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

Abstract

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 |$(\geqslant\,{10}^{12}\,\text{cm}^{-2})$| along at least three types of lines of sight: (i) those with little molecular hydrogen; (ii) those passing through diffuse molecular clouds such as ζ 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 |$(\simeq {10}^{12}\,\text{cm}^{-2})\,\text{of CH}^{+}$| 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 |${10}^{14}\,\text{cm}^{-2}$| 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 |${10}^{14}\,\text{cm}^{-2}$| towards a highly reddened star arises in one thick boundary layer or several boundary layers like those considered for type (ii).