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

TL;DR

This paper evaluates different continuous ellipsoidal shape distributions for interstellar grains, analyzing their suitability for modeling extinction and polarization, and proposes a method to synthesize dielectric functions based on observed absorption data.

Contribution

It compares three shape distributions, assesses their realism, and introduces a method to derive dielectric functions from observational data for better grain shape modeling.

Findings

The CDE distribution includes many extreme shapes, possibly unrealistic.

The CDE2 distribution appears more realistic for interstellar grains.

A method is proposed to synthesize dielectric functions from observational absorption data.

Abstract

Although interstellar grains are known to be aspherical, their actual shapes remain poorly constrained. We assess whether three continuous distributions of ellipsoidal shapes from the literature are suitable for describing the shapes of interstellar grains. Randomly-selected shapes from each distribution are shown as illustrations. The often-used Bohren-Huffman CDE 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 $a_3/a_1 > 33$. The CDE2 distribution includes a much smaller fraction of extreme shapes, and appears to be more realistic. For each of the three CDEs considered, we derive shape-averaged cross sections for extinction and polarization in the Rayleigh limit. 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 in the Rayleigh limit 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.