2011A&A...525A.116G

Using the IRAM 30m telescope we observed molecular absorption lines from CCH and c-C₃H₂ produced by diffuse and translucent clouds along the lines of sight towards massive star forming regions. The same sources are surveyed with Herschel/HIFI as part of the PRISMAS guaranteed time program, for molecular absorption lines due to hydrides and carbon clusters. The background sources are massive star-forming regions (G34.3+0.1, G10.62-0.39, W51, W49N) and Sgr A^*. The line profiles of the CCH and c-C₃H₂ are strikingly similar for all lines of sight, showing that the ratio of the opacities of the probed transitions, (J_Ka_,Kc=2_1,2-1_0,1) for c-C₃H₂ and (J=1-0, F=5/2-3/2) for CCH, is nearly constant along all lines of sight, at τ_CCH∼1.8xτ_c–C3_H2. As a consequence, the ratio of the column densities of CCH and c-C₃H₂ is nearly constant and similar to the value derived earlier for diffuse clouds detected along lines of sight towards extragalactic continuum sources, N(CCH)=(28±1.4) N(c-C₃H₂) (Lucas & Liszt, 2000A&A...358.1069L). PDR models are able to reproduce the observed CCH column densities for the range of physical conditions appropriate for the absorbing matter (n=100-3000cm^–3; A_V=1-5mag) but can neither fit the observed c-C₃H₂ column densities nor the tight correlation between CCH and c-C₃H₂.