The rotation curve of the Galaxy obtained from planetary nebulae and AGB stars

The rotation curve of the Galaxy is obtained from a sample of planetary nebulae and asymptotic giant branch (AGB) stars. The AGB stars are OH/IR stars and carbon-rich stars with large mass-loss rates, with velocities known from radio observations and distances determined from their infrared luminosity. The rotation curve exhibits a steep decrease in the solar vicinity, and a minimum at about 8.5 kpc (adopting R⊙ = 7.9 kpc). We fit the rotation curve with a mass distribution model of the Galaxy, based on the model for star counts in the infrared region of Ortiz & Lépine; the main components are a spherically symmetric density distribution that represents the bulge and the halo, and two exponential disc components with scalelengths 2.6 and 4.5 kpc. A good agreement is found between the star count model and the rotation curve. A minimum is observed at 8.5 kpc; possible explanations are discussed. The surface density of the disc in the solar neighbourhood is 77 M⊙ pc−2,not very different from the value predicted by star counts. This result implies that there is no need for a dark matter component, at least up to a radius of about 12 kpc.