SOURCES OF THE 13-MICRON FEATURE ASSOCIATED WITH OXYGEN-RICH CIRCUMSTELLAR DUST

A search of the database of spectra from the IRAS Low-Resolution Spect rometer has revealed 13 mu m emission features in 187 sources, most of which are associated with stars embedded within oxygen-rich circumste llar dust shells. Our sample is very incomplete, but we estimate that 40%-50% of all sources with oxygen-rich dust on the asymptotic giant b ranch (AGB) exhibit emission features at 13 mu m. The similarities of the Galactic distribution, mean radial velocities, infrared colors, an d other properties of the sample of 13 mu m sources compared to the pa rent population of variables on the AGB suggest that the carrier of th e 13 mu m feature is commonly produced in oxygen-rich dust environment s. A strikingly large fraction of SRb variables show 13 mu m features, approximately 75%-90%, which suggests that the formation and/or survi val of the carrier of the 13 mu m feature may be enhanced by the unusu al pulsational properties of these variables.