1999A&A...351..341C

We report the detection of the 1₀₁-0₀₀ line of linear C₃H₂, an isomer of the ubiquitous cyclic C₃H₂, toward the sources W51E1/E2, W51D, and W49. The line is seen in absorption against the radio continuum of these sources. The column density of l-C₃H₂ is very low, with typical values of a few 10¹¹cm^–2. The abundance ratio between the cyclic and linear isomers of C₃H₂ is found to be 3-5 in the sources where l-C₃H₂ has been detected. Toward Sgr B2 and 3C 123, we derive for the abundance ratio 3σ lower limits of 150 and 1, respectively. The abundance ratio between cyclic and linear C₃H₂ in the diffuse interstellar medium appears to be an order of magnitude lower than the value obtained in TMC1 by Cernicharo et al. (1991a). This difference is probably a consequence of the distinct mechanisms leading to the destruction of the cyclic and linear isomers and/or their progenitor ions. Upper limits to the column density of C₆H, the J=15/2-13/2 lines of which are within the observed frequency range, have also been obtained in the same sources.