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 $\sim \! 2 \, M_{\odot}$, explosion energy $\sim \! 10^{52}$ erg, a circumstellar medium of mass $\sim \! 0.3 \, M_{\odot}$ and radius $\sim \! 1000 \, R_{\odot}$, and a $^{56}$Ni mass of $\sim \! 0.3 \, M_{\odot}$ 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.