The cosmic ray ionization rate in the Galactic disk, as determined from observations of molecular ions

Abstract

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${}^{+}$, H${}_2$O${}^{+}$ and H${}_3^{+}$. For diffuse $atomic$ clouds at Galactocentric distances in the range $R_g\sim 4-9$ kpc, observations of ArH${}^{+}$, OH${}^{+}$, and H${}_2$O${}^{+}$ imply a mean primary CRIR of $(2.2\pm 0.3)\exp [(R_0-R_g)/4.7\mathrm{kpc}]\times 10^{-16}{\mathrm{s}}^{-1}$ per hydrogen atom, where $R_0=8.5$ kpc. Within diffuse $molecular$ clouds observed toward stars in the solar neighborhood, measurements of H${}_3^{+}$ and H${}_2$ imply a primary CRIR of $(2.3\pm 0.6)\times 10^{-16}{\mathrm{s}}^{-1}$ per H atom, corresponding to a total ionization rate per H${}_2$ molecule of $(5.3\pm 1.1)\times 10^{-16}{\mathrm{s}}^{-1},$ 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 $2.9\times 10^{-16}{\mathrm{s}}^{-1}$ per H atom. Our results show marginal evidence that the CRIR in diffuse molecular clouds decreases with cloud extinction, $A_{\mathrm{V}}(\mathrm{tot})$, with a best-fit dependence $\propto A_{\mathrm{V}}(\mathrm{tot}{)}^{-1}$ for $A_{\mathrm{V}}(\mathrm{tot})\ge 0.5$.

48 pages, including 3 tables (at end) and 13 figures. Accepted for publication in the Astrophysical Journal

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