Probing Large-Scale Structure in the wind of EZ Cma/ WR6 through Linear Spectropolarimetric variations across spectral lines.

Luca Fabiani ( Université de Montréal )

Two types of structures can be found in the winds of Wolf-Rayet (WR) stars: clumps on a small scale and CIRs (Corotating Interaction Regions) that form large and coherent structures. EZ Cma (WR6) is a bright WR star known for displaying strong epoch- dependent variations with a period of 3.76 days. As these variations are stable on a timescale of a few weeks and then change in nature, the scenario that is favored by many is that it is a consequence of the presence of CIRs in its wind.

In this work, we use linear polarimetry to probe the wind of this star for additional evidence of the presence of these CIRs. I will present the analysis of 2 sets of 4 consecutive nights of high-resolution spectro-polarimetrics observations of EZ Cma obtained with the ESPaDOnS (Echelle Spectro-Polarimetric Device for the Observations of Stars) spectropolarimeter on the Canada-France-Hawaii Telescope (CFHT) in January 2009 and February 2010.

For both datasets, we find strong linear polarization variations across several HeII lines as well as for lines of other ions such as HeI, CIV, NIV and NV. The general orientation of the excursions in the QU plane caused by these changes varies from night to night. For HeII lines in particular, the polarization variations trace loops in the QU plane that appear to be restricted to the blue side of the line. The variations on the red side seem to be mostly linear in the QU plane. The loops on the blue side of the line are traced either in the clockwise and anticlockwise direction from one night to another.

Variations across spectral lines indicate the presence of some sort of asymmetry in the wind, possibly a CIR. Our data indicate that the changes are due in part to dilution of continuum polarization by unpolarized line flux (i.e. the line effect) but not entirely because the amount of depolarization is clearly not proportional to the line intensity. Some polarization of line photons and/or occultation effects are necessary to explain these complex variations.