  We present the results of the first high angular resolution observations of
SiO maser emission towards the star forming region W51-IRS2 made with the Very
Large Array (VLA) and Very Long Baseline Array (VLBA). Our images of the water
maser emission in W51-IRS2 reveal two maser complexes bracketing the SiO maser
source. One of these water maser complexes appears to trace a bow shock whose
opening angle is consistent with the opening angle observed in the distribution
of SiO maser emission. A comparison of our water maser image with an image
constructed from data acquired 19 years earlier clearly shows the persistence
and motion of this bow shock. The proper motions correspond to an outflow
velocity of 80 km/s, which is consistent with the data of 19 years ago (that
spanned 2 years). We have discovered a two-armed linear structure in the SiO
maser emission on scales of ~25 AU, and we find a velocity gradient on the
order of 0.1 km/s/AU along the arms. We propose that the SiO maser source
traces the limbs of an accelerating bipolar outflow close to an obscured
protostar. We estimate that the outflow makes an angle of <20 degrees with
respect to the plane of the sky. Our measurement of the acceleration is
consistent with a reported drift in the line-of-sight velocity of the W51 SiO
maser source.
