We present photometric and spectroscopic observations of the type Ibn supernova (SN) 2019uo, the second ever SN Ibn with flash ionization (He II, C III, N III) features in its early spectra. SN 2019uo displays a rapid post-peak luminosity decline of 0.1 mag d −1 similar to most of the SNe Ibn, but is fainter (M V max = −18.30 ± 0.24 mag) than a typical SN Ibn and shows a color evolution that places it between SNe Ib and the most extreme SNe Ibn. SN 2019uo shows P-cygni He I features in the early spectra which gradually evolves and becomes emission dominated post peak. It also shows faster evolution in line velocities as compared to most other members of the type Ibn subclass. The bolometric light curve is fairly described by a 56 Ni + circumstellar interaction model.

Rapidly evolving transients, or objects that rise and fade in brightness on timescales two to three times shorter than those of typical Type Ia or Type II supernovae (SNe), have uncertain progenitor systems and powering mechanisms. Recent studies have noted similarities between rapidly evolving transients and Type Ibn SNe, which are powered by ejecta interacting with He-rich circumstellar material (CSM). In this work we present multiband photometric and spectroscopic observations from Las Cumbres Observatory and Swift of four fast-evolving Type Ibn SNe. We compare these observations with those of rapidly evolving transients identified in the literature. We discuss several common characteristics between these two samples, including their light curve and color evolution as well as their spectral features. To investigate a common powering mechanism we construct a grid of analytical model light curves with luminosity inputs from CSM interaction as well as 56Ni radioactive decay. We find that models with ejecta masses of ≈1–3 M ⊙, CSM masses of ≈0.2–1 M ⊙, and CSM radii of ≈20–65 au can explain the diversity of peak luminosities, rise times, and decline rates observed in Type Ibn SNe and rapidly evolving transients. This suggests that a common progenitor system—the core collapse of a high-mass star within a dense CSM shell—can reproduce the light curves of even the most luminous and fast-evolving objects, such as AT 2018cow. This work is one of the first to reproduce the light curves of both SNe Ibn and other rapidly evolving transients with a single model.

We present the discovery and high-cadence follow-up observations of SN2018ivc, an unusual SNe II that exploded in NGC1068 (D=10.1 Mpc). The light curve of SN2018ivc declines piecewise-linearly, changing slope frequently, with four clear slope changes in the first 30 days of evolution. This rapidly changing light curve indicates that interaction between the circumstellar material and ejecta plays a significant role in the evolution. Circumstellar interaction is further supported by a strong X-ray detection. The spectra are rapidly evolving and dominated by hydrogen, helium, and calcium emission lines. We identify a rare high-velocity emission-line feature blueshifted at ∼7800 km s 1 (in Hα, Hβ, Pβ, Pγ, He I, and Ca II), which is visible from day 18 until at least day 78 and could be evidence of an asymmetric progenitor or explosion. From the overall similarity between SN2018ivc and SN1996al, the Hα equivalent width of its parent H IIregion, and constraints from pre-explosion archival Hubble Space Telescope images, we find that the progenitor of SN2018ivc could be as massive as 52 M  but is more likely <12 M . SN2018ivc demonstrates the importance of the early discovery and rapid follow-up observations of nearby supernovae to study the physics and progenitors of these cosmic explosions.