ASASSN-18am/SN 2018gk : An overluminous Type IIb supernova from a massive progenitor

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 $M_V\approx -20$ 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 $\sim 6.0\mathrm{mag}(100d{)}^{-1}$. 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 ${}^{56}\mathrm{Ni}$ mass $M_{\mathrm{Ni}}\sim 0.4M_{\odot }$ and ejecta with high kinetic energy $E_{\mathrm{kin}}=(7-10)\times 10^{51}$ erg. Introducing a magnetar central engine still requires $M_{\mathrm{Ni}}\sim 0.3M_{\odot }$ and $E_{\mathrm{kin}}=3\times 10^{51}$ erg. The high ${}^{56}\mathrm{Ni}$ mass is consistent with strong iron-group nebular lines in its spectra, which are also similar to several SNe Ic-BL with high ${}^{56}\mathrm{Ni}$ yields. The earliest spectrum shows "flash ionisation" features, from which we estimate a mass-loss rate of $\dot{M}\approx 2\times 10^{-4}{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$. 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 $H_{\hat{I}}\pm$, 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 $19-26$ $M_{\odot }$.