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We wish to perform a statistical study of the location and motion ofindividual 22 GHz water maser cloudlets, characterized by sizes that are withina few au, with respect to the radio thermal emission from young stellar objects(YSO). For this purpose, we have been carrying out the Protostellar Outflows atthe EarliesT Stages (POETS) survey of a sample (38) of high-mass YSOs. Thewater maser positions and three-dimensional (3D) velocities were determinedthrough Very Long Baseline Array observations with accuracies of a fewmilliarcsec (mas) and a few km/s, respectively. The position of the ionizedcore of the protostellar wind, marking the YSO, was determined throughsensitive continuum Jansky Very Large Array observations with a typical errorof 20 mas. The statistic of the separation of the water masers from the radiocontinuum shows that 84% of the masers are found within 1000 au from the YSOand 45% of them are within 200 au. Therefore, we can conclude that the 22 GHzwater masers are a reliable proxy for the YSO position. The distribution ofmaser luminosity is strongly peaked towards low values, indicating that abouthalf of the maser population is still undetected with the current Very LongBaseline Interferometry detection thresholds of 50-100 mJy/beam.Next-generation, sensitive radio interferometers will exploit these weak masersfor an improved sampling of the velocity and magnetic fields around the YSOs.The average direction of the water maser proper motions provides astatistically-significant estimate for the orientation of the jet emitted bythe YSO: 55% of the maser proper motions are directed on the sky within anangle of 30 deg from the jet axis. Finally, we show that our measurements of 3Dmaser velocities statistically support models in which water maser emissionarises from planar shocks with propagation direction close to the plane of thesky.

Protostellar Outflows at the EarliesT Stages (POETS). IV. Statistical properties of the 22 GHz H2O masers