Protostellar Outflows at the EarliesT Stages (POETS). IV. Statistical
  properties of the 22 GHz H2O masers
  We wish to perform a statistical study of the location and motion of
individual 22 GHz water maser cloudlets, characterized by sizes that are within
a 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 at
the EarliesT Stages (POETS) survey of a sample (38) of high-mass YSOs. The
water maser positions and three-dimensional (3D) velocities were determined
through Very Long Baseline Array observations with accuracies of a few
milliarcsec (mas) and a few km/s, respectively. The position of the ionized
core of the protostellar wind, marking the YSO, was determined through
sensitive continuum Jansky Very Large Array observations with a typical error
of 20 mas. The statistic of the separation of the water masers from the radio
continuum shows that 84% of the masers are found within 1000 au from the YSO
and 45% of them are within 200 au. Therefore, we can conclude that the 22 GHz
water masers are a reliable proxy for the YSO position. The distribution of
maser luminosity is strongly peaked towards low values, indicating that about
half of the maser population is still undetected with the current Very Long
Baseline Interferometry detection thresholds of 50-100 mJy/beam.
Next-generation, sensitive radio interferometers will exploit these weak masers
for an improved sampling of the velocity and magnetic fields around the YSOs.
The average direction of the water maser proper motions provides a
statistically-significant estimate for the orientation of the jet emitted by
the YSO: 55% of the maser proper motions are directed on the sky within an
angle of 30 deg from the jet axis. Finally, we show that our measurements of 3D
maser velocities statistically support models in which water maser emission
arises from planar shocks with propagation direction close to the plane of the
sky.
