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W.L. Raas (UCLA, Dept. of Physics & Astronomy), S.S. Doeleman, C.J. Lonsdale (MIT-Haystack Observatory, Westford, MA)
We have observed the SiO masers in the Orion-BN/KL star forming region and present VLBI images of the 3mm (86.2 GHz) and 7mm (43.1 GHz) transitions (v=1, J=2-1 and J=1-0 respectively). Emission from both maser lines forms in a distinctive ``X" morphology thought to represent the outlines of a bipolar conical outflow centered on a massive young star. Comparison of the high resolution images, however, reveals that in regions where emission is present, the two transitions are generally not cospatial. The centroid of emission for each maser line coincides with the position of the central star, but the 3mm masers occur at preferentially larger radii from the center than the 7mm masers. This finding, in contrast with expectations from single-dish work, implies that the two masers are excited under different conditions and may be used as sensitive and small scale probes of the outflow environment.
The author(s) of this abstract have provided an email address for comments about the abstract: wraas@ucla.edu