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 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) ≲ -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 ~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. *Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO program 108.2282.001.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- December 2022
- DOI:
- 10.3847/2041-8213/aca672
- arXiv:
- arXiv:2210.01755
- Bibcode:
- 2022ApJ...941L..32K
- Keywords:
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- Supernovae;
- Core-collapse supernovae;
- Ejecta;
- Circumstellar matter;
- Massive stars;
- Late stellar evolution;
- Wolf-Rayet stars;
- 1668;
- 304;
- 453;
- 241;
- 732;
- 911;
- 1806;
- Astrophysics - Solar and Stellar Astrophysics;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- Published, https://iopscience.iop.org/article/10.3847/2041-8213/aca672