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A. P. Boss (Carnegie Institution)
Gas giant planets have been detected in orbit around an increasing number of nearby stars, with most of the planets having larger masses and more eccentric orbits than Jupiter. Two theories have been advanced for the formation of such planets, core accretion and disk instability. Core accretion, the generally accepted mechanism, requires several million years or more to form a gas giant planet in a protoplanetary disk like the solar nebula. Disk instability, on the other hand, can form a gas giant protoplanet in a few hundred years. However, disk instability has previously been thought to be important only in relatively massive disks. New three dimensional, ``locally isothermal'', hydrodynamical models without velocity damping show that a disk instability can form Jupiter-mass clumps, even in a disk with a mass (0.091 solar masses within 20 AU) low enough to be in the range inferred for the solar nebula. The clumps form with initially eccentric orbits, and their survival will depend on their ability to contract to higher densities before they can be tidally disrupted at successive periastrons. Very high spatial resolution is required in order to calculate correctly the formation of these clumps. Because the disk mass in these models is comparable to that apparently required for the core accretion mechanism to operate, the models imply that disk instability could obviate the core accretion mechanism, in the solar nebula and elsewhere. However, it remains to be seen whether disk instability will lead to long-lived clumps in models with a more detailed thermodynamical treatment than these ``locally isothermal'' models. Disk instability models with 3D radiative transfer in the diffusion approximation, detailed equations of state, and dust opacity routines are in progress in order to investigate this concern. This work is partially supported by NASA grant NAG5-3873. The calculations were performed on the Carnegie Alpha Cluster, which is partially supported by NSF MRI grant AST99-76645.