We present Odin observations of the AGB star W Hya in the ground-state transition of ortho-H _ { 2 } O , 1 _ { 10 } -1 _ { 01 } , at 557 GHz .
The line is clearly of circumstellar origin .
Radiative transfer modelling of the water lines observed by Odin and ISO results in a mass-loss rate of ( 2.5 \pm 0.5 ) 10 ^ { -7 } M _ { \odot } yr ^ { -1 } , and a circumstellar H _ { 2 } O abundance of ( 2.0 \pm 1.0 ) 10 ^ { -3 } .
The inferred mass-loss rate is consistent with that obtained from modelling the circumstellar CO radio line emission , and also with that obtained from the dust emission modelling combined with a dynamical model for the outflow .
The very high water abundance , higher than the cosmic oxygen abundance , can be explained by invoking an injection of excess water from evaporating icy bodies in the system .
The required extra mass of water is quite small , on the order of \sim 0.1 M _ { \oplus } .