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 photometric and spectroscopic analyses of the Type Ibn supernova (SN) 2024acyl. It rises to an absolute magnitude peak of $M_0 = -17.58 \pm 0.15$ mag in 10.6 days, and displays a rapid linear post-peak light-curve decline in all bands (e.g. $\gamma_{0-60}(V) = 0.097 \pm 0.002$ mag day$^{-1}$), similar to most SNe Ibn. The optical pseudobolometric light curve peaks at $(3.5\pm 0.8)\times10^{42}$ ergs $^{-1}$, with a total radiated energy of $(5.0 \pm 0.4)\times10^{48}$ erg. The spectra are dominated by a blue continuum at early stages, with narrow P-Cygni He I lines and flash-ionisation emission lines of C III, N III, and He II. The P-Cygni He I 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 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 $E_{\mathrm{k}} = 0.06^{+0.01}{-0.01} \times 10^{51}$ erg, and a $^{56}$Ni mass of $M{\mathrm{Ni}} = 0.018,M_{\odot}$. 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 $R_0 = 17.8^{+3.6}{-3.0}$ AU, and a density of $\rho{\mathrm{CSM}} = (8.3^{+2.7}{-1.2}) \times 10^{-12}$ g cm$^{-3}$. The multi-epoch spectra are well reproduced by the CMFGEN/he4p0 model, corresponding to a He-ZAMS mass of $4,M{\odot}$ (H-ZAMS mass $18.11,M_{\odot}$, pre-SN mass $3.16,M_{\odot}$). 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.