Abstract
The comets, as well as the disks around young stars, show the existence of dust particles composed of silicates. It is known that the thermal emission of silicate has significant features in the infrared wavelengths. We will examine the shape effect of silicate grain on such emission features. That is, a fractal aggregate (BPCA: Ballistic Particle-Cluster Aggregate) is applied for the grain model with irregularly shape. The absorption efficiencies of grain particle are computed by Mie theory for a sphere and by DDA code for the fractal aggregate with the equivalent volume to the sphere. It is found that the emission feature decreases with increasing the size of sphere. On the other hand, even when no significant emission feature at a wavelength of 10 micron appears for a large sphere, the silicate features still exist for a large fractal ag-gregate. The temperature of fractal aggregate is also calculated by using the resulting absorption efficiencies. Consequently, the thermal emission features of dust model are compared with the spectral data observed in comet C/1995 O1 (Hale-Bopp) at a solar distance of 2.9AU. The composition, shape, and mass of dust grains are discussed based on the result of a comparison of the observed spectral data with the model calculations.
