SEARCHING FOR MAGNETIC FIELDS IN 11 WOLF-RAYET STARS: ANALYSIS OF CIRCULAR POLARIZATION MEASUREMENTS FROM ESPaDOnS

With recent detections of magnetic fields in some of their progenitor O stars, combined with known strong fields in their possible descendant neutron stars, it is natural to search for magnetic fields in Wolf-Rayet (WR) stars, despite the problems associated with the presence of winds enhanced by an order of magnitude over those of O stars. We continue our search among a sample of 11 bright WR stars following our introductory study in a previous paper of WR6 = EZ CMa using the spectropolarimeter ESPaDOnS at Canada-France-Hawaii Telescope, most of them in all four Stokes parameters. This sample includes six WN stars and five WC stars encompassing a range of spectral subclasses. Six are medium/long-period binaries and three show corotating interaction regions. We report no definite detections of a magnetic field in the winds in which the lines form (which is about the same distance from the center of the star as it is from the surface of the progenitor O star) for any of the eleven stars. Possible reasons and their implications are discussed. Nonetheless, the data show evidence supporting marginal detections for WR134, WR137, and WR138. According to the Bayesian analysis, the most probable field intensities are B-wind similar to 200, 130, and 80 G, respectively, with a 95.4% probability that the magnetic fields present in the observable parts of their stellar wind, if stronger, does not exceed B-wind(max) similar to 1900 G, similar to 1500 G, and similar to 1500 G, respectively. In the case of non-detections, we report an average field strength upper limit of B-wind(max) similar to 500 G.