Are Fulleranes Responsible for the 21 Micron Feature?

Recent detections of C _60 , C _70 , and C _60 ^+ in space induced extensive studies of fullerene derivatives in circumstellar environments. As the promising fullerene sources, protoplanetary nebulae (PPNe) shows a number of unidentified bands in their infrared spectra, among which a small sample exhibits an enigmatic feature at \sim 21 \mathttμ m. Hydrogenation converts fullerenes into fulleranes, which breaks the symmetry of fullerene molecules and produces new infrared bands. In this work, we investigate the possibility of fulleranes (C _60 H _m ) as the carrier of the 21 \mathttμ m feature in terms of theoretical vibrational spectra of fulleranes. The evidences favoring and disfavoring the fullerane hypothesis are presented. We made an initial guess for the hydrogen coverage of C _60 H _m that may contribute to the 21 \mathttμ m feature.