HyGAL. III. Absorption lines

Column density ratios among these H_2_ tracers are consistent with previous measurements in local diffuse clouds and remain uniform across Galactic environments and velocity intervals. The gas phase oxygen abundance relative to total hydrogen, <X(O)>=N(O)/N(Htotal), is (3.09+/-0.64)x10^-4^, slightly below the elemental solar value but consistent with the previous observations measuring gas-phase abundances. We also find that N(HI) decreases toward the regions where the molecular fraction exceeds f^N^_H2_~0.5, marking the onset of the molecular phase.

While the atomic oxygen abundance remains roughly constant, the abundances of OH, HCO^+^, and CCH increase with the molecular fraction. Gas traced by the HCO^+^ absorption corresponds to higher molecular fractions than that traced by HI and hydride ions, highlighting density variations in the diffuse-to-translucent ISM along different lines of sight.