  In this paper the migration of a 10 Earth-mass planetary core is investigated
at the outer boundary of the dead zone of a protoplanetary disc by means of 2D
hydrodynamic simulations done with the graphics processor unit version of the
FARGO code. In the dead zone, the effective viscosity is greatly reduced due to
the disc self-shielding against stellar UV radiation, X-rays from the stellar
magnetosphere and interstellar cosmic rays. As a consequence, mass accumulation
occurs near the outer dead zone edge, which is assumed to trap planetary cores
enhancing the efficiency of the core-accretion scenario to form giant planets.
Contrary to the perfect trapping of planetary cores in 1D models, our 2D
numerical simulations show that the trapping effect is greatly dependent on the
width of the region where viscosity reduction is taking place. Planet trapping
happens exclusively if the viscosity reduction is sharp enough to allow the
development of large-scale vortices due to the Rossby wave instability. The
trapping is only temporarily, and its duration is inversely proportional to the
width of the viscosity transition. However, if the Rossby wave instability is
not excited, a ring-like axisymmetric density jump forms, which cannot trap the
10 Earth-mass planetary cores. We revealed that the stellar torque exerted on
the planet plays an important role in the migration history as the barycentre
of the system significantly shifts away from the star due to highly
non-axisymmetric density distribution of the disc. Our results still support
the idea of planet formation at density/pressure maximum, since the migration
of cores is considerably slowed down enabling them further growth and runaway
gas accretion in the vicinity of an overdense region.
