{
  "id": "1704.08621",
  "version": "v1",
  "published": "2017-04-27T15:27:02.000Z",
  "updated": "2017-04-27T15:27:02.000Z",
  "title": "Spatially resolved images of reactive ions in the Orion Bar",
  "authors": [
    "Javier R. Goicoechea",
    "S. Cuadrado",
    "J. Pety",
    "E. Bron",
    "J. H. Black",
    "J. Cernicharo",
    "E. Chapillon",
    "A. Fuente",
    "M. Gerin"
  ],
  "comment": "Accepted for publication in A&A letters (4 figures and 5 tables with Appendices)",
  "categories": [
    "astro-ph.GA"
  ],
  "abstract": "We report high angular resolution (4.9\" x 3.0\") images of reactive ions SH+, HOC+, and SO+ toward the Orion Bar photodissociation region (PDR). We used ALMA-ACA to map several rotational lines at 0.8 mm, complemented with multi-line observations obtained with the IRAM 30m telescope. The SH+ and HOC+ emission is restricted to a narrow layer of 2\"- to 10\"-width (~800 to 4000 AU depending on the assumed PDR geometry) that follows the vibrationally excited H2^* emission. Both ions efficiently form very close to the H/H2 transition zone, at a depth of A_V < 1 mag into the neutral cloud, where abundant C+, S+, and H2^* coexist. SO+ peaks slightly deeper into the cloud. The observed ions have low rotational temperatures (T_rot~10-30 K << T_k) and narrow line-widths (~2-3 km/s), a factor of ~2 narrower that those of the lighter reactive ion CH+. This is consistent with the higher reactivity and faster radiative pumping rates of CH+ compared to the heavier ions, which are driven relatively faster toward smaller velocity dispersion by elastic collisions and toward lower T_rot by inelastic collisions. We estimate column densities and average physical conditions from a non-LTE excitation model (n(H2)~10^5-10^6 cm^-3, n(e^-)~10 cm^-3, and T_k~200 K). Regardless of the excitation details, SH+ and HOC+ clearly trace the most exposed layers of the UV-irradiated molecular cloud surface, whereas SO+ arises from slightly more shielded layers.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2017-04-27T15:27:02.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "reactive ion",
      "spatially resolved images",
      "report high angular resolution",
      "orion bar photodissociation region",
      "iram 30m telescope"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 0,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}