Trigonometric Parallaxes of Massive Star-Forming Regions. Vi. Galactic Structure, Fundamental Parameters, and Noncircular Motions

We are using the NRAO Very Long Baseline Array (VLBA) and the Japanese VERA project to measure trigonometric parallaxes and proper motions of masers found in high-mass star-forming regions across the Milky Way. Early results from 18 sources locate several spiral arms. The Perseus spiral arm has a pitch angle of 16 ◦ ±3 ◦ , which favors four rather than two spiral arms for the Galaxy. Combining distances, proper motions, and radial velocities yields complete 3-dimensional kinematic information. We find that star forming regions on average are orbiting the Galaxy ≈ 15 km s −1 slower than expected for circular orbits. By fitting the measurements to a model of the Galaxy, we estimate the distance to the Galactic center R0 = 8.4 ± 0.6 kpc and a circular rotation speed Θ0 = 254 ± 16 km s −1. The ratio Θ0/R0 can be determined to higher accuracy than either parameter individually, and we find it to be 30.3±0.9 km s −1 kpc −1, in good agreement with 1