EVOLUTION OF MAGNETIC FIELDS IN HIGH MASS STAR FORMATION: SUBMILLIMETER ARRAY DUST POLARIZATION IMAGE OF THE ULTRACOMPACT H ii REGION G5.89−0.39

We report high angular resolution (3″) Submillimeter Array (SMA) observations of the molecular cloud associated with the Ultracompact H ii region G5.89 − 0.39. Imaged dust continuum emission at 870 μm reveals significant linear polarization. The position angles (P.A.s) of the polarization vary enormously but smoothly in a region of 2 × 104 AU. Based on the distribution of the P.A.s 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 μm, 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 P.A.s, is clearly disturbed by the expansion of the H ii 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 C17O 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.

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