e present supernova (SN) 2023ufx, a unique Type IIP SN with the shortest known plateau duration (t_PT ∼ 47 days), a luminous V-band peak (M_V = −​​​​​​18.42 ± 0.08 mag), and a rapid early decline rate (s1 = 3.47 ± 0.09 mag (50 days)⁻¹). By comparing observed photometry to a hydrodynamic MESA+STELLA model grid, we constrain the progenitor to be a massive red supergiant with M_ZAMS ∼ 19–25 M_⊙. Independent comparisons with nebular spectral models also suggest an initial He-core mass of ∼6 M_⊙, and thus a massive progenitor. For a Type IIP, SN 2023ufx produced an unusually high amount of nickel (⁵⁶Ni) ∼0.14 ± 0.02 M_⊙, during the explosion. We find that the short plateau duration in SN 2023ufx can be explained with the presence of a small hydrogen envelope (M_Henv ∼ 1.2 M_⊙), suggesting partial stripping of the progenitor. About ∼0.09 M_⊙ of circumstellar material through mass loss from late-time stellar evolution of the progenitor is needed to fit the early time (≲10 days) pseudo-bolometric light curve. Nebular line diagnostics of broad and multipeak components of [O i] λλ6300, 6364, Hα, and [Ca ii] λλ7291, 7323 suggest that the explosion of SN 2023ufx could be inherently asymmetric, preferentially ejecting material along our line of sight.

Multipeaked supernovae with precursors, dramatic light-curve rebrightenings, and spectral transformation are rare, but are being discovered in increasing numbers by modern night-sky transient surveys like the Zwicky Transient Facility. Here, we present the observations and analysis of SN 2023aew, which showed a dramatic increase in brightness following an initial luminous (−17.4 mag) and long (∼100 days) unusual first peak (possibly precursor). SN 2023aew was classified as a Type IIb supernova during the first peak but changed its type to resemble a stripped-envelope supernova (SESN) after the marked rebrightening. We present comparisons of SN 2023aew's spectral evolution with SESN subtypes and argue that it is similar to SNe Ibc during its main peak. P-Cygni Balmer lines are present during the first peak, but vanish during the second peak's photospheric phase, before Hα resurfaces again during the nebular phase. The nebular lines ([O i], [Ca ii], Mg i], Hα) exhibit a double-peaked structure that hints toward a clumpy or nonspherical ejecta. We analyze the second peak in the light curve of SN 2023aew and find it to be broader than that of normal SESNe as well as requiring a very high ⁵⁶Ni mass to power the peak luminosity. We discuss the possible origins of SN 2023aew including an eruption scenario where a part of the envelope is ejected during the first peak and also powers the second peak of the light curve through interaction of the SN with the circumstellar medium.

Nitrogen is produced by CNO-cycling in massive stars, and can be ejected in significant amounts in supernova explosions. While in H-rich SNe, its [N ii] 6548, 6583 emission becomes obscured by strong Hα⁠, in explosions of He stars, this nitrogen emission becomes more visible. We here explore the formation of this line, using the sumo code to compute spectra for a grid of 1D models with parametrized mixing informed from new 2D simulations. Because the mass fraction of nitrogen in the ejecta decreases with larger He-core masses, as more of the He/N zone gets processed by shell helium burning and is lost to winds, the [N ii] luminosity relative to the overall optical flux probes the He-core mass. By comparing to large samples of data, we find that low-mass He cores (⁠M_preSN≲ 3 M_⊙⁠) are exclusively associated with Type IIb SNe, with the exception of Type Ib SN 2007Y. Seeing no strong nitrogen emission in other Type Ib SNe, the implication is either an origin from low-mass stars with the He/N layer (but not the He/C) layer peeled away, or from higher mass He cores. We also see no clear nitrogen emission in Type Ic SNe. We discuss the diagnostic potential of this new line metric, and also dependencies on mass-loss rate and metallicity.