The electron-capture origin of supernova 2018zd

TL;DR

This paper presents strong evidence that supernova 2018zd originated from an electron-capture supernova, providing key insights into stellar evolution and supernova mechanisms in the transitional mass range.

Contribution

It identifies supernova 2018zd as the first supernova with comprehensive evidence supporting an electron-capture origin, based on six distinct indicators.

Findings

Progenitor likely a super-asymptotic giant branch star

Circumstellar material shows chemical enrichment

Explosion energy and nucleosynthesis consistent with electron-capture supernova

Abstract

In the transitional mass range ($\sim$ 8-10 solar masses) between white dwarf formation and iron core-collapse supernovae, stars are expected to produce an electron-capture supernova. Theoretically, these progenitors are thought to be super-asymptotic giant branch stars with a degenerate O+Ne+Mg core, and electron capture onto Ne and Mg nuclei should initiate core collapse. However, no supernovae have unequivocally been identified from an electron-capture origin, partly because of uncertainty in theoretical predictions. Here we present six indicators of electron-capture supernovae and show that supernova 2018zd is the only known supernova having strong evidence for or consistent with all six: progenitor identification, circumstellar material, chemical composition, explosion energy, light curve, and nucleosynthesis. For supernova 2018zd, we infer a super-asymptotic giant branch progenitor based on the faint candidate in the pre-explosion images and the chemically-enriched circumstellar material revealed by the early ultraviolet colours and flash spectroscopy. The light-curve morphology and nebular emission lines can be explained with the low explosion energy and neutron-rich nucleosynthesis produced in an electron-capture supernova. This identification provides insights into the complex stellar evolution, supernova physics, cosmic nucleosynthesis, and remnant populations in the transitional mass range.