Late-time H/He-poor Circumstellar Interaction in the Type Ic Supernova SN 2021ocs: An Exposed Oxygen–Magnesium Layer and Extreme Stripping of the Progenitor*

Abstract Supernova (SN) 2021ocs was discovered in the galaxy NGC 7828 ( z = 0.01911) within interacting system Arp 144 and subsequently classified as a normal Type Ic SN around peak brightness. Very Large Telescope/FORS2 observations nebular phase at 148 days reveal that spectrum is dominated by oxygen magnesium emission lines of different transitions ionization states: O i , [O ], ii iii Mg . Such has no counterpart literature, though it bears few features similar to those some Ibn Icn SNe. Additionally, showed blue color, g − r ) ≲ −0.5 mag, after up late phases, atypical for SN. Together with spectrum, this suggests underwent late-time interaction an H/He-poor circumstellar medium (CSM) resulting from pre-SN progenitor mass loss during its final ∼1000 days. The strong absence C He suggest star’s O–Mg layer exposed, which places most extreme case massive envelope stripping SNe, followed (stripped C–O layer) He-rich This first time such reported literature. emphasizes importance spectroscopy even events, inner ejecta CSM loss.