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

## TL;DR

This study estimates the cosmic-ray ionization rate in the Galactic disk using molecular ion observations and detailed chemical models, revealing a consistent rate around 2-3 x 10^{-16} s^{-1} per hydrogen atom, with potential decrease in dense clouds.

## Contribution

It provides new, detailed estimates of the cosmic-ray ionization rate across different regions of the Galactic disk using molecular ion data and advanced modeling techniques.

## Key findings

- CRIR in diffuse atomic clouds: ~2.2 x 10^{-16} s^{-1} per H atom
- CRIR in diffuse molecular clouds: ~2.3 x 10^{-16} s^{-1} per H atom
- CRIR decreases with cloud extinction, roughly inversely proportional to A_V

## 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 $(2.2 \pm 0.3) \exp [(R_0-R_g)/4.7\,\rm{kpc}] \times 10^{-16} \rm \, s^{-1}$ 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 $(2.3 \pm 0.6) \times 10^{-16}\,\,\rm s^{-1}$ per H atom, corresponding to a total ionization rate per H$_2$ molecule of $(5.3 \pm 1.1) \times 10^{-16}\,\,\rm s^{-1},$ 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 $2.9 \times 10^{-16}\,\,\rm s^{-1}$ per H atom. Our results show marginal evidence that the CRIR in diffuse molecular clouds decreases with cloud extinction, $A_{\rm V}({\rm tot})$, with a best-fit dependence $\propto A_{\rm V}({\rm tot})^{-1}$ for $A_{\rm V}({\rm tot}) \ge 0.5$.