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\nprotostar Cepheus A HW2 with 0.4 mas (0.3 AU) resolution. The main goal of\nthese observations was to follow the evolution of the remarkable water maser\nlinear/arcuate structures found in earlier VLBI observations. Comparing the\ndata of our new epochs of observation with those observed five years before, we\nfind that at "large" scales of > 1" (700 AU) the main regions of maser emission\npersist, implying that both the surrounding medium and the exciting sources of\nthe masers have been relatively stable during that time span. However, at\nsmaller scales of < 0.1" (70 AU) we see large changes in the maser structures,\nparticularly in the expanding arcuate structures R4 and R5. R4 traces a nearly\nelliptical patchy ring of ~ 70 mas size (50 AU) with expanding motions of ~ 5\nmas/yr (15 km/s). This structure is probably driven by the wind of a still\nunidentified YSO located at the centre of the ring (~ 0.18" south of HW2). On\nthe other hand, the R5 expanding bubble structure (driven by the wind of a\npreviously identified YSO located ~ 0.6" south of HW2) is currently dissipating\nin the circumstellar medium and losing its previous degree of symmetry,\nindicating a very short-lived event. In addition, our results reveal, at scales\nof ~ 1" (700 AU), the simultaneous presence of a relatively slow (~ 10-70 km/s)\nwide-angle outflow (opening angle of ~ 102 deg, traced by the masers, and the\nfast (~ 500~km/s) highly collimated radio jet associated with HW2 (opening\nangle of ~ 18 deg, previously observed with the VLA. This simultaneous presence\nof a wide-angle outflow and a highly collimated jet associated with a massive\nprotostar is similar to what is found in some low-mass YSOs. The implications\nof these results in the study of the formation of high-mass stars are\ndiscussed.\n