Abstract
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.
Highlights
Since the discovery of the first bipolar outflows in star-forming regions (Snell et al 1980), significant progress has been made in understanding the large-scale characteristics of such outflows (e.g., Bachiller 1996)
We surmise that the exciting protostar is massive based on an estimate of the mechanical luminosity of the H2O maser outflow: assuming an H2 density for the outflow of ≥ 106 cm−3, an opening angle of 25◦, and flow velocity of ∼ 80 km s−1, we find L ≥ 10 L⊙
We have resolved the structure of the W51-IRS2 SiO maser source, and linked it to a protostellar outflow associated with two long-known sites of intense H2O maser emission
Summary
Since the discovery of the first bipolar outflows in star-forming regions (Snell et al 1980), significant progress has been made in understanding the large-scale characteristics of such outflows (e.g., Bachiller 1996). The large columns of gas and dust that obscure massive protostars hinder traditional optical or infra-red observations of at least the inner ∼ 100 AU of these outflows, where the exciting protostars reside. This has made it difficult to obtain sufficient data to understand well the process of high-mass star formation. The only non-stellar SiO maser source that has been well studied is the one in Orion-KL, where the maser emission traces an outflow within ∼ 100 AU of an obscured massive protostar (Greenhill et al 1998; Doeleman et al 1999)
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