ARE C60 MOLECULES DETECTABLE IN CIRCUMSTELLAR SHELLS OF R CORONAE BOREALIS STARS?

The hydrogen-poor, helium-rich, and carbon-rich character of the gas around R Coronae Borealis (RCB) stars has been suggested to be a site for formation of C-60 molecules. This suggestion is not supported by observations reported here showing that infrared transitions of C-60 are not seen in a large sample of RCB stars observed with the Infrared Spectrograph on the Spitzer Space Telescope. The infrared C-60 transitions are seen, however, in emission and blended with polycyclic aromatic hydrocarbon (PAH) features in spectra of DY Cen and possibly also of V854 Cen, the two least hydrogen-deficient (hydrogen deficiency of only similar to 10-100) RCB stars. The speculation is offered that C-60 (and the PAHs) in the moderately H-deficient circumstellar envelopes may be formed by the decomposition of hydrogenated amorphous carbon but fullerene formation is inefficient in the highly H-deficient environments of most RCBs.