Widespread galactic CF+ absorption: detection toward W49 with the Plateau de Bure Interferometer
Rational Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA, 22903 USA
2 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
3 Observatoire de Paris (CNRS UMR 8112), 61 av. de l’Observatoire, 75014 Paris, France
4 LERMA/LRA, École Normale Supérieure, 24 rue Lhomond, 75005 Paris, France
5 Johns Hopkins University, Baltimore, MD 21218, USA
6 Univ. Bordeaux, LAB, UMR 5804, 33270 Floriac, France
7 CNRS, LAB, UMR 5804, 33270 Floriac, France
Received: 1 April 2015
Accepted: 23 May 2015
Aims. We study the usefulness of CF+ as a tracer of the regions where C+ and H2 coexist in the interstellar medium.
Methods. We used the Plateau de Bure Interferometer to synthesize CF+J = 1–0 absorption at 102.6 GHz toward the core of the distant HII region W49N at l = 43.2°, b = 0.0°, and we modeled the fluorine chemistry in diffuse/translucent molecular gas.
Results. We detected CF+ absorption over a broad range of velocity showing that CF+ is widespread in the H2-bearing Galactic disk gas.
Conclusions. Originally detected in dense gas in the Orion Bar and Horsehead PDR (Photon-Dominated Region) CF+ was subsequently detected in absorption from diffuse and translucent clouds seen toward BL Lac and 3C 111. Here we showed that CF+ is distributed throughout the diffuse and translucent molecular disk gas with N(CF+)/N(H2) = 1.5−2.0 × 10-10, increasing to N(CF+)/N(H2) = 3.5 × 10-10 in one cloud at 39 km s-1 having higher N(H2) ≈ 3 × 1021 cm-2. Models of the fluorine chemistry reproduce the observed column densities and relative abundance of HF, from which CF+ forms, but generally overpredict the column density of CF+ by factors of 1.4–4. We show that a free space photodissociation rate Γ ≳ 10-9 s-1, comparable to that of CH, might account for much of the discrepancy but a recent calculation finds a value about ten times smaller. In the heavily blended and kinematically complex spectra seen toward W49, CF+ absorption primarily traces the peaks of the H2 distribution.
Key words: ISM: molecules
© ESO, 2015