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 similar to 0.8 mas, corresponding to a spatial resolution of 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, 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 similar to 520 AU (similar to 0 ".7). The three-dimensional velocities of the masers have a maximum value 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 masers in a central cluster, with a projected extent of similar to 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.