2017ApJ...845..163N

We have obtained estimates for the cosmic-ray ionization rate (CRIR) in the Galactic disk, using a detailed model for the physics and chemistry of diffuse interstellar gas clouds to interpret previously published measurements of the abundance of four molecular ions: ArH⁺, OH⁺, H₂O⁺, and H₃⁺. For diffuse atomic clouds at Galactocentric distances in the range R_g∼4–9 kpc, observations of ArH⁺, OH⁺, and H₂O⁺ imply a mean primary CRIR of (2.2±0.3) exp[(R₀-R_g)/4.7 kpc]×10^–16 s^–1 per hydrogen atom, where R₀=8.5 kpc. Within diffuse molecular clouds observed toward stars in the solar neighborhood, measurements of H₃⁺ and H₂ imply a primary CRIR of (2.3±0.6)×10^–16 s^–1 per H atom, corresponding to a total ionization rate per H₂ molecule of (5.3±1.1)×10^–16 s^–1, in good accord with previous estimates. These estimates are also in good agreement with a rederivation, presented here, of the CRIR implied by recent observations of carbon and hydrogen radio recombination lines along the sight line to Cas A. Here, our best-fit estimate for the primary CRIR is 2.9×10^–16 s^–1 per H atom. Our results show marginal evidence that the CRIR in diffuse molecular clouds decreases with cloud extinction, A_V(tot), with a best-fit dependence ∝A_V(tot)^–1 for A_V(tot) 0.5.