Velocity Dispersion Profile of the Milky Way Halo
  We present a spectroscopic sample of 910 distant halo stars from the
Hypervelocity Star survey from which we derive the velocity dispersion profile
of the Milky Way halo. The sample is a mix of 74% evolved horizontal branch
stars and 26% blue stragglers. We estimate distances to the stars using
observed colors, metallicities, and stellar evolution tracks. Our sample
contains twice as many objects with R>50 kpc as previous surveys. We compute
the velocity dispersion profile in two ways: with a parametric method based on
a Milky Way potential model, and with a non-parametric method based on the
caustic technique originally developed to measure galaxy cluster mass profiles.
The resulting velocity dispersion profiles are remarkably consistent with those
found by two independent surveys based on other stellar populations: the Milky
Way halo exhibits a mean decline in radial velocity dispersion of -0.38+-0.12
km/s/kpc over 15<R<75 kpc. This measurement is a useful basis for calculating
the total mass and mass distribution of the Milky Way halo.
