C-type shocks in the interstellar medium: Profiles of CH+ and CH absorption lines

Abstract

We have computed optical absorption-line profiles of CH+ and CH, as predicted by a model of a C-type shock propagating in a diffuse interstellar cloud. Both these species are produced in the shock wave in the reaction sequence that is initiated by C+(H2, H)CH+. Whilst CH+ flows at the ion speed, CH, which forms in the dissociative recombination reaction CH+3(e-, H2)CH, flows at a speed which is intermediate between those of the ions and the neutrals. The predicted velocity shift between the CH+ and CH line profiles is found to be no more than approximately 2 km s-1, which is smaller than has previously been assumed. We also investigate OH and HCO+, finding that the correlation between their column densities, recently observed in the diffuse interstellar medium, can be reproduced by the model.