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S. J. Weidenschilling (Planetary Science Institute)
Two competing hypotheses for the origin of jovian-type planets are gravitational instability in a circumstellar disk (Boss, {\it Science} {\bf276}, 1836, 1997), and accumulation of a massive core that accretes gas from the nebula (Pollack {\it et al.}, {\it Icarus} {\bf 124}, 62, 1996). The diversity of orbits among extrasolar planets and our own system suggests that both mechanisms may be effective, perhaps depending on the disk mass. I investigate the core-accretion model, using the PSI multi-zone code (Weidenschilling {\it et al.}, {\it Icarus} {\bf 128}, 429, 1997) to simulate growth of Jupiter's core, without simplifying assumptions used by Pollack {\it et al.} The surface density is not assumed uniform across the feeding zone, and planetesimals may migrate into or out of the feeding zone by gas drag. Gravitational stirring by the embryo is parameterized from Greenzweig and Lissauer ({\it Icarus} {\bf 100}, 440, 1992), including a small but significant increase in inclination at each synodic encounter. While Pollack et al. assumed a Mars-sized "seed body" to initiate core growth, a much smaller (10-3 M\oplus) body is sufficient. The background population of planetesimals (initial size 25 km) grows by mutual collisions. However, the seed body experiences runaway growth and maintains its advantage. It stirs velocities of the smaller bodies, inhibiting runaway of potential competitors. The embryo depletes the local population, creating a local minimum in the surface density. However, gas drag supplies new bodies to the feeding zone, allowing its growth to continue. For the nominal surface density of 10 g/cm2, the core gains about 1 M\oplus per million years. This is too slow to yield a core able to accrete gas during the probable lifetime of the nebula, but a modest increase in surface density allows formation of Jupiter's core. Still under investigation are the minimum "seed" mass required for emergence of a dominant embryo, and the role of collisional fragmentation among the smaller swarm bodies.