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1
Savcheva, Antonia Stefanova. "Magnetically torqued thin accretion disks." Thesis, Massachusetts Institute of Technology, 2006. http://hdl.handle.net/1721.1/36119.
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Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2006.<br>Includes bibliographical references (leaves 61-64).<br>We consider geometrically thin accretion disks around millisecond X-ray pulsars. We start with the Shakura-Sunyaev thin disk model as a basis and modify the disk equations with a magnetic torque from the central neutron star. Disk solutions are computed for a range of neutron star magnetic fields. We also investigate the effect of different equations of state and opacities on the disk solutions. We show that there are indications of thermal instability in some of the disk solutions, especially for the higher values of 3M. We also explain how the time evolution of the disk solutions can be calculated.<br>by Antonia Stefanova Savcheva.<br>S.B.
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Gu, Pin-gao. "Turbulence in Keplerian accretion disks /." Digital version accessible at:, 2000. http://wwwlib.umi.com/cr/utexas/main.
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Isaacs, Sonja [Verfasser]. "Unveiling Accretion Disks - Physical Parameter Eclipse Mapping of Accretion Disks in Dwarf Novae / Sonja Isaacs." München : GRIN Verlag, 2002. http://d-nb.info/1177524554/34.
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Curran, Dian Beard. "Magnetic shearing instabilities in accretion disks /." Digital version accessible at:, 1998. http://wwwlib.umi.com/cr/utexas/main.
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Čemeljić, Miljenko. "Resistive magnetohydrodynamic jets from protostellar accretion disks." [S.l. : s.n.], 2004. http://deposit.ddb.de/cgi-bin/dokserv?idn=974114529.
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Cemeljic, Miljenko. "Resistive magnetohydrodynamic jets from protostellar accretion disks." Phd thesis, Universität Potsdam, 2005. http://opus.kobv.de/ubp/volltexte/2005/209/.
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Astrophysikalische Jets sind ausgedehnte, kollimierte Massenausflüsse von verschiedenen astronomischen Objekten. Zeitabhängige magnetohydrodynamische (MHD) Simulationen der Jet-Entwicklung müssen den Akrretionsprozess in der Scheibe berücksichtigen, da der Jet aus der Scheibenmaterie gespeist wird. Allerdings ist die simultane Berechnung der Entwicklung von Scheibe und Jet schwierig, da die charakteristischen Zeitskalen unterschiedlich sind. Selbstähnliche Modelle zeigten, daß eine Beschreibung der Jetentstehung aus einer Akkretionsscheibe durch rein magnetische Prozesse möglich ist.<br>In this thesis the magnetohydrodynamic jet formation and the effects of magnetic diffusion on the formation of axisymmetric protostellar jets have been investigated in three different simulation sets. The time-dependent numerical simulations have been performed, using the magnetohydrodynamic ZEUS-3D code.
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Caunt, Stuart Edward. "Analytical and numerical models of accretion disks." Thesis, University of Newcastle Upon Tyne, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.265485.
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Nayakshin, Sergei Victor 1969. "Physics of accretion disks with magnetic flares." Diss., The University of Arizona, 1998. http://hdl.handle.net/10150/288916.
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Rapid progress in multi-wavelength observations of Seyfert Galaxies in recent years is providing evidence that X-ray emission in these objects may be produced by magnetic flares occurring above a cold accretion disk. Here we attempt to develop a physically consistent model of accretion disks producing radiation via magnetic flares as well as the optically thick intrinsic disk emission, and apply this model to observations of Active Galactic Nuclei (AGN) and Galactic Black Hole Candidates (GBHCs). The following issues are considered: (1) the pressure equilibrium in the flare region, (2) the reflection and reprocessing of the X-radiation from flares in the underlying disk, (3) the spectra of GBHCs in the context of the model, (4) and the generation of the flares by the disk--the energy budget of the corona. Our results show that: (1) The temperature of the disk atmosphere near active magnetic flares in AGN is in the range 1 - 3 x 10⁵ Kelvin, and that the material is relatively non-ionized. This temperature is in a good agreement with the observed rollover energy in the Big Blue Bump (BBB) of Seyfert 1 Galaxies. We thus suggest that the BBB is simply the X-rays from magnetic flares reprocessed into the X-ray skin of the accretion disk. (2) We suggest an explanation for the recently discovered X-ray Baldwin effect and the controversy over the existence of BBBs in quasars more luminous than typical Seyferts. (3) Due to an ionization instability and much higher X-ray incident flux, we found that the X-ray skin in GBHCs is nearly completely ionized. Using an approximate model to describe this effect, we calculated the reflected/reprocessed spectrum and the resulting corona spectrum simultaneously. We found that the spectrum of GBHCs in their hard state may be explained with this model, with basically the same parameters for magnetic flares as in the AGN case. (4) The magnetic energy transport is shown to be large enough to account for the observed amount of X-rays from Seyferts and GBHCs. We predict that X-ray spectra are hard for accretion rates below the gas-to-radiation transition, and that they are softer above this transition. (5) We collected our results into a diagram that shows how the observational appearance of accreting black holes changes with the accretion rate and the mass of the hole, and compared it with observations of AGN and GBHCs. Our conclusion is that the agreement between theory and observations is very encouraging and we suggest that the physics of magnetic flares is the physics that should be added to the standard accretion disk theory in order to produce a more realistic description of accretion flows with large angular momentum.
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Schultz, Juho. "Studies of accretion disks in X-ray binaries." Helsinki : University of Helsinki, 2005. http://ethesis.helsinki.fi/julkaisut/mat/tahti/vk/schultz/.
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Wang, Zhongxiang 1968. "Multiwavelength studies of accretion disks around compact objects." Thesis, Massachusetts Institute of Technology, 2004. http://hdl.handle.net/1721.1/28648.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2004.<br>Includes bibliographical references (p. 133-149).<br>(cont.) to the XPS in SNR RCW 103. The multiple IR band measurements of 1E 1048.1-5937 provide marginal evidence for spectral flattening, and cannot rule out an accretion disk scenario for AXPs.<br>In this thesis, I present multiwavelength studies of phenomena related to accretion disks around compact objects. The observations were made mainly with ground-based telescopes and the Hubble Space Telescope. I observed several known and candidate ultracompact low-mass X-ray binaries (LMXBs) and found that their optical spectra all show a lack of hydrogen emission lines, supporting the theoretical prediction that mass donors in ultracompact LMXBs must be H-depleted. Time-resolved photometry of the candidate source 4U 1543-624 revealed an 18 minute orbital periodicity, verifying the ultracompact nature of this binary. These studies strongly support the identification of several candidate systems with similar X-ray spectral features as ultracompact binaries. In the ultracompact binary 4U 1820-30, which has the shortest orbital period (685 s) among the known LMXBs, I discovered a 692-s periodicity from its far-ultraviolet (FUV) time series data. I interpret this longer-period FUV signal as a superhump oscillation, arising from a tidal resonance in the accretion disk of an extreme-mass-ratio binary. I also present multiband imaging of the fields surrounding five newly discovered X-ray millisecond pulsars (MSPs) in an effort to identify and study their optical/IR counterparts. For the MSP SAX J1808.4-3658, the optical light curve taken during its 1998 X-ray outburst shows an exponential decay in intensity, roughly following the X-ray light curve early in the outburst. An optical counterpart of XTE J1814-338 was also detected. Finally, optical/IR observations of anomalous X-ray pulsars (AXPs) and X-ray point sources (XPSs) in young supernova remnants (SNRs) identified the IR counterpart to the AXP 1E 1048.1-5937 and a likely IR counterpart<br>by Zhongxiang Wang.<br>Ph.D.
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Klee, J., T. F. Illenseer, M. Jung, and W. J. Duschl. "The impact of numerical oversteepening on the fragmentation boundary in self-gravitating disks." EDP SCIENCES S A, 2017. http://hdl.handle.net/10150/625972.
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Context. Whether or not a self-gravitating accretion disk fragments is still an open issue. There are many different physical and numerical explanations for fragmentation, but simulations often show a non-convergent behavior for ever better resolution. Aims. We aim to investigate the influence of different numerical limiters in Godunov type schemes on the fragmentation boundary in self-gravitating disks. Methods. We have compared the linear and non-linear outcomes in two-dimensional shearingsheet simulations using the VANLE ER and the SUPERBEE limiter. Results. We show that choosing inappropriate limiting functions to handle shock-capturing in Godunov type schemes can lead to an overestimation of the surface density in regions with shallow density gradients. The effect amplifies itself on timescales comparable to the dynamical timescale even at high resolutions. This is exactly the environment in which clumps are expected to form. The effect is present without, but scaled up by, self-gravity and also does not depend on cooling. Moreover it can be backtracked to a well known effect called oversteepening. If the effect is also observed in the linear case, the fragmentation limit is shifted to larger values of the critical cooling timescale.
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Simon, M. N., I. Pascucci, S. Edwards, et al. "TRACING SLOW WINDS FROM T TAURI STARS VIA LOW-VELOCITY FORBIDDEN LINE EMISSION." IOP PUBLISHING LTD, 2016. http://hdl.handle.net/10150/622166.
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Using Keck/HIRES spectra (Delta v similar to 7 km s(-1)) we analyze forbidden lines of [O I] 6300 angstrom, [O I] 5577 angstrom. and [S II] 6731 angstrom. from 33 T Tauri stars covering a range of disk evolutionary stages. After removing a high-velocity component (HVC) associated with microjets, we study the properties of the low-velocity component (LVC). The LVC can be attributed to slow disk winds that could be magnetically (magnetohydrodynamic) or thermally (photoevaporative) driven. Both of these winds play an important role in the evolution and dispersal of protoplanetary material. LVC emission is seen in all 30 stars with detected [O. I] but only in two out of eight with detected [S. II], so our analysis is largely based on the properties of the [O. I] LVC. The LVC itself is resolved into broad (BC) and narrow (NC) kinematic components. Both components are found over a wide range of accretion rates and their luminosity is correlated with the accretion luminosity, but the NC is proportionately stronger than the BC in transition disks. The full width at half maximum of both the BC and NC correlates with disk inclination, consistent with Keplerian broadening from radii of 0.05 to 0.5 au and 0.5 to 5 au, respectively. The velocity centroids of the BC suggest formation in an MHD disk wind, with the largest blueshifts found in sources with closer to face-on orientations. The velocity centroids of the NC, however, show no dependence on disk inclination. The origin of this component is less clear and the evidence for photoevaporation is not conclusive.
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Lin, Min-Kai, and Andrew N. Youdin. "A Thermodynamic View of Dusty Protoplanetary Disks." IOP PUBLISHING LTD, 2017. http://hdl.handle.net/10150/626177.
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Small solids embedded in gaseous protoplanetary disks are subject to strong dust-gas friction. Consequently, tightly coupled dust particles almost follow the gas flow. This near conservation of the dust-to-gas ratio along streamlines is analogous to the near conservation of entropy along flows of (dust-free) gas with weak heating and cooling. We develop this thermodynamic analogy into a framework to study dusty gas dynamics in protoplanetary disks. We show that an isothermal dusty gas behaves like an adiabatic pure gas, and that finite dust-gas coupling may be regarded as effective heating/cooling. We exploit this correspondence to deduce that (1) perfectly coupled, thin dust layers cannot cause axisymmetric instabilities; (2) radial dust edges are unstable if the dust is vertically well-mixed; (3) the streaming instability necessarily involves a gas pressure response that lags behind dust density; and (4) dust-loading introduces buoyancy forces that generally stabilize the vertical shear instability associated with global radial temperature gradients. We also discuss dusty analogs of other hydrodynamic processes (e.g., Rossby wave instability, convective overstability, and zombie vortices) and how to simulate dusty protoplanetary disks with minor tweaks to existing codes for pure gas dynamics.
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Armitage, Philip J., Josh A. Eisner, and Jacob B. Simon. "PROMPT PLANETESIMAL FORMATION BEYOND THE SNOW LINE." IOP PUBLISHING LTD, 2016. http://hdl.handle.net/10150/621505.
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We develop a simple model to predict the radial distribution of planetesimal formation. The model is based on the observed growth of dust to millimeter-sized particles, which drift radially, pile-up, and form planetesimals where the stopping time and dust-to-gas ratio intersect the allowed region for streaming instability-induced gravitational collapse. Using an approximate analytic treatment, we first show that drifting particles define a track in metallicity-stopping time space whose only substantial dependence is on the disk's angular momentum transport efficiency. Prompt planetesimal formation is feasible for high particle accretion rates (relative to the gas, (M) over dot(p)/(M) over dot greater than or similar to 3 x 10(-2) for alpha = 10(-2)), which could only be sustained for a limited period of time. If it is possible, it would lead to the deposition of a broad and massive belt of planetesimals with a sharp outer edge. Numerically including turbulent diffusion and vapor condensation processes, we find that a modest enhancement of solids near the snow line occurs for centimeter-sized particles, but that this is largely immaterial for planetesimal formation. We note that radial drift couples planetesimal formation across radii in the disk, and suggest that considerations of planetesimal formation favor a model in which the initial deposition of material for giant planet cores occurs well beyond the snow line.
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Lin, Min-Kai, and Kaitlin M. Kratter. "ON THE GRAVITATIONAL STABILITY OF GRAVITO-TURBULENT ACCRETION DISKS." IOP PUBLISHING LTD, 2016. http://hdl.handle.net/10150/621384.
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Low mass, self-gravitating accretion disks admit quasi-steady, "gravito-turbulent" states in which cooling balances turbulent viscous heating. However, numerical simulations show that gravito-turbulence cannot be sustained beyond dynamical timescales when the cooling rate or corresponding turbulent viscosity is too large. The result is disk fragmentation. We motivate and quantify an interpretation of disk fragmentation as the inability to maintain gravito-turbulence due to formal secondary instabilities driven by: (1) cooling, which reduces pressure support; and/or (2) viscosity, which reduces rotational support. We analyze the axisymmetric gravitational stability of viscous, non-adiabatic accretion disks with internal heating, external irradiation, and cooling in the shearing box approximation. We consider parameterized cooling functions in 2D and 3D disks, as well as radiative diffusion in 3D. We show that generally there is no critical cooling rate/viscosity below which the disk is formally stable, although interesting limits appear for unstable modes with lengthscales on the order of the disk thickness. We apply this new linear theory to protoplanetary disks subject to gravito-turbulence modeled as an effective viscosity, and cooling regulated by dust opacity. We find that viscosity renders the disk beyond similar to 60 au dynamically unstable on radial lengthscales a few times the local disk thickness. This is coincident with the empirical condition for disk fragmentation based on a maximum sustainable stress. We suggest turbulent stresses can play an active role in realistic disk fragmentation by removing rotational stabilization against self-gravity, and that the observed transition in behavior from gravito-turbulent to fragmenting may reflect instability of the gravito-turbulent state itself.
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Ouyed, Rachid. "Numerical simulations of astrophysical jets from Keplerian accretion disks." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp02/NQ30165.pdf.
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Ouyed, Rachid. "Numerical simulations of astrophysical jets from Kerplerian accretion disks /." *McMaster only, 1996.
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Blackburne, Jeffrey A. "Zooming in on quasar accretion disks using chromatic microlensing." Thesis, Massachusetts Institute of Technology, 2009. http://hdl.handle.net/1721.1/63000.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2009.<br>Cataloged from PDF version of thesis.<br>Includes bibliographical references (p. 217-226).<br>Observing the temperature profiles of accretion disks around black holes is a fundamental test of an important astrophysical process. However, angular resolution limitations have prevented such a measurement for distant quasars. We present a new method for determining the size of quasar accretion disks at a range of wavelengths, thus constraining their temperature profiles. The technique uses single-epoch, multi-wavelength optical and nearinfrared imaging of gravitationally lensed quasars in conjunction with X-ray imaging, and takes advantage of the presence of microlensing perturbations to the magnifications of the lensed images. The dependence of these perturbations on the angular size of the source, combined with the temperature structure of quasar accretion disks, causes the flux ratio anomalies due to microlensing to appear chromatic. This allows us to probe regions of the quasar that are too small to be measured by any other technique. We apply this method to observations of 12 lensed quasars, and measure the size of the accretion disk of each in 8 broadband filters between 0.36 and 2.2 microns (in the observed frame). We find that the overall sizes are larger by factors of 3 to 30 than predicted by the standard thin accretion disk model, and that the logarithmic slope of the wavelength-dependent size is ~ 0.2 on average, much shallower than the predicted slope of 4/3. This implies that the temperature is a steeper function of radius than the thin disk model predicts. With this new approach to determining quasar accretion disk sizes, we are thus able to rule out the standard thin disk model as the source of the (rest-frame) ultraviolet and optical continuum in these bright quasars.<br>by Jeffrey A. Blackburne.<br>Ph.D.
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Schneck, Kristiana E. (Kristiana Elizabeth). "Gravitational influences on magnetic field structure in accretion disks." Thesis, Massachusetts Institute of Technology, 2010. http://hdl.handle.net/1721.1/61266.
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Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2010.<br>Cataloged from PDF version of thesis.<br>Includes bibliographical references (p. 53).<br>Black holes and compact objects are often surrounded by structures known as accretion disks which consist of ionized plasma. Due to the immense forces present in the disk, interesting and complex magnetic field structures can be set up within the disk. The influence of gravity on these structures is explored via a higher-order expansion of the gravitational potential. We consider several cases: the case when the Lorentz force is negligible and the case when the Lorentz force becomes significant in the dynamics of the disk. When the Lorentz force is negligible, we find using the Ferraro Co-rotation Theorem that the strength of the magnetic field increases near the event horizon. As the strength of the Lorentz force increases and it is included in our analysis, we discover that it leads to a periodic "crystal" structure of concentric rings of current. This structure is significantly affected by gravitational forces: we find a solution to the equations of motion that shows a composite structure within the disk. We discuss the general relativistic effects near the event horizon using the Pseudo- Newtonian potential in this limit. In addition, we consider an alternate derivation of the equation of motion describing the behavior of the magnetic field and discuss its consequences.<br>by Kristiana E. Schneck.<br>S.B.
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Down, Emily. "The orientation of accretion disks and jets in quasars." Thesis, University of Oxford, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.572597.
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All massive nearby galaxies, including our own, host supermassive black holes. Active galactic nuclei (AGN) are seen when such black holes accrete, and when they produce powerful jets of synchrotron-emitting plasma, they are termed radio-loud AGN. The close correlation between black hole mass and galaxy bulge mass in elliptical galaxies indicates that AGN feedback may be the key to the regulation of galaxy formation. It is thus necessary to fully understand the structure of AGN, the way that they are fuelled, and their duty cycle, in order to study the feedback processes and get a clear picture of galaxy formation. In this thesis, independent methods are developed to constrain the accretion disk and radio jet angles to the line of sight. H IX emission from a sub-sample of high-redshift quasars is measured from near-infrared spectroscopy and modelled as sums of different components, including the characteristic double-peaked profile which results from a thin, rotating accretion disk. Comparing the models using Bayesian evidence, almost all quasars were found to have infrared spectra consistent with the presence of a disk. The jet inclination angles of the same set of quasars were constrained by fitting a model, including the effect of Doppler boosting and the receding torus model for dust obscuration, to the radio \ spectral energy distribution. The fitted disk and jet angles correlate strongly, and are consistent with a model in which the radio jets are launched orthogonally to the plane of the accretion disk, as expected if the jet is powered by energy drawn from the spin of the black hole. Both disk and jet angles correlate with the observed linear source size, which is a projection effect; when deprojected using the fitted angles, the distribution of source sizes agrees with a scenario in which the sources expand into the surrounding medium at a constant rate up to ~ 1 Mpc and then shut off, probably as the nuclei become quiescent. The accretion disk angle was found to correlate weakly with the low-frequency radio luminosity, which provides direct, albeit tenuous, evidence for the receding torus model.
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Britsch, Markward. "Gravitational instability and fragmentation of self-gravitating accretion disks." [S.l. : s.n.], 2006. http://nbn-resolving.de/urn:nbn:de:bsz:16-opus-69667.
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Pessah, Martin Elias. "Magnetohydrodynamic Turbulence and Angular Momentum Transport in Accretion Disks." Diss., The University of Arizona, 2007. http://hdl.handle.net/10150/194324.
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It is currently believed that angular momentum transport in accretion disks is mediated by magnetohydrodynamic (MHD) turbulence driven by the magnetorotational instability (MRI). More than 15 years after its discovery, an accretion disk model that incorporates the MRI as the mechanism driving the MHD turbulence is still lacking. This dissertation constitutes the first in a series of steps towards establishing the formalism and methodology needed to move beyond the standard accretion disk model and incorporating the MRI as the mechanism enabling the accretion process. I begin by presenting a local linear stability analysis of a compressible, differentially rotating flow and addressing the evolution of the MRI beyond the weak-field limit when magnetic tension forces due to strong toroidal fields are considered. Then, I derive the first formal analytical proof showing that, during the exponential growth of the instability, the mean total stress produced by correlated MHD fluctuations is positive and leads to a net outward flux of angular momentum. I also show that some characteristics of the MHD stresses that are determined during this initial phase are roughly preserved in the turbulent saturated state observed in local numerical simulations. Motivated by these results, I present the first mean-field MHD model for angular momentum transport driven by the MRI that is able to account for a number of correlations among stresses found in local numerical simulations. I point out the relevance of a new type of correlation that couples the dynamical evolution of the Reynolds and Maxwell stresses and plays a key role in developing and sustaining the MHD turbulence. Finally, I address how the turbulent transport of angular momentum depends on the magnitude of the local shear. I show that turbulent MHD stresses in accretion disks cannot be described in terms of shear-viscosity.
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Pinilla, P., L. M. Pérez, S. Andrews, et al. "A Multi-wavelength Analysis of Dust and Gas in the SR 24S Transition Disk." IOP PUBLISHING LTD, 2017. http://hdl.handle.net/10150/624044.
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We present new Atacama Large Millimeter/sub-millimeter Array (ALMA) 1.3 mm continuum observations of the SR 24S transition disk with an angular resolution less than or similar to 0'.18 (12 au radius). We perform a multi-wavelength investigation by combining new data with previous ALMA data at 0.45 mm. The visibilities and images of the continuum emission at the two wavelengths are well characterized by a ring-like emission. Visibility modeling finds that the ring-like emission is narrower at longer wavelengths, in good agreement with models of dust-trapping in pressure bumps, although there are complex residuals that suggest potentially asymmetric structures. The 0.45 mm emission has a shallower profile inside the central cavity than the 1.3 mm emission. In addition, we find that the (CO)-C-13 and (CO)-O-18 (J = 2-1) emission peaks at the center of the continuum cavity. We do not detect either continuum or gas emission from the northern companion to this system (SR 24N), which is itself a binary system. The upper limit for the dust disk mass of SR 24N is less than or similar to 0.12 M-circle plus, which gives a disk mass ratio in dust between the two components of M-dust,M-SR 24S/M-dust,M-SR 24N greater than or similar to 840. The current ALMA observations may imply that either planets have already formed in the SR 24N disk or that dust growth to millimeter sizes is inhibited there and that only warm gas, as seen by rovibrational CO emission inside the truncation radii of the binary, is present.
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Kubsch, Marcus, Tobias F. Illenseer, and Wolfgang J. Duschl. "Accretion disk dynamics alpha-viscosity in self-similar self-gravitating models." EDP SCIENCES S A, 2016. http://hdl.handle.net/10150/614710.
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Aims. We investigate the suitability of alpha-viscosity in self-similar models for self-gravitating disks with a focus on active galactic nuclei (AGN) disks. Methods. We use a self-similar approach to simplify the partial di ff erential equations arising from the evolution equation, which are then solved using numerical standard procedures. Results. We find a self-similar solution for the dynamical evolution of self-gravitating alpha-disks and derive the significant quantities. In the Keplerian part of the disk our model is consistent with standard stationary alpha-disk theory, and self-consistent throughout the self-gravitating regime. Positive accretion rates throughout the disk demand a high degree of self-gravitation. Combined with the temporal decline of the accretion rate and its low amount, the model prohibits the growth of large central masses. Conclusions. alpha-viscosity cannot account for the evolution of the whole mass spectrum of super-massive black holes (SMBH) in AGN. However, considering the involved scales it seems suitable for modelling protoplanetary disks.
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Mulders, Gijs D., Ilaria Pascucci, Carlo F. Manara, et al. "Constraints from Dust Mass and Mass Accretion Rate Measurements on Angular Momentum Transport in Protoplanetary Disks." IOP PUBLISHING LTD, 2017. http://hdl.handle.net/10150/626047.
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In this paper, we investigate the relation between disk mass and mass accretion rate to constrain the mechanism of angular momentum transport in protoplanetary disks. We find a correlation between dust disk mass and mass accretion rate in Chamaeleon I with a slope that is close to linear, similar to the one recently identified in Lupus. We investigate the effect of stellar mass and find that the intrinsic scatter around the best-fit M-dust-M star and M-acc-M star relations is uncorrelated. We simulate synthetic observations of an ensemble of evolving disks using a Monte Carlo approach and find that disks with a constant alpha viscosity can fit the observed relations between dust mass, mass accretion rate, and stellar mass but overpredict the strength of the correlation between disk mass and mass accretion rate when using standard initial conditions. We find two possible solutions. In the first one, the observed scatter in M-dust and M-acc is not primordial, but arises from additional physical processes or uncertainties in estimating the disk gas mass. Most likely grain growth and radial drift affect the observable dust mass, while variability on large timescales affects the mass accretion rates. In the second scenario, the observed scatter is primordial, but disks have not evolved substantially at the age of Lupus and Chamaeleon I owing to a low viscosity or a large initial disk radius. More accurate estimates of the disk mass and gas disk sizes in a large sample of protoplanetary disks, through either direct observations of the gas or spatially resolved multiwavelength observations of the dust with ALMA, are needed to discriminate between both scenarios or to constrain alternative angular momentum transport mechanisms such as MHD disk winds.
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Penna, Robert Francis. "Black Hole Accretion Disks and Jets: Connecting Simulations and Theory." Thesis, Harvard University, 2013. http://dissertations.umi.com/gsas.harvard:10929.
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In recent years, general relativistic magnetohydrodynamic (GRMHD) simulations have produced more realistic models for black hole disks and jets. However the complexity of the simulations has created a disconnect between simulations and theory: it is often unclear whether the simulated physics is correctly described by standard, semi-analytic disk and jet models. In this thesis, we describe new GRMHD simulations of black hole disks and jets. We compare the simulations to standard disk and jet models. We show that GRMHD thin disks are well described by the Novikov-Thorne model, and GRMHD jets are well described by the Blandford-Znajek model. Then, guided by the simulations, we develop two extensions of the standard thin disk model: a radially varying \(\alpha(r)\) viscosity prescription and an analytical disk solution with nonzero stress at the innermost stable circular orbit.<br>Astronomy
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Noguchi, Koichi. "Structure formation through magnetohydrodynamical instabilities in protoplanetary and accretion disks /." Full text (PDF) from UMI/Dissertation Abstracts International, 2001. http://wwwlib.umi.com/cr/utexas/fullcit?p3008407.
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Vanon, Riccardo. "Zonal flows in accretion discs and their role in gravito-turbulence." Thesis, University of Cambridge, 2017. https://www.repository.cam.ac.uk/handle/1810/267991.
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This thesis focuses on the evolution of zonal flows in self-gravitating accretion discs and their resulting effect on disc stability; it also studies the process of disc gravito-turbulence, with particular emphasis given to the way the turbulent state is able to extract energy from the background flow and sustain itself by means of a feedback. Chapters 1 and 2 provide an overview of systems involving accretion discs and a theoretical introduction to the theory of accretion discs, along with potential methods of angular momentum transport to explain the observed accretion rates. To address the issue of the gravito-turbulence self-sustenance, a compressible non-linear spectral code (dubbed CASPER) was developed from scratch in C; its equations and specifications are laid out in Chapter 3. In Chapter 4 an ideal (no viscosities or cooling) linear stability analysis to non-axisymmetric perturbations is carried out when a zonal flow is present in the flow. This yields two instabilities: a Kelvin-Helmholtz instability (active only if the zonal flow wavelength is sufficiently small) and one driven by self-gravity. A stability analysis of the zonal flow itself is carried out in Chapter 5 by means of an axisymmetric linear analysis, using non-ideal conditions. This considers instability due to both density wave modes (which give rise to overstability) and slow modes (which result in thermal or viscous instability) and, thanks a different perturbation wavelength regime, represents an extension to the classical theory of thermal and viscous instabilities. The slow mode instability is found to be aided by high Prandtl numbers and adiabatic index γ values, while quenched by fast cooling. The overstability is likewise stabilised by fast cooling, and occurs in a non-self-gravitational regime only if γ ≲ 1.305. Lastly, Chapter 6 illustrates the results of the non-linear simulations carried out using the CASPER code. Here the system settles into a state of gravito-turbulence, which appears to be linked to a spontaneously-developing zonal flow. Results show that this zonal flow is driven by the slow mode instability discussed in Chapter 5, and that the presence of zonal flows triggers a non-axisymmetric instability, as seen in Chapter 4. The role of the latter is to constrain the zonal flow amplitude, with the resulting zonal flow disruption providing a generation of shearing waves which permits the self-sustenance of the turbulent state.
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Stone, Jordan M., D. P. Marrone, C. D. Dowell, B. Schulz, C. O. Heinke, and F. Yusef-Zadeh. "FAR INFRARED VARIABILITY OF SAGITTARIUS A*: 25.5 hr OF MONITORING WITH HERSCHEL." IOP PUBLISHING LTD, 2016. http://hdl.handle.net/10150/621398.
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Variable emission from Sgr A*, the luminous counterpart to the super-massive black hole at the center of our Galaxy, arises from the innermost portions of the accretion flow. Better characterization of the variability is important for constraining models of the low-luminosity accretion mode powering Sgr A*, and could further our ability to use variable emission as a probe of the strong gravitational potential in the vicinity of the 4 x 10(6) M-circle dot black hole. We use the Herschel Spectral and Photometric Imaging Receiver (SPIRE) to monitor Sgr. A* at wavelengths that are difficult or impossible to observe from the ground. We find highly significant variations at 0.25, 0.35, and 0.5 mm, with temporal structure that is highly correlated across these wavelengths. While the variations correspond to < 1% changes in the total intensity in the Herschel beam containing Sgr. A*, comparison to independent, simultaneous observations at 0.85 mm strongly supports the reality of the variations. The lowest point in the light curves, similar to 0.5 Jy below the time-averaged flux density, places a lower bound on the emission of Sgr. A* at 0.25 mm, the first such constraint on the THz portion of the spectral energy distribution. The variability on few hour timescales in the SPIRE light curves is similar to that seen in historical 1.3 mm data, where the longest time series is available, but the distribution of variations in the sub-mm do not show a tail of large-amplitude variations seen at 1.3 mm. Simultaneous X-ray photometry from XMM-Newton shows no significant variation within our observing period, which may explain the lack of very large submillimeter variations in our data if X-ray and submillimeter flares are correlated.
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Memola, Elisabetta. "Magnetic jets from accretion disks field structure and X-ray emission /." Phd thesis, [S.l.] : [s.n.], 2002. http://pub.ub.uni-potsdam.de/2002/0014/memola.pdf.
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Takeuchi, Mitsuru. "THE SHORT-AND LONG-TERM VARIATIONS OF BLACK HOLE ACCRETION DISKS." 京都大学 (Kyoto University), 1999. http://hdl.handle.net/2433/181953.
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Shidatsu, Megumi. "Observational Studies of Accretion Disks in Black Hole X-ray Binaries." 京都大学 (Kyoto University), 2015. http://hdl.handle.net/2433/199106.
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Hertfelder, Marius [Verfasser], and Wilhelm [Akademischer Betreuer] Kley. "The Boundary Layer of Accretion Disks / Marius Hertfelder ; Betreuer: Wilhelm Kley." Tübingen : Universitätsbibliothek Tübingen, 2017. http://d-nb.info/1165578727/34.
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Raymond, Sean Neylon. "Late-stage accretion and habitability of terrestrial planets /." Thesis, Connect to this title online; UW restricted, 2005. http://hdl.handle.net/1773/5438.
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Meng, Huan Y. A., Peter Plavchan, George H. Rieke, et al. "PHOTO-REVERBERATION MAPPING OF A PROTOPLANETARY ACCRETION DISK AROUND A T TAURI STAR." IOP PUBLISHING LTD, 2016. http://hdl.handle.net/10150/621264.
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Theoretical models and spectroscopic observations of newborn stars suggest that protoplantary disks have an inner "wall" at a distance set by the disk interaction with the star. Around T Tauri stars, the size of this disk hole is expected to be on a 0.1 au scale that is unresolved by current adaptive optics imaging, though some model-dependent constraints have been obtained by near-infrared interferometry. Here we report the first measurement of the inner disk wall around a solar-mass young stellar object, YLW 16B in the rho Ophiuchi star-forming region, by detecting the light-travel time of the variable radiation from the stellar surface to the disk. Consistent time lags were detected on two nights, when the time series in H (1.6 mu m) and K (2.2 mu m) bands were synchronized while the 4.5 mu m emission lagged by 74.5 +/- 3.2 s. Considering the nearly edge-on geometry of the disk, the inner rim should be 0.084 au from the protostar on average, with an error of order 0.01 au. This size is likely larger than the range of magnetospheric truncations and consistent with an optically and geometrically thick disk front at the dust sublimation radius at similar to 1500 K. The widths of the cross-correlation functions between the data in different wavebands place possible new constraints on the geometry of the disk.
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Fang, Min, Jinyoung Serena Kim, Ilaria Pascucci, et al. "NGC 1980 Is Not a Foreground Population of Orion: Spectroscopic Survey of Young Stars with Low Extinction in Orion A." IOP PUBLISHING LTD, 2017. http://hdl.handle.net/10150/623807.
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We perform a spectroscopic survey of the foreground population in Orion. A with MMT/Hectospec. We use these data, along with archival spectroscopic data and photometric data, to derive spectral types, extinction values, and masses for 691 stars. Using the Spitzer Space Telescope data, we characterize the disk properties of these sources. We identify 37 new transition disk (TD) objects, 1 globally depleted disk candidate, and 7 probable young debris disks. We discover an object with a mass of. less than 0.018-0.030 M-circle dot, which harbors a flaring disk. Using the Ha emission line, we characterize the accretion activity of the sources with disks, and confirm that the. fraction of accreting TDs is lower than that of optically thick disks (46% +/- 7% versus 73% +/- 9%, respectively). Using kinematic data from the Sloan Digital Sky Survey and APOGEE INfrared Spectroscopy of the Young Nebulous Clusters program (IN-SYNC), we confirm that the foreground population shows similar kinematics to their local molecular clouds and other young stars in the same regions. Using the isochronal ages, we find that the foreground population has a median age of. around 1-2 Myr, which is similar to that of other young stars in Orion. A. Therefore, our results argue against the presence of a large and old foreground cluster in front of Orion. A.
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Manara, C. F., L. Testi, G. J. Herczeg, et al. "X-shooter study of accretion in Chamaeleon I." EDP SCIENCES S A, 2017. http://hdl.handle.net/10150/625828.
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The dependence of the mass accretion rate on the stellar properties is a key constraint for star formation and disk evolution studies. Here we present a study of a sample of stars in the Chamaeleon I star-forming region carried out using spectra taken with the ESO VLT/X-shooter spectrograph. The sample is nearly complete down to stellar masses (M-star) similar to 0.1 M-circle dot for the young stars still harboring a disk in this region. We derive the stellar and accretion parameters using a self-consistent method to fit the broadband flux-calibrated medium resolution spectrum. The correlation between accretion luminosity to stellar luminosity, and of mass accretion rate to stellar mass in the logarithmic plane yields slopes of 1.9 +/- 0.1 and 2.3 +/- 0.3, respectively. These slopes and the accretion rates are consistent with previous results in various star-forming regions and with different theoretical frameworks. However, we find that a broken power-law fit, with a steeper slope for stellar luminosity lower than similar to 0.45 L-circle dot and for stellar masses lower than similar to 0.3 M-circle dot is slightly preferred according to different statistical tests, but the single power-law model is not excluded. The steeper relation for lower mass stars can be interpreted as a faster evolution in the past for accretion in disks around these objects, or as different accretion regimes in different stellar mass ranges. Finally, we find two regions on the mass accretion versus stellar mass plane that are empty of objects: one region at high mass accretion rates and low stellar masses, which is related to the steeper dependence of the two parameters we derived. The second region is located just above the observational limits imposed by chromospheric emission, at M-star similar to 0.3-0.4 M-circle dot. These are typical masses where photoevaporation is known to be effective. The mass accretion rates of this region are similar to 10(-10) M-circle dot/yr, which is compatible with the value expected for photoevaporation to rapidly dissipate the inner disk.
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Lyra, Wladimir. "Turbulence-Assisted Planetary Growth : Hydrodynamical Simulations of Accretion Disks and Planet Formation." Doctoral thesis, Uppsala universitet, Institutionen för fysik och astronomi, 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-9537.
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The current paradigm in planet formation theory is developed around a hierarquical growth of solid bodies, from interstellar dust grains to rocky planetary cores. A particularly difficult phase in the process is the growth from meter-size boulders to planetary embryos of the size of our Moon or Mars. Objects of this size are expected to drift extremely rapid in a protoplanetary disk, so that they would generally fall into the central star well before larger bodies can form. In this thesis, we used numerical simulations to find a physical mechanism that may retain solids in some parts of protoplanetary disks long enough to allow for the formation of planetary embryos. We found that such accumulation can happen at the borders of so-called dead zones. These dead zones would be regions where the coupling to the ambient magnetic field is weaker and the turbulence is less strong, or maybe even absent in some cases. We show by hydrodynamical simulations that material accumulating between the turbulent active and dead regions would be trapped into vortices to effectively form planetary embryos of Moon to Mars mass. We also show that in disks that already formed a giant planet, solid matter accumulates on the edges of the gap the planet carves, as well as at the stable Lagrangian points. The concentration is strong enough for the solids to clump together and form smaller, rocky planets like Earth. Outside our solar system, some gas giant planets have been detected in the habitable zone of their stars. Their wakes may harbour rocky, Earth-size worlds.
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Hillwig, Todd C. "Analysis of the radial profile emissivity of accretion disks in cataclysmic variables." Virtual Press, 1995. http://liblink.bsu.edu/uhtbin/catkey/941346.
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Zhu, Yucong. "The Bright Side of Black Holes: Radiation From Black Hole Accretion Disks." Thesis, Harvard University, 2015. http://nrs.harvard.edu/urn-3:HUL.InstRepos:17463143.
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An understanding of radiation is paramount for connecting observations of accretion disks with the theory of black holes. In this thesis, we explore via radiative transfer postprocessing calculations the observational signatures of black holes. We investigate disk spectra by analyzing general relativistic magnetohydrodynamic (GRMHD) simulations of accretion disks. For the most part there are no surprises -- the resulting GRMHD spectrum is very close to the analytic Novikov & Thorne (1973) prediction from decades past, except for a small modification in the case of spinning black holes, which exhibit a high-energy power-law tail that is sourced by hot Comptonized gas from within the plunging region of the accretion flow. These conclusions are borne out by both 1D and 3D radiative transfer calculations of the disk. Significant effort was spent in developing from scratch the 3D radiative code that we used for the analysis. The code is named HERO (Hybrid Evaluator for Radiative Objects) and it is a new general purpose grid-based 3D general relativistic radiative solver.<br>Astronomy
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Kawanaka, Norita. "Theory of X-ray emissions from black hole accretion disks with coronae." 京都大学 (Kyoto University), 2008. http://hdl.handle.net/2433/136870.
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Heinemann, Tobias. "The dynamics of spiral density waves in turbulent accretion discs." Thesis, University of Cambridge, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.608944.
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Kimura, Mariko. "Observational and theoretical studies on dwarf-nova outbursts." Kyoto University, 2020. http://hdl.handle.net/2433/253084.
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Medeiros, Lia, Chi-kwan Chan, Feryal Özel, et al. "Variability in GRMHD Simulations of Sgr A*: Implications for EHT Closure Phase Observations." IOP PUBLISHING LTD, 2017. http://hdl.handle.net/10150/625164.
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Closure phases along different baseline triangles carry a large amount of information regarding the structures of the images of black holes in interferometric observations with the Event Horizon Telescope. We use long time span, high cadence, GRMHD+radiative transfer models of Sgr A* to investigate the expected variability of closure phases in such observations. We find that, in general, closure phases along small baseline triangles show little variability, except in the cases when one of the triangle vertices crosses one of the small regions of low visibility amplitude. The closure phase variability increases with the size of the baseline triangle, as larger baselines probe the small-scale structures of the images, which are highly variable. On average, the funnel-dominated MAD models show less closure phase variability than the disk-dominated SANE models, even in the large baseline triangles, because the images from the latter are more sensitive to the turbulence in the accretion flow. Our results suggest that image reconstruction techniques need to explicitly take into account the closure phase variability, especially if the quality and quantity of data allow for a detailed characterization of the nature of variability. This also implies that, if image reconstruction techniques that rely on the assumption of a static image are utilized, regions of the u-v space that show a high level of variability will need to be identified and excised.
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Pacholczyk, A. G., T. F. Stepinski, and W. R. Stoeger. "Active Galactic Nuclei. III. Accretion Flow in an Externally Supplied Cluster of Black Holes." Steward Observatory, The University of Arizona (Tucson, Arizona), 1988. http://hdl.handle.net/10150/623896.
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This third paper in the series modeling QSOs and AGN as clusters of accreting black
holes studies the accretion flow within an externally supplied cluster. Significant radiation
will be emitted by the cluster core, but the black holes in the outer halo, where the flow is
considered spherically symmetric, will not contribute much to the overall luminosity of the
source because of their large velocities relative to the infalling gas, and therefore their small
accretion radii. As a result the scenario discussed in Paper I will refer to the cluster cores,
rather than to entire clusters. This will steepen the high frequency region of the spectrum
unless inverse Compton scattering is effective. In many cases accretion flow in the central
part of the cluster will be optically thick to electron scattering resulting in a spectrum
featuring optically thick radiative component in addition to power -law regimes. The fitting
of these spectra to QSO and AGN observations is discussed, and application to 3C 273 is
worked out as an example.
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Kigure, Hiromitsu. "The study of astrophysical jets and accretion disks by three-dimensional MHD simulations." 京都大学 (Kyoto University), 2006. http://hdl.handle.net/2433/144185.
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Kyoto University (京都大学)<br>0048<br>新制・課程博士<br>博士(理学)<br>甲第12102号<br>理博第2996号<br>新制||理||1446(附属図書館)<br>23938<br>UT51-2006-J97<br>京都大学大学院理学研究科物理学・宇宙物理学専攻<br>(主査)教授 柴田 一成, 助教授 上田 佳宏, 教授 長田 哲也<br>学位規則第4条第1項該当
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Pariev, Vladimir Ivanovich. "Magnetic fields: Their origin and manifestation in accretion disks around supermassive black holes." Diss., The University of Arizona, 2001. http://hdl.handle.net/10150/279820.
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The magnetic field dynamo in the inner part of accretion disks around supermassive black holes in AGNs may be an important mechanism for the generation of magnetic fields in galaxies and in extragalactic space. We consider dynamo with the necessary helicity generation produced by star-disk collisions. Gas heated by a star passing through the disk is buoyant and form rising and expanding plume of plasma. Due to Coriolis forces the flow produced by plumes have coherent helicity. This helicity is the source of alpha effect in the alpha-O dynamo in differentially rotating accretion disk. We apply the mean field dynamo theory to the ensemble of plumes produced by star-disk collisions. We demonstrate the existence of the dynamo and evaluate the growth rate of magnetic field. The estimate of the nonlinear saturated state of the dynamo gives the magnetic field exceeding equipartition with the thermal energy in the accretion disk. Thus, star-disk collision dynamo can be important in generating dynamically significant magnetic fields, which could alter the disk structure and be the source of the energy flow in extragalactic jets. We present results of numerical simulations of the kinematic dynamo produced by star-disk collisions. It was found that for about one star-disk collision per period of rotation of the inner edge of an accretion disk, the typical value of the threshold magnetic Reynolds number is of the order of 100. The generated mean magnetic field has predominantly even parity. We also present theoretical consideration of magnetic dynamo in New Mexico dynamo experiment, which is currently under construction. The experiment utilizes Couette flow and driven jets of liquid sodium to simulate astrophysical alpha-O dynamos in the laboratory. We perform numerical simulations with ideally conducting boundary and evaluate the changes, which vacuum boundary conditions introduce in our numerical results. We also develop the theory of the MHD Ekman boundary layer in differentially rotating conducting fluid. The Ekman layer is formed at the end plates in the experiment. We show that the Ekman layer does not influence the generation of the large scale magnetic field in the experiment.
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Lufkin, Graeme. "Simulations of giant planet migration in gaseous circumstellar disks /." Thesis, Connect to this title online; UW restricted, 2004. http://hdl.handle.net/1773/9698.
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Morgan, Christopher Warren. "Quasar Structure from Microlensing in Gravitationally Lensed Quasars." The Ohio State University, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=osu1205848751.
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Starkey, D., Keith Horne, M. M. Fausnaugh, et al. "SPACE TELESCOPE AND OPTICAL REVERBERATION MAPPING PROJECT.VI. REVERBERATING DISK MODELS FOR NGC 5548." IOP PUBLISHING LTD, 2017. http://hdl.handle.net/10150/622875.
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We conduct a multiwavelength continuum variability study of the Seyfert 1 galaxy NGC 5548 to investigate the temperature structure of its accretion disk. The 19 overlapping continuum light curves (1158 angstrom to 9157 angstrom) combine simultaneous Hubble Space Telescope, Swift, and ground-based observations over a 180 day period from 2014 January to July. Light-curve variability is interpreted as the reverberation response of the accretion disk to irradiation by a central time-varying point source. Our model yields the disk inclination i = 36 degrees +/- 10 degrees, temperature T-1= (44 +/- 6) x 10(3) K at 1 light day from the black hole, and a temperature-radius slope (T proportional to r(-alpha)) of alpha = 0.99 +/- 0.03. We also infer the driving light curve and find that it correlates poorly with both the hard and soft X-ray light curves, suggesting that the X-rays alone may not drive the ultraviolet and optical variability over the observing period. We also decompose the light curves into bright, faint, and mean accretion-disk spectra. These spectra lie below that expected for a standard blackbody accretion disk accreting at L/L-Edd = 0.1.