Herschel Survey of Galactic OH+, H2O+, and H3O+: Probing the Molecular Hydrogen Fraction and Cosmic-Ray Ionization Rate
Journal article, 2015
In diffuse interstellar clouds the chemistry that leads to the formation of the oxygen-bearing ions OH+, H2O+, and H3O+ begins with the ionization of atomic hydrogen by cosmic rays, and continues through subsequent hydrogen abstraction reactions involving H2. Given these reaction pathways, the observed abundances of these molecules are useful in constraining both the total cosmic-ray ionization rate of atomic hydrogen (ζH) and molecular hydrogen fraction (f_H_2). We present observations targeting transitions of OH+, H2O+, and H3O+ made with the Herschel Space Observatory along 20 Galactic sight lines toward bright submillimeter continuum sources. Both OH+ and H2O+ are detected in absorption in multiple velocity components along every sight line, but H3O+ is only detected along 7 sight lines. From the molecular abundances we compute f_H_2 in multiple distinct components along each line of sight, and find a Gaussian distribution with mean and standard deviation 0.042 ± 0.018. This confirms previous findings that OH+ and H2O+ primarily reside in gas with low H2 fractions. We also infer ζH throughout our sample, and find a lognormal distribution with mean log (ζH) = –15.75 (ζH = 1.78 × 10–16 s–1) and standard deviation 0.29 for gas within the Galactic disk, but outside of the Galactic center. This is in good agreement with the mean and distribution of cosmic-ray ionization rates previously inferred from H_3^+ observations. Ionization rates in the Galactic center tend to be 10-100 times larger than found in the Galactic disk, also in accord with prior studies.
Author
N. Indriolo
Johns Hopkins University
University of Michigan
D.A. Neufeld
Johns Hopkins University
M. Gerin
Pierre and Marie Curie University (UPMC)
Ecole Normale Superieure (ENS)
P. Schilke
University of Cologne
A.O. Benz
Swiss Federal Institute of Technology in Zürich (ETH)
B. Winkel
Max Planck Society
K.M. Menten
Max Planck Society
E.T. Chambers
University of Cologne
John H Black
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
S. Bruderer
Max Planck Society
E. Falgarone
Ecole Normale Superieure (ENS)
Pierre and Marie Curie University (UPMC)
B. Godard
LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres
Pierre and Marie Curie University (UPMC)
J. R. Goicoechea
CSIC - Instituto de Ciencia de Materiales de Madrid (ICMM)
H. Gupta
California Institute of Technology (Caltech)
D. C. Lis
California Institute of Technology (Caltech)
LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres
Pierre and Marie Curie University (UPMC)
V. Ossenkopf
University of Cologne
Carina Persson
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
P. Sonnentrucker
Space Telescope Science Institute (STScI)
F. F. S. van der Tak
University of Groningen
Netherlands Institute for Space Research (SRON)
E. F. van Dishoeck
Leiden University
Max Planck Society
M. Wolfire
University of Maryland
F. Wyrowski
Max Planck Society
Astrophysical Journal
0004-637X (ISSN) 1538-4357 (eISSN)
Vol. 800 1 40- 40Subject Categories
Astronomy, Astrophysics and Cosmology
Roots
Basic sciences
DOI
10.1088/0004-637X/800/1/40