Water masers map the complexity of mass loss from evolved stars

Ten years of ALMA imaging of evolved star winds show that the rate, composition and dynamics of mass loss are changeable, inhomogenous and asymmetric. Water masers, in energy states from 200-6000 K,  trace the wind from the pulsation-dominated radio atmosphere, to acceleration through escape velocity, out to  regions where the molecules are photo-dissociated or freeze out. Dozens of transitions are accessible to ALMA, of which at least 7 have been imaged. Maser beaming allows the wind structure to be mapped from the radio atmosphere to hundreds of stellar radii (Rstar) at an order of magnitude higher resolution than is possible with thermal lines. Applying recently-developed models to the overlap or segregation of water (and OH and SiO)  maser transitions reveals the local density, temperature and other physical conditions on Rstar scales. Many water masers are concentrated in dense clumps whilst the less dense surrounding gas is traced by masers such as at 183-GHz and OH lines. These measurements provide input to constrain chemical and kinematic models of thermal lines and dust.

Only 22-GHz masers have been monitored long-term, showing variability linked to the stellar phase but too far from the star to be explained directly by  pulsation shocks, suggesting heating stimulates their collisional pumping. Large variations, sometimes localised, are  seen on other timescales, perhaps due to a companion, variable or episodic directed mass loss -- or simply cloud overlap.   High-resolution observations can reveal shocks, since shocked masers can appear almost as large as the emitting region, in contrast to the usual shrinking due to maser amplification. Shocks may explain the location of some high-energy transitions at tens Rstar, further than expected from the star.

We present studies of VY CMa (RSG) and R Hya (AGB). The former has been mapped in the most water transitions; the CSE of the latter bears the scars of a recent thermal pulse and suggested companion interaction.