A wide-angle outflow with the simultaneous presence of a high-velocity jet in the high-mass Cepheus A HW2 system

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 >rsim1≳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 ~5 mas yr-1 (15 km s-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. © 2010 The Authors. Journal compilation © 2010 RAS.