Polarization light curve modelling of corotating interaction regions in the wind of the Wolf–Rayet star WR 6

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 et al. (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 • ) and for its orientation in the plane of the sky, although in the latter case we obtained two equally acceptable values (ψ = 63 • and ψ = 152 • ) from the polarimetry.Additional line profile variation simulations using the Sobolev approximation for the line transfer allowed us to eliminate the ψ = 152 • solution. With the adopted configuration (i 0 = 166 • and ψ = 63 • ), we were able to reproduce all datasets relatively well with two CIRs located near the stellar equator and always separated by ∼ 90 • 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 (β > ≈ 35 • ) were found to best reproduce the type of spectroscopic variability reported in the literature.

The Galactic WN stars revisited

“…WR 6 is still considered to be single; the polarimetric and photometric variability was rediscussed by St-Louis et al (2018) and attributed to CIRs.…”

A Multiwavelength Search for Intrinsic Linear Polarization in Wolf–Rayet Winds

“…Schmutz & Koenigsberger 2019) or CIRs (e.g. Moffat et al 2018;St-Louis et al 2018). Harries et al (1999) measured the ISP using a different method from ours, and found a very different result of P max = 0.47 ± 0.02% at θ 0 = 164 ± 2 • .…”

Monte Carlo simulations of polarimetric and light variability from corotating interaction regions in hot stellar winds

“…In view of the fact that the polarization curves we obtain using MCRT for CIRs in a spherical wind are quite different from those obtained using analytical models, the fits of the observations of the WR star WR6 presented in St-Louis et al (2018) need to be re-done. The fact that our MCRT curves have a much smaller amplitude and, depending on the optical depth of the wind, have peaks that are shifted compared to those obtained with the analytical models will certainly result in different output parameters, such as the density contrast or the opening angle of the CIR and perhaps even in a different orientation of the stellar axis with respect to our line-of-sight.…”

“…We also present error estimates for different wind densities introduced by varying the random number generator seed. In Section 4 we compare our polarimetric calculations to those obtained both for the optically thin (Ignace et al 2015) and thick (St-Louis et al 2018) limits. Finally, in Section 5 we present a parameter study for polarization and light curve calculations including a CIR and an associated bright gaussian spot.…”

“…Our model polarization curves present clear phase-dependant signatures for one or two CIRs, but for multiple CIRs create more complex behaviours. In St-Louis et al (2018) we extend our model to optically thick winds by accounting for multiple scattering using a "core-halo" approach. This approach defines a pseudo-photosphere beyond the radius of the star, above which the wind can be treated as optically thin.…”