Astrometric VLBI observations of H2O masers in an extreme OH/IR star candidate NSV 17351

The results of astrometric very long baseline interferometry (VLBI) observations toward an extreme OH/IR star candidate NSV 17351 are presented. Using the VERA (VLBI Exploration of Radio Astrometry) VLBI array , we observed 22 GHz H₂O masers of NSV 17351 and derived an annual parallax of 0.247 ± 0.035 mas, which corresponds to a distance of 4.05 ± 0.59 kpc, from the observation. By averaging proper motions of 15 maser spots further, we determined the systemic proper motion of NSV 17351 to be (μ_αcos δ, μ_δ)^avg. = (-1.19 ± 0.11, 1.30 ± 0.19) mas yr^-1. The maser spots spread out over a region of 20 × 30 mas, which can be converted to a spatial distribution of ~80 × 120 au at the source distance. Internal motions of the maser spots suggest an outward-moving maser region with respect to the estimated position of the central star. From single-dish monitoring of the H₂O maser emission, we estimate the pulsation period of NSV 17351 to be 1122 ± 24 d. This is the first report of the periodic activity of NSV 17351, indicating that NSV 17351 could have a mass of ~4 M_⊙. We confirmed that the time variation of H₂O masers can be used as a period estimator of variable OH/IR stars. Furthermore, by inspecting dozens of double-peaked H₂O maser spectra for the last 40 years, we discovered the long-term acceleration in the radial velocity of the circumstellar matter to be 0.17 ± 0.03 km s^-1 yr^-1. We finally determined the position and kinematics in the Milky Way Galaxy and found that NSV 17351 is located in an interarm region between the Outer and Perseus arms. We note that the astrometric VLBI observation toward extreme OH/IR stars shows us a useful sample of the Galactic dynamics.