Massive stars exploding in a He-rich circumstellar medium XII. SN 2024acyl: A fast, linearly declining Type Ibn supernova with early flash-ionisation features
Authors
Y. -Z. Cai
A. Pastorello
K. Maeda
J. -W. Zhao
Z. -Y. Wang
Z. -H. Peng
A. Reguitti
L. Tartaglia
A. V. Filippenko
Y. Pan
G. Valerin
B. Kumar
Z. Wang
M. Fraser
J. P. Anderson
S. Benetti
S. Bose
T. G. Brink
E. Cappellaro
T. -W. Chen
X. -L. Chen
N. Elias-Rosa
A. Esamdin
A. Gal-Yam
M. González-Bañuelos
M. Gromadzki
C. P. Gutiérrez
A. Iskandar
C. Inserra
T. Kangas
E. Kankare
T. Kravtsov
H. Kuncarayakti
L. -P. Li
C. -X. Liu
X. -K. Liu
P. Lundqvist
K. Matilainen
S. Mattila
S. Moran
T. E. Müller-Bravo
T. Nagao
T. Petrushevska
G. Pignata
I. Salmaso
S. J. Smartt
J. Sollerman
M. D. Stritzinger
S. Srivastav
L. -T. Wang
S. -Y. Yan
Y. Yang
Y. -P. Yang
W. Zheng
X. -Z. Zou
L. -Y. Chen
X. -L. Du
Q. -L. Fang
A. Fiore
F. Ragosta
S. Zha
J. -J. Zhang
X. -W. Liu
J. -M. Bai
B. Wang
X. -F. Wang
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.
