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- 40

Subject Categories

Astronomy, Astrophysics and Cosmology

Roots

Basic sciences

DOI

10.1088/0004-637X/800/1/40

More information

Latest update

5/29/2018