2003ApJ...589..319Z

High-quality archival spectra of interstellar absorption from C I toward nine stars, taken with the Goddard High Resolution Spectrograph on the Hubble Space Telescope, were analyzed. Our sample was supplemented by two sight lines, 23 Ori and β¹ Sco, for which the C I measurements of Federman, Welty, & Cardelli were used. Directions with known CH⁺ absorption, but only upper limits on absorption from C₂and CN, were considered for our study. This restriction allows us to focus on regions where CH⁺ chemistry dominates the production of carbon-bearing molecules. Profile synthesis of several multiplets yielded column densities and Doppler parameters for the C I fine-structure levels. Equilibrium excitation analyses, using the measured column densities as well as the temperature from H₂excitation, led to values for gas density. These densities, in conjunction with measurements of CH, CH⁺, C₂, and CN column densities, provided estimates for the amount of CH associated with CH⁺ production, which in turn set up constraints on the present theories for CH⁺ formation in this environment. We found for our sample of interstellar clouds that on average 30%-40% of the CH originates from CH⁺ chemistry, and in some cases it can be as high as 90%. A simple chemical model for gas containing nonequilibrium production of CH⁺ was developed for the purpose of predicting column densities for CH, CO, HCO⁺, CH⁺₂, and CH⁺₃generated from large abundances of CH⁺. Again, our results suggest that nonthermal chemistry is necessary to account for the observed abundance of CH and probably that of CO in these clouds.