Nonthermal Chemistry in Diffuse Clouds with Low Molecular Abundances

Abstract

High-quality archival spectra of interstellar absorption from Ci 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 � 1 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 C2 and 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 H2 excitation, led to values for gas density. These densities, in conjunction with measurements of CH, CH + ,C 2, 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 + ,C H þ , and CH þ generated 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. Subject headings: astrochemistry — ISM: abundances — ISM: molecules — ultraviolet: ISM