Sources of the 13 micron feature associated with oxygen-rich circumstellar dust

A search of the database of spectra from the IRAS Low-Resolution Spectrometer has revealed 13 mu m emission features in 187 sources, most of which are associated with stars embedded within oxygen-rich circumstellar dust shells. Our sample is very incomplete, but we estimate that 40%-50% of all sources with oxygen-rich dust on the asymptotic giant branch (AGB) exhibit emission features at 13 mu m. The similarities of the Galactic distribution, mean radial velocities, infrared colors, and other properties of the sample of 13 mu m sources compared to the parent population of variables on the AGB suggest that the carrier of the 13 mu m feature is commonly produced in oxygen-rich dust environments. A strikingly large fraction of SRb variables show 13 mu m features, approximately 75%-90%, which suggests that the formation and/or survival of the carrier of the 13 mu m feature may be enhanced by the unusual pulsational properties of these variables.