Chemical Analysis of a Diffuse Cloud along a Line of Sight toward W51: Molecular Fraction and Cosmic-Ray Ionization Rate

Abstract

Absorption lines from the molecules OH⁺, H₂O⁺, and H⁺ ₃ have 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 H₂ in the molecular interior of the cloud, which we find to be ζ₂ = (4.8 ± 3.4) × 10^-16 s^-1. 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 epsilon = 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.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.