Tidal streams as probes of the galactic potential

We explore the use of tidal streams from Galactic: satellites for recoverin g the potential of the Milky Way. Our study is motivated both by the discov ery of the first lengthy stellar stream in the halo (Irwin & Totten) and by the prospect of measuring proper motions of stars brighter than 20th magni tude in such a stream with an accuracy of similar to 4 was yr-l, as will be possible with the Space Interferometry Mission (SIM). We assume that the h eliocentric radial velocities of these stars can be determined from support ing ground-based spectroscopic surveys and that the mass and phase-space co ordinates of the Galactic satellite with which they are associated will als o be known to SIM accuracy. Using the results from numerical simulations as trial data sets, we find that, if we assume the correct form for the Galac tic potential, we can predict the distances to the stars as a consequence o f the narrow distribution of energy expected along the streams. We develop an algorithm to evaluate the accuracy of any adopted potential by requiring that the satellite and stars recombine within a Galactic lifetime when the ir current phase-space coordinates are integrated backward. When applied to a four-dimensional,grid of triaxial logarithmic potentials, with varying c ircular velocities, axis ratios, and orientation of the major axis in the d isk plane, the algorithm can recover the parameters used for the Milky Way in a simulated data set to within a few percent using only 100 stars in a t idal stream.