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 supernova discovered to date, with an exceptionally low r-band absolute magnitude of $-16.4$ mag. The pseudo-bolometric light curves peak at $(1-10)\times 10^{42}$ erg s${}^{-1}$, with total radiated energies on the order of $(1-10)\times 10^{48}$ erg. Spectroscopically, these SNe display relatively slow spectral evolution; the early spectra are characterised by a hot blue continuum and prominent He I emission lines. Early spectra show blackbody temperatures exceeding $10000\mathrm{K}$, with a subsequent decline in temperature during later phases. Narrow He I lines, indicative of unshocked circumstellar material (CSM), show velocities of approximately $1000\mathrm{km}{\mathrm{s}}^{-1}$. The spectra suggest that the progenitors of these SNe underwent significant mass loss prior to the explosion, resulting in a He-rich CSM. Light curve modelling yields estimates for the ejecta mass ($M_{\mathrm{ej}}$) in the range $1-3M_{\odot }$, with kinetic energies ($E_{\mathrm{Kin}}$) of $(0.1-1)\times 10^{50}$ erg. The inferred CSM mass ranges from $0.2$ to $1M_{\odot }$. These findings are consistent with expectations for core-collapse events arising from relatively massive, envelope-stripped progenitors.

54 pages, 53 figures, Accepted for publication in Astronomy & Astrophysics