Modeling extinction and infrared emission from fractal dust grains: Fractal dimension as a shape parameter
Modeling extinction and infrared emission from fractal dust grains: Fractal dimension as a shape parameter
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DOI：
10.1086/305809
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发表时间：
1998-07-01
影响因子：
4.9
通讯作者：
Leung, CM
中科院分区：
文献类型：
--
作者：
Fogel, ME;Leung, CM
在天体物理环境中，尘埃颗粒很可能是由随机生长过程形成的，例如，颗粒聚集或簇聚集，并且应该具有不规则形状。然而在天体物理学模型中，球形颗粒经常被假定。我们建立了详细的辐射输运模型来研究非球形尘埃颗粒对星周包层和星际尘埃云光谱的影响。一般来说，非球形晶粒在长波长处具有较大的吸收截面。由于不太紧凑并且具有较低的体积与表面积比，它们通常比它们的球形对应物更冷，通常为10%-20%。因此，具有非球形颗粒的红外源的辐射输运模型将显示峰值通量向更长波长的偏移。此外，10 μ m至20 μ m硅酸盐发射特征的通量比对于较不紧凑的非球形晶粒减小。我们还研究了晶粒形状对星际消光曲线标准模型的影响。此外，我们发现，使用体光学常数的亚微米颗粒引入的误差是显着小于所引入的假设球形颗粒。我们已经确定，第一次，分形维数D作为一个形状参数表征的光学，热，和辐射特性的尘埃颗粒。分形维数D取决于体积与表面积的比率，并且由类似于r（D）的关系N（r）定义，其中N（r）是半径为r的球体内的单体的数量。通常，1 < D < 3，并且更小的D意味着更多的连续性和更不致密的晶粒形态。对于一个给定的质量，晶粒具有相同的D，独立的详细形状，没有显着差异，在其吸收截面，温度和能谱。因此，在涉及不规则形状颗粒的建模现象中，我们只需要引入一个参数，即分形维数D来表征形状。去除不切实际的球形颗粒假设有几个重要的天体物理学意义：（1）基于球形颗粒模型确定尘埃柱密度会导致高估;（2）使用分形颗粒代替球形颗粒的星际消光曲线模型需要更少的元素消耗，通常是三分之一;（3）在演化恒星辐射驱动质量损失的研究中，由于尘埃上的辐射压力敏感地依赖于新形成颗粒的消光截面，因此质量损失率和外流动力学的细节都可能需要修正。
In astrophysical environments, dust grains are most likely formed by stochastic growth processes, e.g., particle aggregation or cluster aggregation, and should have irregular shapes. Yet spherical grains are often assumed in astrophysical models. We have constructed detailed radiation transport models to study the effects of nonspherical dust grains on the spectra of circumstellar envelopes and interstellar dust clouds. In general, nonspherical grains have larger absorption cross sections at long wavelengths. Being less compact and having lower ratios of volume to surface area, they are generally cooler than their spherical counterparts, typically by 10%-20%. Hence radiation transport models of infrared sources with nonspherical grains would show a shift in the peak flux toward longer wavelengths. Furthermore, the flux ratio of 10 mu m to 20 mu m silicate emission features decreases for the less compact nonspherical grains. We have also examined the effects of grain shape on a standard model of the interstellar extinction curve. In addition, we find that the error introduced by using bulk optical constants for submicron particles is significantly smaller than that introduced by assuming spherical grains. We have identified, for the first time, fractal dimension D as a shape parameter characterizing the optical, thermal, and radiative properties of dust grains. The fractal dimension D depends on the ratio of volume to surface area and is defined by the relation N(r) similar to r(D), where N(r) is the number of monomers within a sphere of radius r. In general 1 < D < 3, and a smaller D implies a more filamentary and less compact grain morphology. For a given mass, grains with the same D, independent of detailed shape, show no significant difference in their absorption cross sections, temperatures, and energy spectra. Hence in modeling phenomena involving irregularly shaped grains, we need to introduce just one parameter, the fractal dimension D, to characterize the shape. There are several important astrophysical implications of removing the unrealistic assumption of spherical grains: (1) determination of dust column density based on models assuming spherical grains would lead to an overestimate; (2) models of the interstellar extinction curve using fractal grains instead of spherical grains would require less elemental depletion, typically by one-third; and (3) in the study of radiation-driven mass loss in evolved stars, both the mass-loss rates and details of outflow dynamics may need to be revised since radiation pressure on dust depends sensitively on the extinction cross sections of newly formed grains.