The infrared emission spectra of compositionally inhomogeneous aggregates composed of irregularly shaped constituents
The infrared emission spectra of compositionally inhomogeneous aggregates composed of irregularly shaped constituents
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由不规则形状成分组成的成分不均匀聚集体的红外发射光谱
DOI：
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发表时间：
2008
期刊：
影响因子：
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通讯作者：
A. D. Koter
中科院分区：
文献类型：
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作者：
M. Min;J. Hovenier;L. Waters;L. Waters;A. D. Koter
目标。为了推断尘埃在通过聚集生长的天体物理环境中的性质，需要了解聚集的粒子与辐射相互作用的方式，以及它们发出的热辐射中编码了什么信息。发射特性由聚集体的大小和结构及其成分的组成和形状决定。因此，我们的目标是对由不规则形状的成分组成的组成不均匀的集合体进行计算。此外，我们的目标是开发一种经验配方，以快速而准确的方式计算这种聚集体的光学性质。方法：研究方法。我们对不同组成的不规则形状颗粒的聚集体进行了CDA计算。聚集体的成分被假定为瑞利区(即，它们远小于组分内外的辐射波长)，此外，我们还假设聚集体成分之间的主要相互作用是通过偶极-偶极相互作用。我们计算了含30%无定形碳和70%硅酸盐的聚集体在10μm范围内发射效率的光谱结构，其中含有不同比例的晶态和非晶态组分。结果。我们发现，聚集体各组分的光谱外观有很大的不同，并取决于它们的丰度。最值得注意的是，丰度很低的物质在光谱上看起来就像是在非常小的颗粒中，而更丰富的物质在光谱上似乎存在于更大的颗粒中。基于有效介质近似的思想，我们构造了一种快速的经验近似方法来构造这些聚集体的光谱，几乎完美地再现了更精确的计算。这种新方法的速度足够快，很容易在试图从天文观测中推断尘埃特征的拟合程序中实现。
Aims. In order to deduce properties of dust in astrophysical environments where dust growth through aggregation is high, knowledge of the way aggregated particles interact with radiation, and what information is encoded in the thermal radiation they emit, is needed. The emission characteristics are determined by the size and structure of the aggregate and the composition and shape of their constituents. We thus aim at performing computations of compositionally inhomogeneous aggregates composed of irregularly shaped constituents. In addition we aim at developing an empirical recipe to compute the optical properties of such aggregates in a fast and accurate manner. Methods. We performed CDA computations for aggregates of irregularly shaped particles with various compositions. The constituents of the aggregate are assumed to be in the Rayleigh regime (i.e. they are much smaller than the wavelength of radiation inside and outside the constituents), and in addition we assume that the dominant interaction of the aggregate constituents is through dipole-dipole interactions. We computed the spectral structure of the emission efficiency in the 10 μm region for aggregates with 30% amorphous carbon and 70% silicates by volume with various fractions of crystalline and amorphous components. Results. We find that the spectral appearance of the various components of the aggregate are very different and depend on their abundances. Most notably, materials that have a very low abundance appear spectroscopically as if they were in very small grains, while more abundant materials appear spectroscopically to reside in larger grains. We construct a fast empirical approximate method, based on the idea of an effective medium approximation, to construct the spectra for these aggregates which almost perfectly reproduces the more exact computations. This new method is fast enough to be easily implemented in fitting procedures trying to deduce the dust characteristics from astronomical observations.