Revisiting the OH-CH correlation in diffuse clouds

TL;DR

This study refines the correlation between OH and CH in diffuse clouds using expanded data, verifies oscillator strengths, and explores chemical models, highlighting cosmic ray ionization effects on observed ratios.

Contribution

It provides more accurate column density estimates, confirms theoretical oscillator strengths, and analyzes chemical models including gas-grain interactions and cosmic rays.

Findings

Strong correlation between OH and CH in most diffuse clouds.

Verification of oscillator strengths for key transitions.

Cosmic ray ionization rates influence N(OH)/N(CH) ratios.

Abstract

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 & 3143 Angstrom, and a combination of transitions at 3886 & 3890 Angstrom, 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 & 3082 Angstrom), CH B--X (3886 & 3890 Angstrom) and C--X (3137 & 3143 Angstrom) 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 3 new sightlines can be reproduced primarily by considering different cosmic ray ionization rates.