15min:

NICK INDRIOLO, DAVID A. NEUFELD, Department of Physics & Astronomy, Johns Hopkins University, Baltimore, MD 21218; MARYVONNE GERIN, LERMA, CNRS UMR 8112, 24 rue Lhomond, 75231 Paris Cedex 05, France; THOMAS R. GEBALLE, Gemini Observatory, Hilo, HI 96720.

\hspace0.25in Absorption lines from the molecules OH⁺, H₂O⁺, and H₃⁺ have all 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 (f _{H_2}) in the outskirts of the observed cloud, as well as the product of the cosmic-ray ionization rate of atomic hydrogen and an efficiency factor ( _H). The efficiency factor ( ) describes what fraction of the time ionization of H by cosmic rays eventually leads to OH⁺. H₃⁺ is used to infer the cosmic-ray ionization rate of H₂ ( ₂) in the molecular interior of the cloud. By demanding that the two ionization rates are equal, and taking the value inferred from H₃⁺ to be correct, we determine . This is an important step in the future use of OH⁺ and H₂O⁺ on their own as tracers of the cosmic-ray ionization rate.