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*

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. Very Large Telescope/FORS2 observations in the nebular phase at 148 days 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) less than or similar to -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 similar to 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 a 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. The 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.

Automated summary

SN 2021ocs, a normal Type Ic supernova, was discovered in the galaxy NGC 7828, exhibiting unique oxygen and magnesium emission lines in its spectrum. It also displayed an atypical blue color after the peak, suggesting late-time interaction with a H/He-poor circumstellar medium. This is the first reported case of its kind, highlighting the significance of late-time spectroscopy in studying supernovae.