SN 2016hil-- a Type II supernova in the remote outskirts of an elliptical host and its origin
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SN 2016hil-- a Type II supernova in the remote outskirts of an elliptical host and its origin

  • Author(s): Irani, Ido
  • Schulze, Steve
  • Gal-Yam, Avishay
  • Lunnan, Ragnhild
  • Filippenko, Alexei V
  • Sollerman, Jesper
  • Yang, Yi
  • Brink, Thomas G
  • Zheng, WeiKang
  • Jaeger, Thomas de
  • Nugent, Peter E
  • Kasliwal, Mansi M
  • Fremling, Christoffer
  • Neill, James Don
  • Rebbapragada, Umaa
  • Masci, Frank J
  • Yaron, Ofer
  • et al.

Type II supernovae (SNe) stem from the core collapse of massive ($>8\ M_{\odot}$) stars. Owing to their short lifespan, we expect a very low rate of such events in elliptical host galaxies, where the star-formation rate is low, and which mostly consist of an old stellar population. SN 2016hil (iPTF16hil) is a Type II supernova located in the extreme outskirts of an elliptical galaxy at redshift $z=0.0608$ (projected distance $27.2$ kpc). It was detected near peak brightness ($M_{r} \approx -17$ mag) 9 days after the last nondetection. SN 2016hil has some potentially peculiar properties: while presenting a characteristic spectrum, the event was unusually short lived and declined by $\sim 1.5$ mag in $< 40$ days, following an apparently double-peaked light curve. Its spectra suggest a low metallicity ($Z<0.4\ Z_{\odot}$). We place a tentative upper limit on the mass of a potential faint host at $\log(M/M_{\odot}) =7.27^{+0.43}_{-0.24}$ using deep Keck optical imaging. In light of this, we discuss the possibility of the progenitor forming locally, and other more exotic formation scenarios such as a merger or common-envelope evolution causing a time-delayed explosion. Further observations of the explosion site in the ultraviolet are needed in order to distinguish between the cases. Regardless of the origin of the transient, observing a population of such seemingly hostless Type II SNe could have many uses, including an estimate the number of faint galaxies in a given volume, and tests of the prediction of a time-delayed population of core-collapse SNe in locations otherwise unfavorable for the detection of such events.

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