1999MNRAS.302..499H

We present multi-epoch, intermediate-dispersion linear spectropolarimetry of the pathological Wolf–Rayet star EZ CMa. The continuum polarization shows long-term quasi-periodic variations, in both polarization magnitude (range 0.8 per cent) and position angle (60 °), and we observe changes in the polarization vector at emission-line wavelengths. We argue that the interstellar polarization (ISP) can be estimated from the convergence point of continuum-to-line polarization vectors, and we find that the ISP at 5800 Å is 0.47±0.02 per cent at a position angle of 164 °±2 °.

We subtract the new ISP estimate from our spectra to yield intrinsic polarized line profiles of unprecedented quality. We discuss the characteristics of the profiles, and compare them with synthetic profiles computed using radiative-transfer codes. Measurements of the line profiles provide some evidence for ionization stratification in the wind.

The polarization variation produced by rotational modulation of an azimuthally structured wind is investigated by using a simple single-scattering model. An `orange-segment' wind structure is capable of reproducing the amplitude of the observed polarization variations, although the shape of the polarized light curve cannot be matched using a simple model.