  We report high angular resolution (3") Submillimeter Array (SMA) observations
of the molecular cloud associated with the Ultra-Compact HII region G5.89-0.39.
Imaged dust continuum emission at 870 micron reveals significant linear
polarization. The position angles (PAs) of the polarization vary enormously but
smoothly in a region of 2x10^4 AU. Based on the distribution of the PAs and the
associated structures, the polarized emission can be separated roughly into two
components. The component "x" is associated with a well defined dust ridge at
870 micron, and is likely tracing a compressed B field. The component "o" is
located at the periphery of the dust ridge and is probably from the original B
field associated with a pre-existing extended structure. The global B field
morphology in G5.89, as inferred from the PAs, is clearly disturbed by the
expansion of the HII region and the molecular outflows. Using the
Chandrasekhar-Fermi method, we estimate from the smoothness of the field
structures that the B field strength in the plane of sky can be no more than
2-3 mG. We then compare the energy densities in the radiation, the B field, and
the mechanical motions as deduced from the C^17O 3-2 line emission. We conclude
that the B field structures are already overwhelmed and dominated by the
radiation, outflows, and turbulence from the newly formed massive stars.
