The tight correlation of CCH and c-C3H2in diffuse and translucent clouds

Abstract

Using the IRAM 30 m 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 SgrA ∗ . 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, (JKa,Kc = 21,2−10,1) for c-C3H2 and (J = 1−0,F = 5/2−3/2) for CCH, is nearly constant along all lines of sight, at τCCH ~ 1.8 × τc - C3H2. 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 2000, A&A, 358, 1069). PDR models are able to reproduce the observed CCH column densities for the range of physical conditions appropriate for the absorbing matter (n = 100−3000 cm-3; AV = 1−5 mag) but can neither fit the observed c-C3H2 column densities nor the tight correlation between CCH and c-C3H2.