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

We present VLBA observations of water maser emission associated with thestar-forming region IRAS 21391+5802, which is embedded in a bright rimmedcometary globule in IC1396. The angular resolution of the maps is about 0.8mas, corresponding to a spatial resolution of about 0.6 AU, at an estimateddistance of 750 pc. Proper motions are derived for 10 maser features identifiedconsistently over three epochs, which were separated by intervals of about onemonth. The masers appear in four groups, which are aligned linearly on the sky,roughly along a northeast-southwest direction, with a total separation of about520 AU (about 0.7 arcseconds). The 3-D velocities of the masers have a maximumvalue of about 42 km/s (about 9 AU/yr). The average error on the derived propermotions is about 4 km/s. The overall pattern of proper motions is indicative ofa bipolar outflow. Proper motions of the masers in a central cluster, with aprojected extent of about 20 AU, show systematic deviations from a radialoutflow. However, we find no evidence of Keplerian rotation, as has beenclaimed elsewhere. A nearly circular loop of masers lies near the middle of thecluster. The radius of this loop is 1 AU and the line-of-sight velocities ofthe masers in the loop are within 2 km/s of the systemic velocity of theregion. These masers presumably exist at the radial distance where significantdust condensation occurs in the outflow emanating from the star.Comment: 15 pages, 4 figures. Accepted for publication in the Astrophysical Journal. Version 2.12.00: Astrometric coordinates of maser revise