Expansion of the R4 Water Maser Arc Near Cepheus A HW 2
  We present new (April 2000) MERLIN observations of the H2O masers located near the protostar Cepheus A HW2. The MERLIN observations detect many of the structures found in earlier (1996) VLBA observations of Torrelles and collaborators, and the changed positions of these structures are compatible with the VLBA proper motions and astrometric uncertainties. The radius of curvature of the R4 structure of maser arcs appears to have grown by a factor of two, and the displacement of the arcs between 1996 and 2000 are compatible with expansion about a common center. In addition, the MERLIN observations detect red-shifted masers not previously found; taken with the newly discovered masers, the R4 structure now resembles patchy emission from an elliptical ring. We demonstrate that a simple bow-shock model cannot simultaneously account for the shape and the velocity gradient of the R4 structure. A model involving a slow, hydromagnetic shock propagating into a rotating, circumstellar disk better describes the maser spot kinematics and luminosities. In this model, the central mass is 3 solar masses, and we demonstrate that the mass of the disk is negligible in comparison. The expansion velocity of the post-shock gas, roughly 5 km / s, is slow compared to the average shock velocity (roughly 13 km / s) suggesting that the post-shock gas is magnetically supported with a characteristic field strength of roughly 30 mG. We speculate that the expanding maser rings R4 and R5 may be generated by periodic, instability-driven winds from young stars that periodically send spherical shocks into the surrounding circumstellar material. 
J. F. Gallimore, R. J. Cool, M. D. Thornley, & J. McMullin