Abstract Core-collapse supernovae are explosions of massive stars. While most massive stars end as iron-core-collapse supernovae, less massive stars are expected to explode as electron-capture supernovae (ECSNe), defining the low-mass boundary of core-collapse supernovae. ECSNe were proposed ∼40 yr ago, and first-principles simulations predict their successful explosions with low energies of ∼10 50 erg. Nevertheless, only one convincing candidate, SN 2018zd, has been proposed other than SN 1054, the progenitor of the Crab Nebula. We search for ECSN candidates among Type II SNe from the literature and a public Zwicky Transient Facility sample, using a color-based diagnostic, selecting ten candidates with blue colors at the middle of the plateau. We classify three as gold , for which a spectrum around the middle of the plateau disfavors strong interaction with the circumstellar medium that would make the SN bluer, and seven as silver without such spectra. Comparing the observed multicolor light curves with radiation-hydrodynamical models, we infer the explosion energies, (0.4–1.7) × 10 50 erg for the gold candidates and (0.4–2.7) × 10 50 erg including the silver candidates, consistent with first-principles predictions and the mass-loss rates, 3 × 10 −3 –3 × 10 −2 M ⊙ yr −1 for the gold candidates, which remain similar when the silver candidates are included, and are higher than those expected for the early super-asymptotic-giant-branch phase. The ECSN occurrence ratios among SNe II are inferred as 3 . 0 − 2.9 + 10.6 % and 15 . 7 − 12.7 + 17.3 % from the gold and silver candidates, respectively, which we interpret as lower and upper limits. To robustly identify ECSNe and refine this ratio, spectroscopic follow-ups of ECSN candidates around the middle of the plateau are essential.
Sato et al. (Thu,) studied this question.