HERSCHEL SURVEY OF GALACTIC OH⁺, H₂O⁺, AND H₃O⁺: 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⁺, 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 (ζ_H) and molecular hydrogen fraction (f_H2). 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_H2 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 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 H₃⁺ 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