Tracing the root of the bipolar jet in IRAS 20126+4104: VLBA observations of H2O masers

We present VLBA observations of the H2O masers towards IRAS 20126+4104, a h igh-mass young stellar object which is believed to be associated with a rot ating disk and a bipolar outflow/jet system collimated along the disk axis. A model fit to the positions and velocities of the maser spots demonstrate s that these could arise on a conical surface at the interface between the jet and the surrounding molecular gas. The masers expand from a common cent re which coincides, within the uncertainties, with the position of the embe dded young stellar object: the expansion velocity is less than that measure d on a much larger scale in the SiO(2-1) line and this indicates that the j et is sensitively braked close to its origin by the interaction with the su rrounding medium.