2012ApJ...758...83I

Absorption lines from the molecules OH⁺, H₂O⁺, and H⁺₃have been observed in a diffuse molecular cloud along a line of sight near W51 IRS2. We present the first chemical analysis that combines the information provided by all three of these species. Together, OH⁺ and H₂O⁺ are used to determine the molecular hydrogen fraction in the outskirts of the observed cloud, as well as the cosmic-ray ionization rate of atomic hydrogen. H⁺₃ is used to infer the cosmic-ray ionization rate of H₂in the molecular interior of the cloud, which we find to be ζ₂= (4.8±3.4)x10^–16/s. Combining the results from all three species we find an efficiency factor–defined as the ratio of the formation rate of OH⁺ to the cosmic-ray ionization rate of H–of ε = 0.07±0.04, much lower than predicted by chemical models. This is an important step in the future use of OH⁺ and H₂O⁺ on their own as tracers of the cosmic-ray ionization rate.