{
  "id": "1902.03647",
  "version": "v1",
  "published": "2019-02-10T18:16:45.000Z",
  "updated": "2019-02-10T18:16:45.000Z",
  "title": "The nitrogen carrier in protoplanetary disks",
  "authors": [
    "Klaus M. Pontoppidan",
    "Colette Salyk",
    "Andrea Banzatti",
    "Geoffrey A. Blake",
    "Catherine Walsh",
    "John H. Lacy",
    "Matthew J. Richter"
  ],
  "comment": "Accepted for publication in the Astrophysical Journal",
  "categories": [
    "astro-ph.SR",
    "astro-ph.GA"
  ],
  "abstract": "The dominant reservoirs of elemental nitrogen in protoplanetary disks have not yet been observationally identified. Likely candidates are HCN, NH$_3$ and N$_2$. The relative abundances of these carriers determine the composition of planetesimals as a function of disk radius due to strong differences in their volatility. A significant sequestration of nitrogen in carriers less volatile than N$_2$ is likely required to deliver even small amounts of nitrogen to the Earth and potentially habitable exo-planets. While HCN has been detected in small amounts in inner disks ($<10$ au), so far only relatively insensitive upper limits on inner disk NH$_3$ have been obtained. We present new Gemini-TEXES high resolution spectroscopy of the 10.75 $\\mu$m band of warm NH$_3$, and use 2-dimensional radiative transfer modeling to improve previous upper limits by an order of magnitude to $\\rm [NH_3/H_{nuc}]<10^{-7}$ at 1 au. These NH$_3$ abundances are significantly lower than those typical for ices in circumstellar envelopes ($[{\\rm NH_3/H_{nuc}}]\\sim 3\\times 10^{-6}$). We also consistently retrieve the inner disk HCN gas abundances using archival Spitzer spectra, and derive upper limits on the HCN ice abundance in protostellar envelopes using archival ground-based 4.7 $\\mu$m spectroscopy ([HCN$_{\\rm ice}$]/[H$_2$O$_{\\rm ice}$]$<1.5-9$\\%). We identify the NH$_3$/HCN ratio as an indicator of chemical evolution in the disk, and use this ratio to suggest that inner disk nitrogen is efficiently converted from NH$_3$ to N$_2$, significantly increasing the volatility of nitrogen in planet-forming regions.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2019-02-10T18:16:45.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "protoplanetary disks",
      "nitrogen carrier",
      "upper limits",
      "inner disk hcn gas abundances",
      "gemini-texes high resolution spectroscopy"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 0,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}