  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.
