Using VLBI to Probe the Orion-KL Outflow on AU Scales
  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. 
Sheperd S. Doeleman (1), Colin J. Lonsdale (1), Paul T. Kondratko (2),   C. Read Predmore (3) ((1) MIT - Haystack Observator...