Proper Motion of Water Masers Associated with IRAS 21391+5802: Bipolar Outflow and an AU-Scale Dusty Circumstellar Shell

We present Very Long Baseline Array (VLBA) observations of water maser emission associated with the star-forming region IRAS 21391+5802, which is embedded in a bright-rimmed cometary globule in IC 1396. The angular resolution of the maps is ~0.8 mas, corresponding to a spatial resolution of ~0.6 AU at an estimated distance of 750 pc. Proper motions are derived for 10 maser features identified consistently over three epochs, which were separated by intervals of about 1 month. The masers appear in four groups, which are aligned linearly on the sky, roughly along a northeast-southwest direction, with a total separation of ~520 AU (~0.″7). The three-dimensional velocities of the masers have a maximum value of ~42 km s-1 (~9 AU yr-1). The average error on the derived proper motions is ~4 km s-1. The overall pattern of proper motions is indicative of a bipolar outflow. Proper motions of the masers in a central cluster, with a projected extent of ~20 AU, show systematic deviations from a radial outflow. However, we find no evidence of Keplerian rotation, as has been claimed elsewhere. A nearly circular loop of masers lies near the middle of the cluster. The radius of this loop is 1 AU, and the line-of-sight velocities of the masers in the loop are within 2 km s-1 of the systemic velocity of the region. These masers presumably exist at the radial distance where significant dust condensation occurs in the outflow emanating from the star.