Modeling Variable Linear Polarization Produced by Co-Rotating Interaction Regions (CIRs) Across Optical Recombination Lines of Wolf-Rayet Stars

TL;DR

This paper presents a Monte Carlo radiative transfer model to simulate variable linear polarization caused by Co-rotating Interaction Regions in Wolf-Rayet star winds, matching observed polarization variations and aiding in stellar rotation studies.

Contribution

It introduces a novel Monte Carlo radiative transfer approach combined with a simple CIR model to reproduce polarization features in WR star winds, advancing understanding of wind structures.

Findings

Successfully reproduces polarization amplitude and angle variations for WR 6

Highlights the importance of CIRs for determining stellar rotation periods

Provides a qualitative proof-of-concept for modeling polarization in WR winds

Abstract

Massive star winds are structured both stochastically ("clumps") and often coherently (Co-rotation Interaction Regions, or CIRs). Evidence for CIRs threading the winds of Wolf-Rayet (WR) stars arises from multiple diagnostics including linear polarimetry. Some observations indicate changes in polarization position angle across optical recombination emission lines from a WR star wind but limited to blueshifted Doppler velocities. We explore a model involving a spherical wind with a single conical CIR stemming from a rotating star as qualitative proof-of-concept. To obtain a realistic distribution of limb polarization and limb darkening across the pseudo-photosphere formed in the optically thick wind of a WR star, we used Monte Carlo radiative transfer (MCRT). Results are shown for a parameter study. For line properties similar to WR 6 (EZ CMa; HD 50896), the combination of the MCRT results, a simple model for the CIR, and the Sobolev approximation for the line formation, we were able to reproduce variations in both polarization amplitude and position angle commensurate with observations. Characterizing CIRs in WR~winds has added importance for providing stellar rotation periods since the v sin i values are unobtainable because the pseudo-photosphere forms in the wind itself.