circular polarization (CP) of near-infrared light in star-forming regions using three-dimensional radiative transfer calculations. The simulations trace the change of Stokes parameters at each scattering/absorption process in a dusty gas slab composed of aligned grains. We find that the CP degree enlarges significantly according as the size of dust grains increases and exceeds ∼20 per cent for micron-sized grains. Therefore, if micron-sized grains are dominant in a dusty gas slab, the high CP observed around luminous young stellar objects can be accounted for. The distributions of CP show the asymmetric quadrupole patterns regardless of the grain sizes. Also, we find that the CP degree depends on the relative position of a dusty gas slab. If a dusty gas slab is located behind a star-forming region, the CP reaches ∼60 per cent in the case of 1.0 µm dust grains. Hence, we suggest that the observed variety of CP maps can be explained by different size distributions of dust grains and the configuration of aligned grains.