A Robust Light-Curve Diagnostic for Electron-Capture Supernovae and Low-Mass Fe-Core-Collapse Supernovae

TL;DR

This paper introduces a robust light-curve diagnostic based on color evolution to distinguish electron-capture supernovae from low-mass iron-core-collapse supernovae, supported by synthetic models and applied to SN 2018zd.

Contribution

It presents a new, distance-independent method using color evolution to identify ECSNe, validated with synthetic light curves and applied to real observations.

Findings

ECSNe have shorter, brighter, and bluer plateaus than FeCCSNe.

The proposed color-based criterion successfully identified SN 2018zd as an ECSN.

Multicolor observations are crucial for distinguishing ECSNe from FeCCSNe.

Abstract

Core-collapse supernovae (CCSNe) are the terminal explosions of massive stars. While most massive stars explode as iron-core-collapse supernovae (FeCCSNe), slightly less massive stars explode as electron-capture supernovae (ECSNe), shaping the low-mass end of CCSNe. ECSNe was proposed $\sim 40$ years ago and first-principles simulations also predict their successful explosions. Observational identification and investigation of ECSNe are important for the completion of stellar evolution theory. To date, only one promising candidate has been proposed, SN 2018zd, other than the historical progenitor of the Crab Nebula, SN 1054. We present representative synthetic light curves of low-mass FeCCSNe and ECSNe exploding with energies in circumstellar media (CSM) estimated with theoretically or observationally plausible methods. The plateaus of the ECSNe are shorter, brighter, and bluer than those of the FeCCSNe. To investigate the robustness of their intrinsic differences, we adopted various explosion energies and CSM. Although they may have similar bolometric light-curve plateaus, ECSNe are bluer than FeCCSNe in the absence of strong CSM interaction, illustrating that multicolor observations are essential to identify ECSNe. This provides a robust indicator of ECSNe because the bluer plateaus stem from the low-density envelopes of their super-asymptotic-giant-branch progenitors. Furthermore, we propose a distance-independent method to identify ECSNe: $(g-r)_{t_{\rm PT}/2} < 0.008 \times t_{\rm PT} - 0.4$, i.e., blue $g-r$ at the middle of the plateau $(g-r)_{t_{\rm PT}/2}$, where $t_{\rm PT}$ is the transition epoch from plateau to tail. Using this method, we identified SN 2018zd as an ECSN, which we believe to be the first ECSN identified with modern observing techniques.