Revisiting short-plateau SN 2018gj

TL;DR

This paper presents a hydrodynamic model of SN 2018gj that suggests a large ejected mass and high Ni-56 velocity, challenging previous assumptions based on its short plateau and early gamma-ray escape.

Contribution

It introduces a comprehensive hydrodynamic model for SN 2018gj, emphasizing the role of ejecta asymmetry and clumpiness in type-IIP supernovae, which is a novel approach.

Findings

High ejected mass (~23 Msun) despite short plateau

High Ni-56 velocity explains early gamma-ray escape

Outer ejecta are clumpy, indicating inherent asymmetry

Abstract

We present an alternative model of unusual type-IIP SN 2018gj. Despite the short plateau and early gamma-rays escape seeming to favor low-mass ejecta, our hydrodynamic model requires a large ejected mass (about 23 Msun). The high ejecta velocity, we find from hydrogen lines in early spectra, is 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 Ni-56, whereas the asymmetry of the H-alpha emission is attributed to the asymmetry of the Ni-56 ejecta. The available sample of type-IIP supernovae studied hydrodynamically in a uniform way indicates that the asymmetry of the Ni-56 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.