Herschel Survey of Galactic OH^+, H_2O^+, and H_3O^+: Probing the Molecular Hydrogen Fraction and Cosmic-Ray Ionization Rate

Abstract

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.

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