skip to main content

Title: 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

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 lognormalmore » 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.« less
Authors:
;  [1] ; ;  [2] ; ; ;  [3] ;  [4] ; ;  [5] ; ;  [6] ; ;  [7] ; ;  [8] ;  [9] ;  [10] ;  [11] ;  [12] more »; « less
  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)
  12. SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen (Netherlands)
Publication Date:
OSTI Identifier:
22364227
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 800; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ABSORPTION; COSMIC RADIATION; GAUSS FUNCTION; HYDROGEN; HYDROGEN IONS 3 PLUS; INTERSTELLAR SPACE; IONIZATION; MILKY WAY; MOLECULES; OXONIUM IONS; OXYGEN; PROBES; SPACE