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 60 mag (100 d)1. Owing to the weakening of Hi 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 show significant differences from typical SNe IIb. Using a radiative diffusion model, we find that the light curve requires a high synthesised 56Ni mass 𝑀Ni 04M and ejecta with high kinetic energy 𝐸kin = (7–10) 1051 erg. Introducing a magnetar central engine still requires 𝑀Ni 03M and 𝐸kin = 31051 erg. The high 56Ni mass is consistent with strong iron-group nebular lines in its spectra, which are also similar to several SNe Ic-BL with high 56Ni yields. The earliest spectrum shows “flash ionisation" features, from which we estimate a mass-loss rate of ¤ 𝑀 2 104M yr1. 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 s1 for H, which is remarkably high compared to other SNe II. We estimate an oxygen core mass of 1.8–3.4M using the [O i] luminosity measured from a nebular-phase spectrum, implying a progenitor with a zero-age main sequence mass of 19–26M.
Mon. Not. R. Astron. Soc. 000, 1–23