The far distance to G7.47+0.06 from proper motion measurement of H2O masers

Abstract

Abstract We report on a distance measurement of 22 GHz H2O maser features associated with an ultra-compact H ii region G7.47+0.06 using VERA (VLBI Exploration of Radio Astrometry). Since the source is located farther away than 10 kpc, it turned out to be difficult to derive the distance from annual parallax measurement. Meanwhile, we clearly detected the source's proper motion parallel to the Galactic plane. The proper motion is μ = −5.03 ± 0.07 mas yr−1 and is approaching the Galactic center. We applied a new method to determine the source distance based on absolute proper motions. Considering uncertainties of the Galactic rotation curve and the solar peculiar motion, the detected proper motion leads to a source distance of D = 20 ± 2 kpc, demonstrating that astrometric observation can provide an accurate distance measurement at a 10% level even for sources too distant to measure the annual parallax. Lastly, we scale the physical parameters of the H ii region estimated in a previous paper to be 20 kpc, and show that the H2O maser features are associated with a massive star-forming region corresponding to the spectral type of O5.5.