CH+ in shocks, cloud-intercloud interfaces, and dense photon-dominated regions

A resolution of the long-standing CH+ problem almost certainly will yield considerable insight into processes (e.g. mass and energy transfer at interfaces between plasma in different phases) affecting a wide range of astrophysical environments. CH+ column densities are large (> 1012 cm-2) along at least three types of lines of sight: (i) those with little molecular hydrogen; (ii) those passing through diffuse molecular clouds such as £ Oph, and (iii) those that provide high reddening of the background stars (visual extinctions of about 2-4 mag). This indicates either that one process of ubiquitous importance is responsible for the CH+ production or that a number of different scenarios must be considered. In this paper we explore the latter view. For type (i) we have investigated the production of interstellar CH+ by the destruction, in shocks, of hydrogenated amorphous carbon in grains and the subsequent photoabsorption sequence. We argue that much of the eroded carbon passes through CH+. Maximum column densities (-1012 cm-2) of CH+ are obtained in the absence of H2. For type (ii), we have calculated 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. We argue that such gas exists in turbulent boundary layers at molecular-cloud-intercloud interfaces and has speeds lower than those associated with shocks, which have been advocated as the sources of hot gas containing CH+. We demonstrate that plausible conditions can be adopted which lead to relative abundances of several important species close to those observed. The presence of components of CH associated with CH+ in warm interfaces places fairly stringent constraints on the pressures in the interfaces. For type (iii) we show that CH+ column densities of 1014 cm"2 can be produced in photon-dominated regions only if the conditions in them are similar to those under which hydroxyl maser regions, surrounding young stars, form. A more likely alternative is that a CH+ column density of 1014 cm-2 towards a highly reddened star arises in one thick boundary layer or several boundary layers like those considered for type (ii).