Grain Alignment in OMC 1 as Deduced from Observed Large Circular Polarization

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 southeast 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 µm ice feature, while that of linear polarization increases. This wavelength dependence is different from that predicted in a process of dichroic extinction. – 2 –