MERLIN OBSERVATIONS OF WATER MASER PROPER MOTIONS IN VY-CANIS-MAJORIS

MERLIN observations of the 22-GHz water masers in the circumstellar en velope of the supergiant VY CMa show an ellipsoidal distribution with a maximum extent of 700 mas east-west and 400 mas north-south. Compari son with observations made nine years earlier shows that the majority of maser features have survived and show proper motions throughout the region. The mean change in position is 28 mas and the proper motions are generally directed away from the assumed stellar position, and ten d to be larger for features at greater projected distances. If the H2O maser region is modelled as a partially filled thick spherical shell, and VY CMa is at a distance of 1.5 kpc, then the proper motion veloci ties in the direction of expansion are between 8 km s(-1) at a distanc e of 75 mas from the assumed stellar position and 32 km s(-1) at 360 m as. These velocities are consistent with the H2O maser spectral line v elocities which correspond to a maximum expansion velocity of 36 km s( -1) at 400 mas from the assumed stellar position. These observations a re consistent with radiation pressure on dust providing the force to a ccelerate the stellar wind as it passes through the H2O maser shell. T he H2O maser region is elongated in the same direction as the dusty ne bula around VY CMa. The water masers illuminate the small-scale dynami cs and clumpiness which show the role of dust in driving the outflow. The overall ellipsoidal shape may be due to properties of the dust, su ch as its behaviour in the stellar magnetic field, or to interaction b etween the wind and circumstellar material. Maser monitoring also show s the difference between changes on the time-scale of stellar variabil ity (a few years) and possible stages in the evolution of VY CMa to it s likely fate as a supernova.