Search for Hydrogenated C${}_{60}$ (fulleranes) in Circumstellar Envelopes

Abstract

The recent detection of fullerene (C${}_{60}$) in space and the positive assignment of five diffuse interstellar bands to C${}_{60}^{+}$ reinforce the notion that fullerene-related compounds can be efficiently formed in circumstellar envelopes and be present in significant quantities in the interstellar medium. Experimental studies have shown that C${}_{60}$ can be readily hydrogenated, raising the possibility that hydrogenated fullerenes (or fulleranes, C${}_{60}$H${}_m$, $m=1-60$) may be abundant in space. In this paper, we present theoretical studies of the vibrational modes of isomers of C${}_{60}$H${}_m$. Our results show that the four mid-infrared bands from the C${}_{60}$ skeletal vibrations remain prominent in slightly hydrogenated C${}_{60}$, but their strengths diminish in different degrees with increasing hydrogenation. It is therefore possible that the observed infrared bands assigned to C${}_{60}$ could be due to a mixture of fullerenes and fulleranes. This provides a potential explanation for the observed scatter of the C${}_{60}$ band ratios. Our calculations suggest that a feature around 15$\mu$m due to the breathing mode of heavily hydrogenated C${}_{60}$ may be detectable astronomically. A preliminary search for this feature in 35 C${}_{60}$ sources is reported.

41 pages, 17 figures, accepted for publication in ApJ