Abstract
Through Very Long Baseline Array (VLBA) multi-epoch water maser observations toward the star-forming region Cepheus A, we have recently found and reported an arc structure defining an expanding ( 9km/s) circle of 62AU radius with a precision of one part in a thousand. We have interpreted this structure as an expanding spherical bubble short lived (33 years) ejected from a young stellar object (YSO) located at the center of the circle. This YSO has been recently detected at radio continuum wavelengths. In this paper we describe all these observations, in particular those obtained with the VLBA. We also comment on possible scenarios proposed in the literature to explain the origin of this enigmatic, expanding, spherical bubble.
