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We report the first detection of the ground-state rotational transition ofthe methylidyne cation CH+ towards the massive star-forming region DR21 withthe HIFI instrument onboard the Herschel satellite. The line profile exhibits abroad emission line, in addition to two deep and broad absorption featuresassociated with the DR21 molecular ridge and foreground gas. These observationsallow us to determine a CH+ J=1-0 line frequency of 835137 +/- 3 MHz, in goodagreement with a recent experimental determination. We estimate the CH+ columndensity to be a few 1e13 cm^-2 in the gas seen in emission, and > 1e14 cm^-2 inthe components responsible for the absorption, which is indicative of a highline 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-shockmodels in dense UV-illuminated gas for the emission line, and with those ofturbulent dissipation models in diffuse gas for the absorption lines.

Strong CH+ J=1-0 emission and absorption in DR21