2010A&A...518L.118F

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 ¹²CH⁺J=1-0 line frequency of ν=835137±3MHz, in good agreement with a recent experimental determination. We estimate the CH⁺ column density to be a few 10¹³cm^–2 in the gas seen in emission, and >10¹⁴cm^–2 in the components responsible for the absorption, which is indicative of a high line of sight average abundance [CH⁺]/[H]>1.2x10^–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.