http://arxiv.org/abs/1803.03775
Optical properties of dust aggregates become 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 becomes 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 once multiple scattering becomes important, MFT is not able to reproduce the scattering and absorption opacities. 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 discuss 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.
R. Tazaki and H. Tanaka
Tue, 13 Mar 2018
49/61
Comments: 22 pages, 11 figures; submitted to ApJ
You must be logged in to post a comment.