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
dissociative
h2o+
ions
interstellar oh+
hydrogen
translucent clouds
astrochemistry
h-3(+)
recombination
ISM: molecules
absorption-line
star-forming regions
galactic-center
Author
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) 83Subject Categories
Astronomy, Astrophysics and Cosmology
Roots
Basic sciences
DOI
10.1088/0004-637X/758/2/83