a) Distributions of embryos used to start the N-body simulations. The squares represent the distribution of embryos obtained with the first set of simulations. The circles represent the final results obtained with the semi-analytical model (the second set of simulations). b) Surface density profiles used to distribute 1000 planetesimals to start the N-body simulations. The dashed line represents the surface density for the first set, the solid line the surface density for the second set obtained with the semi-analytical model. A color figure is available in the electronic version.