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 emis sion associated with the star-forming region IRAS 21391+5802, which is embe dded in a bright-rimmed cometary globule in IC 1396. The angular resolution of the maps is similar to 0.8 mas, corresponding to a spatial resolution o f similar to 0.6 AU at an estimated distance of 750 pc. Proper motions are derived for 10 maser features identified consistently over three epochs, wh ich 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-s outhwest direction, with a total separation of similar to 520 AU (similar t o 0 ".7). The three-dimensional velocities of the masers have a maximum val ue of similar to 42 km s(-1) (similar to 9 AU yr(-1)). The average error on the derived proper motions is similar to 4 km s(-1) The overall pattern of proper motions is indicative of a bipolar outflow. Proper motions of the m asers in a central cluster, with a projected extent of similar to 20 AU, sh ow systematic deviations from a radial outflow. However, we find no evidenc e of Keplerian rotation, as has been claimed elsewhere. A nearly circular l oop 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 wit hin 2 km s(-1) of the systemic velocity of the region. These masers presuma bly exist at the radial distance where significant dust condensation occurs in the outflow emanating from the star.