On the CH(+) ion in diffuse interstellar clouds

A survey of 65 lines of sight in the southern Milky Way for the interstellar line of CH(+) at 4232 A yielded 30 detections. The CH(+) column density N(CH+) is shown to be well correlated with the rotationally excited H2 detected by surveys with the Copernicus satellite: log N(CH+)/N(H2)ex inversely proportional to 2500/T(ex), where T(ex) is the excitation temperature of H2 provided by the observed populations in rotational levels J of about 3-5. This correlation is strong evidence that CH(+) is formed in warm gas where the reaction C(+) + H2 yields CH(+) + H is effective. Shocks probably provide the necessary heating.