The 11 Micron Emissions of Carbon Stars

A new classification scheme of the IRAS LRS carbon stars is presented. It comprises the separation of 718probable carbon stars into 12 distinct self-similar spectral groupings. Continuum temperatures are assignedand range from 470 to 5000 K. Three distinct dust species are identifiable: SiC, or:C--H, and MgS. In addi-tion to the narrow 1! + /am emission feature that is commonly attributed to SiC, a broad 11+ #m emissionfeature, that is correlated with the 8.5 and 7.7 /am features, is found and attributed to ct:C--H. SiC and_:C--H band strengths are found to correlate with the temperature progression among the Classes. We find aspectral sequence of Classes that reflects the carbon star evolutionary sequence of spectral types, or alterna-tively developmental sequences of grain condensation in carbon-rich circumstellar shells. If decreasing tem-perature corresponds to increasing evolution, then decreasing temperature corresponds to increasing C/Oresulting in increasing amounts of carbon rich dust, namely ct:C--H. If decreasing the temperature corre-sponds to a grain condensation sequence, then heterogeneous, or induced nucleation scenarios are supported.SiC grains precede _t:C--H and form the nuclei for the condensation of the latter material. At still lowertemperatures, MgS appears to be quite prevalent. No 11.3 /am PAH features are identified in any of the 718carbon stars. However, one of the coldest objects, IRAS 15048-5702, and a few others, displays an 11.9 #memission feature characteristic of laboratory samples of coronene. That feature corresponds to the C--H outof plane deformation mode of aromatic hydrocarbon. This band indicates the presence of unsaturated, sp 3,hydrocarbon bonds that may subsequently evolve into saturated bonds, sp 2, if, and when, the star enters theplanetary nebulae phase of stellar evolution. The effusion of hydrogen from the hydrocarbon grain results inthe evolution in wavelength of this 11.9/am emission feature to the 11.3/am feature.Subject headin#s: circumstellar matter -- infrared: stars -- molecular processes -- stars: carbon --stars: fundamental parameters