Herschel Survey of Galactic OH+, H2O+, and H3O+: Probing the Molecular Hydrogen Fraction and Cosmic-Ray Ionization Rate
Artikel i vetenskaplig tidskrift, 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.

Författare

N. Indriolo

University of Michigan

Johns Hopkins University

D.A. Neufeld

Johns Hopkins University

M. Gerin

Universite Pierre et Marie Curie

École Normale Supérieure

P. Schilke

Universität zu Köln

A.O. Benz

Eidgenössische Technische Hochschule Zürich (ETH)

B. Winkel

Max Planck-institutet

K.M. Menten

Max Planck-institutet

E.T. Chambers

Universität zu Köln

John H Black

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

S. Bruderer

Max Planck-institutet

E. Falgarone

Universite Pierre et Marie Curie

École Normale Supérieure

B. Godard

Universite Pierre et Marie Curie

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

J. R. Goicoechea

CSIC - Instituto de Ciencia de Materiales de Madrid (ICMM)

H. Gupta

CalTech

D. C. Lis

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

CalTech

Universite Pierre et Marie Curie

V. Ossenkopf

Universität zu Köln

Carina Persson

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

P. Sonnentrucker

STScI

F. F. S. van der Tak

SRON Netherlands Institute for Space Research

University of Groningen, Kapteyn Astronomical Institute

E. F. van Dishoeck

Leiden University

Max Planck-institutet

M. Wolfire

University of Maryland

F. Wyrowski

Max Planck-institutet

Astrophysical Journal

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

Vol. 800 40- 40

Ämneskategorier

Astronomi, astrofysik och kosmologi

Fundament

Grundläggande vetenskaper

DOI

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