Weak Elliptical Distortion of the Milky Way Potential traced by Open Clusters

Abstract

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 (R0) = 0.060±0.012 is detected, the Sun is found to be near the minor axis, displaced by 30°±3°. The motion of the clusters is suggested to be on an oval orbit rather than on a circular one.