2025A&A...696L..12D

High-cadence surveys of the sky are revealing that a large fraction of red-supergiant (RSG) stars, which are the progenitors of Type II-Plateau (II-P) supernovae (SNe), explode within circumstellar material (CSM). Such Type II SNe exhibit a considerable diversity, with interaction signatures lasting from hours to days. As a result, they might merge with the Type IIn subclass, for which longer duration interaction typically occurs. To tackle this growing sample of transients and to understand the pre-SN mass loss histories of RSGs, we used the highest quality, spectropolarimetric observations of a young Type IIn SN taken to date. Specifically, we used those of SN 1998S at ∼5 days (d) after explosion. We designed an approach based on a combination of radiation hydrodynamics with HERACLES and polarized radiative transfer with CMFGEN and LONG_POL. The adopted asymmetries are based on a latitudinal, depth- and time-independent scaling of the density of 1D models of Type II SNe interacting with CSM (e.g., model r1w6b with a “wind” mass-loss rate of 0.01 M_⊙ yr^–1 used for SN 2023ixf). For a pole-to-equator density ratio of 5, we find that the polarization reaches (and then remains at each specific level for several days) a maximum value of 1.0, 1.4, and 1.8% as the CSM extent is changed from 6, to 8, and 10 × 10¹⁴ cm. The polarization is independent of wavelength, apart from funnel-shaped depolarizations within the emission lines. Our models imply a significant depolarization at the line cores, which we used to constrain the interstellar polarization of SN 1998S. Our 2D prolate ejecta models with moderate asymmetry offer a good match to the spectropolarimetric observations of SN 1998S at 5 d, supporting a polarization level of about ∼2%. This study provides a framework for interpreting future spectropolarimetric observations of the broad class of Type II SNe with CSM interaction and fostering a better understanding of the origins of their pre-SN mass loss.