Expansion of the R4 H2O maser arc near Cepheus A HW2

We present new (2000 April) 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) Very Long Baseline Array (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 2, and the displacement of the arcs between 1996 and 2000 is compatible with expansion about a common center. In addition, the MERLIN observations detect redshifted 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 M-circle dot, and we demonstrate that the mass of the disk is negligible in comparison. The expansion velocity of the postshock gas, similar to5 km s(-1), is slow compared to the shock velocity, v(S)similar to13 km s(-1), suggesting that the postshock gas is magnetically supported with a characteristic field strength of similar to30 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.