Weak elliptical distortion of the milky way potential traced by open clusters

From photometric observations and star counts, the existence of a bar in the central few kpc of the Galaxy is suggested. It is generally thought that our Galaxy is surrounded by a massive invisible halo. The gravitational potential of the Galaxy is therefore made non-axisymmetric generated by the central triaxial bar, by the outer triaxial halo, and/or by the spiral structures. Selecting nearly 300 open clusters with complete spatial velocity measurements and ages, we were able to construct the rotation curve of the Milky Way within a range of 3 kpc of the Sun. Using a dynamic model for an assumed elliptical disk, a clear weak elliptical potential of the disk with ellipticity of epsilon(R-0) = 0.060 +/- 0.012 is detected, the Sun is found to be near the minor axis, displaced by 30 degrees +/- 3 degrees. The motion of the clusters is suggested to be on an oval orbit rather than on a circular one.