HERSCHEL SURVEY OF GALACTIC OH{sup +}, H{sub 2}O{sup +}, AND H{sub 3}O{sup +}: PROBING THE MOLECULAR HYDROGEN FRACTION AND COSMIC-RAY IONIZATION RATE

Journal Article · Tue Feb 10 04:00:00 EST 2015 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/800/1/40· OSTI ID:22364227

Indriolo, Nick; Neufeld, D. A. ^[1]; Gerin, M.; Falgarone, E. ^[2]; Schilke, P.; Chambers, E. T.; Ossenkopf, V. ^[3]; Benz, A. O. ^[4]; Winkel, B.; Menten, K. M. ^[5]; Black, John H.; Persson, C. M. ^[6]; Bruderer, S.; Van Dishoeck, E. F. ^[7]; Godard, B.; Lis, D. C. ^[8]; Goicoechea, J. R. ^[9]; Gupta, H. ^[10]; Sonnentrucker, P. ^[11]; others, and

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1. Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218 (United States)
2. LERMA, Observatoire de Paris, Ecole Normale Supérieure, PSL Research University, CNRS, UMR8112, F-75014 Paris (France)
3. I. Physikalisches Institut der Universität zu Köln, Zülpicher Str. 77, 50937 Köln (Germany)
4. Institute of Astronomy, ETH Zürich (Switzerland)
5. MPI für Radioastronomie, Bonn (Germany)
6. Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, SE-43992 Onsala (Sweden)
7. Max Planck Institut für Extraterrestrische Physik, Garching (Germany)
8. Sorbonne Universités, UPMC Univ. Paris 06, UMR8112, LERMA, F-75005 Paris (France)
9. Instituto de Ciencias de Materiales de Madrid (CSIC), E-28049 Cantoblanco, Madrid (Spain)
10. California Institute of Technology, Pasadena, CA 91125 (United States)
11. Space Telescope Science Institute, Baltimore, MD 21218 (United States)

In diffuse interstellar clouds the chemistry that leads to the formation of the oxygen-bearing ions OH{sup +}, H{sub 2}O{sup +}, and H{sub 3}O{sup +} begins with the ionization of atomic hydrogen by cosmic rays, and continues through subsequent hydrogen abstraction reactions involving H{sub 2}. Given these reaction pathways, the observed abundances of these molecules are useful in constraining both the total cosmic-ray ionization rate of atomic hydrogen (ζ{sub H}) and molecular hydrogen fraction (f{sub H{sub 2}}). We present observations targeting transitions of OH{sup +}, H{sub 2}O{sup +}, and H{sub 3}O{sup +} made with the Herschel Space Observatory along 20 Galactic sight lines toward bright submillimeter continuum sources. Both OH{sup +} and H{sub 2}O{sup +} are detected in absorption in multiple velocity components along every sight line, but H{sub 3}O{sup +} is only detected along 7 sight lines. From the molecular abundances we compute f{sub H{sub 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{sup +} and H{sub 2}O{sup +} primarily reside in gas with low H{sub 2} fractions. We also infer ζ{sub H} throughout our sample, and find a lognormal distribution with mean log (ζ{sub H}) = –15.75 (ζ{sub H} = 1.78 × 10{sup –16} s{sup –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{sub 3}{sup +} 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.

• OSTI ID: 22364227
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 800; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ABSORPTION
COSMIC RADIATION
GAUSS FUNCTION
HYDROGEN
HYDROGEN IONS 3 PLUS
INTERSTELLAR SPACE
IONIZATION
MILKY WAY
MOLECULES
OXONIUM IONS
OXYGEN
PROBES
SPACE