Polarization of light from fast-rotating Wolf–Rayet stars: Monte Carlo simulations compared to the analytical formula

¹ Department of Theoretical Physics and Astrophysics, Faculty of Science, Masaryk University, Kotlářská 2, Brno, Czech Republic
² Astronomical Institute of the Czech Academy of Sciences, Fričova 298, 251 65 Ondřejov, Czech Republic

^★ Corresponding author: slah@physics.muni.cz

Received: 27 January 2025
Accepted: 27 April 2025

Abstract

Context. Fast-rotating Wolf–Rayet (WR) stars are potential progenitors of long gamma-ray bursts, but observational verification is challenging. Spectral lines from their expanding stellar wind obscure accurate rotational velocity measurements. Intrinsic polarization from wind rotation may help to determine rotational speeds. However, this procedure requires precise wind models.

Aims. Our study aims to investigate the intrinsic polarization due to the rotational distortion of WR winds considering multiplescattering of photons and compare it to a single-scattering model, in which we use an analytical expression of the polarization.

Methods. We studied the polarization signatures resulting from the prolate structure of rotating winds of two WR stars using a 3D Monte Carlo radiative transfer code Hyperion. We estimated the intrinsic polarization resulting from multiple-scattering in WR winds for different rotational velocities, inclination angles, and mass-loss rates.

Results. Our results indicate that at a rotation rate of less than 50% of the critical rate, the intrinsic polarization from multiplescattering is close to that of a single-scattering model. However, at higher rotation velocities, the polarization from multiple-scattering increases with inclination up to 40°, while it decreases for inclinations higher than about 60°. This dependence is inconsistent with the single-scattering model. We also discuss the effect of the mass-loss rate on the polarization and find that the polarization changes linearly with the mass-loss rate. However, it is important to note that the relationship between polarization and mass-loss rate may vary for different types of stars.

Conclusions. The results have implications for future studies of stellar winds and mass loss and may help to improve our understanding of the complex environments of massive stars. Our research offers valuable information on the complex polarization patterns observed in stellar winds, emphasizing the significance of accounting for the influence of multiple-scattering when interpreting observations.