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Abstract
Intermediate resolution ($\Delta\lambda/\lambda\geq 20$) infrared (2.9-13.5 $\mu$m) spectra were obtained for a random, flux-limited sample of sources in the galactic plane ($\vert b\vert < 2\sp\circ$). The spectra fall into four distinct groups: those with strong (NeII) emission, with weak 10-11 $\mu$m features, with broad long-wavelength emission, and with deep silicate absorption. The IRAS (12) - (25) and (25) - (60) colors of the sources show that objects of each spectral group lie near each other in distinct regions of the color-color plane. A large fraction of these sources (40%) show silicate absorption which are among the deepest measured. The sample is shown to be made up of HII regions and embedded B stars, and evolved stars with circumstellar shells which range from optically thin to extremely thick. Three new carbon stars are identified, and one star with what appears to be silicate absorption superposed on silicon carbide emission is presented. Two stars show diffuse shells and emission features centered near 10.5 $\mu$m which are more broad than simple silicate or silicon carbide emission features. Distances and luminosities are derived for the objects, and simple evolutionary scenarios are presented. The spectra are shown to be in agreement with models of late stellar evolution, although some extensions to redder colors and higher mass-loss rates are required.
