A 7 MICRON DUST EMISSION FEATURE IN OXYGEN-RICH CIRCUMSTELLAR SHELLS

New 4 to 8 mum infrared spectroscopic observations of two oxygen-rich stars are presented and combined with IRAS LRS data to span the 4 to 24 mum wavelength range. In the 4 to 8 mum wavelength range, we observe a 7.15 mum (1400 cm-1) emission feature. The new feature at 7.15 mum is not uniquely correlated with any of the sharply defined 10, 11, 13.1, and 19.7 mum emission features that are known to be present in this class of circumstellar shells, but it does appear to be correlated with the spectrally broad dust emission in the 10 to 20 mum spectral region. The feature has not been reported previously in any other astronomical environment. A reinterpretation of prior 4 to 8 mum spectroscopy of alpha Ori and R Cas reveals the presence of the 7.15 mum emission in alpha Ori and possibly in R Cas. The spectrally narrow 19.7 mum emission, that is distinctly different than the relatively broad silicate 18 mum emission feature in oxygen-rich dust shells, is also observed to be present in the LRS spectrum of SAO 197549. The implication of these observations is that a universal astronomical silicate does not exist in oxygen-rich circumstellar shells. This variety is analogous to that observed in interplanetary dust particles and may indicate an intimate relation between the two classes of condensates.