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1. SN 2023zaw: An Ultrastripped, Nickel-poor Supernova from a Low-mass Progenitor

   https://doi.org/10.3847/2041-8213/ad527a  

   Das, Kaustav K; Fremling, Christoffer; Kasliwal, Mansi M; Schulze, Steve; Sollerman, Jesper; Karambelkar, Viraj; Rose, Sam; Anand, Shreya; Andreoni, Igor; Aubert, Marie; et al (June 2024, The Astrophysical Journal Letters)

   Abstract We present SN 2023zaw—a subluminous (M_r= −16.7 mag) and rapidly evolving supernova (t_1/2,r= 4.9 days), with the lowest nickel mass (≈0.002M_⊙) measured among all stripped-envelope supernovae discovered to date. The photospheric spectra are dominated by broad Heiand Ca near-infrared emission lines with velocities of ∼10,000−12,000 km s⁻¹. The late-time spectra show prominent narrow Heiemission lines at ∼1000 km s⁻¹, indicative of interaction with He-rich circumstellar material. SN 2023zaw is located in the spiral arm of a star-forming galaxy. We perform radiation-hydrodynamical and analytical modeling of the lightcurve by fitting with a combination of shock-cooling emission and nickel decay. The progenitor has a best-fit envelope mass of ≈0.2M_☉and an envelope radius of ≈50R_⊙. The extremely low nickel mass and low ejecta mass (≈0.5M_⊙) suggest an ultrastripped SN, which originates from a mass-losing low-mass He-star (zero-age main-sequence mass < 10M_⊙) in a close binary system. This is a channel to form double neutron star systems, whose merger is detectable with LIGO. SN 2023zaw underscores the existence of a previously undiscovered population of extremely low nickel mass (<0.005M_☉) stripped-envelope supernovae, which can be explored with deep and high-cadence transient surveys. 

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2. Characterizing the Rapid Hydrogen Disappearance in SN 2022crv: Evidence of a Continuum between Type Ib and IIb Supernova Properties

   https://doi.org/10.3847/1538-4357/ad710e  

   Dong_董, Yize 一泽; Valenti, Stefano; Ashall, Chris; Williamson, Marc; Sand, David J; Van_Dyk, Schuyler D; Filippenko, Alexei V; Jha, Saurabh W; Lundquist, Michael; Modjaz, Maryam; et al (October 2024, The Astrophysical Journal)

   Abstract We present optical and near-infrared (NIR) observations of SN 2022crv, a stripped-envelope supernova in NGC 3054, discovered within 12 hr of explosion by the Distance Less Than 40 Mpc Survey. We suggest that SN 2022crv is a transitional object on the continuum between Type Ib supernovae (SNe Ib) and Type IIb supernovae (SNe IIb). A high-velocity hydrogen feature (∼ −20,000 to −16,000 km s⁻¹) was conspicuous in SN 2022crv at early phases, and then quickly disappeared. We find that a hydrogen envelope of ∼10⁻³M_⊙can reproduce the observed behavior of the hydrogen feature. The lack of early envelope cooling emission implies that SN 2022crv had a compact progenitor with an extremely low amount of hydrogen. A nebular spectral analysis shows that SN 2022crv is consistent with the explosion of a He star with a final mass of ∼4.5–5.6M_⊙that evolved from a ∼16 to 22M_⊙zero-age main-sequence star in a binary system with ∼1.0–1.7M_⊙of oxygen finally synthesized in the core. In order to retain such a small amount of hydrogen, the initial orbital separation of the binary system is likely larger than ∼1000R_⊙. The NIR spectra of SN 2022crv show a unique absorption feature on the blue side of the Heiline at ∼1.005μm. This is the first time such a feature has been observed in SNe Ib/IIb, and it could be due to Sr II. Further detailed modeling of SN 2022crv can shed light on the progenitor and the origin of the mysterious absorption feature in the NIR. 

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3. SN 2020bio: A Double-peaked, H-poor Type IIb Supernova with Evidence of Circumstellar Interaction

   https://doi.org/10.3847/1538-4357/ace595  

   Pellegrino, C.; Hiramatsu, D.; Arcavi, I.; Howell, D. A.; Bostroem, K. A.; Brown, P. J.; Burke, J.; Elias-Rosa, N.; Itagaki, K.; Kaneda, H.; et al (August 2023, The Astrophysical Journal)

   Abstract We present photometric and spectroscopic observations of SN 2020bio, a double-peaked Type IIb supernova (SN) discovered within a day of explosion, primarily obtained by Las Cumbres Observatory and Swift. SN 2020bio displays a rapid and long-lasting initial decline throughout the first week of its light curve, similarly to other well-studied Type IIb SNe. This early-time emission is thought to originate from the cooling of the extended outer hydrogen-rich (H-rich) envelope of the progenitor star that is shock heated by the SN explosion. We compare SN 2020bio to a sample of other double-peaked Type IIb SNe in order to investigate its progenitor properties. Analytical model fits to the early-time emission give progenitor radius (≈100–1500 R ⊙ ) and H-rich envelope mass (≈0.01–0.5 M ⊙ ) estimates that are consistent with other Type IIb SNe. However, SN 2020bio displays several peculiarities, including (1) weak H spectral features indicating a greater amount of mass loss than other Type IIb progenitors; (2) an underluminous secondary light-curve peak that implies a small amount of synthesized 56 Ni ( M Ni ≈0.02 M ⊙ ); and (3) low-luminosity nebular [O i ] and interaction-powered nebular features. These observations are more consistent with a lower-mass progenitor ( M ZAMS ≈ 12 M ⊙ ) that was stripped of most of its H-rich envelope before exploding. This study adds to the growing diversity in the observed properties of Type IIb SNe and their progenitors. 

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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. 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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