VLBA Observations of G5.89−0.39: OH Masers and Magnetic Field Structure

We present VLBA observations of 1667 MHz OH maser emission from the massivestar formation region G5.89-0.39. The observations were phase referencedallowing the absolute positions of the masers to be obtained. The 1667 MHzmasers have radial velocities that span ~50 km/s but show little evidence oftracing the bipolar molecular outflow, as has been claimed in previous studies.We identify 23 Zeeman pairs through comparison of masers in left and rightcircular polarization. Magnetic field strengths range from -2 mG to +2 mG, andan ordered reversal in magnetic field direction is observed toward the southernregion of the UC HII region. We suggest that the velocity and magnetic fieldstructure of the 1667 MHz masers can be explained in the context of a model inwhich the masers arise in a neutral shell just outside a rapidly exandingionized shell.Comment: 32 pages, 12 figures, accepted for publication in Ap