Scattering and Absorption by Aligned Grains in Circumstellar Environments
Scattering and Absorption by Aligned Grains in Circumstellar Environments
复制标题
星周环境中对齐颗粒的散射和吸收
DOI：
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发表时间：
2002
期刊：
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通讯作者：
M. Wolff
中科院分区：
文献类型：
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作者：
B. Whitney;M. Wolff
我们目前的辐射传输计算显示的偏振效应的散射和吸收排列晶粒。颗粒模型由具有不同排列程度的扁圆或旋转扁长颗粒的尺寸分布组成。为了理解辐射传递效应，我们开始与一个简单的情况下，一个球形信封照明的中心源与恒定的晶粒排列轴在整个信封。非对齐的晶粒在这样的包层中不产生净偏振，而对齐的晶粒产生基本的线偏振和圆偏振。线偏振是由差分消光和散射的竞争效应引起的。偏振态随光深的变化很大，在低光深时散射占主导地位，在高光深时微分消光占主导地位。来自包层的净圆偏振或积分圆偏振为零;然而，穿过被解析星云的圆偏振很大，在星云的“对角线”区域达到±50%。接下来，我们计算轴对称模型的原恒星信封，再次与简化的情况下，恒定的晶粒排列轴整个信封。的偏振图显示的差异，从不对齐的晶粒的情况下，特别是在磁盘中平面，即使是散射光的差分消光导致的偏振矢量对齐垂直于磁盘平面，在许多观察相反。这表明，要么是颗粒在原恒星包层中没有对齐，要么是磁场（假定的对齐机制）没有沿着整个包层和磁盘的磁盘旋转轴对齐。一个明确的测试晶粒排列可能来自原恒星的圆偏振图。排列的颗粒在整个云中产生大的圆偏振值，在这里给出的模型中高达±25%-40%，而非排列的颗粒产生小于1%的最大偏振。在具有排列晶粒的物体中，分析线性和圆偏振图可以探测磁性几何形状。
We present radiative transfer calculations showing the polarization effects of scattering and absorption by aligned grains. The grain model consists of a size distribution of oblate or spinning prolate particles with varying degrees of alignment. To develop an understanding of the radiative transfer effects, we begin with the simple case of a spherical envelope illuminated by a central source with constant grain alignment axis throughout the envelope. Nonaligned grains produce no net polarization in such envelopes, while aligned grains produce substantial linear and circular polarization. The linear polarization results from the competing effects of differential extinction and scattering. The polarization varies strongly with optical depth, with scattering dominating at low optical depth and differential extinction dominating at high optical depth. The net, or integrated, circular polarization from the envelopes is zero; however, the circular polarization across the resolved nebula is large, reaching ±50% in the "diagonal" regions of the nebula. Next we calculate axisymmetric models of protostellar envelopes, again with the simplifying case of constant grain alignment axis throughout the envelope. The polarization maps show differences from the case of nonaligned grains, especially in the disk midplane, where differential extinction of even the scattered light causes the polarization vectors to align perpendicular to the disk plane, in contrast to many observations. This suggests either that grains are not aligned in protostellar envelopes or that the magnetic field (the presumed alignment mechanism) is not aligned along the disk rotational axis throughout the envelope and disk. A definitive test of grain alignment could come from resolved circular polarization maps of protostars. Aligned grains produce large values of circular polarization across the cloud, up to ±25%-40% in the models presented here, whereas nonaligned grains produce maximum polarizations of less than 1%. In objects with aligned grains, analysis of linear and circular polarization maps can probe magnetic geometries.