Are fulleranes responsible for the 21 micron feature?

Abstract

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) show a number of unidentified bands in their infrared spectra, among which a small sample exhibits an enigmatic feature at $\sim21$\,$\mu$m. Hydrogenation of fullerenes can produce fulleranes emitting new infrared bands. In this paper, we investigate the possibility of fulleranes (C$_{60}$H$_m$) as the carrier of the 21\,$\mu$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\,$\mu$m feature.