Electron-capture supernovae (ECSNe) are the explosions of super asymptotic giant branch (SAGB) stars with masses lower than red supergiants – progenitors of normal iron core-collapse supernovae (CCSNe). ECSNe represent the low-mass end of CCSN progenitors with various implications in astrophysics given their expected high abundance. However, they remain elusive with only one promising candidate (SN 2018zd) identified to date due to their complex nature and the lack of complete observations from the early photospheric (∼ 100 days) to late nebular (>200 days) phases, preventing their study as a population. Nebular spectroscopy is a key for identifying and investigating ECSN, as it probes their progenitor mass and distribution of each composition as well as explosive nucleosynthesis. We propose to obtain nebular-phase spectra of 4 ECSN candidates with Subaru/FOCAS ToO observations. The candidates will be selected during S24A from public transient surveys using a novel ECSN diagnostic applied to their photospheric-phase light curves. As the event rate is not high, the ToO observation is necessary not to miss the important opportunity to study the nature of ECSNe.
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