2004ApJ...605..278C

We used archival data acquired with Copernicus to reexamine CO column densities, as self-consistent oscillator strengths are now available. Our focus is on lines of sight containing modest amounts of molecular species. Our resulting column densities are small enough that self-shielding from photodissociation does not occur in the clouds probed by the observations. While our sample shows that the column densities of CO and H₂are related, no correspondence with the CH column density is evident. The case for the CH⁺ column density is less clear. Recent chemical models for these sight lines suggest that CH is mainly a by-product of CH⁺ synthesis in low-density gas. The models are most successful in reproducing the amounts of CO in the densest sight lines. Thus, much of the CO absorption must arise from denser clumps along the line of sight to account for the trend with H₂.