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Measurement of the proper motions of masers provides one of the most precise means available for determining the kinematics of astrophysical objects. Masers may be used to probe extremely small spatial scales, since they are sufficiently compact and intense to be imaged with very long baseline interferometry (VLBI). Multi-epoch VLBI studies can provide a complete three-dimensional picture of source kinematics, assuming that the proper motions measured reflect actual physical motion of the maser-emitting material. This "kinematic" assumption has been made, with little direct supporting evidence, in virtually all past proper motion studies of interstellar masers.
In the course of measuring the proper motions of the OH masers in W3(OH), we have uncovered strong evidence that the kinematic interpretation of these motions is correct. Shapes of individual maser-emitting gas clouds tend to persist from one measurement to the next, despite proper motions that are a sizable fraction of cloud sizes. We will present radio maps of individual maser features illustrating this shape persistence, along with a simple statistical analysis. Similar tests cannot be done for water masers, which are usually not spatially resolved; however, our results provide general reassurance that the study of maser proper motions as a tool for measuring source kinematics and distances is well founded.
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