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In order to deduce properties of dust in astrophysical environments wheredust growth through aggregation is important, knowledge of the way aggregatedparticles interact with radiation, and what information is encoded in thethermal radiation they emit, is needed. The emission characteristics aredetermined by the size and structure of the aggregate and the composition andshape of the constituents. We thus aim at performing computations ofcompositionally inhomogeneous aggregates composed of irregularly shapedconstituents. In addition we aim at developing an empirical recipe to computethe optical properties of such aggregates in a fast and accurate manner. Weperformed CDA computations for aggregates of irregularly shaped particles withvarious compositions. The constituents of the aggregate are assumed to be inthe Rayleigh regime, and in addition we assume that the dominant interaction ofthe aggregate constituents is through dipole-dipole interactions. We computedthe spectral structure of the emission efficiency in the 10 micron region foraggregates with 30% amorphous carbon and 70% silicates by volume with variousfractions of crystalline and amorphous components. We find that the spectralappearance of the various components of the aggregate are very different anddepend on their abundances. Most notably, materials that have a very lowabundance appear spectroscopically as if they were in very small grains, whilemore abundant materials appear, spectroscopically to reside in larger grains.We construct a fast empirical approximate method, based on the idea of aneffective medium approximation, to construct the spectra for these aggregateswhich almost perfectly reproduces the more exact computations. This new methodis fast enough to be easily implemented in fitting procedures.

The infrared emission spectra of compositionally inhomogeneous aggregates composed of irregularly shaped constituents