MINERALOGY OF OXYGEN-RICH DUST SHELLS

Spectra taken with the Short Wavelength Spectrometer on board of the I nfrared Space Observatory of dust shells around evolved oxygen-rich st ars reveal the presence of several emission features at wavelengths be tween 20 and 45 mu m. These features have a range of widths and streng ths, but are all narrow compared to the well-known amorphous silicate bands at 9.7 and 18 mu m. The emission peaks are tentatively identifie d with crystalline forms of silicates such as pyroxenes and olivine. T he emission features tend to be more prominent for objects with cooler dust shells (T < 300 K). This may be due to an intrinsic change in op tical properties of the dust as it cools, or it may be due to an incre ase in the fraction of crystalline silicates compared to amorphous for ms as the mass loss rate increases. The implications for the physics o f dust formation in the outflows of cool giants are briefly discussed.