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

TL;DR

SN 2024acyl is a fast, linearly declining Type Ibn supernova with early flash-ionisation signatures, offering a rare glimpse into ejecta-CSM interaction in a helium-rich environment with residual hydrogen. Using MOSFiT to fit a hybrid Ni+CSM model and CMFGEN spectroscopic modelling, the paper derives a low ejecta mass of $M_{\mathrm{ej}}\approx 0.98\,M_{\odot}$, a small nickel mass $M_{\mathrm{Ni}}\approx 0.017\,M_{\odot}$, and a helium-dominated CSM shell of $M_{\mathrm{CSM}}\approx 0.39\,M_{\odot}$ at $R_0\approx 15.6\,$AU with $\rho_{\mathrm{CSM}}\approx 1.9\times10^{-11}\,\mathrm{g\,cm^{-3}}$, consistent with a low-mass He-star progenitor in an interacting binary and eruptive mass loss. The spectra reveal a transition from narrow He I P-Cygni features and flash-ionisation lines to broad He I emission and Fe-dominated features, supporting sustained ejecta-CSM interaction. Together, these findings imply a continuum between Ibn and IIn events and emphasize binary-interaction channels and eruptive mass loss in shaping Type Ibn observables. While a late-type WR star with hydrogen (WNH) or fallback-driven channels are not entirely ruled out, the data strongly favor a low-mass helium-star progenitor in a close binary with a dense, shell-like CSM.

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 ($3.5\pm0.8) \times 10^{42}$ erg s$^{-1}$, with a total radiated energy of $(5.0\pm0.4) \times 10^{48}$ erg. The spectra are dominated by a blue continuum at early stages, with narrow P-Cygni \Hei~lines and flash-ionisation emission lines of C {\sc iii}, N {\sc iii}, and He {\sc ii}. The P-Cygni \Hei~features gradually evolve and become emission-dominated in late-time spectra. The \Ha~line is detected throughout the entire spectral evolution, which indicates that the CSM is helium-rich with some residual amount of H. Our multiband light-curve modelling yields estimates of the ejecta mass of $M_{ej}$ = $0.98^{+0.30}_{-0.20} \, \msun$, with a kinetic energy of $E_{k} = 0.13^{+0.03}_{-0.02} \times 10^{51}$ erg, and a $^{56}Ni$ mass of $M_{\mathrm{Ni}} = 0.017 \, \msun$. The inferred CSM properties are characterised by a mass of $M_{\rm{CSM}} = 0.39^{+0.04}_{-0.04}$ \msun, an inner radius of $R_0$=$15.6^{+1.9}_{-2.0}$ AU, and a density $ρ_{CSM} = (1.32\pm0.22)\times10^{-11} \, \mathrm{g\,cm^{-3}}$. The multi-epoch spectra are well reproduced by the CMFGEN/ \texttt{he4p0} model, corresponding to a He-ZAMS mass of 4~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.

Figures (13)

• Figure 1: SN 2024acyl in a NOT+ALFOSC coloured image taken with Johnson $B$, $V$, and Sloan $r$ band filters on 2025 January 6. The SN is marked at the crosshair, near the centre of the image.
• Figure 2: Ultraviolet and optical light curves of SN 2024acyl. The dashed vertical line indicates the time of the $o$-band maximum light as the reference epoch. Vertical red lines on the top marks spectral observational epochs. The upper limits are plotted with empty symbols with arrows. The light curves for different filters are shifted with arbitrary constants, reported in the legend. The Mephisto $u$- and $v$-band data points in its unique filter system Chen2024ApJ...971L...2CYang2024ApJ...969..126Y are indicated by $u_{\mathrm{M}}$ and $v_{\mathrm{M}}$ in the legend. Magnitude errors are usually smaller than the symbol size.
• Figure 3: Colour evolution of SN 2024acyl compared with the prototypical Type Ibn SN 2006jc and other fast, linearly declining SNe Ibn. Top panel:$B~-~V$ colours. Bottom panel:$R~-~I$ or $r~-~i$ colours. The colour curves have been corrected for Galactic extinction.
• Figure 4: $V$-band light curves of SN 2024acyl, including the comparison SNe Ibn. Template $V$-band light curves for Type Ibn SNe from Hosseinzadeh2017ApJ...836..158H and Khakpash2024ApJS..275...37K. Owing to data-coverage limitations, SN 2023utc is represented using $r$-band photometry converted to the Vega system.
• Figure 5: Pseudobolometric light curves of SN 2024acyl and the comparison SNe Ibn. The comparison objects have luminosities comparable to that of the "optical" luminosity of SN 2024acyl, integrating from the $B$ to the $I/i$ bands.