On the migration rate of giant planets protocores
          Frédéric Masset, CE-Saclay & UNAM
          Wilhelm Kley, Institute for Astronomy and Astrophysics, Tuebingen
          Gennaro D'Angelo, School of Physics, Exeter

          We present a large set of numerical simulations aimed at investigating a behaviour previously observed in earlier work, namely a significant discrepancy between the linear migration time estimate and its value as obtained from simulations, for planet masses in the range 5-15 earth masses, characteristic of the solid cores of giant planets. For such masses, numerical simulations yield a much longer migration time, and can even display, for some set of parameters, an outward migration. Our simulations show that this offset scales with the gradient of the specific vorticity, increases with the disk viscosity, and has a maximum for a planet mass proportional to H^2.6 (H being the disk thickness). These findings are compatible with non-linear effects associated to the corotation torque acting upon the planet. We tentatively interpret these effects with asymmetries of the horseshoe region.