Multi-Epoch VERA Observations of H2O Masers in OH 43.8−0.1

Abstract

Abstract We report on multi-epoch observations of $\mathrm{H}_2\mathrm{O}$ 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 ${0\rlap {.}{}^{\mathrm {\prime \prime }}7} \times {1\rlap {.}{}^{\mathrm {\prime \prime }}0}$, in addition to a ‘shell-like’ structure with a scale of ${0\rlap {.}{}^{\mathrm {\prime \prime }}3} \times {0\rlap {.}{}^{\mathrm {\prime \prime }}5}$, 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 $\sim 8 \,\mathrm{km} \,\mathrm{s}^{-1}$. 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 $D = 2.8 \pm 0.5 \,\mathrm{kpc}$. This distance is consistent with a near kinematic distance, and rules out a far kinematic distance ($\sim 9 \,\mathrm{kpc}$). 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 $\sim 6.3 \,\mathrm{kpc}$.