Detection of linear C3H2 in absorption toward continuum sources

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 1011 cm−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.