2005PASJ...57..595H

We report on multi-epoch observations of H₂O maser emission in the star-forming region OH 43.8-0.1, carried out with VLBI Exploration of Radio Astrometry. The large-scale maser distributions obtained by single-beam VLBI mapping reveal new maser spots scattered over the area of 0''.7 * 1''.0, in addition to a `shell-like' structure with a scale of 0''.3 * 0''.5, which was mapped previously. Proper motions were also obtained for 43 spots based on 5-epoch monitoring with a time span of 281 days. The distributions of the proper motions show a systematic outflow in the north-south direction with an expansion velocity of ∼ 8 km s^-1. The overall distributions of the maser spots as well as the proper motions are better represented by a bipolar flow plus a central maser cluster with a complex structure, rather than a shell with uniform expansion, such as those found in Cep A R5 and W75N VLA2. The distance to OH 43.8-0.1 was also estimated based on the statistical parallax, yielding D = 2.8 ± 0.5 kpc. This distance is consistent with a near kinematic distance, and rules out a far kinematic distance (∼ 9kpc). Also, the radial velocity of the OH 43.8-0.1 combined with the distance provides a constraint on the flatness of the galactic rotation curve, indicating that there is no systematic difference in the rotation speeds at the Sun and at the position of OH 43.8-0.1, which is located at a galacto-centric radius of ∼ 6.3 kpc.