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The Influence of Gravitationally Unstable Protoplanetary Disks on Type I Migration
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| Title: | The Influence of Gravitationally Unstable Protoplanetary Disks on Type I Migration |
| Other Titles: | 重力不安定原始惑星系円盤のI型惑星移動への影響 |
| Authors: | NGUYEN, Kim Ngan Browse this author |
| Issue Date: | 25-Sep-2020 |
| Publisher: | Hokkaido University |
| Abstract: | The focus of this thesis is the impact of global structure in two types of astrophysicaldisks: galactic and protoplanetary.The first chapter summarises a study on the impact of the global galactic environmenton the physical properties of star-forming clouds. This work was the main focus ofthe Masters thesis and completed for the doctorate. A series of simulations of isolatedgalactic disks with varying background potentials were performed using an AMR hy-drodynamics code. The galactic potentials are expressed by rising, decreasing and flatrotation curves, with the addition of either a massive stellar disk or two-armed spiralpatterns. Results from these simulations explored the role of shear and the gravita-tional stability, Toomre Q, in the fragmentation of the gas disk into clouds. Althoughthe properties of a typical cloud were found to be largely independent of the poten-tial, the production of small and large cloud associations were strongly dependent onthe galactic global structure. The addition of the spiral potential made the greatestdifference to the clouds, successfully sweeping gas into extended structures.In a similar system but on smaller scale, protoplanetary disks share an equivalent setof internal forces and an external global potential. The primary research for this thesisconsiders the instabilities forming in the protoplanetary disks that circle young starsand how this environment affects the evolution of planets forming from the dust andgas.According to current theories, proto-planets are formed as a consequence of either aseries of collisions and mergers between dust and solid material, or through a collapsefrom gravitational instabilities in the protoplanetary disk. However, the young planetis initially embedded within the protoplanetary disk and interacts with its parent gasdisk through the exchange of angular momentum with the surrounding gas. This cancause the orbital radius of the planet to change, leading to a migration through thedisk. Evidence for this phenomenon abounds in exoplanet systems, which have largeplanets on very short orbits that are unlikely to have formed in-situ.Smaller planets undergo what is referred to as "Type I migration", which presentsa major problem for planet formation theories. Numerical estimates of this processtypically indicate a rapid inward migration, resulting in the loss of the planet. Youngplanets are sent into the star within approximately105years, from a distance of 1 AU,well before the protoplanetary disk can evaporate and remove the fatal gas pull fromthe planet (within106−107years). In an attempt to prevent the loss of young planets, various possibilities have beendiscussed, including sharp changes in disk properties to create planet traps, late for-mation of planets after the disk has partially evaporated, and gravitational scatteringby neighboring planets or planetesimals. However, recent ALMA observations haverevealed complex substructures within the protoplanetary disks, including rings andspiral arms which could be results from either Lindblad waves ignited by planets orgravitational instability. Previous work has not typically focused on the effect of suchstructures formed by borderline stability of the protoplanetary disk on the migrationof planets, typically assuming instead that the disk is homogeneous.In this research, we perform a series of simulations of protoplanetary disks with dif-ferent degrees of global structure and follow the migration of a young planet. The iso-lated protoplanetary disk is simulated using ChaNGa; an SPH hydrodynamics code.The disks exist in a variety of stable to borderline stable and unstable states, corre-sponding to different values of Toomre Q.The planet’s migration is strongly affected by instabilities that develop within the pro-toplanetary disk. While the planet’s migration in a homogeneous disk moves inwardsat a steady velocity, structure-rich disks disrupt this migration and cause a irregularmotion through the gas. This is most clearly seen in the torques acting on the planet.These change from a settled constant value in a smooth gas distribution where reso-nance waves can be maintained, to experiencing strong fluctuations due to the forma-tion of spirals and other features. Fragmentation of the disk can lead to scattering ofthe planet and ejection from the system. However, even disks with a Toomre Q>1(representing stability) can still have a strong impact on the planet torques.Overall, this work strongly suggests that massive or structure rich protoplantary diskswill also have an effect on the migration of the planet. This is expected for all proto-planetary disks in their early years and may apply to a wide variety of disks even atlater times. Type I migration is therefore a highly non-linear process and can only beweakly approximated by an analytical expression in the majority of systems. |
| Conffering University: | 北海道大学 |
| Degree Report Number: | 甲第14194号 |
| Degree Level: | 博士 |
| Degree Discipline: | 理学 |
| Examination Committee Members: | (主査) 客員准教授 Elizabeth J. Tasker, 教授 倉本 圭, 教授 小林 達夫, 講師 岡本 崇 |
| Degree Affiliation: | 理学院（宇宙理学専攻） |
| Type: | theses (doctoral) |
| URI: | http://hdl.handle.net/2115/79528 |
| Appears in Collections: | 課程博士 (Doctorate by way of Advanced Course) > 理学院(Graduate School of Science)
学位論文 (Theses) > 博士 （理学）
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