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 μm (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 μm (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 μm 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.