  Precise proper motion measurements (sigma_mu ~ 10 mkas/yr) of the recently
discovered hyper-velocity star (HVS) SDSS J090745.0+024507 would yield
significant constraints on the axis ratios and orientation of a triaxial model
for the Galactic halo. Triaxiality of dark matter halos is predicted by Cold
Dark Matter models of galaxy formation and may be used to probe the nature of
dark matter. However, unless the distance to this star is determined to better
than 10%, these constraints suffer from one-dimensional degeneracies, which we
quantify. We show how proper motion measurements of several HVSs could
simultaneously resolve the distance degeneracies of all such stars and produce
a detailed picture of the triaxial halo. Additional HVSs may be found from
radial velocity surveys or from parallax/proper-motion data derived from GAIA.
High-precision proper-motion measurements of these stars using the Space
Interferometry Mission (SIM PlanetQuest) would substantially tighten the
constraints they yield on the Galactic potential.
