Chemical Analysis of a Diffuse Cloud along a Line of Sight toward W51: Molecular Fraction and Cosmic-Ray Ionization Rate
Journal article, 2012

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 H-2 in the molecular interior of the cloud, which we find to be zeta(2) = (4.8 +/- 3.4) x 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.

cosmic rays




interstellar oh+


translucent clouds




ISM: molecules


star-forming regions



N. Indriolo

Johns Hopkins University

D. A. Neufeld

Johns Hopkins University

M. Gerin

LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres

T. R. Geballe

Gemini Observatory North

John H Black

Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics

K. M. Menten

Max Planck Society

J. R. Goicoechea

Centro de Astrobiologia (CAB)

Astrophysical Journal

0004-637X (ISSN) 1538-4357 (eISSN)

Vol. 758 2 83 (1-8) 83

Subject Categories

Astronomy, Astrophysics and Cosmology


Basic sciences



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