The Motion of Water Masers in the Pre-Planetary Nebula Iras 16342-3814

We present high angular resolution observations, using the Very Long Baseline Array of the NRAO 4 , of the high-velocity water masers toward the "water fountain" pre-planetary nebula, IRAS 16342-3814. The detailed structure of the water masers appears to be that of bow shocks on either side of a highly collimated jet. The proper motions of the water masers are approximately equal to the radial velocities; the three-dimensional velocities are approximately ± 180 km s −1 , which leads to a very short dynamical timescale of ∼100 yrs. Although we do not find direct evidence for precession of the fast collimated jet, there may be indirect evidence for such precession.