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S.J. Kortenkamp (Planetary Science Inst.,& LPL, U. of Arizona.)
Models of planet formation [1-3] indicate that growth of protoplanetary embryos may stall when the largest bodies reach about 2-3 Earth-masses (M\oplus). This is many times smaller than the masses of both Uranus and Neptune and also well below the ~10\,M\oplus size nominally required for subsequent accretion of a massive atmosphere in the core-accretion model of giant planet formation . Numerical modeling of the combined effects of solar nebula gas drag and gravitational scattering of planetesimals by a protoplanet on an eccentric orbit reveals an alternative resonant mode of accretion that may allow embryo growth to continue beyond the 2-3\,M\oplus limit. A significant fraction of planetesimals scattered onto high eccentricity orbits by the protoplanet become temporarily captured in an unusual 1:1 resonance with the protoplanet, where they orbit both the protoplanet and the sun with the same period . These trapped planetesimals are dynamically similar to the so-called quasi-satellites in the restricted 3-body problem [6,7]. Under the continued influence of solar nebula gas drag the fate of these resonant planetesimals ultimately involves deep close-encounters with the protoplanet with velocities at closest approach just marginally above the protoplanet's escape velocity. In addition to a strong gravitational focusing effect, interaction with even a tenuous circumplanetary nebula could further enhance the efficiency with which the protoplanet accretes these resonant planetesimals. REFERENCES:  Lissauer et al., In Neptune and Triton (D.P. Cruikshank, Ed.) U. Arizona Press, Tucson, 37--108, 1995.  Levison & Stewart, Icarus 153, 224--228, 2001.  Inaba & Wetherill, LPSC 32, abstract #1384, 2001.  Pollack et al., Icarus 124, 62--85, 1996.  Kortenkamp & Hamilton, Icarus (in preparation).  Peterson, Ph.D. dissertation, MIT, 1976.  Wiegert et al., AJ 119, 1978--1984, 2000.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.