Revisiting the OH-CH correlation in diffuse clouds
  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.
