The Cosmic-Ray Ionization Rate in the Galactic Disk, as Determined from Observations of Molecular Ions

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 + , , and . For diffuse atomic clouds at Galactocentric distances in the range , observations of ArH + , OH + , and imply a mean primary CRIR of per hydrogen atom, where . Within diffuse molecular clouds observed toward stars in the solar neighborhood, measurements of and H 2 imply a primary CRIR of per H atom, corresponding to a total ionization rate per H 2 molecule of 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 per H atom. Our results show marginal evidence that the CRIR in diffuse molecular clouds decreases with cloud extinction, , with a best-fit dependence for .