Widespread galactic CF+ absorption: detection toward W49 with the Plateau de Bure Interferometer

Aims. We study the usefulness of CF+ as a tracer of the regions where C+ and H-2 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 degrees, b = 0.0 degrees, 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 H-2-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(H-2) = 1.5-2.0 x 10(-10), increasing to N(CF+)/N(H-2) = 3.5 x 10(-10) in one cloud at 39 km s(-1) having higher N(H-2) approximate to 3 x 10(21) 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 Gamma greater than or similar to 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.