{
  "id": "1808.00510",
  "version": "v1",
  "published": "2018-08-01T19:01:11.000Z",
  "updated": "2018-08-01T19:01:11.000Z",
  "title": "Nebular-phase spectra of superluminous supernovae: physical insights from observational and statistical properties",
  "authors": [
    "Matt Nicholl",
    "Edo Berger",
    "Peter K Blanchard",
    "Sebastian Gomez",
    "Ryan Chornock"
  ],
  "comment": "Submitted to ApJ",
  "categories": [
    "astro-ph.HE",
    "astro-ph.SR"
  ],
  "abstract": "We study the spectroscopic evolution of superluminous supernovae (SLSNe) later than 100 days after maximum light. We present new data for Gaia16apd and SN2017egm, and analyse these with a larger sample comprising 41 spectra of 12 events. The spectra become nebular within 2-4 $e$-folding times after light curve peak, with the rate of spectroscopic evolution correlated to the light curve timescale. Emission lines are identified with well-known transitions of oxygen, calcium, magnesium, sodium and iron. SLSNe are differentiated from other Type Ic SNe by a prominent O I $\\lambda$7774 line and higher-ionisation states of oxygen. The iron-dominated region around 5000 \\AA\\ is more similar to broad-lined SNe Ic than to normal SNe Ic. Principal Component Analysis shows that 5 `eigenspectra' capture 75% of the variance, while a clustering analysis shows no clear evidence for multiple SLSN sub-classes. Line velocities are 5000--8000 km/s, and show stratification of the ejecta. O I $\\lambda$7774 likely arises in a dense inner region that also produces calcium emission, while [O I] $\\lambda$6300 comes from further out until 300--400 days. The luminosities of O I $\\lambda$7774 and Ca II suggest significant clumping, in agreement with previous studies. Ratios of [Ca II]$\\lambda$7300/[O I]$\\lambda$6300 favour progenitors with relatively massive helium cores, likely $\\gtrsim 6$ M$_\\odot$, though more modelling is required here. SLSNe with broad light curves show the strongest [O I] $\\lambda$6300, suggesting larger ejecta masses. We show how the inferred velocity, density and ionisation structure point to a central power source.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2018-08-01T19:01:11.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "superluminous supernovae",
      "nebular-phase spectra",
      "physical insights",
      "statistical properties",
      "observational"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 0,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}