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^*

The classification of SN 2021ocs as a Type Ic SN around the light-curve peak (Huber 2021) was obtained using the SNID tool (Blondin & Tonry 2007). Figure 1 shows a comparison between SN 2021ocs and the well-studied Type Ic SNe SN 1994I and SN 2007gr as two of the best matches obtained by SNID. The SN 2021ocs appears similar to normal Type Ic SNe around the light-curve peak. The lack of a narrow Na i D λ λ5889, 5896 absorption line in the spectrum (see SN 1994I, where the narrow absorption line is strong, and weaker in SN 2007gr) suggests that the amount of intervening extinction is minimal. Henceforth, we assume no host galaxy extinction for SN 2021ocs. The reported foreground extinction for NGC 7828 is similarly negligible, as it is at the level of the photometric uncertainty, A_V = 0.086 mag (Schlafly & Finkbeiner 2011, via NED).

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While the light curves of SN 2021ocs are not very well sampled, the earliest data points from ATLAS suggest that it was rising in brightness (Figure 2(a)), and the subsequent epochs indicate that it declined steadily, as is typical for most SESN light curves, though with slight fluctuations. Comparing to the templates of early-time light curves of SNe Ib/c (Taddia et al. 2015), it appears that the peak of the light curve occurred around a week after the brightest point in the photometry, which implies that the classification spectrum was taken within 1–2 weeks from the peak brightness, consistent with the phase determined from spectral matching. The comparison between the light curves and the templates suggests that SN 2021ocs peaked around M ∼ −16.7 mag, which is fainter than most SESNe but still within the range of the previously observed objects (Taddia et al. 2015; Zheng et al. 2022; see also, e.g., Perley et al. 2020, Figure 7). Alternatively, the peak could have occurred during the solar conjunction before the first detections, which would imply that the light curve could be broader and more luminous. However, in this case, the classification spectrum would suggest a considerably older phase. This alternative scenario cannot be ruled out but requires strong additional assumptions on the early spectral evolution in order to be consistent with the classification spectrum.

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In Figure 2(b), the light curves of SN 2021ocs are compared with those of SN 2007gr ¹⁸ and H-poor interacting SNe (see Section 3.3). Assuming typical early SN Ic evolution, SN 2021ocs shows a gradual decline after the light-curve peak. In general, SN 2021ocs shows a slower decline compared to the Ibn/Icn population, although there exist individual objects that exhibit significantly broader light curves (e.g., Ben-Ami et al. 2014; Kool et al. 2021).

While the light-curve peak brightness and width are unlikely to diverge significantly compared to the bulk of SESNe, SN 2021ocs appears to be peculiarly blue from the first detections in the g and r bands up until late phases. ¹⁹ The host-subtracted photometry from ZTF and the FORS2 photometry indicate (g − r) ≲ −0.5 mag throughout the evolution following the light-curve peak. Such a blue color is not normally seen in regular SESNe at epochs postpeak. Indeed, even at 2–3 weeks before maximum light, such a blue color is rare, and the color index usually stays >0 mag throughout the SN evolution (e.g., Taddia et al. 2015; Zheng et al. 2022).

During the decay phase, the light curves show possible flattening leading to a tail phase slope shallower than the ⁵⁶Co decay rate assuming complete γ-ray trapping (Figure 2(b), inset). A decay rate slower than ⁵⁶Co decay indicates that the light curve is not powered solely by radioactive decay and thus may also be powered by other processes, e.g., late-time ejecta–CSM interaction (e.g., Maeda et al. 2015; Dessart et al. 2022) or magnetar spin-down (e.g., Afsariardchi et al. 2021). The blue color is reminiscent of the interaction-powered Type Ibn SNe (see, e.g., Ho et al. 2021, Figure 12), suggesting that a similar powering mechanism is likely to be at play.

The nebular spectrum of SN 2021ocs, obtained +148 days after the o-band peak, shows characteristics not regularly seen in an SESN nebular spectrum. While the latter is typically dominated by strong emission lines of the [O i] λ λ6300, 6364 doublet, [Ca ii] λ λ7292, 7324 doublet, and Ca ii λ λ8498, 8542, 8662 triplet, SN 2021ocs shows a spectrum with more than five emission lines of similar strength across the spectrum (Figure 3). Such a spectrum has never been seen among ∼200 SESN nebular spectra in the literature (Taubenberger et al. 2009; Fang et al. 2022; Prentice et al. 2022), suggesting a very rare occurrence rate of under 1%. It is also markedly different compared to the nebular spectra of Type Ia SNe, which are dominated by Fe-peak elements (e.g., Taubenberger et al. 2013). Strong H emission lines are absent in SN 2021ocs, which supports the initial SESN classification. The emission lines in the spectrum may be attributed to different transitions and ionization states of O, i.e., O i, [O i], [O ii], and [O iii], with similar line widths of ∼6000 km s⁻¹ (FWHM). The commonly seen [O i] λ λ6300, 6364 doublet is present, possibly superposed on a broader base extending to ±12,000 km s⁻¹ that may be attributed to Fe ii (Dessart et al. 2021). The [O ii] λ λ7320, 7330 is present and likely to be blended with the commonly seen [Ca ii] λ λ7292, 7324. It is clear that the [O i] λ λ6300, 6364 and [Ca ii] λ λ7292, 7324 lines are not the strongest lines in the nebular spectrum, which is atypical for SESNe. Furthermore, the flux ratio of [O i] λ λ6300, 6364 to [Ca ii] λ λ7292, 7324 is typically >1 for Type Ic SNe (e.g., Fang et al. 2022), which is not the case in SN 2021ocs.

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In the red part of the spectrum, three permitted O i emission lines are seen at λλ7774, 8446, and 9263. ²⁰ While the O i λ7774 is weak in SESN nebular spectra, it is exceptionally strong in SN 2021ocs, suggesting high-density conditions. This line is possibly contaminated by Mg i λλ7877, 7896 in the red shoulder (Figure 4). The λ8446 line is similarly contaminated by the broad Ca ii triplet on the red side (see comparison with scaled SN 2007gr in Figure 3), although the peak is still clearly prominent. The [O i] λ9263 line is blended with Mg ii λ9224. The peak intensities of the [O i] λ8446 and λ9263 lines are similar to the O i λ7774 line. In typical SESNe, these two redder lines are either very weak or missing. These three O i lines are considered as the most persistent lines of oxygen in the optical and near-infrared regimes, as they appear over a broad range of conditions in spectroscopic experiments (Sansonetti & Martin 2005).

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In the blue, two broad [O iii] emission lines are seen, the λλ4959, 5007 doublet and λ4363. They are accompanied by broader emission lines on the red side (∼4600, 5200 Å), possibly arising from the Fe i/Fe ii complexes (see, e.g., Figure 7 of Dessart et al. 2021), with prominent peaks of Mg i and Mg ii at λλ4481, 4571, and 5170. Broad [O iii] at late phase is also seen in a number of interacting SESNe, e.g., SN 1993J (Matheson et al. 2000) and SN 2014C (Milisavljevic et al. 2015). The Mg appears conspicuously in the spectrum of SN 2021ocs, with other lines at λλ3832 (triplet), 8224 (possible blend with [O i] λ8221), and 9436 (doublet). The Mg emission lines are also seen weakly in some SNe Ibn such as SN 2006jc (Pastorello et al. 2007).

Comparing the nebular spectrum of SN 2021ocs to a number of interacting SNe yields few similarities while the spectrum remains unique and unparalleled in the literature. The blue color and light-curve flattening in SN 2021ocs may be explained by CSM interaction, which warrants a spectral comparison with interacting SNe. Figure 4 shows the SN 2021ocs spectrum compared to interacting SNe of Types IIn, Ibn, and Icn that have relatively good coverage in late phases. ²¹

The strong narrow H emission lines defining the Type IIn SNe (Schlegel 1990) are clearly absent in SN 2021ocs. The SNe Ibn show prominent narrow He emission lines and a rising blue part of the spectrum (Pastorello et al. 2007), which originates from Fe emission (Dessart et al. 2021, 2022). Similarly, this blue rise is also seen in SN 2021ocs, while strong He lines are absent except for the weak He i λ7065 and λ5876, which is blended with the Na i doublet λλ5890, 5896. Also, He i λ5016 may be contaminating the [O iii] doublet λλ4959, 5007. The weak He lines in the spectrum of SN 2021ocs suggest that little He is present.

Similar to SNe IIn and Ibn, SNe Icn (Gal-Yam et al. 2022) also show the Fe bumps at late time. In SNe Icn, the initially strong lines of ionized C disappear at later phases to give way to nebular O i lines in the red part of the spectrum. These O i lines are present in SN 2021ocs, which may suggest a connection to SNe Icn, which are interpreted as explosions of W-R stars within a H/He-deficient CSM. Considering the spectra and light curves, it is possible that SN 2021ocs fits this scenario as well, although the CSM interaction did not occur immediately after the explosion. The expanding ejecta interacted with H/He-poor CSM, and the interaction drove a reverse shock that ionized the O/Mg-rich outer part of the ejecta. Clumping and inhomogeneity in the ejecta, CSM, and ⁵⁶Ni distribution would cause different levels of compression and ionization stages in the gas, which could give rise to the various ionization states seen in the O and Mg emission lines. The absence/weakness of C and He lines sets SN 2021ocs apart from the general population of SNe Ibn and Icn.

Broad [O iii] λλ4959, 5007 emission is usually only seen in very late times in CCSNe, a few years after the explosion, and has been interpreted as evidence of a pulsar/magnetar wind nebula (Chevalier & Fransson 1992; Milisavljevic et al. 2018) alternative to CSM interaction. In this case, the line is also accompanied by [O i] and [O ii], which is interpreted as different ionization layers by photoionization. The [O iii] line is seen in a variety of SESNe, including the normal and superluminous ones (Milisavljevic et al. 2018). This subset of objects that show broad late-time [O iii] curiously display a narrow (∼2000 km s⁻¹) and relatively strong O i λ7774 line in the nebular phase around a half to 1 yr postexplosion, although in these cases, it is neither strong nor accompanied by the same set of lines seen in SN 2021ocs. Figure 5 shows the nebular spectrum of SN 2021ocs compared to such objects. Generally, the agreement is poor; while they show similar line profiles in [O ii] λλ7320, 7330 (sloping blue shoulder, contamination by [Ca ii]) and O i λ7774 (sloping red shoulder, contamination by Mg ii), striking differences are seen in the O i λ8446 line, Mg lines redward of 8000 Å, and the various strong lines in the blue, which all are absent or very weak in the comparison objects. The association of SN 2021ocs with a wind nebula caused by a magnetized central object is therefore weak, and CSM interaction remains as our preferred interpretation of the observed properties in the spectra and light curves. Drawing an analogy with Type Ibn/Icn SNe (e.g., Pastorello et al. 2007; Ben-Ami et al. 2014; Ho et al. 2021; Gal-Yam et al. 2022), SN 2021ocs shows a similarly blue (g − r) color, a rising blue continuum possibly extending to the ultraviolet (see interaction models of Dessart & Hillier 2022), and prominent emission lines of O and Mg, suggesting a similar mechanism of CSM interaction. Even with CSM interaction, SN 2021ocs appears to be underluminous (M_o = −16.7 mag, compared to M_R = −17.9 ± 0.7 mag for SNe Ic in the sample of Zheng et al. 2022). This suggests a small amount of ⁵⁶Ni, which is another similarity to SNe Ibn and Icn.

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In SN 2021ocs, the CSM interaction likely occurred past the light-curve peak. Clearly, if interaction is present, it cannot be strong early on. A delayed interaction may be interpreted as a detached or low-density nearby CSM, which was surrounding the SN progenitor star at the time of explosion. Furthermore, for the interaction to be dominating at late times, the amount of CSM must be small (see, e.g., Dessart et al. 2022). Assuming an ejecta velocity of 10,000 km s⁻¹, if CSM interaction started ∼50 days after the explosion, as inferred from the blue (g − r) color, then the distance traversed by the unimpeded SN ejecta would be ∼4 × 10¹⁵ cm from the progenitor star. This inferred CSM distance is similar to those in other interacting SESNe with detached CSM (e.g., SN 2017dio; Kuncarayakti et al. 2018). In the case of SN 2021ocs, as a Type Ic SN, its progenitor could have been a carbon–oxygen star with a wind velocity of ∼1000 km s⁻¹. If the CSM was formed through such a wind, the mass loss probed by the CSM interaction would have occurred ∼500 days before the SN. At this time, the progenitor star would have just finished the C-shell burning stage and starting Ne burning (Fuller 2017). The short CSM distance and time before the SN explosion indicate that the SN progenitor star could have been surrounded by CSM at the time of explosion, although not as embedded as in the case of SNe Icn/Ibn, which show a rapidly increasing CSM density toward the SN progenitor (steeper than r⁻²; Maeda & Moriya 2022). The immediate circumstellar environment of SN 2021ocs, therefore, was relatively clean compared to those of SNe Icn/Ibn, as the explosion was first seen as a normal Type Ic SN without CSM interaction. This suggests that the CSM density was low closer to the progenitor star, implying a possible CSM distribution in a detached torus or disk, clumps, or a shell, which in any case contains a central cavity. Two possible interpretations arise regarding the progenitor mass loss as it approaches the terminal explosion: (1) a low progenitor mass-loss rate, suggesting that mass loss may have become weaker once the stripping reached the inner O–Mg core, or (2) the progenitor ejected some material that pushed away slower CSM in the vicinity, creating a cavity. In either case, SN 2021ocs represents the most extreme case of envelope stripping where the O–Mg layer of the progenitor is exposed. Within the massive star SN ejecta–CSM interaction case, it is positioned at the end of the sequence of CSM buildup resulting from the progenitor stripping: IIn → Ibn → Icn → SN 2021ocs.