Kinematics of W3(OH) - First proper motions of OH masers from VLBI measurements

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Abstract

The relative proper motions of the 1665 OH maser spots in W3(OH) were measured by comparing data from VLBI experiments at epochs 1978 and 1986. A significant ordered motion with apparent transverse velocities of a few km/s is found which provides 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 is ruled out. The observed pattern of motions suggests expansion from the ultracompact H II region seen in radio continuum emission. Alternatively, a velocity field resembling that expected for a cometary bow shock is consistent with the data. Significant temporal variations of the strengths of maser spots were measured. A map of the magnetic field in the OH maser clusters of W3(OH) is also obtained.

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