The Migration of Gap-Opening Planets is not Locked to Viscous Disk
  Evolution
  Most standard descriptions of Type II migration state that massive,
gap-opening planets must migrate at the viscous drift rate. This is based on
the idea that the disk is separated into an inner and outer region and gas is
considered unable to cross the gap. In fact, gas easily crosses the gap on
horseshoe orbits, nullifying this necessary premise which would set the
migration rate. In this work, it is demonstrated using highly accurate
numerical calculations that the actual migration rate is dependent on disk and
planet parameters, and can be significantly larger or smaller than the viscous
drift rate. In the limiting case of a disk much more massive than the
secondary, the migration rate saturates to a constant which is sensitive to
disk parameters and is not necessarily of order viscous rate. In the opposite
limit of a low-mass disk, the migration rate decreases linearly with disk mass.
Steady-state solutions in the low disk mass limit show no pile-up outside the
secondary's orbit, and no corresponding drainage of the inner disk.
