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

We present the results of the first high angular resolution observations of SiO maser emission toward the star-forming region W51-IRS 2 made with the Very Large Array (VLA) and Very Long Baseline Array (VLBA). Our images of the H2O maser emission in W51-IRS 2 reveal two maser complexes bracketing the SiO maser source. One of these H2O 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 H2O 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-1, 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-1 AU-1 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 less than 20° 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.