SN 2016iet: The Pulsational or Pair Instability Explosion of a Low Metallicity Massive CO Core Embedded in a Dense Hydrogen-Poor Circumstellar Medium. (arXiv:1904.07259v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Gomez_S/0/1/0/all/0/1">Sebastian Gomez</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Berger_E/0/1/0/all/0/1">Edo Berger</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nicholl_M/0/1/0/all/0/1">Matt Nicholl</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Blanchard_P/0/1/0/all/0/1">Peter K. Blanchard</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Villar_V/0/1/0/all/0/1">V. Ashley Villar</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Patton_L/0/1/0/all/0/1">Locke Patton</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Chornock_R/0/1/0/all/0/1">Ryan Chornock</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Leja_J/0/1/0/all/0/1">Joel Leja</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hosseinzadeh_G/0/1/0/all/0/1">Griffin Hosseinzadeh</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cowperthwaite_P/0/1/0/all/0/1">Philip S. Cowperthwaite</a>
We present optical photometry and spectroscopy of SN 2016iet, an
unprecedented Type I supernova (SN) at $z=0.0676$ with no obvious analog in the
existing literature. The peculiar light curve has two roughly equal brightness
peaks ($approx -19$ mag) separated by 100 days, and a subsequent slow decline
by 5 mag in 650 rest-frame days. The spectra are dominated by emission lines of
calcium and oxygen, with a width of only $3400$ km s$^{-1}$, superposed on a
strong blue continuum in the first year, and with a large ratio of $L_{rm
[Ca,II]}/L_{rm [O,I]}approx 4$ at late times. There is no clear evidence
for hydrogen or helium associated with the SN at any phase. We model the light
curves with several potential energy sources: radioactive decay, central
engine, and circumstellar medium (CSM) interaction. Regardless of the model,
the inferred progenitor mass near the end of its life (i.e., CO core mass) is
$gtrsim 55$ M$_odot$ and up to $120$ M$_odot$, placing the event in the
regime of pulsational pair instability supernovae (PPISNe) or pair instability
supernovae (PISNe). The models of CSM interaction provide the most consistent
explanation for the light curves and spectra, and require a CSM mass of
$approx 35$ M$_odot$ ejected in the final decade before explosion. We further
find that SN 2016iet is located at an unusually large offset ($16.5$ kpc) from
its low metallicity dwarf host galaxy ($Zapprox 0.1$ Z$_odot$, $Mapprox
10^{8.5}$ M$_odot$), supporting the PPISN/PISN interpretation. In the final
spectrum, we detect narrow H$alpha$ emission at the SN location, likely due to
a dim underlying galaxy host or an H II region. Despite the overall consistency
of the SN and its unusual environment with PPISNe and PISNe, we find that the
inferred properties of SN,2016iet challenge existing models of such events.
We present optical photometry and spectroscopy of SN 2016iet, an
unprecedented Type I supernova (SN) at $z=0.0676$ with no obvious analog in the
existing literature. The peculiar light curve has two roughly equal brightness
peaks ($approx -19$ mag) separated by 100 days, and a subsequent slow decline
by 5 mag in 650 rest-frame days. The spectra are dominated by emission lines of
calcium and oxygen, with a width of only $3400$ km s$^{-1}$, superposed on a
strong blue continuum in the first year, and with a large ratio of $L_{rm
[Ca,II]}/L_{rm [O,I]}approx 4$ at late times. There is no clear evidence
for hydrogen or helium associated with the SN at any phase. We model the light
curves with several potential energy sources: radioactive decay, central
engine, and circumstellar medium (CSM) interaction. Regardless of the model,
the inferred progenitor mass near the end of its life (i.e., CO core mass) is
$gtrsim 55$ M$_odot$ and up to $120$ M$_odot$, placing the event in the
regime of pulsational pair instability supernovae (PPISNe) or pair instability
supernovae (PISNe). The models of CSM interaction provide the most consistent
explanation for the light curves and spectra, and require a CSM mass of
$approx 35$ M$_odot$ ejected in the final decade before explosion. We further
find that SN 2016iet is located at an unusually large offset ($16.5$ kpc) from
its low metallicity dwarf host galaxy ($Zapprox 0.1$ Z$_odot$, $Mapprox
10^{8.5}$ M$_odot$), supporting the PPISN/PISN interpretation. In the final
spectrum, we detect narrow H$alpha$ emission at the SN location, likely due to
a dim underlying galaxy host or an H II region. Despite the overall consistency
of the SN and its unusual environment with PPISNe and PISNe, we find that the
inferred properties of SN,2016iet challenge existing models of such events.
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