MERLIN polarimetry of the OH maser emission from R Crateris

OH maser emission from the circumstellar envelope of the semiregular variable star R Crt has been imaged in all Stokes parameters using the MERLIN array. The 1665- and 1667-MHz emission has a compact structure with an angular extent less than 0.2 arcsec. The combined 1665- and 1667-MHz data can be fitted with a thin-shell model of angular radius 0.14 +/- 0.04 arcsec. There is a systematic displacement between the blueshifted emission, which tends to lie to the south, and the redshifted emission, which lies mainly to the north. The polarization properties of the blue- and redshifted emission also differ markedly. Linear polarization is observed in the blueshifted 1667-MHz emission only, while there are spatial separations between the maxima of left and right circular polarization for the blueshifted emission. It is suggested that magnetic effects may be responsible both for the polarization asymmetries and for the front-to-back asymmetries in the OH envelope.