THE CARBON-RICH DUST SEQUENCE - INFRARED SPECTRAL CLASSIFICATION OF CARBON STARS

We have developed a classification system for the infrared spectral em ission from carbon stars, using a sample of 96 bright carbon-rich vari ables associated with the asymptotic giant branch. In addition to the stellar contribution, most spectra include the 11.2 mu m emission feat ure from SiC and either a smooth, cool continuum from amorphous carbon or a secondary emission feature at 9.0 mu m. We have identified a car bon-rich dust sequence along which the amorphous carbon component grow s while the 9.0 mu m feature declines in strength. Along this spectral sequence, the proportion of Mira variables increases, as does the per iod of variability, the mass-loss rate, and the thickness of the circu mstellar shell. Thus the carbon-rich dust sequence appears to be an ev olutionary sequence. One class of spectra shows a particularly strong 9.0 mu m feature, enhanced C/O ratio, and several other unusual proper ties that suggest a different sequence, perhaps related to J stars.