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*
H. Kuncarayakti1,2 , K. Maeda3 , L. Dessart4 , T. Nagao1 , M. Fulton5 , C. P. Gutiérrez1,2, M. E. Huber6 ,
D. R. Young5 , R. Kotak1 , S. Mattila1,7, J. P. Anderson8,9 , L. Ferrari10,11, G. Folatelli10,11,12, H. Gao6 , E. Magnier6 ,
K. W. Smith5 , and S. Srivastav5
1 Tuorla Observatory, Department of Physics and Astronomy, FI-20014 University of Turku, Finland
2 Finnish Centre for Astronomy with ESO (FINCA), FI-20014 University of Turku, Finland
3 Department of Astronomy, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
4 Institut d’Astrophysique de Paris, CNRS-Sorbonne Université, 98 bis boulevard Arago, F-75014 Paris, France
5 Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast, BT7 1NN, UK
6 Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
7 School of Sciences, European University Cyprus, Diogenes Street, Engomi, 1516 Nicosia, Cyprus
8 European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Santiago, Chile
9 Millennium Institute of Astrophysics MAS, Nuncio Monsenor Sotero Sanz 100, Off. 104, Providencia, Santiago, Chile
10 Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque S/N, B1900FWA La Plata, Argentina
11 Instituto de Astrofísica de La Plata (IALP), CONICET, Argentina
12 Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583, Japan
Received 2022 October 3; revised 2022 November 19; accepted 2022 November 27; published 2022 December 19
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.
Unified Astronomy Thesaurus concepts: Supernovae (1668); Core-collapse supernovae (304); Ejecta (453);
Circumstellar matter (241); Massive stars (732); Late stellar evolution (911); Wolf-Rayet stars (1806)
Supporting material: data behind figures
1. Introduction
Stripped-envelope supernovae (SESNe) include a broad
variety of events, all of which show little or no hydrogen in the
spectrum. Type Ic SNe are deficient in H and He; hence, the
progenitors are traditionally thought to be Wolf-Rayet (W-R)
stars (e.g., Woosley et al. 1995). The progenitor stars of SESNe
require significant mass loss in order to remove the outer H
envelope. Stellar winds, binary interaction, and eruptions are
among the prominent mechanisms (see, e.g., Smith 2014, for a
review). A highly massive single star (MZAMS 25Me) is
required to form a W-R star through wind stripping. This casts
doubt if there is sufficient number of W-R stars to explain the
observed rate of SESNe, which implies that a significant
number of SESNe must have come from lower-mass stars in
close binary systems (e.g., Yoon et al. 2010; Smith et al. 2011).
The SN progenitor mass loss may result in a circumstellar
medium (CSM), which may interact with the SN ejecta after the
explosion. The SNe interacting with H-free CSM are rare;
nevertheless, modern surveys have discovered a sample of
them, which are now classified into Types Ibn (He-rich CSM)
and Icn (H/He-poor CSM). Some of these objects are thought
to be explosions of genuine massive W-R stars (Pastorello et al.
2007; Smith 2017; Gal-Yam et al. 2022) or less massive stars
(Sanders et al. 2013; Davis et al. 2022; Dessart et al. 2022;
Pellegrino et al. 2022) within a dense CSM.
The SN 2021ocs was discovered by the Asteroid Terrestrial-
impact Last Alert System (ATLAS) survey (Tonry et al. 2018;
Smith et al. 2020) on 2021 May 30 (UTC time used
throughout) as ATLAS21ptp, in the host galaxy NGC 7828
(z = 0.01911, Tully–Fisher distance modulus μ= 34.71 mag;
Theureau et al. 2007, through NED13). The host galaxy is
interacting with a smaller galaxy, NGC 7829, forming the ring
The Astrophysical Journal Letters, 941:L32 (7pp), 2022 December 20 https://doi.org/10.3847/2041-8213/aca672
© 2022. The Author(s). Published by the American Astronomical Society.
* Based on observations collected at the European Organisation for
Astronomical Research in the Southern Hemisphere under ESO program
108.2282.001.
Original content from this work may be used under the terms
of the Creative Commons Attribution 4.0 licence. Any further
distribution of this work must maintain attribution to the author(s) and the title
of the work, journal citation and DOI.
13 The NASA/IPAC Extragalactic Database (NED), http://ned.ipac.caltech.
edu/.
1
galaxy system Arp 144. No meaningful prediscovery upper
limit exists, as the object was emerging from solar conjunction
at the time of discovery. The transient was subsequently
reported to the Transient Name Server (TNS14) by the Zwicky
Transient Facility (ZTF; Bellm et al. 2019) and Pan-STARRS1
(Chambers et al. 2016) surveys as ZTF21abhrpia and PS21hlk,
respectively. Spectral classification reported to the TNS
suggests that SN 2021ocs is a Type Ic SN around 1–2 weeks
after maximum light (Huber 2021). Here we report additional
spectral observations of SN 2021ocs obtained at late time (148
days after light-curve peak).15
2. Observations and Data Reduction
The classification spectrum of SN 2021ocs was obtained on
2021 June 13 using the SNIFS spectrograph (Lantz et al. 2004)
at the 2.2 m University of Hawaii telescope on Maunakea as
part of the Spectroscopic Classification of Astronomical
Transients survey (Tucker et al. 2022). Late-time spectroscopy
of SN 2021ocs was conducted on the night of 2021 October 26
as part of the FORS+ Survey of Supernovae in Late Times
program (FOSSIL; H. Kuncarayakti et al. 2022, in preparation)
using the FORS2 instrument (Appenzeller et al. 1998) attached
to the ESO Very Large Telescope at Cerro Paranal Observa-
tory, Chile. FOSSIL targets all observable core-collapse SNe
(CCSNe) brighter than ∼18.5 mag in the photospheric phase,
which are expected to be ∼21–22 mag or brighter when
observed in the nebular phase, with the goal of obtaining
nebular spectra for a large sample of objects in a magnitude-
limited, unbiased way.
We used FORS2 with grism 300V and the 1 6 slit,
achieving a wide wavelength coverage of 3500–9500Å and a
spectral resolution of R∼ 400 measured from the narrow sky
emission lines. The sky conditions were photometric with
seeing around 0 5 during the length of the integration. The
spectroscopic observations were obtained with 2× 1300 s
exposures accompanied by 20 s g-band imaging under 0 75
seeing conditions. Spectrophotometric standard stars were
observed using the same grism setting. The data were reduced
using the ESOReflex (Freudling et al. 2013) pipeline following
standard procedures. The excellent seeing conditions allowed
reasonable background subtraction during the spectrum
extraction, evidenced by the absence of narrow host galaxy
emission lines.
The g-band photometry of FORS2 was measured with our
own point-spread function photometry code. Synthetic photo-
metry was performed on the nebular spectrum to derive the g
and r synthetic magnitudes; the obtained (g− r) color was then
applied to the g-band photometry from imaging to produce an
r-band magnitude to be used in the light curve. In addition to
the FORS2 gr imaging, photometry was obtained from a
number of sources. Public ATLAS forced photometry in the o
and c bands was obtained from the ATLAS Forced Photometry
server,16 and ZTF g- and r-band photometry was obtained
through the ZTF forced photometry service17 (Masci et al.
2019). Forced photometry data from the Pan-STARRS1 survey
are used, yielding i- and w-band photometry.
3. Results and Discussion
3.1. Early Spectrum and Light Curve
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
λλ5889, 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, AV = 0.086 mag (Schlafly & Finkbeiner 2011,
via NED).
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 detec-
tions, 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
Figure 1. Classification spectrum of SN 2021ocs obtained shortly after the
light-curve peak compared to those of well-observed Type Ic SNe at similar
epochs. The spectra and phases (defined as time relative to the light-curve peak
throughout the paper) of the comparison objects were taken from the WISeREP
repository (Yaron & Gal-Yam 2012), originally from Modjaz et al. (2014).
Prominent absorption features typical of Type Ic SNe are indicated.
14 https://www.wis-tns.org/object/2021ocs
15 The spectrum and light curves are available as the Data behind the Figure.
16 https://fallingstar-data.com/forcedphot/
17 https://ztfweb.ipac.caltech.edu/cgi-bin/requestForcedPhotometry.cgi
2
The Astrophysical Journal Letters, 941:L32 (7pp), 2022 December 20 Kuncarayakti et al.
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.
In Figure 2(b), the light curves of SN 2021ocs are compared
with those of SN 2007gr18 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 photo-
metry 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.
3.2. Nebular Spectrum
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] λλ6300, 6364
doublet, [Ca II] λλ7292, 7324 doublet, and Ca II λλ8498, 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] λλ6300, 6364 doublet is present, possibly super-
posed 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] λλ7320,
7330 is present and likely to be blended with the commonly
seen [Ca II] λλ7292, 7324. It is clear that the [O I] λλ6300,
Figure 2. (a) Light curves of SN 2021ocs in ATLAS o (orange symbols) and c (cyan) with 5 day binning, ZTF 3 day binned 5σ detections and FORS2 in g (green) and
r (red), and Pan-STARRS w (dark purple) and i (light purple) bands. The template SN Ib/c light curves from Taddia et al. (2015) are plotted with dashed lines (blue
for g and red for r). The epochs of the first (classification) and second (nebular) spectra are indicated, along with the radioactive 56Co decay rate assuming complete γ-
ray trapping (0.01 mag day−1). The ATLAS c bandpass corresponds roughly to the g + r bands, while o covers r + i. The Pan-STARRS w band is a white-light
bandpass covering the gri bands. (b) Absolute-magnitude light-curve comparison against well-observed Type Icn SNe 2010mb (Ben-Ami et al. 2014), 2019hgp (Gal-
Yam et al. 2022), and 2021csp (Perley et al. 2022). The SN 2010mb is a peculiar case of Icn-related objects with its broad light curve. A light-curve template of SNe
Ibn (Hosseinzadeh et al. 2017) is also plotted alongside the Ib/c templates and the R-band light curve of SN 2007gr (μ = 29.84 mag; Hunter et al. 2009). The inset
shows a wider phase range covering Pan-STARRS late detections.
(The data used to create this figure are available.)
18 Obtained from the Open Astronomy Catalog API, https://github.com/
astrocatalogs/OACAPI.
19 Note that there is no r-band detection in the ZTF public data stream; see,
e.g., https://alerce.online/object/ZTF21abhrpia.
3
The Astrophysical Journal Letters, 941:L32 (7pp), 2022 December 20 Kuncarayakti et al.
6364 and [Ca II] λλ7292, 7324 lines are not the strongest lines
in the nebular spectrum, which is atypical for SESNe.
Furthermore, the flux ratio of [O I] λλ6300, 6364 to [Ca II]
λλ7292, 7324 is typically >1 for Type Ic SNe (e.g., Fang et al.
2022), which is not the case in SN 2021ocs.
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).
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, 5200Å),
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).
3.3. Interpretation: CSM Interaction or a Pulsar Wind Nebula?
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 inter-
acted 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
Figure 3. Nebular spectrum of SN 2021ocs (red) compared to typical nebular
spectra of SNe Ic and Ia at similar epochs. The comparison spectra are of SNe
2007gr (black; Shivvers et al. 2019) and 2003du (gray; Stanishev et al. 2007),
taken from WISeREP. The spectrum of SN 2021ocs shows a number of O lines
with superposed model lines, assuming a Gaussian profile with
FWHM = 6000 km s−1 (light blue). The flux of the spectrum of SN 2007gr
is scaled to match the r-band magnitude of SN 2021ocs at +148 days; the
spectrum is plotted twice for clarity of the comparison.
(The data used to create this figure are available.)
20 These lines are themselves multiplets of several oxygen transitions.
21 Some objects (i.e., SNe 2006jc and 2019hgp in this case) evolve more
rapidly; thus, the phases in days are shorter compared to the slower-evolving
objects.
4
The Astrophysical Journal Letters, 941:L32 (7pp), 2022 December 20 Kuncarayakti et al.
lines redward of 8000Å, 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 (Mo=−16.7 mag, compared to
MR=−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.
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
Figure 4. Comparison of the SN 2021ocs spectrum with interacting SNe of Types IIn (SN 1997cy; Turatto et al. 2000), Ibn (SN 2006jc; Pastorello et al. 2007), and
Icn (SN 2019hgp, Gal-Yam et al. 2022; SN 2010mb, Ben-Ami et al. 2014). The epochs are relative to maximum light. Spectra were taken from WISeREP. Significant
emission lines are identified.
5
The Astrophysical Journal Letters, 941:L32 (7pp), 2022 December 20 Kuncarayakti et al.
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.
4. Summary
This letter presents photometry and spectroscopy of
SN 2021ocs that yield a peculiar nebular spectrum dominated
by O and Mg emission lines. The unique set of emission lines,
blue continuum and color, and slowly declining light-curve tail
suggest that interaction with a H/He-poor CSM took place in
SN 2021ocs. The absence of signs of interaction in the early
spectrum suggests that the CSM density close to the progenitor
star was low. Comparing with interacting SESNe of Types Ibn
and Icn, SN 2021ocs appears to be more stripped as the deep
O–Mg layer in the progenitor is exposed, and the outer C–O
and He-rich layers are stripped away. The SN 2021ocs poses
yet another challenge to stellar evolution theory regarding the
final phases of evolution of massive stars. With its unique
spectral and photometric behavior, it represents a rare case not
previously considered. Modeling the evolutionary pathways,
mass loss, and explosion of a highly stripped star with an
exposed O–Mg layer will provide insights in comparison with
the observations. Future observations of transients should
consider targets with unusual colors, albeit a spectroscopically
normal appearance, in order to uncover similar peculiar objects.
The anonymous referee is thanked for the useful feedback
that substantially improved the paper. Stephen Smartt,
Masaomi Tanaka, and Lucy McNeill are thanked for the
discussions and suggestions on the paper. H.K. and T.N. were
funded by the Academy of Finland projects 324504 and
328898. The work is supported by the JSPS Open Partnership
Bilateral Joint Research Project between Japan and Finland
(JPJSBP120229923). K.M. acknowledges support from the
Japan Society for the Promotion of Science (JSPS) KAKENHI
grants JP18H05223 and JP20H00174. S.M. acknowledges
support from the Academy of Finland project 350458. R.K.
acknowledges support from the Academy of Finland project
340613. This work has made use of data from the Asteroid
Terrestrial-impact Last Alert System (ATLAS) project. The
ATLAS project is primarily funded to search for near-Earth
asteroids through NASA grants NN12AR55G,
80NSSC18K0284, and 80NSSC18K1575; by-products of the
NEO search include images and catalogs from the survey area.
This work was partially funded by Kepler/K2 grant J1944/
80NSSC19K0112, HST GO-15889, and STFC grants ST/
T000198/1 and ST/S006109/1. The ATLAS science products
have been made possible through the contributions of the
University of Hawaii Institute for Astronomy, the Queenʼs
University Belfast, the Space Telescope Science Institute, the
South African Astronomical Observatory, and the Millennium
Institute of Astrophysics (MAS), Chile. The Pan-STARRS1
Surveys (PS1) and the PS1 public science archive have been
Figure 5. Comparison of the SN 2021ocs spectrum with SNe harboring a pulsar/magnetar wind nebula in the sample of Milisavljevic et al. (2018): SN 2007bi
(SLSN-I; Gal-Yam et al. 2009), iPTF15dtg (Ic with broad light curves; Taddia et al. 2019), SN 2012au (Ib-peculiar; Milisavljevic et al. 2018), and SN 1997dq (Ic-
hypernova; Matheson et al. 2001; Taubenberger et al. 2009). The epochs are relative to maximum light. Spectra were taken from WISeREP. Significant emission lines
are identified.
6
The Astrophysical Journal Letters, 941:L32 (7pp), 2022 December 20 Kuncarayakti et al.
made possible through contributions by the Institute for
Astronomy, the University of Hawaii, the Pan-STARRS
Project Office, the Max Planck Society and its participating
institutes, the Max Planck Institute for Astronomy, Heidelberg
and the Max Planck Institute for Extraterrestrial Physics,
Garching, The Johns Hopkins University, Durham University,
the University of Edinburgh, the Queen’s University Belfast,
the Harvard-Smithsonian Center for Astrophysics, the Las
Cumbres Observatory Global Telescope Network Incorporated,
the National Central University of Taiwan, the Space Telescope
Science Institute, the National Aeronautics and Space Admin-
istration under grant No. NNX08AR22G issued through the
Planetary Science Division of the NASA Science Mission
Directorate, National Science Foundation grant No. AST-
1238877, the University of Maryland, Eotvos Lorand Uni-
versity (ELTE), the Los Alamos National Laboratory, and the
Gordon and Betty Moore Foundation. The ZTF forced
photometry service was funded under Heising-Simons Founda-
tion grant No. 12540303 (PI: Graham). This work was funded
by ANID, Millennium Science Initiative, ICN12_009.
Facilities: VLT:Antu(FORS2), UH:2.2m(SNIFS), PS1.
Software: IRAF (Tody 1986, 1993), ESOReflex (Freudling
et al. 2013).
ORCID iDs
H. Kuncarayakti https://orcid.org/0000-0002-1132-1366
K. Maeda https://orcid.org/0000-0003-2611-7269
L. Dessart https://orcid.org/0000-0003-0599-8407
T. Nagao https://orcid.org/0000-0002-3933-7861
M. Fulton https://orcid.org/0000-0003-1916-0664
M. E. Huber https://orcid.org/0000-0003-1059-9603
D. R. Young https://orcid.org/0000-0002-1229-2499
R. Kotak https://orcid.org/0000-0001-5455-3653
J. P. Anderson https://orcid.org/0000-0003-0227-3451
H. Gao https://orcid.org/0000-0003-1015-5367
E. Magnier https://orcid.org/0000-0002-7965-2815
K. W. Smith https://orcid.org/0000-0001-9535-3199
S. Srivastav https://orcid.org/0000-0003-4524-6883
References
Afsariardchi, N., Drout, M. R., Khatami, D. K., et al. 2021, ApJ, 918, 89
Appenzeller, I., Fricke, K., Fürtig, W., et al. 1998, Msngr, 94, 1
Bellm, E. C., Kulkarni, S. R., Graham, M. J., et al. 2019, PASP, 131, 018002
Ben-Ami, S., Gal-Yam, A., Mazzali, P. A., et al. 2014, ApJ, 785, 37
Blondin, S., & Tonry, J. L. 2007, ApJ, 666, 1024
Chambers, K. C., Magnier, E. A., Metcalfe, N., et al. 2016, arXiv:1612.05560
Chevalier, R. A., & Fransson, C. 1992, ApJ, 395, 540
Davis, K. W., Taggart, K., Tinyanont, S., et al. 2022, arXiv:2211.05134
Dessart, L., & Hillier, D. J. 2022, A&A, 660, L9
Dessart, L., Hillier, D. J., Sukhbold, T., Woosley, S. E., & Janka, H. T. 2021,
A&A, 656, A61
Dessart, L., John Hillier, D., & Kuncarayakti, H. 2022, A&A, 658, A130
Fang, Q., Maeda, K., Kuncarayakti, H., et al. 2022, ApJ, 928, 151
Freudling, W., Romaniello, M., Bramich, D. M., et al. 2013, A&A, 559, A96
Fuller, J. 2017, MNRAS, 470, 1642
Gal-Yam, A., Bruch, R., Schulze, S., et al. 2022, Natur, 601, 201
Gal-Yam, A., Mazzali, P., Ofek, E. O., et al. 2009, Natur, 462, 624
Ho, A. Y. Q., Perley, D. A., Gal-Yam, A., et al. 2021, arXiv:2105.08811
Hosseinzadeh, G., Arcavi, I., Valenti, S., et al. 2017, ApJ, 836, 158
Huber, M. E. 2021, TNSCR, 2021-2064, 1
Hunter, D. J., Valenti, S., Kotak, R., et al. 2009, A&A, 508, 371
Kool, E. C., Karamehmetoglu, E., Sollerman, J., et al. 2021, A&A, 652, A136
Kuncarayakti, H., Maeda, K., Ashall, C. J., et al. 2018, ApJL, 854, L14
Lantz, B., Aldering, G., Antilogus, P., et al. 2004, Proc. SPIE, 5249, 146
Maeda, K., Hattori, T., Milisavljevic, D., et al. 2015, ApJ, 807, 35
Maeda, K., & Moriya, T. J. 2022, ApJ, 927, 25
Masci, F. J., Laher, R. R., Rusholme, B., et al. 2019, PASP, 131, 018003
Matheson, T., Filippenko, A. V., Barth, A. J., et al. 2000, AJ, 120, 1487
Matheson, T., Filippenko, A. V., Li, W., Leonard, D. C., & Shields, J. C. 2001,
AJ, 121, 1648
Milisavljevic, D., Margutti, R., Kamble, A., et al. 2015, ApJ, 815, 120
Milisavljevic, D., Patnaude, D. J., Chevalier, R. A., et al. 2018, ApJL, 864, L36
Modjaz, M., Blondin, S., Kirshner, R. P., et al. 2014, AJ, 147, 99
Pastorello, A., Smartt, S. J., Mattila, S., et al. 2007, Natur, 447, 829
Pellegrino, C., Howell, D. A., Terreran, G., et al. 2022, ApJ, 938, 73
Perley, D. A., Fremling, C., Sollerman, J., et al. 2020, ApJ, 904, 35
Perley, D. A., Sollerman, J., Schulze, S., et al. 2022, ApJ, 927, 180
Prentice, S. J., Maguire, K., Siebenaler, L., & Jerkstrand, A. 2022, MNRAS,
514, 5686
Sanders, N. E., Soderberg, A. M., Foley, R. J., et al. 2013, ApJ, 769, 39
Sansonetti, J. E., & Martin, W. C. 2005, JPCRD, 34, 1559
Schlafly, E. F., & Finkbeiner, D. P. 2011, ApJ, 737, 103
Schlegel, E. M. 1990, MNRAS, 244, 269
Shivvers, I., Filippenko, A. V., Silverman, J. M., et al. 2019, MNRAS,
482, 1545
Smith, K. W., Smartt, S. J., Young, D. R., et al. 2020, PASP, 132, 085002
Smith, N. 2014, ARA&A, 52, 487
Smith, N. 2017, in Handbook of Supernovae, ed. A. W. Alsabti & P. Murdin
(Berlin: Springer), 403
Smith, N., Li, W., Filippenko, A. V., & Chornock, R. 2011, MNRAS,
412, 1522
Stanishev, V., Goobar, A., Benetti, S., et al. 2007, A&A, 469, 645
Taddia, F., Sollerman, J., Fremling, C., et al. 2019, A&A, 621, A64
Taddia, F., Sollerman, J., Leloudas, G., et al. 2015, A&A, 574, A60
Taubenberger, S., Kromer, M., Pakmor, R., et al. 2013, ApJL, 775, L43
Taubenberger, S., Valenti, S., Benetti, S., et al. 2009, MNRAS, 397, 677
Theureau, G., Hanski, M. O., Coudreau, N., Hallet, N., & Martin, J. M. 2007,
A&A, 465, 71
Tody, D. 1986, Proc. SPIE, 627, 733
Tody, D. 1993, in ASP Conf. Ser. 52, Astronomical Data Analysis Software
and Systems II, ed. R. J. Hanisch, R. J. V. Brissenden, & J. Barnes (San
Francisco, CA: ASP), 173
Tonry, J. L., Denneau, L., Heinze, A. N., et al. 2018, PASP, 130, 064505
Tucker, M. A., Shappee, B. J., Huber, M. E., et al. 2022, arXiv:2210.09322
Turatto, M., Suzuki, T., Mazzali, P. A., et al. 2000, ApJ, 534, L57
Woosley, S. E., Langer, N., & Weaver, T. A. 1995, ApJ, 448, 315
Yaron, O., & Gal-Yam, A. 2012, PASP, 124, 668
Yoon, S. C., Woosley, S. E., & Langer, N. 2010, ApJ, 725, 940
Zheng, W., Stahl, B. E., de Jaeger, T., et al. 2022, MNRAS, 512, 3195
7
The Astrophysical Journal Letters, 941:L32 (7pp), 2022 December 20 Kuncarayakti et al.