| The discovery of other planetary systems has led astronomers to revise the Solar Nebula theory, especially to question whether gas giant planets form by core accretion. It seems that Jupiter-size planets formed by pure runaway growth require more than the typical protoplanetary disk lifetime to collect their mass from the surrounding material. Gas giant planet formation by gravitational instabilities (GIs) does no have this problem because planets even several times more massive than Jupiter can be formed in just a few disk orbital periods ( ≲ 1000 yr). In this scenario, the disk succumbs to its own gravity, develops spiral structures and finally fragments into stable, high-density planetary clumps. The efficiency of this planet formation mechanism depends on the detailed physics of protoplanetary disks, such as the thermal conditions of the gas, the role of stellar irradiation, the physical and chemical properties of dust grains and their distribution throughout the disk, to name a few.; GIs are spiral distortions in a self-gravitating disk that appear wherever the local surface density and temperature become favorable for their growth. The restructuring of the disk as it becomes unstable, the thermal processes that sustain the instabilities, and their effect on the long-term evolution of the disk are the studied using 3-D hydrodynamic simulations. These show that the cooling and heating processes balance each other, and as a result the disk asymptotes to quasi-equilibrium within a few orbital periods after the GI onset. The final values of the internal energy and the Toomre Q are independent of cooling time, while the asymptotic mass transport rates are inversely proportional to the cooling time. The formation of dense rings is common in these simulations. GIs fragment into clumps when the cooling time is on the order of the local orbital time, but the clumps are short-lived. External radiation can affect the evolution considerably. GIs are an effective angular momentum and mass transport mechanism, and during their onset, they can reproduce the mass transport rates necessary to trigger FU Ori outbursts.*; *This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation). The CD requires the following system requirements: Adobe Acrobat; Windows MediaPlayer or RealPlayer. |
