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Fig. 2


Example of orbits of objects that are captured in the prograde (top panels) and retrograde (bottom panels) directions in the planetocentric frame. Each dot corresponds to the position of the planetesimal at a given timestep and the arrows mark the direction at which the objects approach the planet. The left row shows the “typical” orbital evolution during the gas drag assisted capture of planetesimals whereas the right row corresponds to the orbital evolution following lower probability events where planetesimals are captured on orbits with a large pericenter and their semimajor axis decays slowly (see Fig. 4; see also, Suetsugu & Ohtsuki 2017). Insets on the top and bottom left panels are close-up views of the trajectories near the pericenter of the orbits with X, Y ∈ [−10;10]. The color of the dots indicates the ablation rate from the surface of the planetesimal. Ablation occurs mainly around the point of closest approach to the planets where the density of gas is the highest. Due to their larger relative velocities with respect to the gas, and hence larger frictional heating, planetesimals on retrograde orbits (bottom) experience more sustained ablation with ablation timescales as short as τablmabl ≲hr. The planetesimals captured in the retrograde directions both reached our imposed cut-off radius of 10 m (at this small size, the objects couple with the gas and end up rotating in the prograde direction which can be more clearly seen in the bottom right panel). However, far from the pericenter of the orbit, ablation rapidly becomes negligible for planetesimals orbiting on both prograde and retrograde orbits (τabl ≳ 107–108 h ).

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