Trigonometric Parallaxes of Massive Star Forming Regions: VI. Galactic
  Structure, Fundamental Parameters and Non-Circular Motions
  We are using the 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 degrees, which favors four
rather than two spiral arms for the Galaxy. Combining positions, 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 slower than expected for circular orbits. By fitting the
measurements to a model of the Galaxy, we estimate the distance to the Galactic
center R_o = 8.4 +/- 0.6 kpc and a circular rotation speed Theta_o = 254 +/- 16
km/s. The ratio Theta_o/R_o can be determined to higher accuracy than either
parameter individually, and we find it to be 30.3 +/- 0.9 km/s/kpc, in good
agreement with the angular rotation rate determined from the proper motion of
Sgr A*. The data favor a rotation curve for the Galaxy that is nearly flat or
slightly rising with Galactocentric distance. Kinematic distances are generally
too large, sometimes by factors greater than two; they can be brought into
better agreement with the trigonometric parallaxes by increasing Theta_o/R_o
from the IAU recommended value of 25.9 km/s/kpc to a value near 30 km/s/kpc. We
offer a "revised" prescription for calculating kinematic distances and their
uncertainties, as well as a new approach for defining Galactic coordinates.
Finally, our estimates of Theta_o and To/R_o, when coupled with direct
estimates of R_o, provide evidence that the rotation curve of the Milky Way is
similar to that of the Andromeda galaxy, suggesting that the dark matter halos
of these two dominant Local Group galaxy are comparably massive.
