Optical properties of dust aggregates : I. Wavelength dependence
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概要
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Optical properties of dust aggregates of astrophysical interest have been investigated by means of the discrete dipole approximation (DDA) to clarify how differences in structure, size and chemical composition of dust aggregates affect their optical properties. Two types of dust aggregates have been treated; one is the Ballistic Particle-Cluster Aggregates (BPCA) whose fractal dimension D ~ 3, and the other is the Ballistic Cluster-Cluster Aggregates (BCCA) for which D ~ 2. Two kinds of minerals have been considered for the constituent particles; silicate and magnetite. The results of the calculations show that the optical properties of dust aggregates are divided into two regimes according to their characteristic size a_c; in the wavelength range λ ≫ λ_c = 2πa_c, the optical properties are qualitatively reproduced by the Rayleigh approximation for a sphere with the same mass and do not depend on the structure of the dust aggregates. The scattering cross sections start to deviate from the Rayleigh regime at about the characteristic wavelength λ_c = 2πa_c, which is a diagnostic for the sizes of dust aggregates, irrespective of the chemical composition of the constituent particles. The wavelength dependence of the absorption cross sections can be interpreted in terms of the optical thickness of the dust aggregates themselves and can be approximated with a simple formula. This formula is useful to determine the dependence of the temperature of dust aggregates in space on their sizes as well as structures, which plays a crucial role in considering the physical and chemical evolution of dust aggregates in space. The wavelength dependence of the asymmetry parameter reflects the size as well as the structure of dust aggregates, independent of the chemical composition of the constituent particles. BPCA are characterized by an enhanced forward scattering in the region of λ ≪ λ_c in comparison with BCCA with the same mass, in spite of the smaller size of BPCA. The applicability of the Maxwell-Garnett mixing rule to dust aggregates is investigated in comparison with the results of the calculations using the discrete dipole approximation.
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