Light Scattering by Fractal Dust Aggregates. II. Opacity and Asymmetry Parameter

TL;DR

This study evaluates various approximation methods for the optical properties of fractal dust aggregates, introduces a modified mean field theory (MMF), and highlights its effectiveness in modeling astrophysical dust opacity and scattering.

Contribution

We develop and validate a modified mean field theory (MMF) that accurately models the optical properties of fractal dust aggregates, improving upon existing methods.

Findings

RGD theory fails with multiple scattering

EMT and DHS are inaccurate for fractal aggregates at short wavelengths

MMF accurately reproduces extinction opacity of dust aggregates

Abstract

Optical properties of dust aggregates are important at various astrophysical environments. To find a reliable approximation method for optical properties of dust aggregates, we calculate the opacity and the asymmetry parameter of dust aggregates by using a rigorous numerical method, the T-Matrix Method (TMM), and then the results are compared to those obtained by approximate methods; the Rayleigh-Gans-Debye (RGD) theory, the effective medium theory (EMT), and the distribution of hollow spheres method (DHS). First of all, we confirm that the RGD theory breaks down when multiple scattering is important. In addition, we find that both EMT and DHS fail to reproduce the optical properties of dust aggregates with fractal dimension of 2 when the incident wavelength is shorter than the aggregate radius. In order to solve these problems, we test the mean field theory (MFT), where multiple scattering can be taken into account. We show that the extinction opacity of dust aggregates can be well reproduced by MFT. However, it is also shown that MFT is not able to reproduce the scattering and absorption opacities when multiple scattering is important. We successfully resolve this weak point of MFT, by newly developing a modified mean field theory (MMF). Hence, we conclude that MMF can be a useful tool to investigate radiative transfer properties of various astrophysical environments. We also point out an enhancement of the absorption opacity of dust aggregates in the Rayleigh domain, which would be important to explain the large millimeter-wave opacity inferred from observations of protoplanetary disks.