We present five epochs of VLBI water maser observations around the massive
protostar Cepheus A HW2 with 0.4 mas (0.3 AU) resolution. The main goal of
these observations was to follow the evolution of the remarkable water maser
linear/arcuate structures found in earlier VLBI observations. Comparing the
data of our new epochs of observation with those observed five years before, we
find that at "large" scales of > 1" (700 AU) the main regions of maser emission
persist, implying that both the surrounding medium and the exciting sources of
the masers have been relatively stable during that time span. However, at
smaller scales of < 0.1" (70 AU) we see large changes in the maser structures,
particularly in the expanding arcuate structures R4 and R5. R4 traces a nearly
elliptical patchy ring of ~ 70 mas size (50 AU) with expanding motions of ~ 5
mas/yr (15 km/s). This structure is probably driven by the wind of a still
unidentified YSO located at the centre of the ring (~ 0.18" south of HW2). On
the other hand, the R5 expanding bubble structure (driven by the wind of a
previously identified YSO located ~ 0.6" south of HW2) is currently dissipating
in the circumstellar medium and losing its previous degree of symmetry,
indicating a very short-lived event. In addition, our results reveal, at scales
of ~ 1" (700 AU), the simultaneous presence of a relatively slow (~ 10-70 km/s)
wide-angle outflow (opening angle of ~ 102 deg, traced by the masers, and the
fast (~ 500~km/s) highly collimated radio jet associated with HW2 (opening
angle of ~ 18 deg, previously observed with the VLA. This simultaneous presence
of a wide-angle outflow and a highly collimated jet associated with a massive
protostar is similar to what is found in some low-mass YSOs. The implications
of these results in the study of the formation of high-mass stars are
discussed.Comment: 28 pages, 7 figures. Animations will be included as supporting
material online (MNRAS web page
