ASASSN-18am/SN 2018gk: an overluminous Type IIb supernova from a massive progenitor. Issue 3 (4th March 2021)

• Record Type: Journal Article
• Authors: Bose, Subhash; Dong, Subo; Kochanek, C S; Stritzinger, M D; Ashall, Chris; Benetti, Stefano; Falco, E; Filippenko, Alexei V; Pastorello, Andrea; Prieto, Jose L; Somero, Auni; Sukhbold, Tuguldur; Zhang, Junbo; Auchettl, Katie; Brink, Thomas G; Brown, J S; Chen, Ping; Fiore, A; Grupe, Dirk; Holoien, T W-S; Lundqvist, Peter; Mattila, Seppo; Mutel, Robert; Pooley, David; Post, R S; Reddy, Naveen; Reynolds, Thomas M; Shappee, Benjamin J; Stanek, K Z; Thompson, Todd A; Villanueva, S; Zheng, WeiKang;
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 503:Issue 3(2021)
• Page Start: 3472
• Page End: 3491
• Publication Date: 2021-03-04
• Subjects: supernovae: general -- supernovae: individual: (ASASSN-18am/SN 2018gk) -- galaxies: individual: WISE J163554.27+400151.8; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stab629
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: 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 MV ≈ −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 ∼6.0 mag (100 d) −1 . Owing to the weakening of H i and the appearance of He i in its later phases, ASASSN-18am is spectroscopically a Type IIb SN with a partially stripped envelope. However, its photometric and spectroscopic evolution shows significant differences from typical SNe IIb. Using a radiative diffusion model, we find that the light curve requires a high synthesized 56 Ni mass $M_{\rm Ni} \sim 0.4\, \rm {M_{\odot }}$ and ejecta with high kinetic energy E kin = (7–10) × 10 51 erg. Introducing a magnetar central engine still requires $M_{\rm Ni} \sim 0.3\, \rm {M_{\odot }}$ and E kin = 3 × 10 51 erg. The high 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 56 Ni yields. The earliest spectrum shows 'flash ionization' features, from which we estimate a mass-loss rate of $\dot{M}\approx 2\times 10^{-4} \, \rm \rm {M_{\odot }}\, yr^{-1}$ . 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 $\rm {\, km\, s^{-1}}$ for Hα, which is remarkably high compared to other SNe II. We estimate an oxygen core mass of 1.8–3.4 M⊙ using the [O i ] luminosity measured from a nebular-phase spectrum, implying a progenitor with a zero-age main-sequence mass of 19–26 M⊙ .