Massive stars exploding in a He-rich circumstellar medium

Cai, Y.-Z., Pastorello, A., Maeda, K., Zhao, J.-W., Wang, Z.-Y., Peng, Z.-H., Reguitti, A., Tartaglia, L., Filippenko, A. V., Pan, Y., Valerin, G., Kumar, B., Wang, Z., Fraser, M., Anderson, J. P., Benetti, S., Bose, S., Brink, T. G., Cappellaro, E., Chen, T. W. and Inserra, C. ORCID: https://orcid.org/0000-0002-3968-4409 2026. Massive stars exploding in a He-rich circumstellar medium. Astronomy & Astrophysics 707 , A157. 10.1051/0004-6361/202558014

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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)×10 42 erg s −1 , with a total radiated energy of (5.0 ± 0.4)×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. The H α 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 × 10 51 erg, and a 56 Ni 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 R 0 =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.

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Schools > Physics and Astronomy
Additional Information:	For the full list of authors see article webpage https://doi.org/10.1051/0004-6361/202558014 License information from Publisher: LICENSE 1: URL: https://creativecommons.org/licenses/by/4.0, Start Date: 2026-03-03
Publisher:	EDP Sciences
ISSN:	0004-6361
Date of First Compliant Deposit:	18 March 2026
Date of Acceptance:	22 January 2026
Last Modified:	18 Mar 2026 12:02
URI:	https://orca.cardiff.ac.uk/id/eprint/185834

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