Search for a Magnetic Field via Circular Polarization in the Wolf-Rayet Star EZ CMa
Abstract
We report on the first deep, direct search for a magnetic field via the circular polarization of Zeeman splitting in a Wolf-Rayet (W-R) star. Using the highly efficient ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope, we observed at three different epochs one of the best W-R candidates in the sky expected to harbor a magnetic field, the bright, highly variable WN4 star EZ CMa = WR6 = HD 50896. We looked for the characteristic circular polarization (Stokes V) pattern in strong emission lines that would arise as a consequence of a global, rotating magnetic field with a split monopole configuration. We also obtained nearly simultaneous linear polarization spectra (Stokes Q and U), which are dominated by electron scattering, most likely from a flattened wind with large-scale corotating structures. As the star rotates with a period of 3.766 days, our view of the wind changes, which in turn affects the value of the linear polarization in lines versus continuum at the ~0.2% level. Depending on the epoch of observation, our Stokes V data were affected by significant crosstalk from Stokes Q and U to V. We removed this spurious signal from the circular polarization data and experimented with various levels of spectral binning to increase the signal-to-noise ratio of our data. In the end, no magnetic field is unambiguously detected in EZ CMa. Assuming that the star is intrinsically magnetic and harbors a split monopole configuration, we find an upper limit of B ~ 100 G for the intensity of its field in the line-forming regions of the stellar wind.
Based on observations obtained at the Canada-France-Hawaii Telescope (CFHT), which is operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii. This research used the facilities of the Canadian Astronomy Data Centre operated by the National Research Council of Canada with the support of the Canadian Space Agency.- Publication:
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The Astrophysical Journal
- Pub Date:
- February 2013
- DOI:
- 10.1088/0004-637X/764/2/171
- Bibcode:
- 2013ApJ...764..171D
- Keywords:
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- magnetic fields;
- polarization;
- stars: early-type;
- stars: Wolf-Rayet;
- techniques: polarimetric