CONSTRAINING THE MILKY WAY POTENTIAL WITH A SIX-DIMENSIONAL PHASE-SPACE MAP OF THE GD-1 STELLAR STREAM

The narrow GD-1 stream of stars, spanning 60 degrees on the sky at a distance of similar to 10 kpc from the Sun and similar to 15 kpc from the Galactic center, is presumed to be debris from a tidally disrupted star cluster that traces out a test-particle orbit in the Milky Way halo. We combine Sloan Digital Sky Survey (SDSS) photometry, USNO-B astrometry, and SDSS and Calar Alto spectroscopy to construct a complete, empirical six-dimensional (6D) phase-space map of the stream. We find that an eccentric orbit in a flattened isothermal potential describes this phase-space map well. Even after marginalizing over the stream orbital parameters and the distance from the Sun to the Galactic center, the orbital fit to GD-1 places strong constraints on the circular velocity at the Sun's radius V-c = 224 +/- 13 km s(-1) and total potential flattening q(Phi) = 0.87(0.04)(+0.07). When we drop any informative priors on V-c, the GD-1 constraint becomes V-c = 221 +/- 18 km s(-1). Our 6D map of GD-1, therefore, yields the best current constraint on V-c and the only strong constraint on q(Phi) at Galactocentric radii near R similar to 15 kpc. Much, if not all, of the total potential flattening may be attributed to the mass in the stellar disk, so the GD-1 constraints on the flattening of the halo itself are weak: q(Phi,halo) > 0.89 at 90% confidence. The greatest uncertainty in the 6D map and the orbital analysis stems from the photometric distances, which will be obviated by GAIA.