SN 2024acyl light curves

We present a photometric and spectroscopic analysis of the Type Ibn supernova (SN) 2024acyl. It rises to an absolute magnitude peak of M_o=-17.58±0.15mag in 10.6 days, and displays a rapid linear post-peak light-curve decline in all bands (e.g., gamma0-60(V)=0.097±0.002mag/d), similar to most SNe Ibn. The optical pseudobolometric light curve peaks at (3.5±0.8)x1042erg/s, with a total radiated energy of (5.0±0.4)x1048erg. The spectra are dominated by a blue continuum at early stages, with narrow P-Cygni HeI lines and flash-ionisation emission lines of CIII, NIII, and HeII. The P-Cygni HeI features gradually evolve and become emission-dominated in late-time spectra. The H-alpha 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 multiband light-curve modelling yields estimates of the ejecta mass of Mej=0.49+0.11-0.09M☉, with a kinetic energy of Ek=0.06+0.01-0.01x1051erg, and a 56Ni mass of MNi=0.018M☉. The inferred CSM properties are characterised by a mass of MCSM= 0.51 (+0.05/-0.04) M_sun, an inner radius of R0=17.8+3.6-3.0AU, and a density rhoCSM=(8.3+2.7-1.2)x10-12g/cm3. The multi-epoch spectra are well reproduced by the CMFGEN/he4p0 model, corresponding to a He-ZAMS mass of 4M☉ (H-ZAMS mass 18.11M☉, pre-SN mass 3.16M☉). 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 (CC) explosion of a late-type Wolf-Rayet (WR) star with hydrogen (WNH), 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.