Fast Blue Optical Transients due to Circumstellar Interaction and the Mysterious Supernova SN 2018gep

Abstract

The discovery of SN 2018gep (ZTF18abukavn) challenged our understanding of the late-phase evolution of massive stars and their supernovae (SNe). The fast rise in luminosity of this SN (spectroscopically classified as a broad-lined Type Ic SN), indicates that the ejecta interacts with a dense circumstellar medium (CSM), while an additional energy source such as ${}^{56}$Ni-decay is required to explain the late-time light curve. These features hint at the explosion of a massive star with pre-supernova mass-loss. In this work, we examine the physical origins of rapidly evolving astrophysical transients like SN 2018gep. We investigate the wave-driven mass-loss mechanism and how it depends on model parameters such as progenitor mass and deposition energy, searching for stellar progenitor models that can reproduce the observational data. A model with an ejecta mass $\sim 2M_{\odot }$, explosion energy $\sim 10^{52}$ erg, a circumstellar medium of mass $\sim 0.3M_{\odot }$ and radius $\sim 1000R_{\odot }$, and a ${}^{56}$Ni mass of $\sim 0.3M_{\odot }$ provides a good fit to the bolometric light curve. We also examine how interaction-powered light curves depend more generally on these parameters, and how ejecta velocities can help break degeneracies. We find both wave-driven mass-loss and mass ejection via pulsational pair-instability can plausibly create the dense CSM in SN 2018gep, but we favor the latter possibility.

22 pages, 28 figures, submitted to Astrophysical Journal at 10 March 2021, accepted for publication at 27 April 2021