2015A&A...579A..12L

We study the usefulness of CF⁺ as a tracer of the regions where C⁺ and H₂ coexist in the interstellar medium. We used the Plateau de Bure Interferometer to synthesize CF⁺ J=1-0 absorption at 102.6GHz 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. We detected CF⁺ absorption over a broad range of velocity showing that CF⁺ is widespread in the H₂-bearing Galactic disk gas. 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 3C111. Here we showed that CF⁺ is distributed throughout the diffuse and translucent molecular disk gas with N(CF⁺/N(H₂)=1.5-2.0x10^–10, increasing to N(CF⁺/N(H₂)=3.5x10^–10 in one cloud at 39km/s having higher N(H₂)=3x10²¹cm^–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^–9s^–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.