A 1 AU Expanding Water Maser Circular Ring in the W75 N(B)-VLA 2 Shell

We have identified and modeled a remarkable water maser "microstructure" of ~1.1 mas size within the expanding shell of ~0"16 size associated with the young stellar object W75 N(B)-VLA 2. The water maser spots of this microstructure present a spatial distribution and line-of-sight velocity components that fit extremely well a circular ring of ~1 AU radius expanding at ~2.5 km s-1. In particular, we have studied the spatio-kinematics and intensity distribution of the maser emission of an expanding ring observed at arbitrary angles. We construct position-velocity diagrams for the maser spots, calculating the maximum intensity as a function of the impact parameter and then comparing the results with the observations. This is the first time that such well-ordered spatio-kinematical behavior of the water masers is observed and modeled at the very small scale of ~1 AU. The VLA 2 maser shell is probably produced by an energetic stellar wind that compresses the ambient medium and drives a shock into it. Water maser emission is excited in flattened structures behind the shocks. We speculate that either these kinds of spatiokinematical microstructures are produced by fluid instabilities within the shocked material or they correspond to nearly round cloudlets (turbulent eddies?) in the ambient medium that were flattened by the expanding shock.