KINEMATICS OF W3(OH) - 1ST PROPER MOTIONS OF OH MASERS FROM VLBI MEASUREMENTS

We have measured the relative proper motions of the 1665 MHz OH maser spots in W3(OH) by comparing data from VLBI (very long baseline interferometric) experiments at epochs 1978 and 1986. Most maser spots are found to persist with similar spatial morphologies, so they can be tracked from one epoch to the next without ambiguity. We find a significant ordered motion with apparent transverse velocities of a few km s-1, providing important direct information about the kinematics in the immediate vicinity of a recently formed massive star. The possibility that the OH maser spots are in a collapsing circumstellar envelope, as suggested by previous investigators, is definitively ruled out. The observed pattern of motions is generally suggestive of expansion from the ultracompact H II region seen in radio continuum emission. Alternatively, because the absolute motion of the maser spots with respect to the ultracompact H II region is not measured, a velocity field resembling that expected for a cometary bow shock is consistent with our data. Significant temporal variations of the strengths of maser spots were measured. By analyzing the line-of-sight velocities of spatially coincident maser spots of opposite circular polarizations in terms of Zeeman splitting, a map of the magnetic field in the OH maser clusters of W3(OH) is also obtained. Magnetic field strengths are typically 4 mG, with significant variations at different positions in the source.