Galactic rotation measurements based on H2O maser astrometry with VERA

We present results of astrometric observations of S269 H₂O maser performed with VERA (VLBI Exploration of Radio Astrometry). We have monitored the positions of S269 H₂O masers for 1 year and successfully detected its parallax to be 189±8 micro-arcsecond. This corresponds to a source distance of 5.28^+0.24_−0.22 kpc, and is the smallest parallax (and thus the largest distance) that has ever been measured by means of annual parallax. Proper motions of S269 H₂O maser were also measured and used to determine the Galactic rotation velocity at the position of S269. Our measurements show that the Galactic rotation velocity at S269 is the same to that at the Sun within 3%, indicating that the Galactic rotation curve is flat out to R~13 kpc.