Massive stars exploding in a He-rich circumstellar medium. XI. Diverse evolution of five Ibn SNe 2020nxt, 2020taz, 2021bbv, 2023utc, and 2024aej

We present the photometric and spectroscopic analysis of five Type Ibn supernovae (SNe): SN,2020nxt, SN,2020taz, SN,2021bbv, SN,2023utc, and SN,2024aej. These events share key observational features and belong to a family of objects similar to the prototypical Type Ibn SN,2006jc. The SNe exhibit rise times of approximately 10 days and peak absolute magnitudes ranging from -16.5 to -19 mag. Notably, SN,2023utc is the faintest Type Ibn SN discovered to date, with an exceptionally low r-band absolute magnitude of -16.4 mag. The pseudo-bolometric light curves peak at $(1-10) $ erg s^-1, with total radiated energies on the order of $(1-10) $ erg. Spectroscopically, these SNe display a relatively slow spectral evolution. The early spectra are characterised by a hot blue continuum and prominent He i emission lines. The early spectra also show blackbody temperatures exceeding $10000 K $, with a subsequent decline in temperature during later phases. Narrow He i lines, which are indicative of unshocked circumstellar material (CSM), show velocities of approximately $1000 km s^ $. The spectra suggest that the progenitors of these SNe underwent significant mass loss prior to the explosion, resulting in a He-rich CSM. Our light curve modelling yielded estimates for the ejecta mass (M_̊m ej) in the range $1-3 M_⊙$ with kinetic energies (E_̊m Kin) of $(0.1-1) $ erg. The inferred CSM mass ranges from $0.2$ to $1 M_⊙$. These findings are consistent with expectations for core collapse events arising from relatively massive envelope-stripped progenitors.