2010ApJ...716.1356M

We derive accurate proper motions of the CH₃ OH 12 GHz masers toward the W3(OH) ultra-compact (UC) H II region, employing seven epochs of VLBA observations spanning a time interval of about 10 yr. The achieved velocity accuracy is of the order of 0.1 km/s, adequate to precisely measure the relative velocities of most of the 12 GHz masers in W3(OH), with amplitude varying in the range 0.3-3 km/s. Toward W3(OH), the most intense 12 GHz masers concentrate in a small area toward the north (the northern clump) of the UC H II region. We have compared the proper motions of the CH₃OH 12 GHz masers with those (derived from literature data) of the OH 6035 MHz masers, emitting from the same region of the methanol masers. In the northern clump, the two maser emissions emerge from nearby (but likely distinct) cloudlets of masing gas with, in general, a rather smooth variation of line-of-sight and sky-projected velocities, which suggests some connection of the environments and kinematics traced by both maser types. The conical outflow model, previously proposed to account for the 12 GHz maser kinematics in the northern clump, does not reproduce the new, accurate measurements of 12 GHz maser proper motions and has to be rejected. We focus on the subset of 12 GHz masers of the northern clump belonging to the "linear structure at P.A. = 130°-140°," whose regular variation of LSR velocities with position presents evidence for some ordered motion. We show that the three-dimensional velocities of this "linear distribution" of 12 GHz masers can be well fitted considering a flat, rotating disk, seen almost edge-on.