2016MNRAS.459.2822M

Based on the analysis of available published data and archival data along 24 sightlines (5 of which are new) we derive more accurate estimates of the column densities of OH and CH towards diffuse/translucent clouds and revisit the typically observed correlation between the abundances of these species. The increase in the sample size was possible because of the equivalence of the column densities of CH derived from a combination of the transitions at 3137 and 3143 Å, and a combination of transitions at 3886 and 3890 Å, which we have demonstrated here. We find that with the exception of four diffuse clouds, the entire source sample shows a clear correlation between the column densities of OH and CH similar to previous observations. The analysis presented also verifies the theoretically predicted oscillator strengths of the OH A-X (3078 and 3082 Å), CH B-X (3886 and 3890 Å) and C-X (3137 and 3143 Å) transitions. We estimate N(H) and N(H₂) from the observed E(B - V) and N(CH) respectively. The N(OH)/N(CH) ratio is not correlated with the molecular fraction of hydrogen in the diffuse/translucent clouds. We show that with the exception of HD 34078 for all the clouds the observed column density ratios of CH and OH can be reproduced by simple chemical models which include gas-grain interaction and gas-phase chemistry. The enhanced N(OH)/N(CH) ratio seen towards the three new sightlines can be reproduced primarily by considering different cosmic ray ionization rates.