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We present the results of the first high angular resolution observations ofSiO maser emission towards the star forming region W51-IRS2 made with the VeryLarge Array (VLA) and Very Long Baseline Array (VLBA). Our images of the watermaser emission in W51-IRS2 reveal two maser complexes bracketing the SiO masersource. One of these water maser complexes appears to trace a bow shock whoseopening angle is consistent with the opening angle observed in the distributionof SiO maser emission. A comparison of our water maser image with an imageconstructed from data acquired 19 years earlier clearly shows the persistenceand motion of this bow shock. The proper motions correspond to an outflowvelocity of 80 km/s, which is consistent with the data of 19 years ago (thatspanned 2 years). We have discovered a two-armed linear structure in the SiOmaser emission on scales of ~25 AU, and we find a velocity gradient on theorder of 0.1 km/s/AU along the arms. We propose that the SiO maser sourcetraces the limbs of an accelerating bipolar outflow close to an obscuredprotostar. We estimate that the outflow makes an angle of <20 degrees withrespect to the plane of the sky. Our measurement of the acceleration isconsistent with a reported drift in the line-of-sight velocity of the W51 SiOmaser source.Comment: 19 pages, 5 figures (including 3 color). Accepted for publication in  ApJ (April 1, 2001 issue

Outflow 20–2000 AU from a High‐Mass Protostar in W51‐IRS 2