The Shape and Scale of Galactic Rotation from Cepheid Kinematics

A catalog of Cepheid variables is used to probe the kinematics of the Galactic disk. Radial velocities are measured for eight distant Cepheids toward l = 300°; these new Cepheids provide a particularly good constraint on the distance to the Galactic center, R 0 . We model the disk with both an axisymmetric rotation curve and one with a weak elliptical component, and find evidence for an ellipticity of 0.043 ± 0.016 near the Sun. Using these models, we derive R 0 = 7.66 ± 0.32 kpc and v circ = 237 ± 12 km s -1 . The distance to the Galactic center agrees well with recent determinations from the distribution of RR Lyrae variables and disfavors most models with large ellipticities at the solar orbit.