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Cool, evolved stars undergo copious mass loss but the details of how thematter is returned to the ISM are still under debate. We investigated thestructure and evolution of the wind at 5 to 50 stellar radii from AsymptoticGiant Branch and Red Supergiant stars. 22-GHz water masers around seven evolvedstars were imaged using MERLIN, at sub-AU resolution. Each source was observedat between 2 and 7 epochs (several stellar periods). We compared our resultswith long-term Pushchino single dish monitoring. The 22-GHz emission is locatedin ~spherical, thick, unevenly filled shells. The outflow velocity doublesbetween the inner and outer shell limits. Water maser clumps could be matchedat successive epochs separated by <2 years for AGB stars, or at least 5 yearsfor RSG. This is much shorter than the decades taken for the wind to cross themaser shell, and comparison with spectral monitoring shows that some featuresfade and reappear. In 5 sources, most of the matched features brighten or dimin concert from one epoch to the next. One cloud in W Hya was caught in the actof passing in front of a background cloud leading to 50-fold, transientamplification. The masing clouds are 1-2 orders of magnitude denser than thewind average and contain a substantial fraction of the mass loss in thisregion, with a filling factor <1%. The RSG clouds are ~10x bigger than thoseround the AGB stars. Proper motions are dominated by expansion, with nosystematic rotation. The maser clouds survive for decades (the shell crossingtime) but the masers are not always beamed in our direction. Radiative effectscause changes in flux density throughout the maser shells on short timescales.Cloud size is proportional to parent star size; clouds have a similar radius tothe star in the 22-GHz maser shell. Stellar properties such as convection cellsmust determine the clumping scale.

Evolved star water maser cloud size determined by star size