  [Abridged] We evaluate the coorbital corotation torque on a migrating
protoplanet. The coorbital torque is assumed to come from orbit crossing fluid
elements which exchange angular momentum with the planet when they execute a
U-turn at the end of horseshoe streamlines. When the planet migrates inward,
the fluid elements of the inner disk undergo one such exchange as they pass to
the outer disk. The angular momentum they gain is removed from the planet, and
this corresponds to a negative contribution to the corotation torque, which
scales with the drift rate. In addition, the material trapped in the coorbital
region drifts radially with the planet giving a positive contribution to the
corotation torque, which also scales with the drift rate. These two
contributions do not cancel out if the coorbital region is depleted, in which
case there is a net corotation torque which scales with the drift rate and the
mass deficit in the coorbital region, and which has same sign as the drift
rate. This leads to a positive feedback on the migrating planet. In particular,
if the coorbital mass deficit is larger than the planet mass, the migration
rate undergoes a runaway which can vary the protoplanet semi-major axis by 50%
over a few tens of orbits. This can happen only if the planet mass is
sufficient to create a dip or gap in its surrounding region, and if the
surrounding disk mass is larger than the planet mass. This typically
corresponds to planet masses in the sub-Saturnian to Jovian mass range embedded
in massive protoplanetary disks. Runaway migration is a good candidate to
account for the orbital characteristics of close orbiting giant planets, most
of which have sub-Jovian masses. Further, we show that in the runaway regime,
migration can be directed outwards.
