Constraining the Milky Way halo potential with the GD-1 stellar stream

We use ESA/Gaia astrometry together with SEGUE and LAMOST measurements of the GD-1 stellar stream to explore the improvement on the Galactic gravitational potential that these new data provide. Assuming a realistic universal model for the dark matter halo together with reasonable models of the baryonic components, we find that the orbital solutions for GD-1 require the circular velocity at the Solar radius to be V-circ(R-circle dot) = 244 +/- 4km s(-1), and also that the density flattening of the dark halo is q(rho) = 0.82(-0.13)(+0.25). The corresponding Galactic mass within 20 kpc was estimated to be M-MW(< 20 kpc) = 2.5 +/- 0.2 x 10(11)M(circle dot). Moreover, Gaia's excellent proper motions also allowed us to constrain the velocity dispersion of the GD-1 stream in the direction tangential to the line of sight to be < 2.30 km s(-1) (95 per cent confidence limit), confirming the extremely cold dynamical nature of this system.