Physical and Chemical Evolution of Dust Aggregate and their Physical Properties in the Primordial Solar Nebula

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The following subject had been investigated during the period supported by this grant.(1) Scattering properties and their dependence on the strusture and the chemical composition of dust aggregates : The discrete dipole approximation was applied to investigate the dependence of the scattering properties on the structure and the chemical composition of dust aggregates. The results show that the behavior of forward scattered light clearly reflects the overall size and the mean chemical composition of dust aggregates, otherwise the backward scattering is sensitive to the roughness of surface layr of dust aggregates.(2) Formation process of dust aggregates in the primordial solar nebula : A method was discussed to treat the formation process and the resulting structure of dust aggregates self-consistently in the primordial solar nebula, but the detailed calculation was not performed.(3) The crystallinity of silicate component in comet dust : On the basis of the Greenberg's interstellar dust model, the origin of crystalline silicate component in cometary dust was investigated ; in comparison with the observation of comet P/Halley the crystalline component was estimated to be the order of 10 to 20% in volume. The estimated value can be realized by the mechanism for the crystallization of silicate component developed in the framework of the evolution from interstellar dust to cometary dust.(4) Formation of dust grains in astrophysical environments ; The formation of dust grains was investigated in circumstellar envelopes of evolved stars. The formation process of core-mantle grains consisting of a SiC core and a carbon mantle was investigated to reproduce the 11.3mum feature observed towards carbon stars. In oxygen-rich AGB stars the observed 13mum feature can be attributed to dust grains consisting of a Al_2O_3 core and a silicate mantle. The possibility of the formation of dust grains in Ia supernovae was discussed.