A model for the optical properties of porous grains

A numerical method, based on an integral representation of Maxwell's equations, is used to calculate scattering properties of model porous aggregates of small particles. It is shown that increasing porosity can lead to enhanced absorption and emission of radiation and a lower albedo. One application is to show how strongly the 9.7-micron emission feature of a piece of astronomical silicate matter depends on its porosity. The integral method is compared with the well-known coupled dipole approach for the case of a solid sphere. The validity of the Maxwell-Garnett effective medium theory is shown for certain cases where the individual particles in the aggregate satisfy the Rayleigh size conditions. 27 refs