Second Epoch Hubble Space Telescope Observations of Kepler's Supernova
  Remnant: The Proper Motions of Balmer Filaments
  We report on the proper motions of Balmer-dominated filaments in Kepler's
supernova remnant using high resolution images obtained with the Hubble Space
Telescope at two epochs separated by about 10 years. We use the improved proper
motion measurements and revised values of shock velocities to derive a distance
to Kepler of 5.1 [+0.8, -0.7] kpc. The main shock around the northern rim of
the remnant has a typical speed of 1690 km/s and is encountering material with
densities of about 8 cm^-3. We find evidence for the variation of shock
properties over small spatial scales, including differences in the driving
pressures as the shock wraps around a curved cloud surface. We find that the
Balmer filaments ahead of the ejecta knot on the northwest boundary of the
remnant are becoming fainter and more diffuse. We also find that the Balmer
filaments associated with circumstellar material in the interior regions of the
remnant are due to shocks with significantly lower velocities and that the
brightness variations among these filaments trace the density distribution of
the material, which may have a disk-like geometry.
