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

Abstract

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)x10(42) erg s(-1), with total radiated energies on the order of (1 - 10)x10(48) 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 10 000 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(-1). 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-ej) in the range 1 - 3 M-circle dot with kinetic energies (E-Kin) of (0.1 - 1)x10(50) erg. The inferred CSM mass ranges from 0.2 to 1 M-circle dot. These findings are consistent with expectations for core collapse events arising from relatively massive envelope-stripped progenitors.