Abstract
We report on proper-motion measurements of H[2]O masers in the massive-star forming region W51A and analyses of the 3-D kinematics of the masers in three maser clusters of W51A (W51 North, Main, and South). Although we have measured more proper motions of H[2]O masers than in previous VLBI observations, we found a systematic motion of masers (an expanding flow) only in W51 North. Fundamentally, the maser kinematics are significantly affected by random motions, implying that several of the young massive stars simultaneously generate outflows and make the maser kinematics around the stars very complicated. We directly estimated the distance to W51 North to be (6.1+/-1.3)kpc based on the model-fitting analyses of the H[2]O maser kinematics.
