Are C60 molecules detectable in circumstellar shells of R Coronae Borealis stars?

TL;DR

This study investigates the presence of C60 molecules in R Coronae Borealis stars' circumstellar shells, finding no evidence in most cases but detecting them in some less hydrogen-deficient stars, suggesting formation depends on hydrogen content.

Contribution

It provides observational evidence that C60 molecules are generally absent in highly hydrogen-deficient RCB stars but can form in less hydrogen-deficient environments, challenging previous formation theories.

Findings

C60 infrared transitions are not observed in most RCB stars.

C60 is detected in some less hydrogen-deficient RCB stars.

Fullerene formation may depend on hydrogen content in circumstellar environments.

Abstract

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 C60 molecules. This suggestion is not supported by observations reported here showing that infrared transitions of C60 are not seen in a large sample of RCB stars observed with the Infrared Spectrograph on the Spitzer Space Telescope. The infrared C60 transitions are seen, however, in emission and blended with PAH-features in spectra of DY Cen and possibly also of V854 Cen, the two least hydrogen-deficient (hydrogen deficiency of only ~10-100) RCB stars. The speculation is offered that C60 (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.