U-ORIONIS - THE EVOLUTION AND PROPER MOTION OF THE OH MASER ENVELOPE

The Jodrell Bank MERLIN array was used to map the OH 1665- and 1667-MHz masers in the circumstellar envelope of U Orionis in 1984, 1986 and 1987. the OH masers lie within a region of extent 130 x 180 AU2, and have complex distributions which are different for each line and polarization. The masers show clumpy structures with a typical clump size of approximately 25 AU. The strongest emission is from a ring of radius 60 AU which we interpret as part of an expanding, tilted torus. Proper motions have been measured for the OH masers on the ring. The ring has a proper motion of 5.4 +/- 1.4 mas yr-1 corresponding to a transverse expansion velocity of approximately 7 km s-1 for the period-luminosity distance to U Orionis of 260 pc. This expansion velocity is also indicated by a compact blueshifted maser spot which lies near the projected stellar position. The intensity of the blueshifted feature increases strongly near stellar maximum, showing that the maser is unsaturated. The intensities of the masers on the ring vary to a smaller degree and closely follow the stellar variations. OH maser emission is also detected from a redshifted filament which appears to project over a considerable radial depth in the envelope and varies in intensity in an erratic fashion.