2000A&A...361..273H

We present high signal-to-noise, multi-epoch linear spectropolarimetry of the dust producing Wolf-Rayet+O star binary WR 137 obtained over the period 1993-1998 at the William Herschel Telescope and the Pine Bluff Observatory. These observations display the `line effect' (a reduction in polarization magnitude through the emission lines) which is an unambiguous signature of Wolf-Rayet wind asphericity. We estimate the foreground polarization by fitting the spectra with a Serkowski law plus straight-line continua. The continuum polarization magnitude is variable (although we find no correlation with binary phase), while the position angle of the intrinsic vector is remarkably constant. It is shown that the position angle of the intrinsic polarization is perpendicular to the extended dust emission observed by Marchenko et al. (1999ApJ...522..433M). Numerical simulations of electron and dust scattering in WR 137 are presented. It is shown that the deviations from spherical symmetry caused by wind-wind interaction in the binary are insufficient to explain the observed polarization variability, and that the lack of polarization attributable to the dust formation episode places a strong constraint on its location.