Physical chemical control of molecular line profiles of CH+, CH and CN in single homogeneous parcels of interstellar clouds

In this paper we investigate how chemical reactions may play a role in controlling the translational energy distributions of CH+, CH and CN. Using photochemical kinetic models, we have examined the chemistry of these molecules in homogeneous parcels under steady-state conditions in diffuse and translucent clouds. We show that while the CH+ and CN kinetic energy distributions are relatively invariant in different chemical conditions, those of CH show wide variability and surprising complexity under various cloud conditions. The CH line profile is dependent upon the H-2 volume density fraction in a local parcel. The profile is predicted to be composed of a narrow and a broad Gaussian component, with the contribution of the broad component increasing with decreasing H-2 fraction. This effect is a consequence of the competition between the reaction of CH with an H atom and energy loss by inelastic collision with H and H-2. We compare our predictions with recent observations of the ultra-high-resolution optical spectra of CH+, CH and CN. We examine several sight-lines that appear to display one low-turbulence cloud component, which allow the interpretation of the line structure with our chemical reaction dynamics models. Our results are consistent with the observations with respect to quantitative widths and the compound nature of the CH lines.