Electron-capture Supernova Candidates from Light Curves: Implications for Their Progenitors and Explosion Properties

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 about 40 years ago, and first-principles simulations predict their successful explosions with low energies of about 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 circumstellar-medium interaction 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) times10 50 erg for the gold candidates and (0.4-2.7) times10 50 erg including the silver candidates, consistent with first-principles predictions and the mass-loss rates, 3 times10-3 - 3 times 10-2 M yr-1 for the gold candidates, which remain similar when the silver candidates are included, higher than those expected for the early super-asymptotic-giant-branch phase. The ECSN occurrence ratios among SNe II are inferred as 3.0+10.6-2.9 and 15.7+17.3-12.7 % 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.