Massive stars exploding in a He-rich circumstellar medium

XII. SN 2024acyl: A fast, linearly declining Type Ibn supernova with early flash-ionisation features

¹ Yunnan Observatories, Chinese Academy of Sciences Kunming 650216, P.R. China
² International Centre of Supernovae, Yunnan Key Laboratory Kunming 650216, P.R. China
³ INAF – Osservatorio Astronomico di Padova Vicolo dell’Osservatorio 5 35122 Padova, Italy
⁴ Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku Kyoto 606-8502, Japan
⁵ South-Western Institute for Astronomy Research, Yunnan University Kunming 650500, P.R. China
⁶ Yunnan Key Laboratory of Survey Science, Yunnan University, Kunming Yunnan 650500, P.R. China
⁷ School of Astronomy and Space Science, University of Chinese Academy of Sciences Beijing 100049, P.R. China
⁸ National Astronomical Observatories, Chinese Academy of Sciences Beijing 100101, P.R. China
⁹ School of Electronic Science and Engineering, Chongqing University of Posts and Telecommunications Chongqing 400065, P.R. China
¹⁰ INAF – Osservatorio Astronomico di Brera Via E. Bianchi 46 23807 Merate (LC), Italy
¹¹ INAF – Osservatorio Astronomico d’Abruzzo Via M. Maggini snc 64100 Teramo, Italy
¹² Department of Astronomy, University of California Berkeley CA 94720-3411, USA
¹³ School of Physics, O’Brien Centre for Science North, University College Dublin Belfield Dublin 4, Ireland
¹⁴ European Southern Observatory Alonso de Córdova 3107 Casilla 19 Santiago, Chile
¹⁵ Millennium Institute of Astrophysics (MAS) Nuncio Monseñor Sòtero Sanz 100 Providencia Santiago 8320000, Chile
¹⁶ Department of Physics and Astronomy, Aarhus University Ny Munkegade 120 DK-8000 Aarhus C, Denmark
¹⁷ Graduate Institute of Astronomy, National Central University 300 Jhongda Road 32001 Jhongli, Taiwan
¹⁸ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n E-08193 Barcelona, Spain
¹⁹ Xinjiang Astronomical Observatory, Chinese Academy of Sciences, Urumqi Xinjiang 830011, P.R. China
²⁰ Department of Particle Physics and Astrophysics, Weizmann Institute of Science 76100 Rehovot, Israel
²¹ Institut d’Estudis Espacials de Catalunya (IEEC) 08860 Castelldefels Barcelona, Spain
²² Astronomical Observatory, University of Warsaw Al. Ujazdowskie 4 00-478 Warszawa, Poland
²³ Cardiff Hub for Astrophysics Research and Technology, School of Physics & Astronomy, Cardiff University, Queens Buildings The Parade Cardiff CF24 3AA, UK
²⁴ Finnish Centre for Astronomy with ESO (FINCA), Quantum, University of Turku Vesilinnantie 5 FI-20014 Turku, Finland
²⁵ Tuorla Observatory, Department of Physics and Astronomy, University of Turku FI-20014 Turku, Finland
²⁶ Department of Physics & Astronomy, University of Turku Vesilinnantie 5 Turku FI-20014, Finland
²⁷ The Oskar Klein Centre, Department of Astronomy, Stockholm University AlbaNova SE-10691 Stockholm, Sweden
²⁸ Nordic Optical Telescope, Aarhus Universitet, Rambla José Ana Fernández Pérez 7, local 5, E-38711 San Antonio Breña Baja Santa Cruz de Tenerife, Spain
²⁹ Department of Physics and Astronomy, University of Turku FI-20014 Turku, Finland
³⁰ School of Sciences, European University Cyprus Diogenes Street Engomi 1516 Nicosia, Cyprus
³¹ School of Physics and Astronomy, University of Leicester University Road Leicester LE1 7RH, UK
³² School of Physics, Trinity College Dublin, The University of Dublin Dublin 2, Ireland
³³ Instituto de Ciencias Exactas y Naturales (ICEN), Universidad Arturo Prat Iquique, Chile
³⁴ Department of Physics and Astronomy, University of Turku FI-20014 Turku, Finland
³⁵ Aalto University Metsähovi Radio Observatory Metsähovintie 114 02540 Kylmälä, Finland
³⁶ Aalto University Department of Electronics and Nanoengineering PO BOX 15500 FI-00076 AALTO, Finland
³⁷ Center for Astrophysics and Cosmology, University of Nova Gorica Vipavska 11c 5270 Ajdovščina, Slovenia
³⁸ Instituto de Alta Investigación, Universidad de Tarapacá Casilla 7D Arica, Chile
³⁹ INAF – Osservatorio Astronomico di Capodimonte Salita Moiariello 16 80131 Napoli, Italy
⁴⁰ Department of Physics, University of Oxford, Denys Wilkinson Building Keble Road Oxford OX1 3RH, UK
⁴¹ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast Belfast BT7 1NN, UK
⁴² Department of Physics, Tsinghua University Beijing 100084, P.R. China
⁴³ National Astronomical Observatory of Japan, National Institutes of Natural Sciences 2-21-1 Osawa Mitaka Tokyo 181-8588, Japan
⁴⁴ Dipartimento di Fisica “Ettore Pancini”, Università di Napoli Federico II Via Cinthia 9 80126 Naples, Italy
⁴⁵ Purple Mountain Observatory, Chinese Academy of Sciences Nanjing 210023, P.R. China

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Received: 7 November 2025
Accepted: 22 January 2026

Abstract

We present a photometric and spectroscopic analyses of the Type Ibn supernova (SN) 2024acyl. It rises to an absolute magnitude peak of M_o = −17.58 ± 0.15 mag in 10.6 days, and displays a rapid linear post-peak light-curve decline in all bands (e.g. γ_0 − 60(V) = 0.097 ± 0.002 mag day⁻¹), similar to most SNe Ibn. The optical pseudobolometric light curve peaks at (3.5 ± 0.8)×10⁴² erg s⁻¹, with a total radiated energy of (5.0 ± 0.4)×10⁴⁸ 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 Hα 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_ej = 0.49^+0.11_−0.09 M_⊙ with a kinetic energy of E_k = 0.06^+0.01_−0.01 × 10⁵¹ erg, and a ⁵⁶Ni mass of M_Ni = 0.018 M_⊙. The inferred CSM properties are characterised by a mass of M_CSM = 0.51^+0.05_−0.04 M_⊙, an inner radius of R₀=17.8^+3.6_−3.0 AU, and a density of ρ_CSM = (8.3^+2.7_−1.2) × 10⁻¹² g cn⁻³. The multi-epoch spectra are well reproduced by the CMFGEN/ he4p0 model, corresponding to a He-ZAMS mass of 4 M_⊙ (H-ZAMS mass 18.11 M_⊙, pre-SN mass 3.16 M_⊙). 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.