SILICATE EMISSION AT 10 MICRONS IN VARIABLES ON THE ASYMPTOTIC GIANT BRANCH

We show that the observed shapes of the silicate emission profile at 10 mu m are described by a sequence from the classic, narrow silicate emission feature peaking at 10 mu m to a broad, low-contrast feature which peaks longward of 11 mu m. We use this silicate dust sequence as the basis for a new system to classify the silicate emission feature, apply it to spectra taken by the IRAS Low-Resolution Spectrometer (LRS) for a flux-limited sample of variable stars on the asymptotic giant branch (AGE), and compare our system to previous classification systems of the LRS spectra. Our results suggest that either AGE stars produce multiple shells during their evolution or that spectra from oxygen-rich dust shells do not evolve from broad to narrow emission.