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Published online by Cambridge University Press: 06 January 2014
We use line-of-sight velocities of more than 4000 red clump stars observed from MMT/Hectospec to derive the Galactic rotation curve between 8 and 13 kpc in Galactocentric radius to better than 10 km s−1. A three-component (bulge+disk+halo) with an additional massive ring of 1.66×1010 M⊙ at about 11.6 kpc provides an excellent fit with the observation.