Nested, asymmetric H–He circumstellar shells in the Type Icn/Ibn SN 2024abvb

Anderson, J. P., Aster, C., Bulla, M., Chen, T.-W., Fraser, M., Galbany, L., Gutiérrez, C. P., Inserra, C. ORCID: https://orcid.org/0000-0002-3968-4409, Killestein, T., Leloudas, G., Lyman, J. D., Maeda, K., Maguire, K., Mason, E., Moriya, T., Pastorello, A., Taubenberger, S., Pursiainen, M. and Wichern, H. 2026. Nested, asymmetric H–He circumstellar shells in the Type Icn/Ibn SN 2024abvb. Astronomy & Astrophysics 710 , A35. 10.1051/0004-6361/202659241

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Abstract

Context. Interacting transients probe mass loss in the final stages of stellar evolution; however, the geometry and timing of multi-episode mass loss remain poorly constrained. SN 2024abvb is a nearby interacting event with transitional Ibn/Icn spectroscopic properties and multi-epoch polarimetry, offering a rare opportunity to study structured circumstellar material (CSM). Aims. We aim to characterise the kinematics, composition, and geometry of the CSM around SN 2024abvb and to identify plausible progenitor and ejection scenarios that can produce the observed spectro-polarimetric evolution. Methods. We present high-resolution (VLT/UVES and VLT/X-Shooter) optical/NIR spectroscopy across several epochs, complemented by broadband polarimetry and spectropolarimetry (VLT/FORS2 and NOT/ALFOSC). Line identifications, velocity decompositions, and polarimetric time series were used to trace multiple kinematic components and changes in scattering geometry. Results. The high-resolution spectra reveal multiple narrow CSM components composed of He, C, and O with absorption minima at ∼150 − 400 km s −1 and additional faster material up to ∼2000 km s −1 . Low-velocity Balmer absorptions are present, indicating distant H-rich material, a first in Type Ibn/Icn supernovae. Polarimetry shows a marked evolution ( P ∼ 1% near the peak, ≲0.5% after ∼1 week, rising to ∼1.5% after ∼20 d with ∼50° position-angle rotation and to ∼4% after ∼30 d, stronger in the blue), implying a time-variable, wavelength-dependent scattering and obscuration component. The combination of kinematics and polarimetric behaviour is consistent with multiple, concentric toroidal shells with differing orientations and partial dust content.

Item Type:	Article
Date Type:	Published Online
Status:	Published
Schools:	Schools > Physics and Astronomy
Additional Information:	License information from Publisher: LICENSE 1: URL: https://creativecommons.org/licenses/by/4.0, Start Date: 2026-05-29
Publisher:	EDP Sciences
ISSN:	0004-6361
Date of First Compliant Deposit:	10 June 2026
Date of Acceptance:	7 April 2026
Last Modified:	04 Aug 2026 21:24
URI:	https://orca.cardiff.ac.uk/id/eprint/187507

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