  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.
