OH MASER EMISSION IN THE CIRCUMSTELLAR ENVELOPES OF U-HERCULIS, R-CASSIOPEIAE AND W-HYDRAE

MERLIN maps are presented of the OH 1665- and 1667-MHz masers in the c ircumstellar envelopes of the Mira variables U Her and R Cas, and the semiregular variable W Hya. For U Her, approximately 95 per cent of th e OH maser emission comes from a partially filled thick shell, at radi i between 50 and 135 au from the star. Across this shell, the logarith mic velocity gradient, epsilon, is in the range 0 < epsilon < 0.3. The OH masers of U Her have a core-halo structure. The strongest and most polarized emission occurs from approximately 10 core components with angular sizes < 0.03 arcsec. Weaker emission occurs from halo structur es with angular sizes < 1.0 arcsec. In R Cas, the OH masers have a clu mpy distribution with no obvious centre of symmetry. The circumstellar envelope of this source may be partially disrupted by a binary compan ion star. Three possible envelope geometries are discussed for R Cas. In the preferred model, the masers are located within an incomplete th ick shell at a mean radius of approximately 110 au, with a logarithmic velocity gradient in the range 0 < epsilon < 1.0. For W Hya, MERLIN d etected only the stronger emission from the front of the envelope, wit h a maximum detected OH extent of 60 au. For all three sources, the lo cations of the OH masers are consistent with standard models for the e xcitation of mainline OH, and with the estimated infrared mass-loss ra tes. We suggest that stellar mass loss from type I sources is intrinsi cally chaotic and clumpy.