On the Shapes of Interstellar Grains: Modeling Infrared Extinction and Polarization by Spheroids and Continuous Distributions of Ellipsoids
On the Shapes of Interstellar Grains: Modeling Infrared Extinction and Polarization by Spheroids and Continuous Distributions of Ellipsoids
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DOI：
10.3847/1538-4357/abddb7
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发表时间：
2021-03
期刊：
影响因子：
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通讯作者：
B. Draine;B. Hensley
中科院分区：
文献类型：
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作者：
B. Draine;B. Hensley
虽然星际颗粒是已知的非球面，但它们的实际形状仍然没有得到很好的限制。我们评估了文献中三个连续分布的椭球体(CDE)是否适合描述星际颗粒的形状。从每个分布中随机选择的形状显示为插图。通常使用的Bohren-Huffman CDE包括非常大一部分极端形状：整整10%的随机抽签的轴比为a 3/a 1>；19.7，5%的抽签具有3/a 1>；33。CDE2分布包含的极端形状比例要小得多，似乎是最逼真的。对于所考虑的三个CDE中的每一个，我们都得到了瑞利极限下的消光和偏振的形状平均截面。最后，我们描述了一种“合成”假设形状或形状分布的介电函数的方法，如果瑞利极限下每个颗粒体积的实际吸收截面已知的话。这种合成介电函数预测了偏振的波长依赖关系，然后可以与观测结果进行比较，以限制颗粒形状。
Although interstellar grains are known to be aspherical, their actual shapes remain poorly constrained. We assess whether three continuous distributions of ellipsoids (CDEs) from the literature are suitable for describing the shapes of interstellar grains. Randomly selected shapes from each distribution are shown as illustrations. The often-used Bohren–Huffman CDE includes a very large fraction of extreme shapes: fully 10% of random draws have axial ratio a 3/a 1 > 19.7, and 5% have a 3/a 1 > 33. The CDE2 distribution includes a much smaller fraction of extreme shapes, and appears to be the most realistic. For each of the three CDEs considered, we derive shape-averaged cross sections for extinction and polarization in the Rayleigh limit. Finally, we describe a method for “synthesizing” a dielectric function for an assumed shape or shape distribution if the actual absorption cross sections per grain volume in the Rayleigh limit are known from observations. This synthetic dielectric function predicts the wavelength dependence of polarization, which can then be compared to observations to constrain the grain shape.