Using VLBI to Probe the Orion-KL Outflow on AU Scales

Astrophys.J. 607 (2004) 361-368 We present the first contemporaneous 43GHz and 86GHz VLBI images of the v=1 J=2-1 and J=1-0 SiO masers in the Orion-KL nebula. Both maser species exhibit the same general morphology of earlier J=1-0 maser images which appear to trace the edges of a bi-polar conical outflow. Surprisingly, the J=2-1 masers form further from the central protostar than the J=1-0 masers, a fact not readily explained by current SiO maser pumping models. The average magnitude of offsets between corresponding regions of the two masing transitions is approximately 14% of the total radial extent of the SiO maser emission. This offset indicates that each transition must trace different physical conditions.