Optical properties of the silicate dust grains in the envelopes around asymptotic giant branch stars
We have investigated the optical properties of the silicate dust grains in the envelopes around oxygen-rich asymptotic giant branch (AGB) stars, paying close attention to the infrared observations of the stars and the laboratory-measured optical data of the candidate dust grain materials. We have compared the radiative transfer model results with the observed spectral energy distributions of the stars including the IRAS PSC and IRAS LRS data. We find that the opacity function at lambda > 13 mu m for the OH/IR stars with thick dust shells is different from the opacity for the stars with thin dust shells. This may be the effect of some changes of optical constants of the dust grains depending on the temperature. From the opacity functions, we derive two sets of optical constants for the warm and cool silicate dust grains. The optical constants satisfy the Kramers-Kronig relation and produce opacity functions that fit the observations of AGB stars better than previous works. From the opacity functions, we have calculated the Planck mean values of the optical efficiency factors.