3-D kinematics of water masers in the W 51A region

We report on proper-motion measurements of water masers in the massive-star forming region W 51A and analyses of the 3-D kinematics of the masers in three maser clusters of W 51A (W 51 North, Main, and South). In W 51 North, we found a clear expanding flow having an expansion velocity of similar to 70kms(-1), which indicates deceleration. The originating point of the flow coincides within 0."1 with a silicon-monoxide maser source near the H II region W 51d. In W 51 Main, no systematic motion was found over the whole velocity range (-58 km(s-1) less than or equal to V-LSR < 158kms(-1)), although a stream motion was previously reported in a limited range of the Doppler velocity (54kms(-1) less than or equal to VLSR < 68kms(-1)). Multiple driving sources of outflows are thought to explain the kinematics of W 51 Main. In W 51 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 the distance to W 51 North to be 6.1+/-1.3kpc based on a model-fitting method adopting a radially expanding flow.