{
  "id": "1611.03783",
  "version": "v1",
  "published": "2016-11-11T17:04:44.000Z",
  "updated": "2016-11-11T17:04:44.000Z",
  "title": "Formation and Fractionation of CO (carbon monoxide) in diffuse clouds observed at optical and radio wavelengths",
  "authors": [
    "Harvey S. Liszt"
  ],
  "comment": "Accepted for The Astrophysical Journal",
  "categories": [
    "astro-ph.GA"
  ],
  "abstract": "We modelled \\HH\\ and CO formation incorporating the fractionation and selective photodissociation affecting CO when \\AV\\ $\\la2$mag. UV absorption measurements typically have N(\\cotw)/N(\\coth) $\\approx 65$ that are reproduced with the standard UV radiation and little density dependence at n(H) $\\approx32-1024\\pccc$: Densities n(H) $\\la256\\pccc$ avoid overproducing CO. Sightlines observed in mm-wave absorption and a few in UV show enhanced \\coth\\ by factors of 2-4 and are explained by higher n(H) $\\ga256\\pccc$ and/or weaker radiation. The most difficult observations to understand are UV absorptions having N(\\cotw)/N(\\coth) $>$100 and N(CO)$\\ga10^{15}\\pcc$. Plots of \\WCO\\ vs. N(CO) show that \\WCO\\ remains linearly proportional to N(CO) even at high opacity owing to sub-thermal excitation. \\cotw\\ and \\coth\\ have nearly the same curve of growth so their ratios of column density/integrated intensity are comparable even when different from the isotopic abundance ratio. For n(H)$\\ga128\\pccc$, plots of \\WCO\\ vs N(CH) are insensitive to n(H), and \\WCO/N(CO)$\\approx1\\Kkms/(10^{15}~{\\rm CO}\\pcc)$: This compensates for small CO/\\HH\\ to make \\WCO\\ more readily detectable. Rapid increases of N(CO) with n(H), N(H) and N(\\HH) often render the CO bright, ie a small CO-\\HH\\ conversion factor. For n(H) $\\la64\\pccc$ CO enters the regime of truly weak excitation where \\WCO $\\propto$n(H)N(CO). \\WCO\\ is a strong function of the average \\HH\\ fraction and models with \\WCO=1\\Kkms\\ fall in the narrow range \\mfH2\\=0.65-0.8, or \\mfH2\\=0.4-0.5 at \\WCO\\=0.1\\Kkms. The insensitivity of easily-detected CO emission to gas with small \\mfH2\\ implies that even deep CO surveys using broad beams may not discover substantially more emission.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2016-11-11T17:04:44.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "diffuse clouds",
      "radio wavelengths",
      "carbon monoxide",
      "fractionation",
      "standard uv radiation"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 0,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}