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ZTF 18aaqeasu (SN2018byg): A Massive Helium-shell Double Detonation on a Sub-Chandrasekhar-mass White Dwarf

  • Author(s): De, Kishalay
  • Kasliwal, Mansi M
  • Polin, Abigail
  • Nugent, Peter E
  • Bildsten, Lars
  • Adams, Scott M
  • Bellm, Eric C
  • Blagorodnova, Nadia
  • Burdge, Kevin B
  • Cannella, Christopher
  • Cenko, S Bradley
  • Dekany, Richard G
  • Feeney, Michael
  • Hale, David
  • Fremling, U Christoffer
  • Graham, Matthew J
  • Ho, Anna YQ
  • Jencson, Jacob E
  • Kulkarni, SR
  • Laher, Russ R
  • Masci, Frank J
  • Miller, Adam A
  • Patterson, Maria T
  • Rebbapragada, Umaa
  • Riddle, Reed L
  • Shupe, David L
  • Smith, Roger M
  • et al.

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© 2019. The American Astronomical Society. All rights reserved. The detonation of a helium shell on a white dwarf (WD) has been proposed as a possible explosion triggering mechanism for SNe Ia. Here, we report ZTF 18aaqeasu (SN 2018byg/ATLAS 18pqq), a peculiar Type I supernova, consistent with being a helium-shell double-detonation. With a rise time of ≈18 days from explosion, the transient reached a peak absolute magnitude of M R ≈ -18.2 mag, exhibiting a light curve akin to sub-luminous SN 1991bg-like SNe Ia, albeit with an unusually steep increase in brightness within a week from explosion. Spectra taken near peak light exhibit prominent Si absorption features together with an unusually red color (g - r ≈ 2 mag) arising from nearly complete line blanketing of flux blueward of 5000 . This behavior is unlike any previously observed thermonuclear transient. Nebular phase spectra taken at and after ≈30 days from peak light reveal evidence of a thermonuclear detonation event dominated by Fe-group nucleosynthesis. We show that the peculiar properties of ZTF 18aaqeasu are consistent with the detonation of a massive (≈0.15 ) helium shell on a sub-Chandrasekhar mass (≈0.75 ) WD after including mixing of ≈0.2 of material in the outer ejecta. These observations provide evidence of a likely rare class of thermonuclear supernovae arising from detonations of massive helium shells.

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