Emission features in IRAS LRS spectra of M Mira variables

A total of 291 M Mira variables with IRAS low-resolution spectrometer (LRS) spectra that have 8-micron fluxes greater than 15 Jy have been analyzed. After subtraction of a 2500 K blackbody energy distribution from the spectrum, the remaining difference spectra can be classified into seven groups, depending on the shape of their spectral emission features around 10 microns: Sil, Sil(+) Sil(2+), S, three-component, 'broad', and no feature. The spectral emission features are interpreted as being produced primarily by amorphous silicates with differing amounts of crystalline olivine. A weak broad feature at 9-13 microns is possibly produced by aluminum oxide and/or a size distribution of silicate particles that favors larger grains. The 8-22-micron IR excess, defined as the fraction of energy above the 2500 K continuum, shows only a slight tendency to correlate with period and no tendency to correlate with mass loss rate. Maser emission from OH and H2O is much more frequently detected in stars that show the Sil or Sil(+) emission feature, but SiO maser emission is detected with the same percentage in all emission feature classes with the exception of the S feature. Stars showing the S feature appear to be closely related to the MS and S stars.