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 b oth senses of circular polarization at 1612 MHz with an angular resolu tion of 0.14 arcsec and a velocity resolution of 0.36 km s(-1) using t he MERLIN array. The maser emission arises in an envelope of inner rad ius similar to 1000 au and outer radius similar to 1900 au expanding a t velocities of 20 and 22 km s(-1) respectively. The envelope structur e is complex, but most of the 1612-MHz data can be accounted for by an axisymmetric radial outflow in which the masers are concentrated towa rd 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 m asers. Several maser features exhibiting complete circular polarizatio n have been detected. The maser features with circular polarization hi gher 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 configu ration. 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 o f the circumstellar disc. Such a configuration suggests a bipolar flow in which mass loss may be magnetically channelled.