Determining the Galactic mass distribution using tidal streams from globular clusters

We discuss how to use tidal streams from globular clusters to measure the mass distribution of the Milky Way. Recent proper-motion determinations for globular clusters from plate measurements and Hipparcos astrometry provide several good candidates for Galactic mass determinations in the intermediate halo far above the Galactic disk, including Pal 5, NGC 4147, NGC 5024 (M53), and NGC 5466; the remaining Hipparcos clusters provide candidates for measurements several kiloparsecs above and below the disk. These clusters will help determine the profile and shape of the inner halo. To aid this effort, we present two methods of mass determination: (1) a generalization of rotation-curve mass measurements, yielding the mass and potential from complete position-velocity observations for stream stars, and (2) a simple chi(2) estimator, which can be used when only projected positions and radial velocities are known for stream stars. We illustrate the use of the second method using simulated tidal streams from Pal 5, and we find that fairly accurate mass determinations are possible for relatively poor data sets, although current proper-motion uncertainties represent the limiting factor. Follow-up observations of clusters with proper-motion determinations may reveal tidal streams; obtaining radial velocity measurements can give useful measurements of the mass distribution in the inner Galaxy.