A new infrared band in interstellar and circumstellar clouds:: C4 or C4H?

We report on the detection with the Infrared Space Observatory of a molecular band at 57.5 mum (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 nu(5) bending mode of C-4 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, nu(7), is poorly known (130-226 cm(-1)). If the carrier is C-4, the derived maximum abundance is nearly similar to that found for C-3 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.