Abundance of CH+ in Translucent Molecular Clouds: Problems for Shock Models?

Abstract

The large abundance of CH+ in diffuse clouds has been a mystery for more than 50 years (Dalgarno 1976). Many different explanations have been proposed, but only (shock) models with a substantial column of warm gas appear capable of approaching the observed column densities (Elitzur and Watson 1978). In these models, CH+ is formed in the warm postshock gas through the reaction C+ + H2 → CH+ + H, which is endoergic by 4650 K. Although the most sophisticated MHD shock models are consistent with various observational aspects of CH+, they require substantial “fine-tuning” of the parameters (Draine and Katz 1986; Hartquist et al. 1990). In addition, they predict a shift in velocity between CH+ and other molecules found in the cold quiescent gas, which is not observed in recent data (e.g. Crawford 1989). In order to test further the shock models, we have searched for CH+ in a number of translucent clouds (AV ≍ 1-5 mag), which have H2 column densities that are up to an order of magnitude larger than the clouds studied so far (cf. Souza 1979). Observations of the chemically related molecules CH, C2, CN and CO have been obtained as well.