Analytical modeling of polarization signals arising from confined circumstellar material in Type II supernovae

¹ Department of Physics and Astronomy, University of Turku FI-20014 Turku, Finland
² Aalto University Metsähovi Radio Observatory Metsähovintie 114 02540 Kylmälä, Finland
³ Aalto University Department of Electronics and Nanoengineering P.O. BOX 15500 FI-00076 AALTO, Finland
⁴ National Astronomical Observatory of Japan, National Institutes of Natural Sciences 2-21-1 Osawa Mitaka Tokyo 181-8588, Japan
⁵ Department of Astronomy, Kyoto University Kitashirakawa-Oiwake-cho Sakyo-ku Kyoto 606-8502, Japan
⁶ Department of Astronomy, School of Science, The University of Tokyo 7-3-1 Hongo Bunkyo-ku Tokyo 113-0033, Japan

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 24 September 2025
Accepted: 2 January 2026

Abstract

Context. Recent observations of Type II supernovae (SNe) have brought a challenge to our understanding of the final evolutionary stage of massive stars. The early-time spectra and light curves of Type II SNe suggest that a significant fraction are surrounded by dense circumstellar material (CSM), referred to as confined CSM, which likely results from enhanced mass-loss episodes in the final stages of the progenitor’s evolution. However, the mechanism driving this mass loss remains uncertain.

Aims. To address this problem, we aim to study the spatial distribution of the confined CSM, which carries important information about the mechanism.

Methods. We analytically calculated the polarization signals created by electron scatterings within disk-like confined CSM and applied the results to the case of SN 2023ixf.

Results. The calculated polarization angle remains fixed at the angle aligned with the CSM disk axis and is insensitive to the disk parameters. The calculated polarization degree evolves over a timescale of ≲10 days, depending on the disk parameters: it remains constant or increases slightly while the unshocked CSM is optically thick, peaks as it becomes optically thin, and drops to zero when the shock reaches the disk’s outer edge. We also find that the time evolution of the polarization in Type II SNe with confined CSM can be used for estimating the CSM parameters. In particular, the maximum degree and the rise time are strongly connected to the values of the viewing angle and the opening angle of the CSM disk, while the duration and the decline time are sensitive to the values of the mass and extension of the CSM disk. We demonstrate that the time evolution of the polarization of SN 2023ixf can be explained with a disk-like CSM with the following parameters: the viewing angle of θ_obs ≳ 40 degrees, the half-opening angle of the disk of θ_disk ∼ 50 − 60 degrees, the CSM mass of M_csm ∼ 2 × 10⁻³ M_⊙, and the outer edge of the CSM disk of r_out ∼ 3 × 10¹⁴ cm.

Conclusions. This information provides a strong constraint on the mechanism to create the confined CSM. Moreover, the observed alignment between the explosion asymmetry and the CSM disk of SN 2023ixf may point to a shared origin of these structures, possibly associated with the progenitor star itself rather than with companion interaction. Further early-time polarimetry of Type II SNe will be crucial for clarifying the underlying mechanism by probing the diversity of confined CSM. In addition, since this method can be applied to polarization calculations from an arbitrary shape of aspherical scattering-dominated photospheres, it enables us to study the geometries of a wide range of objects with scattering-dominated photospheres.