The Shapes of the Circumstellar 'Silicate' Features,
AIR FORCE GEOPHYSICS LAB HANSCOM AFB MA
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Around oxygen-rich stars we find that the spectra of most long-period variables LPV show an excess infrared emission which is attributed to circumstellar silicate dust grains. These grains produce emission features at about 10 and 18 micrometers due to bending and stretching modes of Si0 respectively. It has been known Forrest, Gillett and Stein 1975 that the spectral energy distribution of the 10 micrometers emission shows variations from star to star. With the availability of many IRAS Low Resolution Spectra LRS in the 9-22 micrometers region of M stars, we can now study the 10-micrometers feature to determine its uniformity or lack thereof. For this analysis we assume that the 8-22 micrometers emission from these stars is produced by a the stellar photosphere, b a continuum emission from the dust grains and c a strongly wavelength dependent dust grain emission term. By representing the first two terms with blackbody energy distributions and subtracting them from the observed spectrum, we are left with a remaining strongly wavelength dependent emission feature which we call the excess silicate or 10 micrometers emission.