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

¹ European Southern Observatory, Alonso de Córdova 3107 Vitacura, Casilla, 19001, Santiago, Chile
² Cardiff Hub for Astrophysics Research and Technology, School of Physics & Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK
³ Department of Physics and Earth Science, University of Ferrara, Via Saragat 1, I-44122 Ferrara, Italy
⁴ INFN, Sezione di Ferrara, Via Saragat 1, I-44122 Ferrara, Italy
⁵ INAF, Osservatorio Astronomico d’Abruzzo, Via Mentore Maggini snc, 64100 Teramo, Italy
⁶ Graduate Institute of Astronomy, National Central University, 300 Jhongda Road, 32001 Jhongli, Taiwan
⁷ School of Physics, University College Dublin, LMI Main Building, Beech Hill Road, Dublin 4, D04 C1P1, Ireland
⁸ Institut d’Estudis Espacials de Catalunya (IEEC), Edifici RDIT, Campus UPC, 08860 Castelldefels, (Barcelona), Spain
⁹ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, E-08193 Barcelona, Spain
¹⁰ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
¹¹ DTU Space, Department of Space Research and Space Technology, Technical University of Denmark, Elektrovej 327, 2800 Kgs., Lyngby, Denmark
¹² Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto, Kyoto 606-8502, Japan
¹³ School of Physics, Trinity College Dublin, College Green, Dublin 2, Ireland
¹⁴ INAF-OATS, Via G.B. Tiepolo 11, 34143 Trieste, Italy
¹⁵ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁶ INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁷ Technical University of Munich, TUM School of Natural Sciences, Physics Department, James-Franck-Str. 1, 85741 Garching, Germany
¹⁸ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild Str. 1, D85741 Garching, Germany

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Received: 30 January 2026
Accepted: 7 April 2026

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.