ASASSN-18am/SN 2018gk : An overluminous Type IIb supernova from a massive progenitor. (arXiv:2007.00008v2 [astro-ph.HE] UPDATED)

ASASSN-18am/SN 2018gk : An overluminous Type IIb supernova from a massive progenitor. (arXiv:2007.00008v2 [astro-ph.HE] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Bose_S/0/1/0/all/0/1">Subhash Bose</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dong_S/0/1/0/all/0/1">Subo Dong</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kochanek_C/0/1/0/all/0/1">C. S. Kochanek</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Stritzinger_M/0/1/0/all/0/1">M. D. Stritzinger</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ashall_C/0/1/0/all/0/1">Chris Ashall</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Benetti_S/0/1/0/all/0/1">Stefano Benetti</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Falco_E/0/1/0/all/0/1">E. Falco</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Filippenko_A/0/1/0/all/0/1">Alexei V. Filippenko</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pastorello_A/0/1/0/all/0/1">Andrea Pastorello</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Prieto_J/0/1/0/all/0/1">Jose L. Prieto</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Somero_A/0/1/0/all/0/1">Auni Somero</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sukhbold_T/0/1/0/all/0/1">Tuguldur Sukhbold</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zhang_J/0/1/0/all/0/1">Junbo Zhang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Auchettl_K/0/1/0/all/0/1">Katie Auchettl</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Brink_T/0/1/0/all/0/1">Thomas G. Brink</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Brown_J/0/1/0/all/0/1">J. S. Brown</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Chen_P/0/1/0/all/0/1">Ping Chen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fiore_A/0/1/0/all/0/1">A. Fiore</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Grupe_D/0/1/0/all/0/1">Dirk Grupe</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Holoien_T/0/1/0/all/0/1">T. W. -S. Holoien</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lundqvist_P/0/1/0/all/0/1">Peter Lundqvist</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Mattila_S/0/1/0/all/0/1">Seppo Mattila</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Mutel_R/0/1/0/all/0/1">Robert Mutel</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pooley_D/0/1/0/all/0/1">David Pooley</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Post_R/0/1/0/all/0/1">R. S. Post</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Reddy_N/0/1/0/all/0/1">N. Reddy</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Reynolds_T/0/1/0/all/0/1">Thomas M. Reynolds</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shappee_B/0/1/0/all/0/1">Benjamin J. Shappee</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Stanek_K/0/1/0/all/0/1">K. Z. Stanek</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Thompson_T/0/1/0/all/0/1">Todd A. Thompson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Villanueva_S/0/1/0/all/0/1">S. Villanueva Jr.</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zheng_W/0/1/0/all/0/1">WeiKang Zheng</a>

ASASSN-18am/SN 2018gk is a newly discovered member of the rare group of
luminous, hydrogen-rich supernovae (SNe) with a peak absolute magnitude of $M_V
approx -20$ mag that is in between normal core-collapse SNe and superluminous
SNe. These SNe show no prominent spectroscopic signatures of ejecta interacting
with circumstellar material (CSM), and their powering mechanism is debated.
ASASSN-18am declines extremely rapidly for a Type II SN, with a
photospheric-phase decline rate of $sim6.0~rm mag~(100 d)^{-1}$. Owing to the
weakening of HI and the appearance of HeI in its later phases, ASASSN-18am is
spectroscopically a Type IIb SN with a partially stripped envelope. However,
its photometric and spectroscopic evolution show significant differences from
typical SNe IIb. Using a radiative diffusion model, we find that the light
curve requires a high synthesised $rm ^{56}Ni$ mass $M_{rm Ni}
sim0.4~M_odot$ and ejecta with high kinetic energy $E_{rm kin} = (7-10)
times10^{51} $ erg. Introducing a magnetar central engine still requires
$M_{rm Ni} sim0.3~M_odot$ and $E_{rm kin}= 3times10^{51} $ erg. The high
$rm ^{56}Ni$ mass is consistent with strong iron-group nebular lines in its
spectra, which are also similar to several SNe Ic-BL with high $rm ^{56}Ni$
yields. The earliest spectrum shows “flash ionisation” features, from which we
estimate a mass-loss rate of $ dot{M}approx 2times10^{-4}~rm
M_odot~yr^{-1} $. This wind density is too low to power the luminous light
curve by ejecta-CSM interaction. We measure expansion velocities as high as $
17,000 $ km/s for $H_alpha$, which is remarkably high compared to other SNe
II. We estimate an oxygen core mass of $1.8-3.4$ $M_odot$ using the [OI]
luminosity measured from a nebular-phase spectrum, implying a progenitor with a
zero-age main sequence mass of $19-26$ $M_odot$.

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