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1. Massive stars exploding in a He-rich circumstellar medium: XI. Diverse evolution of five Ibn SNe 2020nxt, 2020taz, 2021bbv, 2023utc, and 2024aej

   https://doi.org/10.1051/0004-6361/202554768  

   Wang, Z-Y; Pastorello, A; Cai, Y-Z; Fraser, M; Reguitti, A; Lin, W-L; Tartaglia, L; Andrew_Howell, D; Benetti, S; Cappellaro, E; et al (August 2025, Astronomy & Astrophysics)

   We present the photometric and spectroscopic analysis of five Type Ibn supernovae (SNe): SN 2020nxt, SN 2020taz, SN 2021bbv, SN 2023utc, and SN 2024aej. These events share key observational features and belong to a family of objects similar to the prototypical Type Ibn SN 2006jc. The SNe exhibit rise times of approximately 10 days and peak absolute magnitudes ranging from −16.5 to −19 mag. Notably, SN 2023utc is the faintest Type Ibn SN discovered to date, with an exceptionally lowr-band absolute magnitude of −16.4 mag. The pseudo-bolometric light curves peak at (1 − 10)×10⁴²erg s⁻¹, with total radiated energies on the order of (1 − 10)×10⁴⁸erg. Spectroscopically, these SNe display a relatively slow spectral evolution. The early spectra are characterised by a hot blue continuum and prominent He Iemission lines. The early spectra also show blackbody temperatures exceeding 10 000 K, with a subsequent decline in temperature during later phases. Narrow He Ilines, which are indicative of unshocked circumstellar material (CSM), show velocities of approximately 1000 km s⁻¹. The spectra suggest that the progenitors of these SNe underwent significant mass loss prior to the explosion, resulting in a He-rich CSM. Our light curve modelling yielded estimates for the ejecta mass (M_ej) in the range 1 − 3 M_⊙with kinetic energies (E_Kin) of (0.1 − 1)×10⁵⁰erg. The inferred CSM mass ranges from 0.2 to 1 M_⊙. These findings are consistent with expectations for core collapse events arising from relatively massive envelope-stripped progenitors. 

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2. When IIb Ceases To Be: Bridging the Gap between IIb and Short-plateau Supernovae

   https://doi.org/10.3847/1538-4357/ae3a71  

   Farah, Joseph R; Howell, D Andrew; Hiramatsu, Daichi; McCully, Curtis; Andrews, Moira; Newsome, Megan; Padilla_Gonzalez, Estefania; Pellegrino, Craig; Berger, Edo; Blanchard, Peter; et al (February 2026, The Astrophysical Journal)

   Abstract Hydrogen-rich supernovae (SNe) span a range of hydrogen envelope masses at core collapse, producing diverse light curves from extended plateaus in Type IIP SNe to double-peaked Type IIb SNe (SNe IIb). Recent simulations predict a continuous sequence of light-curve morphologies as hydrogen is removed, with short-plateau (SP; plateau durations ≈50–70 days) SNe emerging as a transitional class. However, the observational boundary between types IIb and SP remains poorly defined, and thus far unobserved. We report on extensive photometric and spectroscopic follow-up of SN 2023wdd and SN 2022acrv, two candidate transitional events on the low-mass end of the SP class. Both exhibit weak, double-peaked light curves, which we interpret as exceptionally short plateaus (10–20 days), and hybrid spectral features: persistent Hαabsorption with HeIcontamination, but without the helium dominance characteristic of SNe IIb. Using analytic shock-cooling models and numerical light-curve fitting, we estimate H-rich envelope masses of ∼0.6–0.8M_⊙—significantly larger than canonical IIb values (≲0.1M_⊙) but consistent with the ∼0.9M_⊙threshold predicted for short-plateau behavior. Although the progenitor radii inferred from analytic and numerical methods differ by factors of 2–5, envelope mass estimates are consistent across approaches. Comparisons to well-studied Type IIb (SN 2016gkg, SN 2022hnt), SP (SN 2023ufx, SN 2006ai, SN 2016egz, SN 2006Y), and Type II (SN 2023ixf, SN 2013ej) SNe suggests a monotonic relationship between hydrogen envelope mass and plateau length, consistent with analytic and numerical expectations. These findings provide additional evidence for a continuous distribution of envelope stripping in H-rich core-collapse progenitors, and place SN 2023wdd and SN 2022acrv along the IIb–SP boundary. 

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3. The Extremely Metal-poor SN 2023ufx: A Local Analog to High-redshift Type II Supernovae

   https://doi.org/10.3847/1538-4357/ad8448  

   Tucker, Michael A; Hinkle, Jason; Angus, Charlotte R; Auchettl, Katie; Hoogendam, Willem B; Shappee, Benjamin; Kochanek, Christopher S; Ashall, Chris; de_Boer, Thomas; Chambers, Kenneth C; et al (November 2024, The Astrophysical Journal)

   Abstract We present extensive observations of the Type II supernova (SN II) SN 2023ufx, which is likely the most metal-poor SN II observed to date. It exploded in the outskirts of a low-metallicity (Z_host∼ 0.1Z_⊙) dwarf (M_g= −13.39 ± 0.16 mag,r_proj∼ 1 kpc) galaxy. The explosion is luminous, peaking atM_g≈ −18.5 mag, and shows rapid evolution. Ther-band (pseudobolometric) light curve has a shock-cooling phase lasting 20 (17) days followed by a 19 (23) day plateau. The entire optically thick phase lasts only ≈55 days following explosion, indicating that the red supergiant progenitor had a thinned H envelope prior to explosion. The early spectra obtained during the shock-cooling phase show no evidence for narrow emission features and limit the preexplosion mass-loss rate to $\dot{M}\lesssim {10}^{-3}$M_⊙yr⁻¹. The photospheric-phase spectra are devoid of prominent metal absorption features, indicating a progenitor metallicity of ≲0.1Z_⊙. The seminebular (∼60–130 days) spectra reveal weak Feii, but other metal species typically observed at these phases (Tiii, Scii, and Baii) are conspicuously absent. The late-phase optical and near-infrared spectra also reveal broad (≈10⁴km s⁻¹) double-peaked Hα, Pβ, and Pγemission profiles suggestive of a fast outflow launched during the explosion. Outflows are typically attributed to rapidly rotating progenitors, which also prefer metal-poor environments. This is only the second SN II with ≲0.1Z_⊙and both exhibit peculiar evolution, suggesting a sizable fraction of metal-poor SNe II have distinct properties compared to nearby metal-enriched SNe II. These observations lay the groundwork for modeling the metal-poor SNe II expected in the early Universe. 

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4. SN 2024iss: A double-peaked Type IIb supernova with evidence of circumstellar interaction

   https://doi.org/10.1051/0004-6361/202557843  

   Chen, Liyang; Wang, Xiaofeng; Wu, Qinyu; Andrews, Moira; Farah, Joseph; Ochner, Paolo; Reguitti, Andrea; Filippenko, Alexei V; Brink, Thomas G; Zhang, Jujia; et al (June 2026, Astronomy & Astrophysics)

   Aims.We present optical, ultraviolet, and X-ray observations of supernova (SN) 2024iss, a Type IIb SN that shows a prominent double-peaked light curve. Methods.We modeled the first peak with a semianalytical shock-cooling model and the X-ray emission with a free-free model. We also compared the envelope radius and mass-loss rate with those of other Type IIb SNe to explore the relationships between the progenitor envelope and the circumstellar material. Results.The shock-cooling peak in theV-band light curve reachedM_V = −17.33 ± 0.26 mag, while the⁵⁶Ni-powered second peak attainedM_V = −17.43 ± 0.26 mag. Early spectra show a photospheric velocity of approximately 19 400 km s⁻¹at 3.82 days from the HαP Cygni profile. The Balmer lines persist for at least more than 87 days after the explosion, which is characteristic of hydrogen-rich ejecta. Modeling the first light-curve peak with the shock-cooling model suggests an extended hydrogen envelope with a mass of 0.11 ± 0.04 M_⊙and a radius of 244 ± 43 R_⊙. Fitting the second light-curve peak with an Arnett-like model indicates a typical⁵⁶Ni mass of 0.117 ± 0.013 M_⊙and a relatively low ejecta mass of 1.27 ± 0.34 M_⊙. X-ray observations revealed bright thermal bremsstrahlung emission and indicate a mass-loss rate of 1.6 × 10⁻⁵M_⊙yr⁻¹, which is similar to that of SN 1993J. Conclusions.Supernova 2024iss occupies a transitional position between the two subclasses of extended and compact Type IIb SNe. Its envelope radius and preexplosion mass-loss rate appear to be consistent with the correlation observed in the broader sample. The observational properties of SN 2024iss are compatible with a binary-interaction scenario being the dominant mechanism for envelope stripping. 

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5. Early Shock Cooling Observations and Progenitor Constraints of Type IIb Supernova SN 2024uwq

   https://doi.org/10.3847/2041-8213/adfe52  

   Subrayan, Bhagya M; Sand, David J; Bostroem, K Azalee; Jha, Saurabh W; Ravi, Aravind P; Schwab, Michaela; Andrews, Jennifer E; Hosseinzadeh, Griffin; Valenti, Stefano; Dong_董, Yize 一泽; et al (September 2025, The Astrophysical Journal Letters)

   Abstract We present early multiwavelength photometric and spectroscopic observations of the Type IIb supernova SN 2024uwq, capturing its shock-cooling emission phase and double-peaked light-curve evolution. Early spectra reveal broad Hα(v ∼ 15,500 km s⁻¹) and HeIP Cygni profiles of similar strengths. Over time the HeIlines increase in strength while the Hαdecreases, consistent with a hydrogen envelope (M_env = 0.7–1.35M_⊙) overlying helium-rich ejecta. Analytic modeling of early shock cooling emission and bolometric light analysis constrains the progenitor to a partially stripped star with radiusR = 10–60R_⊙, consistent with a blue/yellow supergiant with an initial zero-age main-sequence mass of 12–20M_⊙likely stripped via binary interaction. SN 2024uwq occupies a transitional position between compact and extended Type IIb supernovae, highlighting the role of binary mass transfer efficiency in shaping a continuum of stripped-envelope progenitors. Our results underscore the importance of early UV/optical observations to characterize shock breakout signatures critical to map the diversity in evolutionary pathways of massive stars. Upcoming time-domain surveys, including Rubin Observatory’s LSST and UV missions like ULTRASAT and UVEX, will revolutionize our ability to systematically capture these early signatures, probing the full diversity of stripped progenitors and their explosive endpoints. 

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