Maser Spot Shape Persistence: Evidence for the Kinematic Interpretation of Proper Motions

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Abstract
Intrinsic morphologies of interstellar OH maser-emitting gas condensations have been measured, for the first time, at two distinct epochs. The shapes and orientations of these maser condensations, in the vicinity of the ultracompact H II region W3(OH), are remarkably persistent from one epoch to the next despite significant position shifts. These observations provide the first direct evidence that the motions measured are due to actual physical movement of discrete clumps of maser-emitting matter, rather than to some sort of nonkinematic effect, such as traveling excitation phenomena or chance realignments of coherency paths through the masing gas. The kinematic assumption is crucial to astrophysical applications of maser proper-motion measurements, including distance determinations and studies of source dynamics. The shapes of the OH maser spots in W3(OH) show a tendency to be elongated in a direction parallel to the shock front delineated by radio continuum maps of the H II region, supporting the picture in which masers are formed in compressed gas behind the shock front.