INFRARED-SPECTRA AND CIRCUMSTELLAR EMISSION OF IRAS SOURCES WITH 10-MICRON SILICATE ABSORPTION

Near-infrared photometry was carried out in the J, H, K, and L bands for 33 IRAS sources with a 10-mum silicate absorption feature. These observed sources mostly comprised two groups of stars which showed very different spectral characteristics from each other between 1 and 100 mum. One comprised OH/IR stars, which generally showed infrared photometric spectra roughly similar to blackbody spectra; the other comprised young stellar objects, showing much broader spectral energy distributions than black-body spectra. Correlations between the 10-mum spectral feature and the spectral energy distribution were separately examined extensively for OH/IR stars and young stellar objects. On one hand, based on a statistical analysis of the OH/IR stars, we found that OH/IR stars generally show the 10-mum absorption feature in the 1.5 mag less than or similar to [12] - [60] less than or similar to 4.5 mag range, where the absorption depth is correlated with [12] - [60]. On the other hand, the young stellar objects show a different behavior from that of the OH/IR stars regarding the 10-mum absorption strength. A few OH/IR stars with [12] - [60] > 4.5 mag show broad spectral energy distributions which are very different from usual OH/IR stars. The spectral difference in some OH/IR stars is likely to be due to a difference in the structure of the circumstellar dust envelope related to the evolution from asymptotic giant branch stars toward planetary nebulae.