Massive stars exploding in a He-rich circumstellar medium. XII. SN 2024acyl: A fast, linearly declining Type Ibn supernova with early flash-ionisation features

We present a photometric and spectroscopic analyses of the Type Ibn supernova (SN) 2024acyl. It rises to an absolute magnitude peak of M_ o = -17.58 ± 0.15 , mag in 10.6 days, and displays a rapid linear post-peak light-curve decline in all bands (e.g. γ_ 0-60 (V$) = 0.097 ± 0.002$ mathrm mag , -1 ), similar to most SNe Ibn. The optical pseudobolometric light curve peaks at (3.5 erg s 10^ 42 -1 $, with a total radiated energy of $(5.0 $ erg. The spectra are dominated by a blue continuum at early stages, with narrow P-Cygni ̋ei lines and flash-ionisation emission lines of C 10^ 48 iii , N iii , and He ii . The P-Cygni ̋ei features gradually evolve and become emission-dominated in late-time spectra. The ̋a 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_ ej = 0.49^ +0.11 _ -0.09 , with a kinetic energy of E_ k = 0.06^ +0.01 _ -0.01 10^ 51 , , and a erg 56 Ni mass of M_ Ni = 0.018 , The inferred CSM properties are characterised by a mass of M_ CSM = 0.51^ +0.05 _ -0.04 an inner radius of R_0=17.8^ +3.6 _ -3.0 AU, and a density of $ρ_ CSM = (8.3_ -1.2 ^ +2.7 ) , -12 -3 . The multi-epoch spectra are well reproduced by the CMFGEN/ 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. he4p0