The Carbon-rich Dust Sequence: Infrared Spectral Classification of Carbon Stars

We have developed a classi-cation system for the infrared spectral emission from carbon stars, using a sample of 96 bright carbon-rich variables associated with the asymptotic giant branch. In addition to the stellar contribution, most spectra include the 11.2 km emission feature from SiC and either a smooth, cool continuum from amorphous carbon or a secondary emission feature at 9.0 km. We have identi-ed a carbon-rich dust sequence along which the amorphous carbon component grows while the 9.0 km feature declines in strength. Along this spectral sequence, the proportion of Mira variables increases, as does the period of variability, the mass-loss rate, and the thickness of the circumstellar shell. Thus the carbon-rich dust sequence appears to be an evolutionary sequence. One class of spectra shows a particu- larly strong 9.0 km feature, enhanced C/O ratio, and several other unusual properties that suggest a di†erent sequence, perhaps related to J stars.