CH+ in shocks, cloud- intercloud interfaces and dense photondominated regions.

The paper investigates the production of interstellar CH(+) by the destruction, in shocks, of hydrogenated amorphous carbon in grains and the subsequent photoabsorption sequence. It is argued that much of the eroded carbon passes through CH(+). Maximum column densities (about 10 exp 12/sq cm) of CH(+) are obtained in the absence of H2. The relative abundances of CH(+), CH, OH, and various v = 0 rotationally excited levels of H2 in hot diffuse molecular gas in which the velocity distributions are all Maxwellian are calculated. It is argued that such gas exists in turbulent boundary layers at molecular-cloud-intercloud interfaces and has speeds lower than those associated with shocks. It is demonstrated that plausible conditions can be adopted which lead to relative abundances of several important species close to those observed. It is shown that CH(+) column densities of 10 exp 14/sq cm can be produced in photon-dominated regions only if the conditions in them are similar to those under which the hydroxyl maser regions surrounding young stars form.