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The CH$^+$ ion is a key species in the initial steps of interstellar carbonchemistry. Its formation in diverse environments where it is observed is notwell understood, however, because the main production pathway is so endothermic(4280 K) that it is unlikely to proceed at the typical temperatures ofmolecular clouds. We investigation CH$^+$ formation with the firstvelocity-resolved spectral mapping of the CH$^+$ $J=1-0, 2-1$ rotationaltransitions, three sets of CH $\Lambda$-doubled triplet lines, $^{12}$C$^+$ and$^{13}$C$^+$, and CH$_3$OH 835~GHz E-symmetry Q branch transitions, obtainedwith Herschel/HIFI over $\approx$12 arcmin$^2$ centered on the Orion BN/KLsource. We present the spatial morphologies and kinematics, cloud boundaryconditions, excitation temperatures, column densities, and $^{12}$C$^+$ opticaldepths. Emission from C$^+$, CH$^+$, and CH is indicated to arise in thediluted gas, outside of the explosive, dense BN/KL outflow. Our models showthat UV-irradiation provides favorable conditions for steady-state productionof CH$^+$ in this environment. Surprisingly, no spatial or kinematiccorrespondences of these species are found with H$_2$ S(1) emission tracingshocked gas in the outflow. We propose that C$^+$ is being consumed by rapidproduction of CO to explain the lack of C$^+$ and CH$^+$ in the outflow, andthat fluorescence provides the reservoir of H$_2$ excited to higherro-vibrational and rotational levels. Hence, in star-forming environmentscontaining sources of shocks and strong UV radiation, a description of CH$^+$formation and excitation conditions is incomplete without including theimportant --- possibly dominant --- role of UV irradiation.

Herschel/HIFI Spectral Mapping of C$^+$, CH$^+$, and CH in Orion BN/KL: The Prevailing Role of Ultraviolet Irradiation in CH$^+$ Formation