The infrared spectral classification of oxygen-rich dust shells

This paper presents infrared spectral classifications for a flux-limited sa mple of 635 optically identified oxygen-rich variables including supergiant s and sources on the asymptotic giant branch (AGB). Several classes of spec tra from oxygen-rich dust exist, and these can be arranged in a smoothly va rying sequence of spectral shapes known as the silicate dust sequence. Clas sification based on this sequence reveals several dependencies of the dust emission on the properties of the central star. Nearly all S stars show bro ad emission features from alumina dust, while most of the supergiants exhib it classic features from amorphous silicate dust. Mira variables with symme tric light curves generally show broad alumina emission, while those with m ore asymmetric light curves show classic silicate emission. These differenc es may arise from differences in the photospheric C/O ratio.