2002ApJ...564..813M

In this paper, two epochs (1994 February and 1998 December) of Very Long Baseline Array observations of the 12 GHz CH₃OH masers toward the ultracompact (UC) H II region W3(OH) are compared. The 12 GHz maser emission is observed in three separated clumps (<500 AU in size), located north, south, and at the center of the UC H II region. The general structure of the 12 GHz maser emission has remained remarkably constant, with the maser spots identified in the 1994 epoch maps having similar relative positions (within a few milliarcseconds), shapes, and line-of-sight velocities (within a few tenths of a kilometer per second) as in the 1998 epoch maps. Cross-correlating the intensity distribution of the corresponding spots between the two epochs, we measure relative proper motions of amplitude in the range 1-7 km.s^–1. The spots of the southern clump move relative to the reference feature (located in the northern clump) with high velocities (~4 km.s^–1) and have similar directions of motion. This suggests that the whole southern clump moves relative to the northern clump. The amplitude of the proper motions of the 12 GHz masers agrees well with the values measured for the 1.665 GHz OH masers in W3(OH). The different spatial distribution of the 1.665 and 12 GHz maser spots prevents one from performing a detailed comparison of the gas motion traced by the two maser transitions. The 12 GHz CH₃OH masers of the northern clump are clearly associated with both the 6.035 GHz OH and 6.7 GHz CH₃OH masers. In particular, the same ``linear structure'' (~200 mas in size at P.A.=141°) is traced by the most intense spots of the three maser species, and they all show a common regular variation of the line-of-sight velocities along it. We fit the measured three-dimensional velocities and the sky-projected positions of the 12 GHz maser spots of the northern clump with an ``expanding conical model'', in which the spots lie on the surface of a bipolar cone and are accelerated radially outward, spiraling with a constant angular velocity around the cone axis. The model suggests that geometrical conditions may play an important selective role in the observation of the strong 12 GHz masers since the strongest spots are predicted at positions on the conical surface where the cone generators are approximately parallel (within a few degrees) to the line of sight.