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R.S. Gomes, D.E. Santos Jr. (Observatorio Nacional - Brazil)
In the primordial stage of a forming planetary system, solid material is supposed to orbit in a gas rich medium causing aerodynamic drag on planetesimals. This drag leads to a circularization and shrinkage of their orbits. A protoplanet already formed may halt the radial decay of planetesimals at resonance sites through a mechanism known as resonance trapping. Eccentricities of trapped objects are always higher than those of nontrapped ones. Because random eccentric orbits mutually intersect at points where the relative velocity is usually high, one is led to conclude that impacts between planetesimals trapped in resonance are catastrophic yielding fragmentation and further escape from resonance of the smaller pieces since they suffer a drag force too strong to be compensated by the planet's resonant gravitation effect. However, the orbits of planetesimals trapped in a common resonance are not random but there are constraints among their orbital elements, thus implying not necessarily high impact velocities. We determine what are the real collision velocities of these planetesimals for the main resonances and several planetesimals sizes (drag rates). We notice that planetesimals suffering similar drag rates collide with low relative velocities for a low planetary eccentricity. High planetary eccentricity and too different drag rates may however lead to disruptive impacts. We discuss for which conditions resonance trapping of planetesimals can help or hamper their accretion process.
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