SN 2022acko: JWST Low-Luminosity SN II-P

• The paper demonstrates that SN 2022acko’s JWST spectroscopy provides unprecedented insights into ejecta geometry and progenitor mass tensions in a low-luminosity Type II-P event.
• It reveals clear evidence of compact CO formation and chemically asymmetric ejecta, establishing a benchmark for studying mixing and molecule formation in supernovae.
• It highlights a divergence between direct progenitor detections and shock-cooling/nebular modeling, illustrating the challenges in accurately estimating red supergiant progenitor masses.

SN 2022acko is a nearby, hydrogen-rich, low-luminosity Type II-P supernova in the barred spiral galaxy NGC 1300. It is notable on several distinct fronts: it was the first core-collapse supernova to receive JWST spectroscopy at plateau phase, the first Type II supernova spectroscopically observed by JWST, and the first low-mass SN II followed with JWST into the nebular phase for a direct study of molecule formation, inner-ejecta chemistry, and ejecta geometry (Shahbandeh et al., 2024, Medler et al., 30 Jun 2026). Across ultraviolet, optical, near-infrared, and mid-infrared datasets, SN 2022acko has emerged as a benchmark case for a low-energy, low-velocity SN II whose progenitor was probably a red supergiant near the lower core-collapse mass boundary, but for which direct-imaging, shock-cooling, and nebular inferences do not fully agree (Dyk et al., 2023, Teixeira et al., 4 Sep 2025).

1. Discovery, classification, and global observables

SN 2022acko was discovered by the DLT40 survey on 2022 December 6.2 and classified within about a day as a young Type II-P supernova in NGC 1300 (Dyk et al., 2023, Shahbandeh et al., 2024). The host redshift adopted in multiple studies is $z=0.00526$, and several analyses use a PHANGS distance of $18.99\pm2.85$ Mpc, often rounded to $19.0\pm2.9$ Mpc (Dyk et al., 2023, Shahbandeh et al., 2024, Teixeira et al., 4 Sep 2025). A later optical study adopts $19.8\pm2.8$ Mpc as a final working distance after comparing NED/Tully-Fisher, PHANGS, EPM, and SCM estimates (Han et al., 29 Dec 2025).

Photometrically, SN 2022acko occupies the low-luminosity end of the SN II distribution. Early optical/UV work reports a peak absolute magnitude of $V=-15.4$ mag and a plateau length of $\sim115$ d (Bostroem et al., 2023). Another analysis gives $M_V^{\rm max}=-15.5\pm0.3$ mag, $M_V^{50}=-15.2\pm0.3$ mag, and a $V$-band plateau decline rate $s_2=0.54\ {\rm mag\ (100\ d)^{-1}}$ (Han et al., 29 Dec 2025). A bolometric reconstruction extending to $18.99\pm2.85$0 d yields a plateau-transition timescale of $18.99\pm2.85$1 d and a nebular decline rate of about $18.99\pm2.85$2, steeper than the $18.99\pm2.85$3 expected for full $18.99\pm2.85$4Co trapping (Teixeira et al., 4 Sep 2025). The radioactive tail implies a modest $18.99\pm2.85$5Ni mass: $18.99\pm2.85$6 in one study and $18.99\pm2.85$7 in another (Teixeira et al., 4 Sep 2025, Han et al., 29 Dec 2025).

These observables consistently place SN 2022acko among low-luminosity or transitional Type II-P events rather than among normal-luminosity plateau supernovae. Several papers explicitly compare it to SN 2005cs, SN 2008in, SN 2009N, and SN 2012A, with SN 2008in and SN 2009N often serving as the closest optical analogues (Bostroem et al., 2023, Teixeira et al., 4 Sep 2025, Han et al., 29 Dec 2025).

2. Early ultraviolet, optical, and plateau-phase infrared behavior

The earliest uniquely important dataset is the HST/STIS ultraviolet sequence at 5.2, 6.0, 7.3, 18.9, and 20.8 d after explosion, with the first three FUV spectra earlier than any previous Type IIP/L FUV observation (Bostroem et al., 2023). These spectra showed that the first-week ultraviolet behavior was dominated by strong Doppler-broadened metal features, in sharp contrast to the relatively featureless early optical spectra. CMFGEN modeling identified $18.99\pm2.85$8 as the principal UV sculptor, with further contributions from $18.99\pm2.85$9, $19.0\pm2.9$0, $19.0\pm2.9$1, $19.0\pm2.9$2, $19.0\pm2.9$3, $19.0\pm2.9$4, $19.0\pm2.9$5, $19.0\pm2.9$6, $19.0\pm2.9$7, $19.0\pm2.9$8, $19.0\pm2.9$9, $19.8\pm2.8$0, $19.8\pm2.8$1, and $19.8\pm2.8$2 (Bostroem et al., 2023). The UV flux faded rapidly because of ejecta cooling plus line blanketing, and the dataset quantified how strongly early bolometric estimates depend on UV coverage: on day 5, only 54% of the 1150–10150 Å flux lay redward of the $19.8\pm2.8$3 band, while 96% lay redward of $19.8\pm2.8$4 (Bostroem et al., 2023).

Optical spectra beginning at 1.5 d revealed an additional early-time peculiarity. A broad emission feature peaking near 4600 Å, described as the “ledge” feature, was present at 1.5 and 2.5 d, and a weak narrow H$19.8\pm2.8$5 component was isolated at 1.5 d with $19.8\pm2.8$6 after instrumental correction (Han et al., 29 Dec 2025). That work interprets the ledge as blueshifted He II $19.8\pm2.8$7 formed in ionized ejecta and the narrow H$19.8\pm2.8$8 as a possible flash-ionized circumstellar feature; with an early ejecta velocity of $19.8\pm2.8$9, the disappearance of the narrow feature within $V=-15.4$0 d implies highly confined circumstellar material within $V=-15.4$1 cm (Han et al., 29 Dec 2025). By contrast, the day-5 HST UV study reports no symmetric emission lines and takes the broad absorption and P-Cygni structure to indicate that there was no optically thick CSM at that epoch (Bostroem et al., 2023). This suggests that any CSI was weak and extremely short-lived rather than extended.

At plateau phase, JWST provided a nearly continuous $V=-15.4$2 SED at $V=-15.4$3 d by combining NIRSpec, MIRI/MRS, JWST photometry, and ground-based optical/NIR data (Shahbandeh et al., 2024). The NIRSpec epoch was 50.18–50.19 d, the MIRI/MRS epoch 55.57 d, and the nearly simultaneous JWST photometry occurred at $V=-15.4$4 d (Shahbandeh et al., 2024). The spectrum was hydrogen-dominated, with roughly 30 H I features spanning Balmer, Paschen, Brackett, Pfund, and Humphreys series, alongside signatures attributed to CNO-cycle products and s-process elements such as Sc II and Ba II (Shahbandeh et al., 2024). The continuum was consistent with a blackbody of $V=-15.4$5 K, the NIR/MIR continuum was described as dominated by Thomson and free-free emission, and there was no convincing infrared excess (Shahbandeh et al., 2024).

The hydrogen line kinematics at this stage were relatively low for a Type II SN. Absorption velocities ranged from about 2100 to 4900 km s$V=-15.4$6, with lower transitions tending to show higher velocities, and relative line shifts within H series were used to infer a shallow density gradient $V=-15.4$7 with $V=-15.4$8 (Shahbandeh et al., 2024). Later comparative work with SN 2024ggi and SN 2023ixf retained the same basic picture: SN 2022acko occupied the under-luminous, narrower-lined, lower-velocity end of the emerging JWST-observed SN II sample (Baron et al., 24 Jul 2025).

3. Progenitor identification and the mass-estimate tension

SN 2022acko is also central to progenitor studies because JWST enabled the first localization of a supernova progenitor system in pre-explosion HST images (Dyk et al., 2023). A post-explosion JWST/NIRCam image from 2023 January 25, 52 d after explosion, was registered to archival HST F160W and F814W images, and an object consistent with the SN position was isolated with reasonable confidence (Dyk et al., 2023). In that analysis, the candidate was detected only in F814W and F160W, with $V=-15.4$9 mag and $\sim115$0 mag, while all bluer HST bands and pre-explosion Spitzer images yielded only limits (Dyk et al., 2023). BPASS endpoint comparisons favored a low-mass red supergiant, with the 7.7 $\sim115$1 single-star model providing the best match; an 8 $\sim115$2 model was somewhat too luminous in F814W, while the 7.6 $\sim115$3 super-AGB endpoint was a very poor fit (Dyk et al., 2023). That study therefore argued against an electron-capture/SAGB progenitor and placed the initial mass near $\sim115$4, while allowing values up to about $\sim115$5 if a larger SCM distance were adopted (Dyk et al., 2023).

A later, more extensive study combined deep pre-explosion imaging, Gemini/GSAOI astrometry, shock-cooling fits, bolometric-tail modeling, and optical nebular spectroscopy (Teixeira et al., 4 Sep 2025). In that work the pre-explosion counterpart was detected at ACS/F814W $\sim115$6 AB mag and WFC3/IR F160W $\sim115$7 AB mag, with a chance coincidence probability of $\sim115$8 (Teixeira et al., 4 Sep 2025). A two-point blackbody fit yielded $\sim115$9 K and $M_V^{\rm max}=-15.5\pm0.3$0, but the temperature was regarded as implausibly cool for an RSG; the direct-detection luminosity was instead summarized as implying a red-supergiant-like progenitor with $M_V^{\rm max}=-15.5\pm0.3$1, while MARCS+wind+dust models constrained the maximum luminosity allowed by the IRAC limits to about $M_V^{\rm max}=-15.5\pm0.3$2 (Teixeira et al., 4 Sep 2025).

Indirect methods push higher. Shock-cooling analysis with the MSW23 formalism yielded $M_V^{\rm max}=-15.5\pm0.3$3, $M_V^{\rm max}=-15.5\pm0.3$4, $M_V^{\rm max}=-15.5\pm0.3$5, $M_V^{\rm max}=-15.5\pm0.3$6, and $M_V^{\rm max}=-15.5\pm0.3$7 MJD, implying $M_V^{\rm max}=-15.5\pm0.3$8-10 $M_V^{\rm max}=-15.5\pm0.3$9 when compared with MIST tracks (Teixeira et al., 4 Sep 2025). Optical nebular spectra at 275, 396, and 612 d then gave a broad progenitor constraint of roughly $M_V^{50}=-15.2\pm0.3$0, with the abstract slightly narrowing the preferred range to $M_V^{50}=-15.2\pm0.3$1 (Teixeira et al., 4 Sep 2025). An independent optical/UV study likewise argued that the overall observed properties were consistent with $M_V^{50}=-15.2\pm0.3$2-10 $M_V^{50}=-15.2\pm0.3$3, $M_V^{50}=-15.2\pm0.3$4 erg, $M_V^{50}=-15.2\pm0.3$5, and $M_V^{50}=-15.2\pm0.3$6 (Han et al., 29 Dec 2025).

The central tension can therefore be summarized as follows.

Method	Inferred progenitor scale
Direct detection	$M_V^{50}=-15.2\pm0.3$7 or $M_V^{50}=-15.2\pm0.3$8
Shock cooling	$M_V^{50}=-15.2\pm0.3$9
Nebular spectroscopy	$V$0

Published interpretations do not treat this as a data-scarcity problem. The preferred explanation is that current shock-cooling and nebular analyses remain model-limited, with distance uncertainty, flux calibration, sparse pre-explosion SED coverage, and systematic effects in spectral rescaling all contributing materially to the disagreement (Teixeira et al., 4 Sep 2025). A plausible implication is that SN 2022acko is a strong example of how direct and indirect progenitor constraints can diverge even for a comparatively well-observed Type II-P.

4. Nebular-phase infrared spectroscopy and ejecta geometry

The second and third JWST nebular spectra, obtained at $V$1 and $V$2 d, transformed SN 2022acko from a plateau-phase benchmark into a geometry and chemistry case study (Medler et al., 30 Jun 2026). The core infrared dataset combined MIRI medium-resolution spectroscopy over roughly $V$3 at both epochs with a $V$4 d NIRSpec spectrum over $V$5 at $V$6, plus Keck/NIRES near-infrared data (Medler et al., 30 Jun 2026). The central instrumental point was that MIRI/MRS resolved MIR blends that had been heavily blended in Spitzer-era data, especially the $V$7 complex separating [Co II] 10.521 from [Ni II] 10.682 and the $V$8 region disentangling [Co I] 12.255 from Hu$V$9 12.372 (Medler et al., 30 Jun 2026).

Even at nebular phases, the spectra remained rich in hydrogen. Identified H features included Paschen, Brackett, Pfund, and Humphreys lines across the NIR and MIR, while He I was seen chiefly at 1.083 and 2.059 $s_2=0.54\ {\rm mag\ (100\ d)^{-1}}$0 (Medler et al., 30 Jun 2026). The intermediate-mass elements included C, O, Mg, Si, Ca, Ar, and Ne, and the iron-group inventory was exceptionally extensive, with [Fe I], [Fe II], [Fe III], [Co I], [Co II], [Co III], [Ni I], [Ni II], and [Ni III] transitions explicitly identified (Medler et al., 30 Jun 2026). Between $s_2=0.54\ {\rm mag\ (100\ d)^{-1}}$1 and $s_2=0.54\ {\rm mag\ (100\ d)^{-1}}$2 d, IME and IGE forbidden lines strengthened relative to the continuum, [Ar II] 6.985 and [Fe II] 17.936 emerged or strengthened markedly, [Ne II] 12.813 became more flat-topped, and the overall SED faded without developing an infrared dust excess (Medler et al., 30 Jun 2026). The [Ni II] 6.636 line remained strong at both epochs and was especially prominent relative to [Ar II], which was taken to suggest unusually rapid exposure of deeper ejecta and/or a high stable-Ni yield (Medler et al., 30 Jun 2026).

Single-Gaussian profile fitting in velocity space produced one of the paper’s main physical results. After correction for a local peculiar velocity of $s_2=0.54\ {\rm mag\ (100\ d)^{-1}}$3, the IMEs showed characteristic $s_2=0.54\ {\rm mag\ (100\ d)^{-1}}$4 and offset velocities $s_2=0.54\ {\rm mag\ (100\ d)^{-1}}$5, whereas the IGEs showed broader $s_2=0.54\ {\rm mag\ (100\ d)^{-1}}$6 but smaller offsets $s_2=0.54\ {\rm mag\ (100\ d)^{-1}}$7 (Medler et al., 30 Jun 2026). Helium was somewhat distinct, with $s_2=0.54\ {\rm mag\ (100\ d)^{-1}}$8 and offset $s_2=0.54\ {\rm mag\ (100\ d)^{-1}}$9, while the H envelope at $18.99\pm2.85$00-369 d was confined within $18.99\pm2.85$01 and most H centroids lay near zero after peculiar-motion correction (Medler et al., 30 Jun 2026). Individual IME offsets were often large, such as [Ne II] 12.813 at $18.99\pm2.85$02 and $18.99\pm2.85$03, and [Ar II] 6.985 at $18.99\pm2.85$04 and $18.99\pm2.85$05 (Medler et al., 30 Jun 2026).

The authors interpret this systematic separation between IMEs and H/He/IGEs as a chemically asymmetric ejecta structure. Their preferred explanation is a fallback-fed bipolar outflow or wind from an accretion disk around the compact remnant, with Rayleigh-Taylor instabilities contributing secondarily but importantly, especially at composition interfaces reaching the C/O and He shells (Medler et al., 30 Jun 2026). Superposed on this is a global bulk shift: the median corrected centroid offset across IME and IGE lines is $18.99\pm2.85$06, derived from Monte Carlo resampling of the observed offset distribution (Medler et al., 30 Jun 2026). This was interpreted as a true net ejecta momentum offset and hence as evidence for a natal neutron-star kick in the opposite direction by momentum conservation (Medler et al., 30 Jun 2026).

5. Molecule formation, compact CO, and the absence of SiO and dust

The earliest JWST infrared spectrum at $18.99\pm2.85$07 d provided a stringent pre-molecule baseline. Using MOLFIT, built on molecular-band and spherical radiative-transfer modules in HYDRA, the plateau-phase study modeled the CO fundamental band around $18.99\pm2.85$08 and derived a CO mass upper limit of $18.99\pm2.85$09 (Shahbandeh et al., 2024). The paper emphasized that the fundamental band is about $18.99\pm2.85$10 times more constraining than the first overtone, and that the non-detection implied little or no substantial mixing between the H envelope and the C/O core by that epoch (Shahbandeh et al., 2024). It also found no evidence for dust in the ISM or CSM and no warm-dust component at $18.99\pm2.85$11 d (Shahbandeh et al., 2024).

By $18.99\pm2.85$12 and $18.99\pm2.85$13 d, that baseline had changed qualitatively. CO was clearly detected, with the first overtone seen at $18.99\pm2.85$14 and a stronger fundamental band at $18.99\pm2.85$15, while SiO fundamental emission near $18.99\pm2.85$16 remained undetected and the MIRI continuum still showed no convincing warm-dust excess (Medler et al., 30 Jun 2026). The nebular CO analysis used MOFAT, the MOlecular Fitting Analysis Tool, a seven-parameter iterative framework built on HYDRA molecular opacities with non-LTE corrections (Medler et al., 30 Jun 2026). MOFAT assumes a large-scale spherical geometry but allows clumpy spheroidal substructures; its fitted parameters include $18.99\pm2.85$17, $18.99\pm2.85$18, $18.99\pm2.85$19, $18.99\pm2.85$20, $18.99\pm2.85$21, and, for clumped models, $18.99\pm2.85$22, $18.99\pm2.85$23, and $18.99\pm2.85$24, with the emergent intensity explicitly discussed through $18.99\pm2.85$25 (Medler et al., 30 Jun 2026).

For the conservative spherical sequence, the best fits gave

$18.99\pm2.85$26

at $18.99\pm2.85$27 d, and

$18.99\pm2.85$28

at $18.99\pm2.85$29 d (Medler et al., 30 Jun 2026). These values imply continued CO growth and cooling in a very centrally concentrated molecular zone. At $18.99\pm2.85$30 d, however, a clumped distribution was required to reproduce the overtone+fundamental profiles satisfactorily; the preferred clumped fit improved the match by about 10% and had

$18.99\pm2.85$31

(Medler et al., 30 Jun 2026). The paper stresses that clumped and spherical masses are not directly interchangeable because the clumps approach a semi-optically thick regime in the overtone, modifying the emergent flux (Medler et al., 30 Jun 2026).

The molecular zone is therefore exceptionally compact. Across the accepted fits, $18.99\pm2.85$32–500 $18.99\pm2.85$33, $18.99\pm2.85$34–1000 $18.99\pm2.85$35, and the density slope steepens from $18.99\pm2.85$36 to $18.99\pm2.85$37–10 as CO accumulates in the coolest inner regions (Medler et al., 30 Jun 2026). Relative to SN 2024ggi, SN 2022acko forms about an order of magnitude less CO, and its CO-forming region is much more centrally confined: $18.99\pm2.85$38, $18.99\pm2.85$39 versus SN 2024ggi’s $18.99\pm2.85$40 and $18.99\pm2.85$41 (Medler et al., 30 Jun 2026). Yet the inferred clump scale is similar, with $18.99\pm2.85$42 in SN 2022acko compared with $18.99\pm2.85$43 in SN 2024ggi, and the clumps are similarly prolate ($18.99\pm2.85$44 versus $18.99\pm2.85$45) (Medler et al., 30 Jun 2026).

The broader interpretation given in the nebular paper is that low-mass Type II SNe can form CO but do so less efficiently than more massive H-rich explosions, likely because their carbon-oxygen layers are more compact and centrally confined (Medler et al., 30 Jun 2026). In SN 2022acko, this lower molecule yield is accompanied by no detectable SiO and no dust continuum through $18.99\pm2.85$46 d. The sharp, symmetric nebular H profiles and lack of an infrared dust excess were taken to imply that dust formation in such low-mass Type II events is delayed relative to normal Type II SNe, perhaps substantially delayed or absent altogether on these timescales (Medler et al., 30 Jun 2026).

6. Comparative position and continuing interpretive issues

SN 2022acko now serves as a comparative anchor across several SN II subproblems. At early times, it established the first JWST core-collapse baseline for a plateau-phase SN II with many hydrogen lines but no pre-existing molecules or dust (Shahbandeh et al., 2024). In re-reduced comparisons with SN 2024ggi and SN 2023ixf, it remained the faintest and slowest of the three, with $18.99\pm2.85$47 mag versus $18.99\pm2.85$48 mag for SN 2024ggi and $18.99\pm2.85$49 mag for SN 2023ixf; the line-velocity comparison was used to reinforce a luminosity-velocity trend across the sample (Baron et al., 24 Jul 2025). At MIR wavelengths, all three objects are hydrogen-dominated beyond $18.99\pm2.85$50, but SN 2022acko’s lower signal-to-noise makes all but the strongest H lines difficult to distinguish in the early JWST dataset (Baron et al., 24 Jul 2025).

Relative to historical comparison objects, SN 2022acko appears less mixed than SN 1987A at comparable recombination-phase epochs. The plateau-phase infrared analysis explicitly contrasts SN 1987A, where H was mixed inward to about 800 km s$18.99\pm2.85$51 and C/O-rich material outward to about 3000 km s$18.99\pm2.85$52, with SN 2022acko, whose hydrogen lines remained in the roughly 2000–5000 km s$18.99\pm2.85$53 range and whose lack of CO at $18.99\pm2.85$54 d was taken to indicate substantially weaker H-envelope/C/O-core mixing (Shahbandeh et al., 2024). Later CO formation then traced molecule growth in a compact inner zone rather than early mixed-out core material (Medler et al., 30 Jun 2026).

Several controversies remain open but are now sharply defined. The first concerns the progenitor mass: direct imaging favors $18.99\pm2.85$55–8 or $18.99\pm2.85$56, whereas shock-cooling and nebular methods favor $18.99\pm2.85$57–10 and $18.99\pm2.85$58–15 $18.99\pm2.85$59 (Dyk et al., 2023, Teixeira et al., 4 Sep 2025). The second concerns the earliest circumstellar environment: one study argues for signs of early CSI, a compact CSM radius of $18.99\pm2.85$60 cm, and a mass-loss rate of $18.99\pm2.85$61 under a spherically symmetric wind assumption (Han et al., 29 Dec 2025), whereas the early UV paper finds no evidence for optically thick CSM by day 5 (Bostroem et al., 2023). These positions are not mutually exclusive; they are compatible with a very confined, rapidly fading interaction region. A third issue is whether the nebular asymmetry requires a fallback-fed bipolar outflow specifically or whether Rayleigh-Taylor instability can account for more of the observed structure; the nebular paper explicitly allows both, while preferring a bipolar component for the IME/IGE kinematic separation (Medler et al., 30 Jun 2026).

Taken together, the published record presents SN 2022acko as a low-luminosity Type II-P from a red-supergiant progenitor near the lower core-collapse mass boundary, but also as a methodologically important object in its own right. It links first-week FUV line blanketing, first-epoch JWST plateau-phase infrared spectroscopy, direct progenitor localization with JWST astrometry, resolved nebular MIR line kinematics, compact CO formation, and persistent non-detections of SiO and dust into a single time-domain case study (Bostroem et al., 2023, Shahbandeh et al., 2024, Dyk et al., 2023, Medler et al., 30 Jun 2026). This suggests that SN 2022acko will remain a reference event for assessing how low-mass H-rich explosions differ from more massive Type II SNe in progenitor structure, mixing, molecule formation, and prompt dust production.