Grain Alignment in OMC1 as Deduced from Observed Large Circular Polarization

Abstract

The properties of polarization in scattered light by aligned ellipsoidal grains are investigated with the Fredholm integral equation method (FIM) and the T-matrix method (Tmat), and the results are applied to the observed circular polarization in OMC1. We assume that the grains are composed of silicates and ellipsoidal (oblate, prolate, or tri-axial ellipsoid) in shape with a typical axial ratio of 2:1. The angular dependence of circular polarization p_c on directions of incident and scattered light is investigated with spherical harmonics and associated Legendre polynomials. The degree of circular polarization p_c also depends on the Rayleigh reduction factor R which is a measure of imperfect alignment. We find that p_c is approximately proportional to R for grains with |m|x_{eq} < 3 - 5, where x_{eq} is the dimensionless size parameter and m is the refractive index of the grain. Models that include those grains can explain the observed large circular polarization in the near infrared, ~15%, in the south-east region of the BN object (SEBN) in OMC1, if the directions of incidence and scattering of light is optimal, and if grain alignment is strong, i.e. R > 0.5. Such a strong alignment cannot be explained by the Davis-Greenstein mechanism; we prefer instead an alternative mechanism driven by radiative torques. If the grains are mixed with silicates and ice, the degree of circular polarization p_c decreases in the 3 micron ice feature, while that of linear polarization increases. This wavelength dependence is different from that predicted in a process of dichroic extinction.Comment: 27 pages, including 14 figures and 3 table