A New Infrared Band in Interstellar and Circumstellar Clouds: C₄ or C₄H?

Abstract

We report on the detection with the Infrared Space Observatory of a molecular band at 57.5 μm (174 cm^-1) in carbon-rich evolved stars and in Sgr B2. Taking into account the chemistry of these objects, the most likelihood carrier is a carbon chain. We tentatively assign the band to the ν₅ bending mode of C₄ for which a wavenumber of 170-172.4 cm^-1 has been derived in matrix experiments by Withey et al. An alternate carrier might be C₄H, although the frequency of its lowest energy vibrational bending mode, ν₇, is poorly known (130-226 cm^-1). If the carrier is C₄, the derived maximum abundance is nearly similar to that found for C₃ in the interstellar and circumstellar media by Cernicharo et al. Hence, tetra-atomic carbon could be one of the most abundant carbon chain molecules in these media.