Monte Carlo Simulations of Polarimetric and Light Variability from Corotating Interaction Regions in Hot Stellar Winds

TL;DR

This study employs 3D Monte Carlo radiative transfer simulations to analyze how corotating interaction regions in hot stellar winds cause observable polarimetric and light variability, revealing complex effects of multiple scattering and localized bright spots.

Contribution

It introduces a detailed 3D Monte Carlo model for CIR-induced variability, improving upon previous analytical methods especially in optically thick conditions.

Findings

Monte Carlo simulations show significant differences from analytical models in optically thick cases.

Bright spots at the CIR base cause small polarization excess but can dominate light curves.

Multiple scattering effects are crucial for accurate modeling of polarization in stellar winds.

Abstract

We use a 3D Monte Carlo radiative transfer code to study the polarimetric and photometric variability from stationary corotating interaction regions (CIR) in the wind of massive stars. Our CIRs are approximated by Archimedean spirals of higher (or lower) density formed in a spherical wind originating from the star and we also made allowance for a bright gaussian spot at the base of the CIR. Comparing results from our code to previous analytical calculations in the optically thin case, we find differences which we attribute mainly to a better estimation of the total unpolarized flux reaching the observer. In the optically thick case, the differences with the analytical calculations are much larger, as multiple scattering introduces extra complexities including occultation effects. The addition of a gaussian spot does not alter the shape of the polarization curve significantly but does create a small excess in polarization. On the other hand, the effect can be larger on the light curve and can become dominant over the resulting CIR, depending on the spot parameters and density of the wind.