MODELING COMPOSITE AND FLUFFY GRAINS - THE EFFECTS OF POROSITY

Recent studies of interplanetary and interstellar dust provide evidenc e that cosmic dust grains are fluffy, composite objects, highlighting the need for models of the electromagnetic scattering by these grains. Effective medium theory (EMT) with Mie-type series solutions has been used to explore the effects of porosity which would be important in c omposite dust particles. While this indirect approach is both flexible and computationally efficient, it is not necessarily a good approxima tion. The need for EMT and its rather restrictive assumptions may be c ircumvented through a direct computation of the scattering properties via finite element methods, such as the discrete dipole approximation (DDA). Recently, the utility of the DDA method has been advanced signi ficantly through improvements in theory, in numerical algorithms, and in computer hardware. Extensive calculations with the DDA method are u sed here to examine more directly the effects of porosity. A particula r emphasis is placed upon developing a valid methodology. For both sol id and porous targets we establish both numerical and physical converg ence properties over the range of size parameter that is required for our study. DDA cross sections for grains with a range of porosity are compared to those computed by the EMT/series expansion technique to ex amine the applicability of several mixing rules, including two extensi ons of the Bruggeman rule. We show that for particles with Rayleigh va cuum inclusions, the extension proposed by Rouleau & Martin is quite s uccessful. We also investigate the effects of larger, non-Rayleigh vac uum inclusions for various levels of porosity and find that they can b e significant.