Revisiting short-plateau SN 2018gj

Abstract

We present an alternative model of unusual type-IIP SN 2018gj. Despite the short plateau and early gamma-ray escape seeming to favor low-mass ejecta, our hydrodynamic model requires a large ejected mass (≈23 \(M_{\odot }\)). The high ejecta velocity, we find from hydrogen lines in early spectra, is among the 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-ray escape is found to be due to the high velocity of ⁵⁶Ni, whereas the asymmetry of the H\(\alpha \) emission is attributed to the asymmetry of the ⁵⁶Ni ejecta. The available sample of type-IIP supernovae studied hydrodynamically in a uniform way indicates that the asymmetry of the ⁵⁶Ni ejecta is probably their intrinsic property. Hydrogen lines in the 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.