Polarization structure of the OH 1612-MHz maser envelope of VX Sagittarii

OH maser emission from the supergiant star VX Sgr has been mapped in both senses of circular polarization at 1612 MHz with an angular resolution of 0.14 arcsec and a velocity resolution of 0.36 km s(-1) using the MERLIN array. The maser emission arises in an envelope of inner radius similar to 1000 au and outer radius similar to 1900 au expanding at velocities of 20 and 22 km s(-1) respectively. The envelope structure is complex, but most of the 1612-MHz data can be accounted for by an axisymmetric radial outflow in which the masers are concentrated toward a thick circumstellar disc. One Zeeman pair was found that implies a magnetic field strength of -1.1 mG at the location of the 1612-MHz masers. Several maser features exhibiting complete circular polarization have been detected. The maser features with circular polarization higher than 50 per cent are preferentially located within the north-east and south-west quadrants of the envelope. The non-random distribution of highly polarized features suggests a dipole magnetic field configuration. The projection of the magnetic axis in the plane of the sky is 30 +/- 20 degrees, which is quite similar to that of the polar axis of the circumstellar disc. Such a configuration suggests a bipolar flow in which mass loss may be magnetically channelled.