{
  "id": "1111.6936",
  "version": "v1",
  "published": "2011-11-29T19:10:43.000Z",
  "updated": "2011-11-29T19:10:43.000Z",
  "title": "Investigating the Cosmic-Ray Ionization Rate in the Galactic Diffuse Interstellar Medium through Observations of H3+",
  "authors": [
    "Nick Indriolo",
    "Benjamin J. McCall"
  ],
  "comment": "48 pages, 19 figures, 4 tables, accepted for publication in ApJ",
  "journal": "ApJ 745 (2012) 91",
  "categories": [
    "astro-ph.GA"
  ],
  "abstract": "Observations of H3+ in the Galactic diffuse interstellar medium (ISM) have led to various surprising results, including the conclusion that the cosmic-ray ionization rate (zeta_2) is about 1 order of magnitude larger than previously thought. The present survey expands the sample of diffuse cloud sight lines with H3+ observations to 50, with detections in 21 of those. Ionization rates inferred from these observations are in the range (1.7+-1.3)x10^-16 s^-1<zeta_2<(10.6+-8.2)x10^-16 s^-1 with a mean value of zeta_2=(3.5^+5.3_-3.0)x10^-16 s^-1. Upper limits (3 sigma) derived from non-detections of H3+ are as low as zeta_2<0.4x10^-16 s^-1. These low upper-limits, in combination with the wide range of inferred cosmic-ray ionization rates, indicate variations in zeta_2 between different diffuse cloud sight lines. A study of zeta_2 versus N_H (total hydrogen column density) shows that the two parameters are not correlated for diffuse molecular cloud sight lines, but that the ionization rate decreases when N_H increases to values typical of dense molecular clouds. Both the difference in ionization rates between diffuse and dense clouds and the variation of zeta_2 among diffuse cloud sight lines are likely the result of particle propagation effects. The lower ionization rate in dense clouds is due to the inability of low-energy (few MeV) protons to penetrate such regions, while the ionization rate in diffuse clouds is controlled by the proximity of the observed cloud to a site of particle acceleration.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2011-11-29T19:10:43.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "galactic diffuse interstellar medium",
      "cosmic-ray ionization rate",
      "diffuse cloud sight lines",
      "molecular cloud sight lines",
      "observations"
    ],
    "tags": [
      "journal article"
    ],
    "publication": {
      "doi": "10.1088/0004-637X/745/1/91",
      "journal": "The Astrophysical Journal",
      "year": 2012,
      "month": "Jan",
      "volume": 745,
      "number": 1,
      "pages": 91
    },
    "note": {
      "typesetting": "TeX",
      "pages": 48,
      "language": "en",
      "license": "arXiv",
      "status": "editable",
      "inspire": 1079273,
      "adsabs": "2012ApJ...745...91I"
    }
  }
}