用弹道聚集体模拟多孔尘埃颗粒I：几何和光学性质
Modeling Porous Dust Grains with Ballistic Aggregates I: Geometry and Optical Properties
摘要：
本文研究了球形单体无规弹道聚集体对光的散射和吸收。我们提出了一个一般措施的孔隙率的不规则颗粒。三种不同类别的弹道骨料被认为是具有不同程度的孔隙度。散射和吸收截面的计算，使用离散偶极子近似（DDA），为三种成分的颗粒（50%硅酸盐和50%石墨; 50%硅酸盐和50%无定形碳;和100%硅酸盐），从0.1微米到4微米的波长。对于固定的颗粒质量，增加孔隙率增加消光在短波长，但减少消光在波长长相比，总的聚集体尺寸。散射和吸收截面对单体尺寸不敏感，只要组成单体与入射波长相比是小的。我们比较我们的准确DDA结果与其他两个近似：分析多层球（MLS）模型和有效介质理论（EMT）。对于高孔隙率和/或吸收性材料，MLS模型不提供弹道聚集体的散射和吸收的良好近似。EMT方法为这些聚集体提供了比MLS模型更好的近似，与DDA结果相比，消光和散射截面的典型差异小于20%，对于本研究中探测的所有类型、组合物和波长。
英文摘要：
We investigate the scattering and absorption of light by random ballistic aggregates of spherical monomers. We present a general measure for the porosity of an irregular particle. Three different classes of ballistic aggregates are considered, with different degrees of porosity. Scattering and absorption cross sections are calculated, using the discrete dipole approximation (DDA), for grains of three compositions (50% silicate and 50% graphite; 50% silicate and 50% amorphous carbon; and 100% silicate), for wavelengths from 0.1 micron to 4 micron. For fixed particle mass, increased porosity increases the extinction at short wavelengths, but decreases the extinction at wavelengths long compared to the overall aggregate size. Scattering and absorption cross sections are insensitive to monomer size as long as the constituent monomers are small compared with the incident wavelength. We compare our accurate DDA results with two other approximations: the analytical multi-layer sphere (MLS) model and effective medium theory (EMT). For high porosity and/or absorptive materials, the MLS model does not provide a good approximation for scattering and absorption by ballistic aggregates. The EMT method provides a much better approximation than the MLS model for these aggregates, with a typical difference less than 20% in extinction and scattering cross sections compared with DDA results, for all types, compositions and wavelengths probed in this study.
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