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

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 $\sim 6.3 \,\mathrm{kpc}$.

Masers and star formation

“…Water masers are frequently seen in outflows from both high-mass and low-mass YSOs (Honma et al 2005;Moscadelli, Cesaroni, & Rioja 2005;Moscadelli et al 2006). These jets are seen in deceleration (Imai et al 2002) and often have substructure on AU scales Furuya et al 2005;Uscanga et al 2005).…”

Absolute Proper Motions of H2O Masers Away from the Galactic Plane Measured with VERA in the “Superbubble” Region NGC 281

“…The average of the maser motions is expected to trace the systemic motion of the molecular cloud on the H I loop, which then yields a direct measure of the superbubble expansion. In order to detect the systemic motion of the NGC 281 West cloud on the H I loop, we therefore conducted high-resolution observations of the H 2 O maser source in the cloud with VERA, a Japanese VLBI array dedicated to phase-referencing VLBI astrometry (e.g., Honma et al 2000;Kobayashi et al 2003;Honma et al 2005;Imai et al 2006). In this paper, we report on our successful detection of the absolute proper motion of the H 2 O maser source, which reveals the motion of this region away from the Galactic plane, most likely having originated in the Galactic plane and been blown out by a superbubble blowout.…”