Herschel/HIFI Spectral Mapping of C+, CH+, and CH in Orion BN/KL: The Prevailing Role of Ultraviolet Irradiation in CH+ Formation
Résumé
The CH+ ion is a key species in the initial steps of interstellar carbon chemistry. Its formation in diverse environments where it is observed is not well understood, however, because the main production pathway is so endothermic (4280 K) that it is unlikely to proceed at the typical temperatures of molecular clouds. We investigate the formation of this highly reactive molecule with the first velocity-resolved spectral mapping of the CH+ J = 1-0, 2-1 rotational transitions, three sets of CH Λ-doubled triplet lines, 12C+ and 13C+ {}2{P}3/2{{--}}2{P}1/2, and CH3OH 835 GHz E-symmetry Q-branch transitions, obtained with Herschel/HIFI over a region of ≈12 arcmin2 centered on the Orion BN/KL source. We present the spatial morphologies and kinematics, cloud boundary conditions, excitation temperatures, column densities, and 12C+ optical depths. Emission from all of C+, CH+, and CH is indicated to arise in the diluted gas, outside the explosive, dense BN/KL outflow. Our models show that UV irradiation provides favorable conditions for steady-state production of CH+ in this environment. Surprisingly, no spatial or kinematic correspondences of the observed species are found with H2 S(1) emission tracing shocked gas in the outflow. We propose that C+ is being consumed by rapid production of CO to explain the lack of both C+ and CH+ in the outflow. Hence, in star-forming environments containing sources of shocks and strong UV radiation, a description of the conditions leading to CH+ formation and excitation is incomplete without including the important—possibly dominant—role of UV irradiation.