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

TL;DR

SN 2021ocs, a type-Ic supernova, shows unique late-time spectral features indicating extreme stripping of its progenitor's envelope and interaction with a hydrogen/helium-poor circumstellar medium, revealing insights into massive star evolution.

Contribution

This is the first report of a type-Ic supernova with exposed oxygen-magnesium layers and late-time interaction with a H/He-poor circumstellar medium.

Findings

Spectral dominance of oxygen and magnesium emission lines in nebular phase.

Evidence of late-time interaction with H/He-poor circumstellar medium.

SN 2021ocs exhibits the most extreme envelope stripping among interacting supernovae.

Abstract

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