| Abstract : | We investigate the correlation between OH and H2 column densities in diffuse Galactic clouds, in order to identify potential molecular tracers of interstellar H2. For this, we analyse near-UV spectra extracted from the ESO/VLT archives towards twenty-four sightlines (eleven of them new) with known ?(H2), along with eleven sightlines with no H2 information. We explore correlations between the column densities of OH, CH, H2 and OH+ (detected along three new sightlines) for the entire sample. The derived abundances of OH and CH relative to H2, ∼ 10$^{−7}$ and ∼ 10$^{−8}$ respectively, are consistent with values in literature. Over an extended range of H2 column density, ?(OH) shows significant correlation with both ?(H2) and ?(CH), following the relations ?(OH) = (8.81±0.90) × 10$^{−8}$ × ?(H2), and ?(OH) = (2.61±0.17) × ?(CH) respectively. Comparison with predictions of numerical models indicate that OH absorption arises from diffuse gas (nH ∼ 50 cm$^{−3}$) illuminated by radiation fields ∼ 0.5–5 G0, while CH is associated with higher density of 500 cm$^{−3}$. We posit that the apparent dichotomy in the properties of the diffuse clouds giving rise to OH and CH absorption could be due to either (a) the presence of multiple clouds along the line-of-sight, that are spectroscopically unresolved or, (b) density gradients along the line-of-sight but within a single cloud. Thus, we conclude that OH is indeed a reliable tracer for H2 over a large range of ?(H2), with the near-UV absorption features arising from low-density diffuse molecular clouds. |
