A Highly Collimated Water Maser Bipolar Outflow in the Cepheus A HW3d Massive Young Stellar Object

We present the results of multi-epoch very long baseline interferometry (VLBI) water (H2O) maser observations carried out with the VLBI Exploration of Radio Astrometry toward the Cepheus A HW3d object. We measured for the first time relative proper motions of the H2O maser features, whose spatio-kinematics traces a compact bipolar outflow. This outflow looks highly collimated and expanding through similar to 280 AU (400 mas) at a mean velocity of similar to 21 km s(-1) (similar to 6 mas yr(-1)) without taking into account the turbulent central maser cluster. The opening angle of the outflow is estimated to be similar to 30 degrees. The dynamical timescale of the outflow is estimated to be similar to 100 years. Our results provide strong support that HW3d harbors an internal massive young star, and the observed outflow could be tracing a very early phase of star formation. We also have analyzed Very Large Array archive data of 1.3 cm continuum emission obtained in 1995 and 2006 toward Cepheus A. The comparative result of the HW3d continuum emission suggests the possibility of the existence of distinct young stellar objects in HW3d and/or strong variability in one of their radio continuum emission components.