The radially expanding molecular outflow of VX Sagittarii

We used MERLIN to observe 22-GHz H(2)O maser emission from the red supergiant VX Sgr in 1994 and 1999. The masers lie in a region 300-400 mas in diameter. The angular size and the velocity distribution of the masers are similar at both epochs, although the total flux density in 1999 is only half of that seen in 1994. The maser emission is resolved into clouds of average diameter (11.5 +/- 4.0) mas which we infer are 300 times denser than the surrounding wind. The cloud radius is comparable to the stellar radius. We have previously found an order of magnitude variation in the size of H(2)O clouds around other evolved stars of masses 1-20 M(.), but the average size is consistently proportional to stellar size. 43 out of 92 distinct maser clouds observed in 1994 were matched with clouds seen in 1999. The mean change in position is 10.7 mas. The radial component of proper motion is directed away from the centre of expansion and increases with increasing angular separation. There is no significant rotational proper motion. The expansion proper motions are consistent with the maser Doppler velocities if VX Sgr is at (1.8 +/- 0.5) kpc. The maser distribution and kinematics suggest a spheroidal thick shell in which the stellar wind is undergoing radial acceleration, from 10 km s(-1) at the inner edge of the maser shell to 20 km s(-1) at the outer edge. We suggest a model for the asymmetric appearance of the maser shell and compare this with the magnetic field direction deduced from OH 1612-MHz maser observations. The maser expansion velocities have doubled between 1983 and 1994, suggesting the entire H(2)O maser shell has become more effectively accelerated, on a timescale much longer than the light-crossing time for the maser shell and much shorter than the wind-crossing time.