Optical properties of circumstellar silicate grains

The present study investigates the optical properties of circumstellar silicate dust grains around oxygen-rich giant stars, with the emphasis placed on infrared observations of OH/IR stars. The optical constants are deduced from available astronomical and laboratory data. The deduced opacities at longer wavelengths for OH/IR stars are higher than the ones for M-type Miras possibly because of the change of optical constants depending on the temperature of the dust grains. Absorption and scattering efficiencies are evaluated for various grain size distributions and shapes. The results of detailed radiative transfer model calculations based on the present dust parameters are compared with observational data. The Planck mean values incorporating substantial far-infrared absorption are also calculated. (P.D.)