{
  "id": "1708.03548",
  "version": "v1",
  "published": "2017-08-11T14:08:41.000Z",
  "updated": "2017-08-11T14:08:41.000Z",
  "title": "Carbon and Nitrogen Abundance Ratio In the Broad Line Region of Tidal Disruption Events",
  "authors": [
    "Chenwei Yang",
    "Tinggui Wang",
    "Gary J. Ferland",
    "Liming Dou",
    "Hongyan Zhou",
    "Ning Jiang",
    "Zhenfeng Sheng"
  ],
  "comment": "10 pages, 5 figures",
  "categories": [
    "astro-ph.GA",
    "astro-ph.HE"
  ],
  "abstract": "The rest-frame UV spectra of three recent tidal disruption events (TDEs), ASASSN-14li, PTF15af and iPTF16fnl display strong nitrogen emission lines but weak or undetectable carbon lines. In these three objects, the upper limits of the C III] $\\lambda 1908$/N III] $\\lambda 1750$ ratio are about two orders of magnitude lower than those of quasars, suggesting a high abundance ratio of [N/C]. With detailed photoionization simulations, we demonstrate that $C^{2+}$ and $N^{2+}$ are formed in the same zone, so the C III]/N III] ratio depends only moderately on the physical conditions in the gas and weakly on the shape of the ionizing continuum. There are smaller than $0.5$ dex variations in the line ratio over wide ranges of gas densities and ionization parameters at a given metallicity. This allows a robust estimate of the relative abundance ratio nitrogen to carbon. We derive a relative abundance ratio of [N/C]$>1.5$ for ASASSN-14li, and even higher for PTF15af and iPTF16fnl. This suggests that the broad line region in those TDE sources is made of nitrogen-enhanced core material that falls back at later times. Based on stellar evolution models, the lower limit of the disrupted star should be larger than 0.6M$_{\\sun}$. The chemical abundance of the line emitting gas gives a convincing evidence that the flares origin from stellar tidal disruptions. The coincidence of the weakness of the X-ray emission with the strong broad absorption lines in PTF15af, iPTF16fnl and the strong X-ray emission without such lines in ASASSN-li14 are in analogy to quasars with and without broad absorption lines.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2017-08-11T14:08:41.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "broad line region",
      "tidal disruption events",
      "nitrogen abundance ratio",
      "strong nitrogen emission lines",
      "display strong nitrogen emission"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 10,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}