{
  "id": "1704.03877",
  "version": "v1",
  "published": "2017-04-12T18:00:00.000Z",
  "updated": "2017-04-12T18:00:00.000Z",
  "title": "The cosmic ray ionization rate in the Galactic disk, as determined from observations of molecular ions",
  "authors": [
    "David A. Neufeld",
    "Mark G. Wolfire"
  ],
  "comment": "48 pages, including 3 tables (at end) and 13 figures. Accepted for publication in the Astrophysical Journal",
  "categories": [
    "astro-ph.GA"
  ],
  "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 $A_{\\rm V}({\\rm tot}) \\ge 0.5$.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2017-04-12T18:00:00.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "cosmic ray ionization rate",
      "galactic disk",
      "molecular ions",
      "primary crir",
      "observations"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 48,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}