Porous interstellar grains

Recent observations of stellar composition suggest that elements in the Sun are significantly more abundant than in other stars. The reduction in the available element budget implies a drastic revision in current models of in terstellar dust, Theoretical models are therefore exploring fluffy, porous physical structure for the grain material. Since a detailed exact treatment of extinction cross-sections is mandatory for a correct understanding of t he nature of interstellar dust, we present a technique based on the multipo le expansions of the electromagnetic field, which has proven to be general, flexible and powerful in treating scattering of light by porous, composite , arbitrarily shaped particles. The results of this study speak in favour o f core-mantle structures characterized by the presence of porosities.