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J. E. Chambers (Carnegie Institution of Washington)
The oligarchic growth phase of planet formation led to substantial changes in the sun's protoplanetary nebula. When oligarchic growth began, the largest bodies were the size of asteroids. When it ended, the inner Solar System contained bodies comparable to Mercury and Mars. In the outer Solar System, oligarchic growth may have directly formed the solid cores of the giant planets.
Most models of oligarchic growth fall into two categories: (i) simple analytic models with a bimodal mass distribution (planetesimals and planetary embryos), which assume planetesimal random velocities reach equilibrium between excitation by embryos and damping due to gas drag, (ii) highly detailed models which divide objects between many mass bins, and explicitly calculate dynamical and collisional interactions between different bins.
I will describe a new semi-analytic model for oligarchic growth that retains much of the simplicity of the analytic models, but incorporates more physics from the detailed models. Damping and excitation of planetesimal velocities are calculated separately, and eccentricities and inclinations vary independently. The model includes simplified treatments of planetesimal fragmentation, inward drift of planetesimals due to gas drag, and enhancement in embryo collision cross-sections due to atmospheres. Embryo-embryo collisions are also included at no extra computational cost.
Planetesimal fragmentation, and non-equilibrium random velocities greatly speed up the growth of planetary embryos during the early part of oligarchic growth. At later times, the presence of embryo atmospheres also promotes rapid growth. When embryo-embryo collisions are included, growth rates increase by 50 percent. The new model predicts that 0.1-Earth-mass protoplanets form at 1 AU and 10-Earth-mass cores form at 5 AU in 0.1 and 1 million years respectively, in a nebula where the local surface density of solids is 7 g/cm squared at each of these locations.
This work was supported by the NASA Origins and TPF Foundation Science programmes.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.