Planetesimal and Protoplanet Dynamics in a Turbulent Protoplanetary Disk

Accretion Disks;Instabilities;Numerical methods;Magnetohydrodynamics (MHD);Planetary systems: formation;Protoplanetary disks;Turbulence

Yang, Chao-Chin

关键词: Accretion Disks;    Instabilities;    Numerical methods;    Magnetohydrodynamics (MHD);    Planetary systems: formation;    Protoplanetary disks;    Turbulence;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/17009/Yang_Chao-Chin.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

In a young protoplanetary disk, planetary objects undergo orbital migration due to the gravitational torques exerted by their surrounding gas.It has been an outstanding issue that an earth-sized planet embedded in a gas-rich environment may rapidly migrate into its host star.In this dissertation, I study some of the processes that affect the orbital migration of planetary objects and their roles on planet formation scenarios.Sharp density features in protoplanetary disks, for instance at the edge of a magnetically dead zone, have recently been proposed as effective barriers to slow down or even stop the problematically fast migration of planetary cores into their central star. Density features on a radial scale approaching the disk vertical scale height might not exist, however, since they could be Rayleigh (or more generally Solberg-Høiland) unstable. Stability must be checked explicitly in one-dimensional viscous accretion disk models because these instabilities are artificially eliminated in the process of reducing the full set of axisymmetric equations. The disk thermodynamics, via the entropy stratification, and its vertical structure also influence stability when sharp density features are present. We propose the concept of Rayleigh adjustment for viscous disk models: any density feature that violates Rayleigh stability (or its generalization) should be diffused radially by hydrodynamical turbulence on a dynamical time-scale, approaching marginal stability in a quasi-continuous manner.Due to the gravitational influence of density fluctuations in the gas disk subject to magneto-rotational instability, planetesimals and protoplanets undergo diffusive radial migration as well as changes of other orbital properties. The magnitude of the effect on particle orbits has important consequences for planet formation scenarios.To accurately measure the gravitational influence of turbulent density fluctuations on particle orbits, numerical simulations capturing both large-scale and small-scale coherent structures are required.Using local shearing boxes with various resolutions up to 64 points per disk scale height and horizontal sizes up to 16 scale heights, we systematically study the corresponding density structure and particle orbit evolution.We consider ideal magnetized disks with isothermal equation of state, and compare disks with and without vertical stratification.We find that although the results converge with resolution for fixed box dimensions, the response of the particles to the gravity of the turbulent gas strongly depends on the horizontal box dimension.Our results indicate the dominance of large-scale density structures, which are closely related to recently discussed zonal flow models of protoplanetary disks.Based on heuristic arguments, some implications may be drawn from the measurements of our local models.The radial diffusive migration of protoplanets induced by magneto-rotational turbulence may be unimportant compared to secular migration.Kilometer-sized planetesimals moving in magneto-rotational turbulence may not suffer from mutual collisional destruction, except for those in the inner region of a young protoplanetary disk.Before these results can be considered valid, though, it will be necessary to elucidate the discrepancies that have appeared between global and local models.