ON THE ELLIPTICITY OF THE GALACTIC DISK

We investigate two kinds of constraints on the ellipticity of the Gala ctic disk: local constraints, based on the kinematics of the solar nei ghborhood (such as the Oort constants, the axis ratio of the velocity ellipsoid, the vertex deviation, and the radial velocity of the local standard of rest relative to the Galactic center), and global constrai nts (H I tangent point velocities, the velocity fields of distant carb on stars, Cepheids, and H II regions, and the kinematics of distant H I). Local and global constraints independently suggest that the Sun li es near the minor axis of the potential of an elliptical Galactic disk , with equipotential axis ratio 0.9. The kinematic data are consistent with a flat rotation curve of constant ellipticity, with a mean rotat ion speed of 200 km s(-1). In this model, the present velocity of the local standard of rest is 180 km s(-1).