On the Shapes of Interstellar Grains: Modeling Extinction and Polarization by Speroids and Continuous Distributions of Ellipsoids

B. T. Draine, Brandon S. Hensley

Published 2017-10-24Version 1

Although interstellar grains are known to be aspherical, their actual shapes remain poorly constrained. We assess whether three distributions of ellipsoidal shapes from the literature, employed principally for their convenient mathematical properties, are suitable for describing the shapes of interstellar grains. Randomly-selected shapes from each distribution are shown as illustrations. The often-used BHCDE distribution includes a very large fraction of extreme shapes: fully 10% of random draws have axial ratio a_3/a_1 > 19.7, and 5% have a_3/a_1 > 33. Of the three distributions considered, the CDE2 appears to be most realistic. For each of the three CDEs considered, we derive shape-averaged cross sections for extinction and polarization. Finally, we describe a method for "synthesizing" a dielectric function for an assumed shape or shape distribution if the actual absorption cross sections per grain volume are known from observations. This synthetic dielectric function predicts the wavelength dependence of polarization, which can then be compared to observations to constrain the grain shape.