We report the first detection of the ground-state rotational transition of the methylidyne cation CH ^ { + } towards the massive star-forming region DR 21 with the HIFI instrument onboard the Herschel satellite .
The line profile exhibits a broad emission line , in addition to two deep and broad absorption features associated with the DR 21 molecular ridge and foreground gas .
These observations allow us to determine a ^ { 12 } CH ^ { + } J =1–0 line frequency of \nu =835137 \pm 3 MHz , in good agreement with a recent experimental determination .
We estimate the CH ^ { + } column density to be a few 10 ^ { 13 } cm ^ { -2 } in the gas seen in emission , and > 10 ^ { 14 } cm ^ { -2 } in the components responsible for the absorption , which is indicative of a high line of sight average abundance [ CH ^ { + } ] / [ H ] > 1.2 \times 10 ^ { -8 } .
We show that the CH ^ { + } column densities agree well with the predictions of state-of-the-art C-shock models in dense UV-illuminated gas for the emission line , and with those of turbulent dissipation models in diffuse gas for the absorption lines .