3.1. Line-dependent polarization structure

The alternative representation of the same data in the Stokes q–u plane is shown in Figure 2. Data points are color-coded by wavelength, from red (receding velocities) to blue (approaching velocities), with the line center marked by a circled point. This representation highlights the contrasting behavior between the two sides of the line. The red-shifted points, corresponding to constant $\theta_{\text{red}}$, align along straight lines, whose slope defines $\theta_{\text{red}}$ (shown as black lines in figure 2). In contrast, the blue-shifted points deviate from this linear relation and trace closed “hook-shaped” loops, as first reported by Schulte-Ladbeck et al. (Reference Schulte-Ladbeck, Nordsieck and Taylor1991); Schulte-Ladbeck et al. (Reference Schulte-Ladbeck, Nordsieck, Taylor, Bjorkman, Magalhães and Wolff1992).

Figure 2.

q-u plot for the four nights of the 2009 Heii $\lambda 6560 A$ line organized in a clockwise direction. The black lines have been derived from the mean polarization angle of the red part of the lines $\theta_{red}$. Typical 1 $\sigma$ error bars are show as a black cross. A circular arrow indicates the direction of rotation for each night across the line starting by its bluest point.

The observed loops alternate in their sense of rotation from night to night, switching between clockwise and counterclockwise orientations when followed from the bluest to the reddest wavelengths. This systematic alternation reveals a rotating, large-scale structure that modulates the polarized flux as different regions of the wind come into view. A natural explanation is provided by the model of Ignace et al. (Reference Ignace, Bjorkman, Chené, Erba, Fabiani, Moffat, Sincennes and St-Louis2023), in which a CIR sweeps across the visible photosphere, intermittently obscuring the limb polarization. In that framework, the reversal of the loop’s rotation sense occurs as the CIR passes through phase 0, moving from one side of the line of sight to the other. The two successive reversals we detect within each four-night observing run therefore indicate that at least two CIRs are simultaneously present, ensuring continuous modulation across the star’s 3.76-day period, since a single CIR would affect the blue-side polarization only during half of the cycle (Ignace et al. Reference Ignace, Bjorkman, Chené, Erba, Fabiani, Moffat, Sincennes and St-Louis2023).

3.2. Formation radii and diagnostic power

Figure 3 displays the radial emissivity distributions for representative emission lines computed from PoWR models of WR 6. Recombination lines such as He ii and N v form close to the pseudo-photosphere, within the region where CIR-induced occultation can modulate the observed polarization. By contrast, He i lines form farther out in the wind, beyond the CIR-affected zone, while fluorescence–pumped lines such as C iv $\lambda\lambda$ $5801-12 A$ and N iv $\lambda 7111 A$ follow a different excitation pathway. Their upper levels are populated by UV continuum pumping rather than recombination, leaving the optical transitions optically thin. As they do not absorb polarized continuum photons, these lines remain insensitive to CIR-induced occultation and therefore do not exhibit blue-side polarization signatures. These differences in excitation and opacity explain why blue-side variations and loop patterns occur only in recombination lines, while He i and fluorescence–pumped transitions show only symmetric depolarization. Our analysis of WR 6’s linear spectropolarimetric variability provides new evidence that large-scale, co-rotating structures modulate the wind geometry. The contrasting polarimetric behavior of the red and blue wings of its emission lines reveals two complementary diagnostics. From the red wing, we can recover the intrinsic continuum polarization under the assumption of pure dilution, while the blue wing carries the signature of large-scale structures crossing our line of sight. The alternating orientation of the Q–U loops, observed consistently across all nights, supports a scenario involving at least two CIRs rotating with the star, each periodically obscuring part of the photosphere. In contrast, the absence of blue-side variations in He I and fluorescence lines points to formation regions unaffected by such occultation, either because they are located farther out in the wind or because their excitation mechanism differs fundamentally. Taken together, these results suggest that CIR-induced asymmetries may represent a recurrent phenomenon among early-type WN stars, rather than a peculiarity of WR 6. Continued time-resolved spectropolarimetric monitoring will help determine whether these structures persist, evolve, or reconfigure over time, offering valuable constraints on the rotational and magnetic dynamics that govern massive-star outflows.

Figure 3.

Illustration of $\xi$ (as defined in Hillier et al. (1987)) showing the origin of some of the strongest lines of WR6. The dashed line is Nv $\lambda 4945 A$ (7-6) which we can’t investigated. This figure is based on models calculated with the $Potsdam Wolf-Rayet$ (PoWR) model atmosphere code.^a The blue zone is shown as a indication of the area influenced by CIRs. $^{a}$ http://www.astro.physik.uni-potsdam.de/PoWR/