{
  "id": "2101.03732",
  "version": "v1",
  "published": "2021-01-11T07:32:47.000Z",
  "updated": "2021-01-11T07:32:47.000Z",
  "title": "Roll of Polycyclic Aromatic Hydrocarbons on the Cosmic-Ray ionization rate in the Galaxy",
  "authors": [
    "Gargi Shaw",
    "G. J. Ferland"
  ],
  "comment": "Accepted for publication in ApJ, 31 pages, 22 figures",
  "categories": [
    "astro-ph.GA"
  ],
  "abstract": "The cosmic-ray ionization rate ($\\zeta$, s$^{-1}$) plays an important role in the interstellar medium. It controls ion-molecular chemistry and provides a source of heating. Here we perform a grid of calculations using the spectral synthesis code CLOUDY along nine sightlines towards, HD 169454, HD 110432, HD 204827, $\\lambda$ Cep, X Per, HD 73882, HD 154368, Cyg OB2 5, Cyg OB2 12. The value of $\\zeta$ is determined by matching the observed column densities of H$_3^+$ and H$_2$. The presence of polycyclic aromatic hydrocarbons (PAHs) affects the free electron density, which changes the H$_3^+$ density and the derived ionization rate. PAHs are ubiquitous in the Galaxy, but there are also regions where PAHs do not exist. Hence, we consider clouds with a range of PAH abundances and show their effects on the H$_3^+$ abundance. We predict an average cosmic-ray ionization rate for H$_2$ ($\\zeta$(H$_2$))= (7.88 $\\pm$ 2.89) $\\times$ 10$^{-16}$ s$^{-1}$ for models with average Galactic PAHs abundances, (PAH/H =10$^{-6.52}$), except Cyg OB2 5 and Cyg OB2 12. The value of $\\zeta$ is nearly 1 dex smaller for sightlines toward Cyg OB2 12. We estimate the average value of $\\zeta$(H$_2$)= (95.69 $\\pm$ 46.56) $\\times$ 10$^{-16}$ s$^{-1}$ for models without PAHs.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2021-01-11T07:32:47.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "polycyclic aromatic hydrocarbons",
      "cyg ob2",
      "spectral synthesis code cloudy",
      "average cosmic-ray ionization rate",
      "average galactic pahs abundances"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 31,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}