Optical Properties of Nonspherical Particles of Size Comparable to the Wavelength of Light: Application to Comet Dust
Optical Properties of Nonspherical Particles of Size Comparable to the Wavelength of Light: Application to Comet Dust
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尺寸与光波长相当的非球形颗粒的光学特性：在彗星尘埃中的应用
DOI：
10.1006/icar.1998.6066
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发表时间：
1999
期刊：
影响因子：
--
通讯作者：
P. Yanamandra
中科院分区：
文献类型：
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作者：
Pacilnavati A. Yatlatllarldra;P. Yanamandra
摘要为了解释观测到的彗星尘埃的光学性质，对非球形粒子进行了散射计算。我们重点研究了彗星尘埃对颗粒形状敏感的两个光学特性：相角≤21°的负线极化和11.2 μm的硅酸盐发射特征。采用离散偶极子近似（DDA）方法计算了非球形硅酸盐和与波长相当大小的吸收粒子的散射矩阵。具有多种形状和尺寸参数X eq ~ 2.5的硅酸盐颗粒，对应的线性尺寸为0.5 ~ 1.0 μm，在小相角处可以产生负的线极化，而碳颗粒在相角为90°时产生强的正极化最大值。与波长相比，硅酸盐和碳质材料的混合物在较小的尺度上消除了这个尺寸范围内的负极化；然而，宏观上硅酸盐和碳的混合物在低相角（≤21°）时可以产生观察到的负线性极化，在相角为90°时可以产生最大正极化。在彗星中观测到的11.2 μm热发射峰的位置，归因于结晶橄榄石，强烈依赖于颗粒形状，即使颗粒比波长小得多，并且可以与我们的模型四面体或中等伸长砖的各向异性富镁橄榄石相匹配。球体和极端形状，如圆盘或针状，似乎被排除在外。大约20%的结晶橄榄石和80%的无序橄榄石再现了观测到的10 μm和1.2μm的彗星的光谱，例如P/Halley， Bradfield 1987 XXIX, Mueller， Levy 1990 XX和C/1995 O1 （Hale-Bopp）。这项研究是对彗星尘埃进行现实建模的重要的第一步，彗星尘埃是由非球形单体组成的聚集体，其尺寸与入射辐射的波长相当。
Abstract Scattering calculations for nonspherical particles have been carried out in order to explain observed optical properties of cometary dust. We focused on two optical properties of cometary dust sensitive to particle shape: negative linear polarization at phase angles ≤21° and the 11.2-μm silicate emission feature. The discrete dipole approximation (DDA) method was employed to compute the scattering matrix for nonspherical silicate and absorbing particles of size comparable to the wavelength. Silicate particles with a variety of shapes and size parameter X eq ∼2.5, corresponding to a linear dimension of 0.5–1.0 μm, can produce negative linear polarization at small phase angles, whereas carbon particles produce a strong positive maximum of polarization near phase angles of 90°. Mixtures of silicate and carbonaceous material, on a scale small compared to the wavelength, eliminate the negative polarization in this size range; however, macroscopic mixtures of silicate and carbon could yield the observed negative linear polarization at low phase angles (≤21°) and a maximum positive polarization at phase angle of 90°. The position of the 11.2-μm thermal emission peak observed in comets, attributed to crystalline olivine, depends strongly on particle shape even for particles much smaller than the wavelength and can be matched with anisotropic Mg-rich olivine for our model tetrahedra or moderately elongated bricks. Spheres and extreme shapes, such as disks or needles, appear to be ruled out. Approximately 20% crystalline olivine and 80% disordered olivine reproduces the observed spectra of comets with comparable peaks at 10 and 11.2μm, e.g., P/Halley, Bradfield 1987 XXIX, Mueller, Levy 1990 XX, and C/1995 O1 (Hale–Bopp). This study is an essential first step toward realistic modeling of comet dust as aggregates composed of nonspherical monomers having dimensions comparable to the wavelength of incident radiation.