Infrared properties of circumstellar dust envelopes of oxygen-rich asymptotic giant branch stars

Models of oxygen-rich circumstellar dust envelopes around red giant stars are calculated for a range of mass-loss rate up to 7 x 10-4 solar mass/yr. A new generalized two-stream Eddington approximation was employed in the radiative transfer calculation for spherical circumstellar envelopes with nongray dust grains. Calculating infrared spectral energy distributions of the models with various sets of input parameters, observational infrared characteristics such as IRAS properties are presented as a function of physical conditions of the circumstellar dust envelope. Through a direct comparison of infrared observational data with the computation results, we can determine the physical status of the circumstellar dust envelope of each individual oxygen-rich (AGB) star. Observational data at wavelengths shorter than about 20 micrometers can well determine two physical parameters, the stellar mass-loss rate and the grain temperature at the inner boundary of the circumstellar dust envelope. Observational data at wavelengths longer than 60 micromaters can give an indication of the mass-loss history in a timescale longer than several hundred years. It is possible to investigate AGB evolution with mass loss through model analyses of the infrared data of a number of AGB stars in various evolutionary stages.