The Infrared Spectral Classification of Oxygen-rich Dust Shells

This paper presents infrared spectral classifications for a flux-limited sample of 635 optically identified oxygen-rich variables including supergiants and sources on the asymptotic giant branch (AGB). Several classes of spectra from oxygen-rich dust exist, and these can be arranged in a smoothly varying sequence of spectral shapes known as the silicate dust sequence. Classification based on this sequence reveals several dependencies of the dust emission on the properties of the central star. Nearly all S stars show broad emission features from alumina dust, while most of the supergiants exhibit classic features from amorphous silicate dust. Mira variables with symmetric light curves generally show broad alumina emission, while those with more asymmetric light curves show classic silicate emission. These differences may arise from differences in the photospheric C/O ratio.