Title:Distance and location of the Sagittarius arm traced by trigonometric parallaxes of massive star-forming 
regions

Abstract:
We report measurements of the trigonometric parallaxes of H2O maser sources in massive star-forming 
regions in the Sagittarius spiral arm.  We newly measured parallax distances to 4 star-forming regions in 
the arm using the Japanese VERA array and the NRAO Very Long Baseline Array (VLBA), including 
G14.33‑0.64 that most likely traces the near side of the Sagittarius arm (at ~1kpc) and the famous W51 
region located near the tangency of the Sagittarius arm.  Our distance to W51 yields an estimate of the 
distance to the Galactic center of Ro = 8.3 ± 0.46 (statistical) ±1.0 (systematic) kpc by geometry.  We 
will explore the pitch angle and motion of the Sagittarius arm by incorporating all star-forming regions in the 
arm whose parallaxes have been measured using VERA and the VLBA.