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Published February 10, 2015 | Published + Submitted
Journal Article Open

Herschel Survey of Galactic OH^+, H_2O^+, and H_3O^+: 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^+, H_2O^+, and H_3O^+ begins with the ionization of atomic hydrogen by cosmic rays, and continues through subsequent hydrogen abstraction reactions involving H_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_H) and molecular hydrogen fraction (ƒ_H_2). We present observations targeting transitions of OH^+, H_2O^+, and H_3O^+ made with the Herschel Space Observatory along 20 Galactic sight lines toward bright submillimeter continuum sources. Both OH^+ and H_2O^+ are detected in absorption in multiple velocity components along every sight line, but H_3O^+ is only detected along 7 sight lines. From the molecular abundances we compute ƒ_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 H_2O^+ primarily reside in gas with low H_2 fractions. We also infer ζ_H throughout our sample, and find a lognormal distribution with mean log (ζ_H) = –15.75 (ζ_H = 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.

Additional Information

© 2015 American Astronomical Society. Received 2014 August 13; accepted 2014 November 25; published 2015 February 6. Support for this work was provided by NASA through an award issued by JPL/Caltech. N.I. and D.A.N. are funded by NASA Research Support Agreement No. 1393741 provided through JPL. J.R.G. thanks the Spanish MINECO for funding support under grants CSD2009-00038 and AYA2012- 32032. The authors thank Vincent Fish for providing digital copies of Hi spectra from his 2003 paper, and the anonymous referee for insightful comments and suggestions. HIFI has been designed and built by a consortium of institutes and university departments from across Europe, Canada and the United States under the leadership of SRON Netherlands Institute for Space Research, Groningen, The Netherlands and with major contributions from Germany, France and the US. Consortium members are: Canada: CSA, U.Waterloo; France: CESR, LAB, LERMA, IRAM; Germany: KOSMA, MPIfR, MPS; Ireland, NUI Maynooth; Italy: ASI, IFSI-INAF,Osservatorio Astrofisico di Arcetri-INAF; Netherlands: SRON, TUD; Poland: CAMK, CBK; Spain: Observatorio Astronmico Nacional (IGN), Centro de Astrobiologa (CSIC-INTA). Sweden: Chalmers University of Technology - MC2, RSS & GARD; Onsala Space Observatory; Swedish National Space Board, Stockholm University— Stockholm Observatory; Switzerland: ETH Zurich, FHNW; USA: Caltech, JPL, NHSC.

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Published - 0004-637X_800_1_40.pdf

Submitted - 1412.1106v1.pdf


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August 22, 2023