{
  "id": "2404.03112",
  "version": "v1",
  "published": "2024-04-03T23:25:18.000Z",
  "updated": "2024-04-03T23:25:18.000Z",
  "title": "On the Formation of the W-shaped O II Lines in Spectra of Type I Superluminous Supernovae",
  "authors": [
    "Sei Saito",
    "Masaomi Tanaka",
    "Paolo A. Mazzali",
    "Stephan Hachinger",
    "Kenta Hotokezaka"
  ],
  "comment": "19 pages, 11 figures, accepted for publication in ApJ",
  "categories": [
    "astro-ph.HE",
    "astro-ph.SR"
  ],
  "abstract": "H-poor superluminous supernovae (SLSNe-I) are characterized by O II lines around 4,000 - 4,500 A in pre-/near-maximum spectra, so-called W-shaped O II lines. As these lines are from relatively high excitation levels, they have been considered a sign of non-thermal processes, which may give a hint of power sources of SLSNe-I. However, the conditions for these lines to appear have not been understood well. In this work, we systematically calculate synthetic spectra to reproduce observed spectra of eight SLSNe-I, parameterizing departure coefficients from the nebular approximation in the SN ejecta (expressed as b_neb). We find that most of the observed spectra can be reproduced well with b_neb ~< 10, which means that no strong departure is necessary for the formation of the W-shaped O II lines. We also show that the appearance of the W-shaped O II lines is sensitive to temperature; only spectra with temperatures T ~ 14,000 - 16,000 K can produce the W-shaped O II lines without large departures. Based on this, we constrain the non-thermal ionization rate near the photosphere. Our results suggest that spectral features of SLSNe-I can give independent constraints on the power source through the non-thermal ionization rates.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2024-04-03T23:25:18.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "superluminous supernovae",
      "non-thermal ionization rate",
      "power source",
      "relatively high excitation levels",
      "independent constraints"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 19,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}