Widespread Galactic CF+ absorption: detection toward W49 with the
  Plateau de Bure Interferometer
  To study the usefulness of \CFP\ as a tracer of the regions where C\p\ and
\HH\ coexist in the interstellar medium. We used the Plateau de Bure
Interferometer to synthesize \CFP\ J=1-0 absorption at 102.6 GHz toward the
core of the distant HII region W49N at l = 43.2\degr, b=0.0\degr, and we
modeled the fluorine chemistry in diffuse/translucent molecular gas. We
detected \CFP\ absorption over a broad range of velocity showing that \CFP\ is
widespread in the \HH-bearing Galactic disk gas. Originally detected in dense
gas in the Orion Bar and Horsehead PDR, \CFP\ was subsequently detected in
absorption from diffuse and translucent clouds seen toward \bll\ and 3C111.
Here we showed that \CFP\ is distributed throughout the diffuse and translucent
molecular disk gas with N(\CFP)/N(\HH) $= 1.5-2.0\times10^{-10}$, increasing to
N(\CFP)/N(\HH) $= 3.5\times10^{-10}$ in one cloud at 39 \kms\ having higher
N(\HH) $\approx 3\times10^{21}\pcc$. Models of the fluorine chemistry reproduce
the observed column densities and relative abundance of HF, from which \CFP\
forms, but generally overpredict the the column density of \CFP\ by factors of
1.4-4. We show that a free space photodissociation rate $\Gamma \ga
10^{-9}\ps$, 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, \CFP\
absorption primarily traces the peaks of the \HH\ distribution.
