Abstract
We present optical features characteristic of the shape of composite dust particles, such as grain-aggregates formed under cosmic or atmospheric conditions. The discussion develops along two examples of realistic models for the formation of the composite particles, and constant refractive indices. That way, particular features exhibited in the optical cross section behaviors result from the particle structure only. The role of the ratio between optical particle cross section and the corresponding cross section of the coated sphere of same composition and volume is highlighted. Limited wavelength ranges are defined where the coated sphere model can be used to obtain the effective radius or the volume composition of the particle. Other wavelength domains are very dependent on the particle formation mechanisms, then giving constraints on the possible formation processes. Therefore, such an approach is able to provide inverse methods to obtain the actual values of physical parameters from definite features of optical quantities.
Highlights
Statement of the Problem In the interstellar medium, strong optical extinction of dust particles in the ultraviolet region is interpreted as the presence of nanometer sized particles, while characteristics in the visible range indicate micrometer sized particles
Similar inference arose for other cosmic dust particles (Weidenschilling and Cuzzi, 1993)
Agreement between observational data and the simple model in suitable parameter ranges, allows determination of the relevant parameters characterizing the model, while discrepancies are used to understand the geometry of the scatterers, as they correspond to shape effects
Summary
The starting point is the suggestion by Mathis (Mathis, 1996) to consider the ratios between the optical cross sections of randomly oriented oblate spheroids of given shape and the corresponding cross sections of the sphere of equal volume. 3.1 Composition The present article is devoted to explaining a method, not to fitting current data, the precise values of the parameters are not so important and we consider here typical values in order to keep the analysis simple Following this idea, the volume fractions will be: 20% of A and 80% of B, and the respective refractive indices: mA = 1.9 + 0.5 i ; mB = 1.3 + 10−3 i. R PB B PA process: B-grains condensate first in clusters through the Reaction-limited Particle-Cluster Aggregation process (RPCA) since bonds between adjoining B-grains are weak (Jullien and Botet, 1987) These clusters are compact with rough surface, see Fig. 2(a). Consist in a rough compact core of B, coated with a thin shell of A
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