VLBA imaging of the 3mm SiO maser emission in the disk-wind from the
  massive protostellar system Orion Source I
  We present the first images of the 28SiO v=1, J=2-1 maser emission around the
closest known massive young stellar object Orion Source I observed at 86 GHz
(3mm) with the VLBA. These images have high spatial (~0.3 mas) and spectral
(~0.054 km/s) resolutions. We find that the 3mm masers lie in an X-shaped locus
consisting of four arms, with blue-shifted emission in the south and east arms
and red-shifted emission in the north and west arms. Comparisons with previous
images of the 28SiO v=1,2, J=1-0 transitions at 7mm (observed in 2001-2002)
show that the bulk of the J=2-1 transition emission follows the streamlines of
the J=1-0 emission and exhibits an overall velocity gradient consistent with
the gradient at 7mm. While there is spatial overlap between the 3mm and 7mm
transitions, the 3mm emission, on average, lies at larger projected distances
from Source I (~44 AU compared with ~35 AU for 7mm). The spatial overlap
between the v=1, J=1-0 and J=2-1 transitions is suggestive of a range of
temperatures and densities where physical conditions are favorable for both
transitions of a same vibrational state. However, the observed spatial offset
between the bulk of emission at 3mm and 7mm possibly indicates different ranges
of temperatures and densities for optimal excitation of the masers. We discuss
different maser pumping models that may explain the observed offset. We
interpret the 3mm and 7mm masers as being part of a single wide-angle outflow
arising from the surface of an edge-on disk rotating about a
northeast-southwest axis, with a continuous velocity gradient indicative of
differential rotation consistent with a Keplerian profile in a high-mass
proto-binary.
