2013MNRAS.431.1077A

Silicon monoxide maser emission has been detected in the circumstellar envelopes of many evolved stars in various vibrationally excited rotational transitions. It is considered a good tracer of the wind dynamics close to the photosphere of the star. We have investigated the polarization morphology in the circumstellar envelope of an asymptotic giant branch star, R Cassiopeiae. We mapped the linear and circular polarization of SiO masers in the v = 1, J = 1-0 transition. The linear polarization is typically a few tens of per cent, while the circular polarization is a few per cent. The fractional polarization tends to be higher for emission of lower total intensity. We found that, in some isolated features, the fractional linear polarization appears to exceed 100 per cent. We found the Faraday rotation is not negligible but is ∼ 15°, which could produce small-scale structure in polarized emission, whilst the total intensity is smoother and partly resolved out. The polarization angles vary considerably from feature to feature, but there is a tendency to favour the directions parallel or perpendicular to the radial direction with respect to the star. In some features, the polarization angle abruptly flips 90°. We found that our data are in the regime gΩ ≫ R ≫ Γ, which indicates that the model of Goldreich, Keeley & Kwan can be applied and the polarization angle flip is caused when the magnetic field is at close to 55° to the line of sight. The polarization angle configuration is consistent with a radial magnetic field, although other configurations are not excluded.