1994A&AS..107..445H

Using IRAS observational data, the best-fitting models are searched for oxygen-rich AGB stars in various evolutionary stages from optically visible red giant stars to OH/IR stars with heavy mass loss. The model infrared spectral energy distributions of the red giant stars with circumstellar dust envelopes are calculated with the use of a radiative transfer code. It is found that the oxygen-rich IRAS AGB stars surrounded by circumstellar dust envelopes are divided into three groups, which can be clearly distinguished by their mass loss rate and the grain temperature at the inner boundary of the circumstellar dust envelopes. One group corresponds to Mira type objects with a moderate mass loss rate and a grain formation temperature of about 500 K, the second to OH/IR star type objects with a large mass loss rate and a high grain formation temperature, and the third to M stars with a very cold grain temperature at the inner boundary, indicating that the moderate mass loss stopped a considerable time ago in these objects. The higher grain formation temperature of the OH/IR star type objects indicates a grain formation point closer to the stellar surface, suggesting an effective acceleration of dust grains by radiation pressure. Such an effective acceleration can play an important role in the mass loss phenomena of OH/IR stars with large mass loss rates. From the model fitting results, it is suggested that an AGB star undergoes several dozen phases of moderate mass loss of {dot}(M)∼10^–7-10^–6M_☉/yr for a period of 10³-10⁴years with interruptions of 10³-10⁴years during the ordinary Mira variable phase before becoming an OH/IR star with a mass loss rate exceeding 10^–5M_☉/yr and a lifetime of about 10³-10⁴years.