Proper motions of water vapour masers and bipolarout ow from NML

MERLIN observations of the 22-GHz H 2 O masers in the circumstellar envelope of the supergiant NML Cyg show an irregular ring of emission 200 mas across, plus a pair of outlying features to the NW and SE nearly 600 mas apart. Comparison with maps at previous epochs reveals proper motions of these outlying features, of 1:90:4 mas yr ?1. This corresponds to a transverse velocity of 194 km s ?1 at a distance of 2 kpc, which is consistent with the radial velocity range of the 22-GHz emission. The remaining masers which form the irregular ring are modelled as an expanding and accelerating thick shell. The NW-SE symmetry axis of the H 2 O masers is aligned with a NW-SE asymmetry which occurs in the OH 1612-MHz masers on a larger scale of 3 arcsec. It appears that this is an internally driven eeect, most likely a bipolar outtow which is more highly collimated near the star (H 2 O masers) and becomes less collimated at greater distances (OH masers). The results are compared with bipolar outtows seen in other evolved objects.