Abstract | dc.description.abstract | 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
5 yr before, we find that at ‘large’ scales of 1 arcsec (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 arcsec (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∼5mas yr−1 (15 kms−1), consistent with previous results of
Gallimore and collaborators. This structure is probably driven by the wind of a still unidentified
YSO located at the centre of the ring (∼0.18 arcsec south of HW2). On the other hand, the
R5 expanding bubble structure (driven by the wind of a previously identified YSO located
∼0.6 arcsec 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 arcsec (700 au), the simultaneous presence of a relatively slow (∼10–
70 km s−1) wide-angle outflow (opening angle of ∼102◦), traced by the masers, and the fast
(∼500 km s−1) highly collimated radio jet associated with HW2 (opening angle of ∼18◦),
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. There are indications that the primary wind(s) from HW2 could be rotating.
The implications of these results in the study of the formation of high-mass stars are discussed. | es_CL |