Tidal Streams as Probes of the Galactic Potential

We explore the use of tidal streams from Galactic satellites to recover the potential of the Milky Way. Our study is motivated both by the discovery of the first lengthy stellar stream in the halo (Irwin & Totten 1998) and by the prospect of measuring proper motions of stars brighter than 20th magnitude in such a stream with an accuracy of ∼ 4μas/yr, as will be possible with the Space Interferometry Mission (SIM). We assume that the heliocentric radial velocities of these stars can be determined from supporting ground-based spectroscopic surveys, and that the mass and phase-space coordinates of the Galactic satellite with which they are associated will also be known to SIM accuracy. From this information we can predict the distances to the stars as a consequence of the narrow distribution of angular momenta expected along the streams. Our results imply that the circular velocity and flattening of the Milky Way can be inferred to within ∼ 2.5% using the radial velocities and SIM measurements of proper motions of only 100 stars in a tidal stream.