Discovery of interstellar acetylene

Absorption by interstellar acetylene (C2H2) has been detected in the spectra of three IR sources embedded in molecular clouds. One or more lines in the 13.7 micron nu5 vibration-rotation band were detected toward GL 2591, W3 IRS 5, and OMC-1 IRc2. The observations require C2H2 abundances of about 0.0003 to 0.003 of CO. Such abundances can be explained by models in which the molecular clouds are chemically unevolved or in which gas phase abundances have been enhanced by recent evaporation of grain mantles. 17 refs