从大圆极化率推断OMC1中的晶粒取向
Grain Alignment in OMC1 as Deduced from Observed Large Circular Polarization
摘要：
利用Fredholm积分方程法和T矩阵法研究了椭球面排列颗粒散射光的偏振特性，并将结果应用于OMC 1中观测到的圆偏振。我们假设，颗粒组成的硅酸盐和椭圆形（扁球形，长形，或三轴椭球）的形状与一个典型的轴比为2：1。本文用球谐函数和勒让德多项式研究了圆偏振度p_c与入射光和散射光方向的关系。圆偏振度p_c还取决于瑞利衰减因子R，其是不完美对准的度量。我们发现，对于|m| x_{eq} < 3 - 5的晶粒，p_c近似与R成正比，其中x_{eq}是无量纲尺寸参数，m是晶粒的折射率。如果光的入射和光的散射方向是最佳的，并且如果晶粒排列是强的，即R > 0.5，则包括这些晶粒的模型可以解释在OMC 1中BN物体（SEBN）的东南区域中在近红外中观察到的~ 15%的大圆偏振。这种强的排列不能用戴维斯-格林斯坦机制来解释;我们更倾向于用辐射力矩驱动的另一种机制。如果颗粒与硅酸盐和冰混合，则在3微米冰特征中圆偏振度p_c减小，而线偏振度p_c增大。这种波长依赖性不同于二向色性消光过程中预测的波长依赖性。
英文摘要：
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.
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