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

Abstract

We present a photometric and spectroscopic analysis of the Type Ibn supernova (SN) 2024acyl. It rises to an absolute magnitude peak of about -17.58 mag in 10.6 days, and displays a rapid linear post-peak light-curve decline in all bands, similar to most SNe Ibn. The optical pseudobolometric light curve peaks at $(3.5\pm 0.8)\times 10^{42}$ erg s${}^{-1}$, with a total radiated energy of $(5.0\pm 0.4)\times 10^{48}$ 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. Our multi-band light-curve modelling yields estimates of the ejecta mass of $M_{\mathrm{ej}}=0.49_{-0.09}^{+0.11}$ 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 $M_{\mathrm{CSM}}=0.51_{-0.04}^{+0.05}$ M${}_{\odot }$, an inner radius of $R_0=17.8_{-3.0}^{+3.6}$ AU, and a density of $\ddot{I}{}_{\mathrm{CSM}}=(8.3_{-1.2}^{+2.7})\times 10^{-12}$ 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. In addition, a channel of core-collapse explosion of a late-type Wolf-Rayet star with H, or an Ofpe/WN9 star with fallback accretion, cannot be entirely ruled out.

19 pages, 12 figures. Accepted for publication in Astronomy and Astrophysics