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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 (≈0.002 M _⊙ ) measured among all stripped-envelope supernovae discovered to date. The photospheric spectra are dominated by broad He i and Ca near-infrared emission lines with velocities of ∼10,000−12,000 km s ^−1 . The late-time spectra show prominent narrow He i emission 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 ≈0.2 M _☉ and an envelope radius of ≈50 R _⊙ . The extremely low nickel mass and low ejecta mass (≈0.5 M _⊙ ) suggest an ultrastripped SN, which originates from a mass-losing low-mass He-star (zero-age main-sequence mass < 10 M _⊙ ) 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 (

We report observations of the optical counterpart of the long gamma-ray burst (GRB) GRB 230812B and its associated supernova (SN) SN 2023pel. The proximity ( z = 0.36) and high energy ( E _γ _,iso ∼ 10 ^53 erg) make it an important event to study as a probe of the connection between massive star core collapse and relativistic jet formation. With a phenomenological power-law model for the optical afterglow, we find a late-time flattening consistent with the presence of an associated SN. SN 2023pel has an absolute peak r -band magnitude of M _r = −19.46 ± 0.18 mag (about as bright as SN 1998bw) and evolves on quicker timescales. Using a radioactive heating model, we derive a nickel mass powering the SN of M _Ni = 0.38 ± 0.01 M _⊙ and a peak bolometric luminosity of L _bol ∼ 1.3 × 10 ^43 erg s ^−1 . We confirm SN 2023pel’s classification as a broad-line Type Ic SN with a spectrum taken 15.5 days after its peak in the r band and derive a photospheric expansion velocity of v _ph = 11,300 ± 1600 km s ^−1 at that phase. Extrapolating this velocity to the time of maximum light, we derive the ejecta mass M _ej = 1.0 ± 0.6 M _⊙ and kinetic energy ${E}_{\mathrm{KE}}={1.3}_{-1.2}^{+3.3}\times {10}^{51}\,\mathrm{erg}$ . We find that GRB 230812B/SN 2023pel has SN properties that are mostly consistent with the overall GRB-SN population. The lack of correlations found in the GRB-SN population between SN brightness and E _γ _,iso for their associated GRBs across a broad range of 7 orders of magnitude provides further evidence that the central engine powering the relativistic ejecta is not coupled to the SN powering mechanism in GRB-SN systems.

Eruptive mass loss of massive stars prior to supernova (SN) explosion is key to understanding their evolution and end fate. An observational signature of pre-SN mass loss is the detection of an early, short-lived peak prior to the radioactive-powered peak in the lightcurve of the SN. This is usually attributed to the SN shock passing through an extended envelope or circumstellar medium. Such an early peak is common for double-peaked Type IIb SNe with an extended hydrogen envelope but uncommon for normal Type Ibc SNe with very compact progenitors. In this paper, we systematically study a sample of 14 double-peaked Type Ibc SNe out of 475 Type Ibc SNe detected by the Zwicky Transient Facility. The rate of these events is ∼3%–9% of Type Ibc SNe. A strong correlation is seen between the peak brightness of the first and the second peak. We perform a holistic analysis of this sample’s photometric and spectroscopic properties. We find that six SNe have ejecta mass less than 1.5 M _⊙ . Based on the nebular spectra and lightcurve properties, we estimate that the progenitor masses for these are less than ∼12 M _⊙ . The rest have an ejecta mass >2.4 M _⊙ and a higher progenitor mass. This sample suggests that the SNe with low progenitor masses undergo late-time binary mass transfer. Meanwhile, the SNe with higher progenitor masses are consistent with wave-driven mass loss or pulsation-pair instability-driven mass-loss simulations.

We present a sample of 34 normal Type II supernovae (SNe II) detected with the Zwicky Transient Facility, with multiband UV light curves starting at t ≤ 4 days after explosion, and X-ray observations. We characterize the early UV-optical color, provide empirical host-extinction corrections, and show that the t > 2 day UV-optical colors and the blackbody evolution of the sample are consistent with shock cooling (SC) regardless of the presence of “flash ionization” features. We present a framework for fitting SC models that can reproduce the parameters of a set of multigroup simulations up to 20% in radius and velocity. Observations of 15 SNe II are well fit by models with breakout radii 10 ^14 cm breakout radius. However, these fits predict an early rise during the first day that is too slow. We suggest that these large-breakout events are explosions of stars with an inflated envelope or with confined circumstellar material (CSM). Using the X-ray data, we derive constraints on the extended (∼10 ^15 cm) CSM density independent of spectral modeling and find that most SN II progenitors lose $\dot{M}\lt {10}^{-4}{M}_{\odot }\,{\mathrm{yr}}^{-1}$ up to a few years before explosion. We show that the overall observed breakout radius distribution is skewed to higher radii due to a luminosity bias. We argue that the ${66}_{-22}^{+11} \% $ of red supergiants (RSGs) explode as SNe II with breakout radii consistent with the observed distribution of RSGs, with a tail extending to large radii, likely due to the presence of CSM.

We present observations of SN 2022joj, a peculiar Type Ia supernova discovered by the Zwicky Transient Facility. SN 2022joj exhibits an unusually red g _ZTF − r _ZTF color at early times and a rapid blueward evolution afterward. Around maximum brightness, SN 2022joj shows a high luminosity ( ${M}_{{g}_{\mathrm{ZTF}},\max }\simeq -19.7$ mag), a blue broadband color ( g _ZTF − r _ZTF ≃ −0.2 mag), and shallow Si ii absorption lines, consistent with those of overluminous, SN 1991T-like events. The maximum-light spectrum also shows prominent absorption around 4200 Å, which resembles the Ti ii features in subluminous, SN 1991bg-like events. Despite the blue optical-band colors, SN 2022joj exhibits extremely red ultraviolet minus optical colors at maximum luminosity ( u − v ≃ 0.6 mag and uvw 1 − v ≃ 2.5 mag), suggesting a suppression of flux at ∼2500–4000 Å. Strong C ii lines are also detected at peak. We show that these unusual spectroscopic properties are broadly consistent with the helium-shell double detonation of a sub-Chandrasekhar mass ( M ≃ 1 M _⊙ ) carbon/oxygen white dwarf from a relatively massive helium shell ( M _s ≃ 0.04–0.1 M _⊙ ), if observed along a line of sight roughly opposite to where the shell initially detonates. None of the existing models could quantitatively explain all the peculiarities observed in SN 2022joj. The low flux ratio of [Ni ii ] λ 7378 to [Fe ii ] λ 7155 emission in the late-time nebular spectra indicates a low yield of stable Ni isotopes, favoring a sub-Chandrasekhar mass progenitor. The significant blueshift measured in the [Fe ii ] λ 7155 line is also consistent with an asymmetric chemical distribution in the ejecta, as is predicted in double-detonation models.