3-D Kinematics of Water Masers in the W51A Region

We report proper motion measurements of water masers in the massive-star forming region W 51A and the analyses of the 3-D kinematics of the masers in three maser clusters of W51A (W51 North, Main, and South). In W 51 North, we found a clear expanding flow that has an expansion velocity of ∼70 km s and indicates deceleration. The originating point of the flow coincides within 0.1 with a silicon-monoxide maser source near the HII region W 51d. In W51 Main, no systematic motion was found in the whole velocity range (158 km s ≤ VLSR < −58 km s) although a stream motion was reported previously in a limited range of the Doppler velocity (54 km s ≤ VLSR < 68 km s). Multiple driving sources of outflows are thought to explain the kinematics of W51 Main. In W51 South, an expansion motion like a bipolar flow was marginally visible. Analyses based on diagonalization of the variance-covariance matrix of maser velocity vectors demonstrate that the maser kinematics in W 51 North and Main are significantly tri-axially asymmetric. We estimated a distance to W51 North to be 6.1±1.3 kpc on the basis of the model fitting method adopting a radially expanding flow.