Calculating cross sections of composite interstellar grains

Interstellar grains may be composite collections of particles of distinct materials, including voids agglomerated together. We determine the various optical cross sections of such composite grains, given the optical properties of each constituent, using an approximate model of the composite grain. We assume it consists of many concentric spherical layers of the various materials, each with a specified volume fraction. In such a case the usual Mie theory can be generalized and the extinction, scattering, and other cross sections determined exactly. We find that the ordering of the materials in the layering makes some difference to the derived cross sections, but averaging over the various permutations of the order of the materials provides rapid convergence as the number of shells (each of which is filled by all of the materials proportionately to their volume fractions) is increased. Three shells, each with one layer of a particular constituent material, give a very satisfactory estimate of the average cross section produced by larger numbers of shells. We give the formulae for the Rayleigh limit (small size parameter) for multilayered spheres and use it to propose an "effective medium theory" (EMT), in which an average optical constant is taken to represent the ensemble of materials. Multilayered models are used to compare the accuracies of several EMTs already in the literature. EMTs are worse for predicting scattering cross sections than extinction, and considerably worse for predicting g, the mean cosine of the angle of scattering. However, the angular distribution of the scattered radiation depends sensitively on the assumed grain geometry and should be taken with caution for any grain theory. Our computation is vastly simpler than discrete multipole calculations and may be easily applied for practical modeling of the extinction and scattering properties of interstellar grains.