We present high-cadence optical and ultraviolet light curves of the normal Type Ia supernova (SN) 2021aefx, which shows an early bump during the first two days of observation. This bump may be a signature of interaction between the exploding white dwarf and a nondegenerate binary companion, or it may be intrinsic to the white dwarf explosion mechanism. In the case of the former, the short duration of the bump implies a relatively compact main-sequence companion star, although this conclusion is viewing-angle dependent. Our best-fit companion-shocking and double-detonation models both overpredict the UV luminosity during the bump, and existing nickel-shell models do not match the strength and timescale of the bump. We also present nebular spectra of SN 2021aefx, which do not show the hydrogen or helium emission expected from a nondegenerate companion, as well as a radio nondetection that rules out all symbiotic progenitor systems and most accretion disk winds. Our analysis places strong but conflicting constraints on the progenitor of SN 2021aefx; no current model can explain all of our observations.

Context. The mass of synthesised radioactive material is an important power source for all supernova (SN) types. In addition, the difference of 56Ni yields statistics are relevant to constrain progenitor paths and explosion mechanisms. Aims. Here, we re-estimate the nucleosynthetic yields of 56Ni for a well-observed and well-defined sample of stripped-envelope SNe (SE-SNe) in a uniform manner. This allows us to investigate whether the observed hydrogen-rich–stripped-envelope (SN II–SE SN) 56Ni separation is due to real differences between these SN types or because of systematic errors in the estimation methods. Methods. We compiled a sample of well-observed SE-SNe and measured 56Ni masses through three different methods proposed in the literature: first, the classic “Arnett rule”; second the more recent prescription of Khatami & Kasen (2019, ApJ, 878, 56) and third using the tail luminostiy to provide lower limit 56Ni masses. These SE-SN distributions were then compared to those compiled in this article. Results. Arnett’s rule, as previously shown, gives 56Ni masses for SE-SNe that are considerably higher than SNe II. While for the distributions calculated using both the Khatami & Kasen (2019, ApJ, 878, 56) prescription and Tail 56Ni masses are offset to lower values than “Arnett values”, their 56Ni distributions are still statistically higher than that of SNe II. Our results are strongly driven by a lack of SE-SN with low 56Ni masses, that are, in addition, strictly lower limits. The lowest SE-SN 56Ni mass in our sample is of 0.015 M⊙, below which are more than 25% of SNe II. Conclusions. We conclude that there exist real, intrinsic differences in the mass of synthesised radioactive material between SNe II and SE-SNe (types IIb, Ib, and Ic). Any proposed current or future CC SN progenitor scenario and explosion mechanism must be able to explain why and how such differences arise or outline a bias in current SN samples yet to be fully explored.

We present the densely sampled early light curve of the Type II supernova (SN) 2023ixf, first observed within hours of explosion in the nearby Pinwheel Galaxy (Messier 101; 6.7 Mpc). Comparing these data to recently updated models of shock-cooling emission, we find that the progenitor likely had a radius of 410 ± 10 R ⊙. Our estimate is model dependent but consistent with a red supergiant. These models provide a good fit to the data starting about 1 day after the explosion, despite the fact that the classification spectrum shows signatures of circumstellar material around SN 2023ixf during that time. Photometry during the first day after the explosion, provided almost entirely by amateur astronomers, does not agree with the shock-cooling models or a simple power-law rise fit to data after 1 day. We consider the possible causes of this discrepancy, including precursor activity from the progenitor star, circumstellar interaction, and emission from the shock before or after it breaks out of the stellar surface. The very low luminosity (−11 mag > M > −14 mag) and short duration of the initial excess lead us to prefer a scenario related to prolonged emission from the SN shock traveling through the progenitor system.

We present high-cadence optical, ultraviolet (UV), and near-infrared data of the nearby (D ≈ 23 Mpc) Type II supernova (SN) 2021yja. Many Type II SNe show signs of interaction with circumstellar material (CSM) during the first few days after explosion, implying that their red supergiant (RSG) progenitors experience episodic or eruptive mass loss. However, because it is difficult to discover SNe early, the diversity of CSM configurations in RSGs has not been fully mapped. SN 2021yja, first detected within ≈ 5.4 hours of explosion, shows some signatures of CSM interaction (high UV luminosity and radio and x-ray emission) but without the narrow emission lines or early light-curve peak that can accompany CSM. Here we analyze the densely sampled early light curve and spectral series of this nearby SN to infer the properties of its progenitor and CSM. We find that the most likely progenitor was an RSG with an extended envelope, encompassed by low-density CSM. We also present archival Hubble Space Telescope imaging of the host galaxy of SN 2021yja, which allows us to place a stringent upper limit of ≲ 9 M ☉ on the progenitor mass. However, this is in tension with some aspects of the SN evolution, which point to a more massive progenitor. Our analysis highlights the need to consider progenitor structure when making inferences about CSM properties, and that a comprehensive view of CSM tracers should be made to give a fuller view of the last years of RSG evolution.

We present photometric and spectroscopic data of SN 2018lab, a low-luminosity Type IIP supernova (LLSN) with a V-band peak luminosity of −15.1 ± 0.1 mag. SN 2018lab was discovered by the Distance Less Than 40 Mpc (DLT40) SN survey only 0.73 days post-explosion, as determined by observations from the Transiting Exoplanet Survey Satellite (TESS). TESS observations of SN 2018lab yield a densely sampled, fast-rising, early-time light curve likely powered by ejecta–circumstellar medium (CSM) interaction. The blueshifted, broadened flash feature in the earliest spectra (<2 days) of SN 2018lab provides further evidence for ejecta–CSM interaction. The early emission features in the spectra of SN 2018lab are well described by models of a red supergiant progenitor with an extended envelope and a close-in CSM. As one of the few LLSNe with observed flash features, SN 2018lab highlights the need for more early spectra to explain the diversity of the flash feature morphology of Type II SNe.