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

TL;DR

This study uses multi-epoch VLBI water maser observations to reveal a highly collimated bipolar outflow in the Cepheus A HW3d massive young stellar object, indicating an early star formation phase.

Contribution

First measurement of proper motions of water masers in HW3d, demonstrating a compact bipolar outflow and providing insights into early massive star formation.

Findings

Bipolar outflow traced by water masers is highly collimated.

Outflow expands at ~21 km/s over ~280 AU.

Dynamical age of outflow is approximately 100 years.

Abstract

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 (VERA) 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 ~ 280 AU (400 mas) at a mean velocity of ~ 21 km/s (~ 6 mas/yr) without taking into account the turbulent central maser cluster. The opening angle of the outflow is estimated to be ~ 30{\circ}. The dynamical time-scale of the outflow is estimated to be ~ 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 (VLA) 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 (YSOs) in HW3d and/or strong variability in one of their radio continuum emission components.