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

TL;DR

SN 2024abvb reveals a complex circumstellar environment shaped by episodic, binary-driven mass loss, exhibiting multiple narrow CSM components with velocities up to ~2000 km s^-1 and a pronounced, time-evolving polarisation that becomes blue-dominated at late times. The authors combine high-resolution optical/NIR spectroscopy with multi-epoch polarimetry to identify concentric, tilted toroidal CSM shells containing He, C, and O, plus distant H-rich material, and to infer a low optical depth wind-like CSM. The data favor a massive-star/binary CE or post-CE evolution, producing nested shells whose changing geometry and possible dust content explain the observed spectral evolution and the late-time polarization. This study demonstrates how detailed spectro-polarimetric monitoring can constrain progenitor channels and mass-loss histories in interacting transients, with implications for the late-stage evolution of stripped stars and binary interactions.

Abstract

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). We aim to characterise the kinematics, composition and geometry of the CSM around SN 2024abvb and to identify plausible progenitor/ejection scenarios that can produce the observed spectro-polarimetric evolution. 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 are used to trace multiple kinematic components and changes in scattering geometry. The high-resolution spectra reveal multiple narrow CSM components composed of He, C and O with absorption minima at $\sim150 - 400$ km s$^{-1}$ and additional faster material up to $\sim2000$ \kms. Low-velocity Balmer absorptions are present, indicating distant H-rich material, a first in SNe Ibn/Icn. Polarimetry shows a marked evolution ($P\sim1\%$ near peak, $\lesssim0.5\%$ after $\sim1$ week, rising to $\sim1.5\%$ at $\sim20$ d with $\sim50^\circ$ position-angle rotation and to $\sim4\%$ at $\sim30$ d, stronger in the blue), implying a time-variable, wavelength-dependent scattering/obscuration component. The combination of kinematics and polarimetric behaviour is consistent with multiple, concentric toroidal shells with differing orientations and partial dust content.

Figures (20)

• Figure 1: ATLAS, Pan-STARRS and public ZTF, BlackGEM and GOTO photometric data, with uncertainties, of SN 2024abvb. Open symbols denote limits. Phase is with respect to the maximum light. INTEL data and ancillary polarimetric data are shown with vertical lines. Note that a further XShooter spectrum was taken at $\sim 55$d post maximum when the SN was no longer visible in the optical.
• Figure 2: Cutout from the Legacy Surveys covering the position of SN 2024abvb (in the centre of the red circle) and its host galaxy. The image is oriented North up, East to the left, and covers approximately 1 on each side.
• Figure 3: Cumulative Distribution Function of the observed offsets from the host galaxy nucleus for different SN types 2021ApJS..255...29S. The location of SN 2024abvb lies far beyond the typical offset seen for SNe Ibn.
• Figure 4: Spectral sequence for SN 2024abvb. The spectra have been corrected for redshift and Milky Way reddening, and are vertically offset by arbitrary amounts for clarity. The colour of each spectrum indicates the instrument used. Vertical dashed lines mark the rest wavelengths of the strongest spectral features. Caution is advised when interpreting the UVES spectrum, as many apparent features are due to the echelle order pattern.
• Figure 5: Regions of our spectra covering H$\beta$ (left panel) and H$\alpha$ (right panel). In both cases, we see time evolving narrow absorptions with velocity between 100 and 400 km s$^{-1}$ with respect to the rest wavelength of the line. In the right panel, we also plot the atmospheric transmission, demonstrating that these lines are not associated with any telluric features.