HYPERSONIC ACCELERATION AND TURBULENCE OF H2O MASERS IN W49N

Velocity turbulence and hypersonic acceleration of H2O maser features appear in their distributions in position and velocity. These conclusions are based on five epochs of astrometric VLBI observations of W49N, particularly on the fourth epoch, when more than 250 features were detected. Correlation functions indicate that physical maser features have sizes of 1 AU and velocity scales of 0.5 km s-1. Between 1 and 300 AU, maser features show power-law correlation functions suggestive of velocity turbulence. At scales of 300 AU, masers are organized into arcs: associations of features oriented perpendicular to the direction of outflow. Velocities of features, including proper motion as well as Doppler velocity, show a minimum of about 20 km s-1. The high Doppler velocities of some features, long observed spectroscopically, represent high physical velocities. High-velocity features lie at the periphery of the maser cluster. The association of high- and low-velocity features on scales of 300 AU, much less than the 10(4) AU diameter of the cluster, suggests that high-velocity features are accelerated in situ. These observations can be understood in a picture where an energetic stellar wind strikes ambient material to produce masers behind shocks. Fluid instabilities in the shocked material, such a the instability of C-shocks discovered by Wardle, may then produce the strong turbulence observed between masers. Ram pressure from this wind could accelerate high-velocity features to the observed hypersonic velocities, as suggested by Strelnitskii & Sunyaev.