Grain Alignment in OMC1 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 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.
