Results of astrometric very long baseline interferometry (VLBI) observations
towards an extreme OH/IR star candidate NSV17351 are presented. We used the
VERA (VLBI Exploration of Radio Astrometry) VLBI array to observe 22\,GHz
H2O masers of NSV17351. We derived an annual parallax of 0.247±0.035 mas
which corresponds to a distance of 4.05±0.59 kpc. By averaging the proper
motions of 15 maser spots, we obtained the systemic proper motion of NSV17351
to be (μαcosδ,μδ)avg= (−1.19
± 0.11, 1.30 ± 0.19) mas\,yr−1. The maser spots spread out over a
region of 20 mas × 30 mas, which can be converted to a spatial
distribution of ∼80 au ×∼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
NSV17351 to be 1122±24 days. This is the first report of the periodic
activity of NSV17351, indicating that NSV17351 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 from the last 40 years,
we detected a long-term acceleration in the radial velocity of the
circumstellar matter to be 0.17±0.03 km\,s−1\,yr−1 Finally, we
determined the position and kinematics of NSV17351 in the Milky Way Galaxy and
found that NSV17351 is located in an interarm region between the Outer and
Perseus arms. We note that astrometric VLBI observations towards extreme OH/IR
stars are useful samples for studies of the Galactic dynamics.Comment: Published in the Publications of the Astronomical Society of Japa