Through the study of accretion onto the young, low-mass stars known as T Tauri Stars (TTS), we can better understand the formation of our solar system. Gas is funneled along stellar magnetic field lines into magnetospheric accretion columns where it reaches free-fall velocities and shocks at the stellar surface, generating emission that carries information about the inner regions of the protoplanetary disk. Accretion is a variable process, with characteristic timescales ranging from minutes to years. In this dissertation, I use simulations, models, and observations to provide insight into the driving forces of mass accretion rate variability on timescales of minutes to weeks and the structure of the inner disk. Using hydrodynamic simulations, I find that steady-state, transonic accretion occurs naturally in the absence of any other source of variability. If the density in the inner disk varies smoothly in time with roughly day-long time-scales (e.g., due to turbulence), traveling shocks develop within the accretion column, which lead to rapid increases in the accretion luminosity followed by slower declines. I present the largest Hubble Space Telescope (HST) spectral variability study of TTS to date. I infer mass accretion rates and accretion column surface coverage using newly updated accretion shock models. I find typical changes in the mass accretion rate of order 10% and moderate changes in the surface coverage for most objects in the sample on week timescales. Individual peculiar epochs are further discussed. I find that the inner disk is inhomogeneous and that dust may survive near the magnetic truncation radius. Next, I link 2-minute cadence light curves from the Transiting Exoplanet Survey Satellite (TESS) to accretion using ground-based U-band photometry. Additional HST observations for one target enable more detailed connections between TESS light curves and accretion. I also use the TESS light curves to identify rotation periods and patterns of quasi-periodicity. Finally, I connect hydrodynamic simulations, accretion shock models, and stellar rotation to predict signatures of a turbulent inner disk. I generate light curves from these models to make comparisons to previous month-long photometric monitoring surveys of TTS using metrics of light curve symmetry and periodicity.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18797号 / 理博第4055号 / 新制||理||1583(附属図書館) / 31748 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)准教授 上田 佳宏, 教授 太田 耕司, 教授 嶺重 慎 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

Accretion disks around black holes with gas radiating out parts of its gravitational potential energy have long served as objects of both theoretical and observational studies. By solving the structure equations of the disk it is possible to predict the outgoing radia- tive flux and the observed spectrum of the disk and test the validity of the theory against direct observations. The standard thin disk model (Shakura-Sunyaev, Novikov-Thorne) shows, however, a still unexplained non-negligible deviance in the observed spectrum at higher mass accretion rates. To amend to the set of proposed explanations, in this thesis we examine the effect of the magnetic pressure on the trapping of some of the internal energy generated by viscous dissipation processes in the disk and advecting this energy to the black hole. A phenomenological description of heat advection mediated by a highly heterogenous magnetic field will be given, as well as its effect on the spectrum and observed effective temperature. 1

An aircraft may experience inlight ice accretion and corresponding reductions in performance and control when the vehicle encounters clouds of super-cooled water droplets. In order to study anti-icing coatings, the EADS-IW Surface Engineering Group is building a refrigerated wind tunnel. Several variations of droplet delivery systems were explored to determine the most effective way to introduce mono-dispersed droplets into the wind tunnel. To investigate this flow, timeurate, unsteady viscous flow simulations were performed using the Loci/CHEM flow solver with a multi-scale hybrid RANS/LES turbulence model. A Lagrangian droplet model was employed to simulate the movement of water droplets in the wind tunnel. It was determined that the droplet delivery system required pressure relief to properly orient the flow inside the droplet delivery tube. Additionally, a streamlined drop tube cross-section was demonstrated to reduce turbulence in the wake and decrease the variability in droplet trajectories in the test section.

Researchers have been developing techniques to predict inlight icing in order to determine aircraft behavior under different icing conditions. A key component of the techniques is the mesh generation strategy. Automated meshing facilitates numerical simulation of ice accretion on realistic aircraft configurations by deforming the surface and volume meshes in response to the evolving ice shape. The objective of this research is to validate an ice accretion strategy for wings, using a previously developed meshing strategy. The intent is to investigate the effect of varying numerical parameters, on the predicted ice shape. Using this framework, results are simulated for rime and glaze ice accretions on a rectangular planform wing with a constant GLC-305 airfoil section. The number of time steps is shown to have a significant effect on the ice shape, depending on the icing time and conditions. Decreasing the height smoothing parameters generally improves the ice shape accuracy.

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The mechanism by which massive stars form is not nearly as well understood as it is for lower mass stars. For instance, at the onset of massive star formation, it is still not clear whether the mass for a given massive star comes from the turbulent collapse of a dense core (i.e McKee & Tan, 2003) or whether the star continues to accrete material from the cores environment as it grows (i.e. Bonnell et al., 1998). From this point, it is suggested that the cold, massive core (an Infrared Dar Cloud) begins to heat up and form a Hot Core. Later in its protostellar evolution, an HII region forms from the ionizing radiation being produced by the massive star. How, or even whether, accretion onto the massive protostar can continue in the presence of the large outward thermal and radiation pressures from the star is also quite uncertain. Can the star continue to accrete ionized gas (i.e. Keto & Wood, 2006)? Are the accretion rates high enough early on to account for the final observed masses (i.e. Klaassen et al., 2006)? Or, is there some way of minimizing the radiation pressure affecting the infalling gas (i.e. McKee & Ostriker, 2007, and references therein). Here, we present observations which suggest that there is a statistically significant, although short, period in which rotation and infall of molecular gas (which powers a bipolar outflow) continue after the formation of an HII region. This continued infall of material is seen on both large and small scales, and appears to be continuing to produce outflows in many of the sources observed in this study. That it is not seen in all sources suggests that this stage is short lived. / Thesis / Doctor of Philosophy (PhD)

The origin of cosmic magnetic fields is a important area of astrophysics. The process by which they are created falls under the heading of dynamo theory, and is the topic of this thesis. Our focus for the location of where these magnetic fields operate is one the most ubiquitous objects in the universe, the accretion disk. By studying the accretion disk and the dynamo process that occurs there we wish to better understand both the accretion process and the dynamo process in stars and galaxies as well. We analyse the output from a stratified zero net flux shearing box simulation performed using the ATHENA MHD code in collaboration with Shane Davis. The simulation has turbulence which is naturally forced by the presence of a linear instability called the magnetorotational instability (MRI). We utilise Fourier filtering and the tools of mean field dynamo theory to establish a connection between the calculated EMF and the model predictions of the dynamically quenched alpha model. We find a positive correlation for both components parallel to the large scale magnetic field and the azimuthal components. We have explored many aspects of the theory including additional contributions from magnetic buoyancy and an effect arising from the large scale shear and the current density. We also directly measure the turbulent correlation time for the velocity and magnetic fields both large scale and small. We can also observe the effects of the dynamo cycle, with the azimuthal component of the large scale magnetic field flipping sign in this analysis. We find a positive correlation between the divergence of the eddy scale magnetic helicity flux and the component of the electromotive force parallel to the large scale magnetic field. This correlation directly links the transfer of magnetic helicity to the dynamo process in a system with naturally driven turbulence. This highlights the importance of magnetic helicity and its conservation even in a system with triply periodic boundary conditions. / Thesis / Doctor of Philosophy (PhD)

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