Volume 525, January 2011
|Number of page(s)||9|
|Section||Interstellar and circumstellar matter|
|Published online||06 December 2010|
The tight correlation of CCH and c-C3H2 in diffuse and translucent clouds ⋆
LERMA, UMR 8112 du CNRS, Observatoire de Paris, École Normale Supérieure, UPMC
e-mail: firstname.lastname@example.org; email@example.com; firstname.lastname@example.org; email@example.com
2 Centre for Astronomy, Nicolaus Copernicus University, Gagarina 11, 87-100 Toruń, Poland
3 Astronomical Observatory of the Jagiellonian University, Orla 171, Kraków, Poland
4 Laboratoire d’Astrophysique de Grenoble, Université Joseph Fourier and CNRS UMR 5571, Observatoire de Grenoble, France
Accepted: 27 September 2010
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.
Key words: ISM: clouds / ISM: molecules
© ESO, 2010
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