Understanding the launching mechanism of winds and jets remains one of the fundamental challenges in astrophysics. The``Protostellar Outflows at the Earliest Stages'' (POETS) survey has recently mapped the 3D velocity field of the protostellar winds in a sample (37) of luminous young stellar objects (YSO) at scales of 10-100au via very long baseline interferometry (VLBI) observations of the 22GHz water masers. In most of the targets, the distribution of the 3D maser velocities can be explained in terms of a magnetohydrodynamic (MHD) disk-wind (DW). We have performed multi-epoch Very Long Baseline Array (VLBA) observations of the 22GHz water masers in the YSO IRAS 21078+5211, the most promising MHD DW candidate from the POETS survey, to determine the 3D velocities of the gas flowing along several wind streamlines previously identified at a linear resolution of ~1au. By exploiting the 3D kinematic information of the masers, the launch radii of the streamlines are determined with an accuracy of ~1au and lie in the range of 10-50au. At increasing larger distances along the jet, the outflowing gas speeds up while it collimates close to the jet axis. Magneto-centrifugal launching in a radially extended MHD DW appears the only viable process to explain the fast (up to 60~km/s) and collimated (down to 10 deg) velocities of the wind in correspondence of launch radii ranging between 10 and 50au. The results obtained in IRAS 21078+5211 demonstrate that VLBI observations of the 22GHz water masers can reliably determine the launching mechanism of protostellar winds.

Cone search capability for table J/A+A/690/A81/table1 (Water maser parameters)