Evolution of magnetic fields in high mass star formation: Submillimeter array dust polarization image of the ultracompact H II Region G5.89-0.39

Abstract

We report high angular resolution (3) Submillimeter Array (SMA) observations of the molecular cloud associated with the Ultracompact Hii 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 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 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.