Polarization Light Curve Modeling of Corotating Interaction Regions in the Wind of the Wolf-Rayet Star WR 6

TL;DR

This study models the polarization light curves of the Wolf-Rayet star WR 6 using a simplified analytical approach to understand the impact of Corotating Interaction Regions (CIRs) on its observed variability, successfully fitting multiple datasets.

Contribution

It introduces a modified analytical model for CIRs in stellar winds that accounts for multiple scattering and fits extensive observational data of WR 6 with consistent parameters.

Findings

Two CIRs near the stellar equator reproduce the data well.

Inclination angle of 166° and azimuth of 63° fit the polarimetric data.

Density contrasts less than two and large opening angles match observed variability.

Abstract

The intriguing WN4b star WR6 has been known to display epoch-dependent spectroscopic, photometric and polarimetric variability for several decades. In this paper, we set out to verify if a simplified analytical model in which Corotating Interaction Regions (CIRs) threading an otherwise spherical wind is able to reproduce the many broadband continuum light curves from the literature with a reasonable set of parameters. We modified the optically thin model we developed in Ignace, St-Louis & Proulx-Giraldeau (2015) to approximately account for multiple scattering and used it to fit 13 separate datasets of this star. By including two CIRs in the wind, we obtained reasonable fits for all datasets with coherent values for the inclination of the rotation axis ($i_0=166^{\circ}$) and for its orientation in the plane of the sky, although in the latter case we obtained two equally acceptable values ($\psi=63^{\circ}$ and $\psi=152^{\circ}$) from the polarimetry. Additional line profile variation simulations using the Sobolev approximation for the line transfer allowed us to eliminate the $\psi=152^{\circ}$ solution. With the adopted configuration ($i_0=166^{\circ}$ and $\psi=63^{\circ}$), we were able to reproduce all datasets relatively well with two CIRs located near the stellar equator and always separated by $\sim 90^{\circ}$ in longitude. The epoch-dependency comes from the fact that these CIRs migrate along the surface of the star. Density contrasts smaller than a factor of two and large opening angles for the CIR ($\beta \geq 35^{\circ}$) were found to best reproduce the type of spectroscopic variability reported in the literature.