Analysis of Galactic Rotation from Masers based on a Nonlinear Oort
  Model
  Based on data on Galactic masers with measured trigonometric parallaxes, we
have tested a nonlinear model of Galactic rotation using generalized Oort
formulas. This model is shown to yield pretty good results up to heliocentric
distances of 3--4 kpc. The main feature of the method is the possibility of
estimating the solar Galactocentric distance Ro. This distance has been found
by analyzing approximately 60 masers to be Ro=8.3+/-0.4 kpc. Our study of the
three-dimensional kinematics of more than 100 masers based on the
Ogorodnikov--Milne model has shown that significant nonlinearities are present
only in the xy plane (rotation around the Galactic z axis) due to the
peculiarities of the Galactic rotation curve. No significant linear dependences
have been found in the xz and yz planes. We show the presence of a wave in the
velocities w as a function of coordinate x or distance R with a wavelength of 3
kpc and an amplitude of 10 km/s. The wave is particularly prominent in the
Local and Perseus arms.
