Light scattering by fractal dust aggregates: I. Angular dependence of scattering

TL;DR

This paper investigates how porous dust aggregates in protoplanetary disks scatter light, comparing rigorous and approximate methods to understand the influence of hierarchical structure on scattering properties.

Contribution

It demonstrates the applicability of the RGD theory for modeling light scattering by porous dust aggregates, highlighting its advantages over EMT in certain cases.

Findings

Hierarchical structure affects scattering intensity at different angles.

EMT significantly underestimates backward scattering in BCCAs.

RGD theory effectively models scattering for certain aggregate types.

Abstract

In protoplanetary disks, micron-sized dust grains coagulate to form highly porous dust aggregates. Because the optical properties of these aggregates are not completely understood, it is important to investigate how porous dust aggregates scatter light. In this study, the light scattering properties of porous dust aggregates were calculated using a rigorous method, the T-matrix method, and the results were then compared with those obtained using the Rayleigh-Gans-Debye (RGD) theory and Mie theory with the effective medium approximation (EMT). The RGD theory is applicable to moderately large aggregates made of nearly transparent monomers. This study considered two types of porous dust aggregates, ballistic cluster-cluster agglomerates (BCCAs) and ballistic particle-cluster agglomerates (BPCAs). First, the angular dependence of the scattered intensity was shown to reflect the hierarchical structure of dust aggregates; the large-scale structure of the aggregates is responsible for the intensity at small scattering angles, and their small-scale structure determines the intensity at large scattering angles. Second, it was determined that the EMT underestimates the backward scattering intensity by multiple orders of magnitude, especially in BCCAs, because the EMT averages the structure within the size of the aggregates. It was concluded that the RGD theory is a very useful method for calculating the optical properties of BCCAs.