Other
Scientific paper
Sep 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.1209b&link_type=abstract
American Astronomical Society, DPS meeting #28, #12.09; Bulletin of the American Astronomical Society, Vol. 28, p.1112
Other
Scientific paper
The discovery of the suspected extrasolar planets 47 UMa B and 55 rho (1) Cnc C, with masses substantially greater than that of Jupiter, motivates a re-evaluation of mechanisms for giant planet formation. Here we consider forming giant planets through the gravitational instability of a protoplanetary disk. Radiative hydrodynamical calculations of the thermal structure of an axisymmetric protoplanetary disk with a mass of ~ 0.14 M_sun orbiting a solar-mass star predict that the outer disk may be cool enough ( ~ 100 K) to become gravitationally unstable. This possibility has now been investigated with a fully three dimensional hydrodynamics code. Growth of nonaxisymmetry occurs within a few rotation periods of the outer disk (P_o ~ 30 yrs); the nonaxisymmetry is large enough to result in disk evolution through gravitational torques within ~ 10(4) yrs. After about 10 P_o, the dominant m = 1 and m = 2 modes saturate at an amplitude greater than 1 -- by this time, a 6-Jupiter-mass clump of gas has formed around 8 AU, accompanied by a 0.4-Jupiter-mass clump at the same orbital radius, but 180(o) away in azimuth. The clumps are gravitationally bound and so should form giant planets. The hot inner disk remains nearly axisymmetric throughout, suggesting a ``best of both worlds'' scenario where formation of terrestrial planets occurs through collisional accumulation in the hot inner nebula, while rapid formation of giant gaseous protoplanets occurs in the cool outer nebula through gravitational instability. However, the gravitational instability depends on the thermodynamics assumed for the azimuthal density variations, and on the Toomre Q parameter for the disk (Q < 1 implies instability). For isothermal azimuthal variations, the instability is damped when the outer disk is hotter than about 150 K (Qmin ~ 1.1). Even if the variations occur adiabatically with an effective gamma > 1, the instability can still occur, provided that Qmin ~ 1. Clumpy accretion of molecular cloud gas by the disk may be sufficient to drive Q < 1 over a short time scale and trigger an episode of giant planet formation in a marginally stable disk.
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