Abstract
Low-Resolution Spectrograph spectra obtained by IRAS have been studied for a sample of about 20 bright M Mira variables with known mass-loss rates. It is found that the spectra of stars with asymmetric light curves show the well-known 9.7 microns and 20 microns silicate emission features while stars with more symmetric light curves show much weaker broad 12 microns and 20 microns emission features, presumably also due to some kind of silicate dust. The spectra of 80 percent of the stars in the sample further show a narrow 8 microns emission feature that is attributed to the v = 1-0 vibration-rotation band of SiO. Most of the stars that show this band are known to be SiO maser stars and the presence of the molecular band in emission supports a collisional pump model for the SiO maser.
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