Radial surface density profiles for cases discussed in Fig. 5 (planet-to-star mass ratios q are given by labels in individual panels). We plot the initial azimuthal average (solid grey curve), the azimuthal average at t_orb = 40000 P_orb for trapped planets and at t_orb = 9000 P_orb for q = 9 × 10⁻⁵ (black solid curve), azimuthal averages taken over the span of the front (θ > 0) and rear (θ < 0) horseshoe regions (red and blue solid curves, respectively), and radial cuts Σ(r, θ = π/3) and Σ(r, θ = −π/3) (orange and purple dashed curves, respectively). We point out that the Hill sphere material is excluded from calculations of all azimuthal averages. The instantaneous position of the planet is marked by the black arrow. Unlike in Fig. 2, the vertical scale is linear. The spread of the curves (notably the orange and red ones) indicates the development of the front-rear asymmetry in the co-rotation region.