MAGNETIC FIELDS IN EVOLVED STARS: IMAGING THE POLARIZED EMISSION OF HIGH-FREQUENCY SiO MASERS

We present Submillimeter Array observations of high-frequency SiO masers around the supergiant VX Sgr and the semi-regular variable star W Hya. The J = 5-4, v = 1(28)SiO and v = 0(29)SiO masers of VX Sgr are shown to be highly linearly polarized with a polarization from similar to 5% to 60%. Assuming the continuum emission peaks at the stellar position, the masers are found within similar to 60 mas of the star, corresponding to similar to 100 AU at a distance of 1.57 kpc. The linear polarization vectors are consistent with a large-scale magnetic field, with position and inclination angles similar to that of the dipole magnetic field inferred in the H2O and OH maser regions at much larger distances from the star. We thus show for the first time that the magnetic field structure in a circumstellar envelope can remain stable from a few stellar radii out to similar to 1400 AU. This provides further evidence supporting the existence of large-scale and dynamically important magnetic fields around evolved stars. Due to a lack of parallactic angle coverage, the linear polarization of masers around W Hya could not be determined. For both stars, we observed the (SiO)-Si-28 and (SiO)-Si-29 isotopologues and find that they have a markedly different distributions and that they appear to avoid each other. Additionally, emission from the SO 5(5)-4(4) line was imaged for both sources. Around W Hya, we find a clear offset between the red-and blueshifted SO emission. This indicates that W Hya is likely host to a slow bipolar outflow or a rotating disk-like structure.