Revisiting short-plateau SN 2018gj

Abstract

We present an alternative model of unusual type IIP SN 2018gj. Despite the short plateau and early gamma-rays escape seem to favor low-mass ejecta, our hydrodynamic model requires the large ejected mass (≈23 M⊙). The high ejecta velocity, we find from hydrogen lines in early spectra, turns out among crucial constraints on the hydrodynamic model. We recover the wind density that rules out a notable contribution of the circumstellar interaction to the bolometric luminosity. The early radioactive gamma-rays escape is found to be due to the high velocity of 56 Ni, whereas the asymmetry of the Hα emission is attributed to the asymmetry of the 56 Ni ejecta. The available sample of type IIP supernovae studied hydrodynamically in a uniform way indicates that the asymmetry of the 56 Ni ejecta is probably their intrinsic property. Hydrogen lines in early spectra of SN 2018gi and SN 2020jfo are found to imply a clumpy structure of the outer ejecta. With two already known similar cases of SN 2008in and SN 2012A we speculate that the clumpiness of the outer ejecta is inherent to type IIP supernovae related to the red supergiant explosion.