FULLERENES AND PROTO-FULLERENES IN INTERSTELLAR CARBON DUST

Laboratory spectra of hydrogenated amorphous carbon (HAC) particles prepared under a variety of conditions show spectral features at 7.05, 8.5, 17.4, and 18.9 mu m (1418, 1176, 575, and 529 cm(-1)) that have been associated with emission from C-60 molecules. These lines occur in the spectra even though C-60 molecules as such are not present in our samples. It appears that these four spectral lines in HAC can instead be associated with precursor molecules or "proto-fullerenes" (PFs) that subsequently react to yield C-60. We develop a model tracing the evolution and de-hydrogenation of HAC dust and show that the observation of an emission feature at 16.4 mu m (610 cm (1)) in astronomical spectra signals the presence of the pentagonal carbon rings required for the formation of fullerenes. We suggest that the set of four IR emission lines previously identified with C-60 in many objects that also show the 16.4 mu m feature and other polycyclic aromatic hydrocarbon bands arise from PFs rather than C-60. Tc1 is an example of a source in which de-hydrogenation has proceeded to the point where only fullerenes are present.