Modeling composite and fluffy grains. II. Porosity and phase functions

Michael J. Wolff, Space Science Institute
Geoffrey C. Clayton, Louisiana State University
Steven J. Gibson, University of Calgary


The evidence from observation and theory is mounting that cosmic dust grains possess a fluffy and composite nature. The computational simplicity of using effective medium theory (EMT) with series solution cross sections to model such grains has lead some workers to disregard the validity criterion inherent in the various EMT mixing rules. Fortunately, the recent availability of robust electromagnetic scattering finite element codes allows one to explore the applicability of the various mixing rules through direct computation. Building on the methodology and results of a previous paper, we extend our calculations to the angular distribution of scattered light. We establish numerical and physical convergence properties for both solid and porous targets. The discrete dipole approximation (DDA) scattering properties are compared to those computed by the EMT/series solution technique in order to examine further the applicability of several mixing rules. For grains with vacuum inclusions in the Rayleigh limit, we show that the EMT/series solution approach is generally valid even for the particle phase functions, especially in the case of a polydispersion (size distribution) of particles. We also investigate the impact of inclusions that are clearly non-Rayleigh in nature and find that they can be significant and are not reproducible by the mixing rules considered. Possible ramifications of the results are explored through the simple example of radiation pressure in late-type stellar and brown dwarf environments. © 1993. The American Astronomical Society. All rights reserved.