Massive stars exploding in a He-rich circumstellar medium

We present a photometric and spectroscopic analyses of the Type Ibn supernova (SN) 2024acyl. It rises to an absolute magnitude peak of Mo = −17.58 ± 0.15 mag in 10.6 days, and displays a rapid linear post-peak light-curve decline in all bands (e.g. γ0 − 60(V) = 0.097 ± 0.002 mag day−1), similar to most SNe Ibn. The optical pseudobolometric light curve peaks at (3.5 ± 0.8)×1042 erg s−1, with a total radiated energy of (5.0 ± 0.4)×1048 erg. The spectra are dominated by a blue continuum at early stages, with narrow P-Cygni He <ns0:sc>I</ns0:sc> lines and flash-ionisation emission lines of C <ns0:sc>III</ns0:sc>, N <ns0:sc>III</ns0:sc>, and He <ns0:sc>II</ns0:sc>. The P-Cygni He <ns0:sc>I</ns0:sc> features gradually evolve and become emission-dominated in late-time spectra. The Hα line is detected throughout the entire spectral evolution, which indicates that the circumstellar material (CSM) is helium-rich with some residual amount of hydrogen. Our multi-band light-curve modelling yields estimates of the ejecta mass of $ M_{\mathrm{ej}} = 0.49^{+0.11}_{-0.09} \, M_{\odot} $ with a kinetic energy of $ E_{\mathrm{k}} = 0.06^{+0.01}_{-0.01} \times 10^{51}\,\mathrm{erg} $, and a 56Ni mass of MNi = 0.018 M⊙. The inferred CSM properties are characterised by a mass of $ M_{\mathrm{CSM}} = 0.51^{+0.05}_{-0.04}\, M_{\odot} $, an inner radius of R0=$ 17.8^{+3.6}_{-3.0} $ AU, and a density of $ \rho_{\mathrm{CSM}} = (8.3_{-1.2}^{+2.7})\times10^{-12} \, \mathrm{g\,cm}^{-3} $. The multi-epoch spectra are well reproduced by the CMFGEN/ <ns0:monospace>he4p0</ns0:monospace> model, corresponding to a He-ZAMS mass of 4 M⊙ (H-ZAMS mass 18.11 M⊙, pre-SN mass 3.16 M⊙). These findings are consistent with a scenario of an SN powered by ejecta-CSM interaction originating from a low-mass helium star that evolved within an interacting binary system where the CSM with some residual hydrogen may originate from the mass-transfer process. We also discuss an extreme scenario involving the possible merger of a helium white dwarf. In addition, a channel of core-collapse explosion of a late-type Wolf-Rayet (WR) star with hydrogen, or a transitional star between an Of and a WR type (e.g. an Ofpe/WN9 star) with fallback accretion cannot be entirely ruled out.