FULL POLARIZATION STRUCTURE OF THE OH MAIN-LINE MASER ENVELOPES OF W HYDRAE

Simultaneous MERLIN observations of the OH 1665- and 1667-MHz maser Li nes in the circumstellar envelope of the semiregular star W Hya have b een taken in all Stokes parameters. The 1665-MHz emission comes from t wo elongated clusters located 80 au from the star. The 1667-MHz emissi on arises in an incomplete shell of radius 130 au, with the blueshifte d features located in the northern part of the envelope and the redshi fted components clustered south of the centre. The circularly polarize d maser components exhibit spatial separation along the north-south di rection. The linearly polarized components were found from the near si de of the envelope. Their polarization position angles indicate that t he projected axis of the magnetic field at PA similar or equal to -20 degrees is consistent with spatial segregation of circular polarizatio n. The intensity of the magnetic field, estimated from a tentative mea surement of Zeeman splitting, is about 0.6 mG at the location of the 1 667-MHz emission, with the field pointing away from the observer. A sm all change of position angles of linear polarization observed in both maser lines is interpreted as a weak Faraday effect in the maser regio ns with an electron density of about 2 cm(-3). The overall polarizatio n structure of the envelope suggests an ellipsoidal or weak bipolar ge ometry. In such a configuration, the circumstellar magnetic held may e xert a nonnegligible influence on mass loss. The velocity held in the circumstellar envelope recovered from observations of SiO, H2O, OH and CO lines at five radial distances reveals a logarithmic velocity grad ient of 0.25 and 0.21 in the 1665- and 1667-MHz maser regions respecti vely. The acceleration within tens of stellar radii cannot be explaine d by the classical model of radiation pressure on dust.