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1
Grundstrom, Erika Dawn. "Hot Stars with Disks." Digital Archive @ GSU, 2007. http://digitalarchive.gsu.edu/phy_astr_diss/19.
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The evolutionary paths of the massive O and B type stars are often defined by angular momentum transformations that involve circumstellar gas disks. This circumstellar gas is revealed in several kinds of observations, and here I describe a series of investigations of the hydrogen line emission from such disk using detailed studies of five massive binaries and a survey of 128 Be stars. By examining three sets of spectra of the active mass-transfer binary system RY Scuti, I determined masses of 7.1 +/- 1.2 M_sun for the bright supergiant and 30.0 +/- 2.1 M_sun for the massive companion that is hidden by an accretion torus. I also present a cartoon model of the complex mass flows in the system. Using optical spectroscopy and X-ray flux data, I investigated the mass transfer processes in four massive X-ray binaries (a massive B star with mass flowing onto a compact, neutron star companion). The B-supergiant system LS I +65 010 transfers mass via stellar winds. I find the X-ray flux modulates with the orbital period. In the other three X-ray binary systems (LS I +61 303, HDE 245770, and X Persei), an outflowing circumstellar disk is responsible for the mass transfer, and in all three systems, the disk appears to be truncated by gravitational interactions with the compact companion. The disk in the microquasar system LS I +61 303 is limited in radius by the periastron separation and an increase in both H-alpha equivalent width and X-ray flux following periastron may be due to a density wave in the disk induced by tidal forces. Observations of HDE 245770 document what appears to be the regeneration of a circumstellar disk. The disk of X Persei appears to have grown to near record proportions and the X-ray flux has dramatically increased. Tidal interaction may generate a spiral density wave in the disk and cause an increase in H-alpha equivalent width and mass transfer to the compact companion. During the course of the analysis of the X-ray binaries, I developed numerical models for estimating the size of the Be star disks using just the H-alpha equivalent width. Finally, I present the results of a three year spectroscopic survey of both the H-alpha and H-gamma regions of 128 Be stars. I find that the median fractional variation in the equivalent width of the disk emission lines is 15% over a two year period. I also find that two-thirds of the sample displays evidence of Fe II emission or absorption resulting from surrounding circumstellar material. Many candidates for non-radial pulsation and binary systems are also found. Spectra and notes for all of the sample stars are presented in an appendix.
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Muranushi, Takayuki. "Lightning in Protoplanetary Disks." 京都大学 (Kyoto University), 2013. http://hdl.handle.net/2433/180374.
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Cavallius, Maria. "Gas in Debris Disks." Licentiate thesis, Stockholms universitet, Institutionen för astronomi, 2020. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-186017.
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Chiang, Eugene Goldreich Peter Martin. "Circumstellar and circumplanetary disks /." Diss., Pasadena, Calif. : California Institute of Technology, 2000. http://resolver.caltech.edu/CaltechETD:etd-09172008-151500.
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Pohlen, Michael. "The radial structure of galactic stellar disks surface photometric study on disk galaxies /." [S.l.] : [s.n.], 2001. http://deposit.ddb.de/cgi-bin/dokserv?idn=964128535.
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Lobo, Gomes Aiara [Verfasser], and Hubert [Akademischer Betreuer] Klahr. "Planet-disk interactions in non-isothermal disks / Aiara Lobo Gomes ; Betreuer: Hubert Klahr." Heidelberg : Universitätsbibliothek Heidelberg, 2015. http://d-nb.info/1180502523/34.
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Li, Guijun, and 李桂君. "Development of recording technology with FePt recording media and magnetic tunnel junction sensors with conetic alloy." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2013. http://hub.hku.hk/bib/B50899776.
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With highly demanding requirement in current emerging cloud storage and personal computers, hard disk drive recording with high stability and high volume has attached much attention in industry and academy. Recording media and recording head feasible for future high-density recording are both crucial to utilize magnetic recording with 1T bit/in2 recording density.
Recoding media with FePt for high density and high stability was investigated in this thesis using FePt polymers with imprinting methods and FePt thin films with ion-beam bombardment technologies. The FePt polymers can be patterned using imprint at micro-and nano-scales. The micro-and nano-patterns could be retained on substrates after sintering at high temperatures. The high magnetic coercivity was proved with line and dot patterns at different scales.
Recording heads with Al2O3based magnetic tunneling junction sensors were also studied in thesis. The magnetic tunneling junction sensors were proved to work stable at different temperatures varying from -30oC to 100oC. The long time running test up to 100 hours also proved the stability of the magnetic tunneling junction sensors working in extreme temperatures.
Withstate-of-art patterning and depositing technologies, new ideas about using FePt polymer to work as magnetic recording media and using ion beam bombardments to tune the FePt magnetic properties were verified. The feasibility of using Al2O3 based magnetic tunneling junction sensors as recording head was also discussed.published_or_final_version
Electrical and Electronic Engineering
Doctoral
Doctor of Philosophy
8
Bastos, Martins Ledo Hugo Ricardo. "A census of nuclear stellar disks in early-type galaxies." Thesis, University of Hertfordshire, 2016. http://hdl.handle.net/2299/17184.
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In this thesis we explored the use of nuclear stellar discs as tracers of the merging history of early-type galaxies. These small structures, just a few tens to a few hundreds of parsecs across, are a common but poorly studied feature of early-type galaxies. They are formed during or shortly after merging events due to the infall of gas, which settles in a disc and leads to the formation of new stars. Initial simulations showed that they should not survive a following major merger and could, therefore, be used to trace the epoch when their host galaxies experienced their last major merger event. We produced the first census of nuclear discs and established that their incidence is 20%, fairly independent of the host-galaxy mass or galactic environment. Furthermore, we have more than doubled the sample of nuclear discs with known photometric properties, finding that they give a hint of possessing different characteristics from those of large, galactic discs. Using these nuclear discs as clocks for the assembly history of galaxies requires dating their stellar populations. By combining the use of integral-field spectroscopy with the a priori knowledge of the relative bulge- and disc-light contribution to the observed spectra, as determined by a photometric disc-bulge decomposition, we have shown that it is possible to reduce the degeneracies that affect the study of two superimposed populations and thus that the age of stellar discs can be measured more precisely. To illustrate our method, we present VLT-VIMOS data for NGC 4458, a low-mass slowly rotating early-type galaxy with a disc that we found to be at least 5-6 Gyr old. The presence of such an old central disc in such a small, slowly-rotating and, mostly likely, round galaxy is particularly puzzling and presents a challenge to existing models. Disc fragility is central to our studies and we have expanded the limited initial simulations to study it in more detail. By means of N-body simulations, we have reproduced the final stages of a galaxy encounter by exposing a nuclear disc rotating in the gravitational potential of its host bulge and central supermassive black hole to the impact of a secondary massive black hole. We explored not only major mergers (1:1 mass ratio), but also large minor mergers (1:5 and 1:10), across a variety of collision angles, and assessed the survival of the disc, as perceived by current observational limits, both for photometry and spectroscopy. As expected, the discs do not survive a major merger whereas it is in general possible to detect their presence after a 1:5 or 1:10 encounter, in particular when looking at kinematic signatures with spectroscopy. This thesis has demonstrated that nuclear discs constitute both a common and accurate tool for constraining the assembling history of nearby early-type galaxies. The advent of more sensitive integral-field spectrographs, such as MUSE, will make measuring the stellar age of nuclear discs not only more precise, but also more economical in terms of telescope time. This will allow embarking on a more systematic age dating campaign for nuclear stellar discs across a wider range of type, mass and galactic environments for their host galaxies. Combining such a census with a larger set of numerical simulations aimed at calibrating better the range of merger event that would erase any photometric or kinematic signature of a nuclear disc, should finally allow us to put firm constraints on the merging history of early-type galaxies.
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Dånmark, Anders. "Volume measurement of wood disks." Thesis, Uppsala universitet, Bildanalys och människa-datorinteraktion, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-212199.
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At the Department of Forest Products at Swedish University of Agricultural Sciences different metrics for wood are used. The volume of wood disks' is measured using archimedes principle.There are concerns of how accurate this measurement is and a different measuringsystem is wanted. This thesis has investigated the possibility of measuring the disks' volumes with imageanalysis. The recovery error should be less than 1% of the actual volume. In general, there are two methods for recovering an object using imageanalysis, active and passive methods. Compairing active and passive methods, active methods usually require simple algorithms but more expensive equipment compared to passive methods. Different methods for measuring objects' volumes have been evaluated and the choosen method was ``shape from silhouette''. Shape from silhouette is a passive method, only using the silhouette of anobject from multiple views to recover the objects volume. Passive methods have one drawback, they can only recover the visual hull of an object and the wood disks can be slightly concave. Due to the questionable accuracy of the current measurement method it was still deemed as possible to achieve at least equal performance. When the volume measuring algorithm was developed it was first tested in two simulations using on a sphere to determine its performance with different voxel sizes and different number of images. The algorithm performed well and an error of less than 1 % was achieved with a sphere. A third simulation was performed using a simulated wood disk, which is a much more complex object, and 5 % accuracy was achieved. Finally, an experiment on real images was performed. This experiment did, however, fail due to the low quality imaging setup. The conclusion of this thesis is that itis not possible to achieve less than 1 % accuracy of the recovered volume using the shape from silhouette technique.
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Yoachim, Peter. "Thick disks in external galaxies /." Thesis, Connect to this title online; UW restricted, 2007. http://hdl.handle.net/1773/5444.
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Pichon, Christophe. "Dynamics of self-gravitating disks." Thesis, University of Cambridge, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.319818.
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BAPTISTA, PAULA MONTEIRO. "TILINGS OF DISKS WITH HOLES." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2006. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=9177@1.
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COORDENAÇÃO DE APERFEIÇOAMENTO DO PESSOAL DE ENSINO SUPERIORCoberturas de um disco quadriculado com buracos D são contados de acordo com volume (na variável formal q) e fluxo (em p1, p2, ..., pN). Consideramos propriedades algébricas dos resultados gerados pela função F (p1, p2, ..., pN, q). Para números fixos p2, ..., pN, q > 0 o polinômio f(p1) = F(p1, p2, ..., pN, q) tem todas as raízes reais (e negativas).
Tilings of a quadriculated disk with holes D are counted according to vol- ume (in the formal variabel q) and flux (in p1; p2;... pN). We consider algebraic properties of the resulting generating function D (p1; p2; ...; pk; q). For p1; p2; ...; bpi; ...; pn; q > 0 the polynomial f(pi) = D (p1; p2; ... ; pi; ...; pn; q) has all roots real numbers (and negative).
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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.Includes bibliographical references (leaves 61-64).
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.
by Antonia Stefanova Savcheva.
S.B.
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Lemke, Bastian. "Freiheitsgrade beim Einsatz Verteilter Disks." [S.l. : s.n.], 2008. http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-58078.
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Li, Yinan. "Tree indexing on flash disks /." View abstract or full-text, 2009. http://library.ust.hk/cgi/db/thesis.pl?CSED%202009%20LI.
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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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Roberge, Aki. "Ultraviolet spectroscopy of circumstellar disks." Available to US Hopkins community, 2002. http://wwwlib.umi.com/dissertations/dlnow/3068203.
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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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Józsa, Gyula István Géza. "Grand design warps in galactic disks." [S.l.] : [s.n.], 2005. http://deposit.ddb.de/cgi-bin/dokserv?idn=979515076.
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Schütz, Oliver. "High-resolution studies of protoplanetary disks." [S.l. : s.n.], 2005. http://deposit.ddb.de/cgi-bin/dokserv?idn=974449792.
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Tazaki, Ryo. "Multiwavelength polarimetric properties of protoplanetary disks." 京都大学 (Kyoto University), 2017. http://hdl.handle.net/2433/225407.
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Dzanovic, Dajana. "Evolution of galactic disks and spheroids." Thesis, Durham University, 2004. http://etheses.dur.ac.uk/3062/.
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In this Thesis the structural properties of galaxies using quantitative 2-dimensional bulge to disk decompositions are analysed across a range of environments and red- shifts. The study of morphological properties of galaxies is undergoing a significant progress since large galaxy surveys such as the Sloan Digital Sky Survey are producing a vast amounts of data enabling properties such as galaxy structure to be analysed on a statistical level. The wide-field imaging available today makes the studies of the nearby and high redshift environments comparable on a similar physical scale and depth. The advances in the theoretical studies and understanding the physics behind processes that govern galaxy formation and evolution make studies such as this one important since they provide basic estimates of galaxy structure that can be used to constrain the current theoretical models. By comparing the structural properties of bright cluster galaxies between z ~ 0.5 and present we are able to confirm that galaxies have undergone a high degree of evolution although the exact processes that govern this evolution still remain unresolved. In order to account for the evolution of z ~ 0.5 cluster spirals into present-day SOs a significant number of spiral galaxies (factor of 3) must have had their disks faded to make them drop .out of our magnitude limited sample. However, the structural properties between the faded-disk spirals and present day SOs are not consistent. One possibility is that the progenitor galaxies might have been accreted recently from the field and that these have also been previously pre-processed. The study of the structural properties of field galaxies has resulted in a potential evidence that galaxies in the field environment have an additional structural component or structural property that cannot be accounted for by the current 2- dimensional decomposition methods. This study has found that disks contribute about 3 times as much light as bulges to the total luminosity density of the local universe. Tighter constraints in terms of bulge and disk mass densities would help to constrain the current theoretical predictions.
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Creasey, Peter Edward. "Simulating supernova feedback in galaxy disks." Thesis, Durham University, 2012. http://etheses.dur.ac.uk/6349/.
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In this thesis I examine supernova feedback in hydrodynamical simulations of galaxy disks. Understanding this process entails the numerical evaluation of cooling in radiative shocks, and we present a set of simulations using two widely used numerical schemes: smoothed particle hydro- dynamics and adaptive mesh refinement. We obtain a similarity solution for a shock-tube problem in the presence of radiative cooling, and test how well the solution is reproduced. We interpret our findings in terms of a resolution criterion, and apply it to realistic simulations of cosmological accretion shocks onto galaxy halos, cold accretion and thermal feedback from supernovae or active galactic nuclei. To avoid numerical overcooling of accretion shocks onto halos that should develop a hot corona requires a particle or cell mass resolution of 10^6 M⊙, which is within reach of current state-of-the-art simulations. At this mass resolution, thermal feedback in the interstellar medium of a galaxy requires temperatures of supernova or AGN driven bubbles to be in excess of 10^7 K at densities of n_H = 1.0 cm−3, in order to avoid spurious suppression of the feedback by numerical overcooling. In order to improve sub-grid models of feedback we perform a series of numerical experiments to investigate how supernova explosions shape the interstellar medium in a disk galaxy and power a galactic wind. We model a simplified ISM, including gravity, hydrodynamics, radiative cooling above 10^4 K, and star formation that reproduces the Kennicutt-Schmidt relation. By simulating a small patch of the ISM in a tall box perpendicular to the disk, we obtain sub-parsec resolution allowing us to resolve individual supernova events. We run a large grid of simulations in which we vary gas surface density, gas fraction, and star formation rate in order to investigate the dependencies of the mass loading, β ≡ dot M_wind / dot M_star. In the cases with the most effective outflows we observe a β of 4, however in other cases we find β<<1. We find that outflows are more efficient in disks with lower surface densities or gas fractions. A simple model in which the warm clouds are the barriers that limit the expansion of the blast wave reproduces the scaling of outflow properties with disk parameters at high star formation rates. We extend the scaling relations derived from an ISM patch to infer an effective mass loading for a galaxy with an exponential disk, finding that the mass loading depends on circular velocity as β ∝ V −α with α ≈ 2.5 for a model which fits the Tully-Fisher relation. Such a scaling is often assumed in phenomenological models of galactic winds in order to reproduce the flat faint end slope of the mass function. Our normalisation is in approximate agreement with observed estimates of the mass loading for the Milky Way. Finally, we extend these simulations to follow the ejecta produced by these SNe, allowing us to track the distribution of metals as they are mixed into the different phases of the ISM and swept out into a galactic wind. Such calculations are important both directly in predicting the enrichment of the intergalactic medium, but also with the sister problem of understanding the enrichment of the host galaxies and the mass-metallicity relation, owing to the unique role that supernovae are believed to play both as the sources of galactic winds and the sources of galactic metals. We study the dependence of the amount of metals released per unit of star formation, β_Z ≡ dot M_z / dot M_star, and the fraction of metals released, β_Z / y. We include thermal and momentum feedback from massive stars and find these make a less significant contribution to the galactic wind than SNe. We build up a model of galactic chemical evolution and we demonstrate that these models are compatible with the metallicity distributions of faint stars and compare to closed box models of chemical evolution. We infer metal retention fractions from the observed data, although this may be complicated by recycling in the galaxy halos. We compare these rates to the fraction of metals ejected in the simulations and demonstrate approximate agreement, although the simulation data has considerable scatter, primarily due to the stochastic nature of the feedback in the limited volumes of the simulations.
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Walsh, Catherine. "The chemical structure of protoplanetary disks." Thesis, Queen's University Belfast, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.534583.
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Harris, Robert Jason. "Protoplanetary Disks in Multiple Star Systems." Thesis, Harvard University, 2013. http://dissertations.umi.com/gsas.harvard:11194.
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Most stars are born in multiple systems, so the presence of a stellar companion may commonly influence planet formation. Theory indicates that companions may inhibit planet formation in two ways. First, dynamical interactions can tidally truncate circumstellar disks. Truncation reduces disk lifetimes and masses, leaving less time and material for planet formation. Second, these interactions might reduce grain-coagulation efficiency, slowing planet formation in its earliest stages.Astronomy
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Chan, Siew Ngan. "Analysis of disks with friction loading." Thesis, University of Liverpool, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.240397.
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Fitzgerald, Alan. "DATA SECURITY IN SOLID STATE DISKS." International Foundation for Telemetering, 2005. http://hdl.handle.net/10150/604786.
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ITC/USA 2005 Conference Proceedings / The Forty-First Annual International Telemetering Conference and Technical Exhibition / October 24-27, 2005 / Riviera Hotel & Convention Center, Las Vegas, NevadaSolid state disk characteristics make them ideal for data collection in both harsh environments and secure telemetry application. In comparison to their magnetic counterparts, solid state disks are faster, more reliable, extremely durable and, with changing economies and geometries, more affordable and available in higher capacities than ever before. This paper will discuss solid state disk storage, access controls, and data elimination in relation to various telemetry scenarios. The reader will be introduced to the operational considerations of solid state disk data security and the underlying technical concepts of how these are implemented.
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Stolker, T., C. Dominik, M. Min, A. Garufi, G. D. Mulders, and H. Avenhaus. "Scattered light mapping of protoplanetary disks." EDP SCIENCES S A, 2016. http://hdl.handle.net/10150/622692.
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Context. High-contrast scattered light observations have revealed the surface morphology of several dozen protoplanetary disks at optical and near-infrared wavelengths. Inclined disks offer the opportunity to measure part of the phase function of the dust grains that reside in the disk surface which is essential for our understanding of protoplanetary dust properties and the early stages of planet formation. Aims. We aim to construct a method which takes into account how the flaring shape of the scattering surface of an optically thick protoplanetary disk projects onto the image plane of the observer. This allows us to map physical quantities (e.g., scattering radius and scattering angle) onto scattered light images and retrieve stellar irradiation corrected images (r(2)-scaled) and dust phase functions. Methods. The scattered light mapping method projects a power law shaped disk surface onto the detector plane after which the observed scattered light image is interpolated backward onto the disk surface. We apply the method on archival polarized intensity images of the protoplanetary disk around HD 100546 that were obtained with VLT/SPHERE in the R' band and VLT/NACO in the H and K-s bands. \Results. The brightest side of the r(2)-scaled R-0 band polarized intensity image of HD 100546 changes from the far to the near side of the disk when a flaring instead of a geometrically flat disk surface is used for the r(2)-scaling. The decrease in polarized surface brightness in the scattering angle range of similar to 40 degrees-70 degrees is likely a result of the dust phase function and degree of polarization which peak in different scattering angle regimes. The derived phase functions show part of a forward scattering peak, which indicates that large, aggregate dust grains dominate the scattering opacity in the disk surface. Conclusions. Projection effects of a protoplanetary disk surface need to be taken into account to correctly interpret scattered light images. Applying the correct scaling for the correction of stellar irradiation is crucial for the interpretation of the images and the derivation of the dust properties in the disk surface layer.
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Ozkan, Musa. "Boundary layer transition over rotating disks." Thesis, University of Warwick, 2016. http://wrap.warwick.ac.uk/87170/.
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This thesis summarizes results of a combined numerical and experimental study investigating the effects of surface roughness, and of the geometry of the ow domain (confinement) on the boundary{layer transition over rotating disks. Numerically, a three{dimensional enclosed cavity ow in a rotor{stator flow configuration is simulated. The effects induced by surface roughness of the rotor disk and the effects induced by the stator geometry enclosing the ow domain are investigated. The steady{state velocity pro les of the boundary{layer ow on the rotating disk are obtained, subjected to a linear stability analysis and compared to relevant data from the literature. Experimentally, the ow over rotating disks is studied for smooth disk surfaces and for disks with concentric grooves representing distributed roughness. The disks are mounted submerged inside a water {filled tank. Due to the surrounding perimeter wall of the tank and the liquid surface this arrangement resembles the classic rotor{stator flow configuration. Comprehensive measurements of the boundary{layer ow and its laminar{turbulent transition were performed by means of an hot{ lm anemometer. The computational results suggest that, for the rotor{stator ow investigated, the roughness{induced effects are very similar to the geometry{induced effects, both in nature and magnitude. This suggests that it may be di cult to distinguish between both effects in experiments where the ow domain is restricted. Nevertheless, in comparison to previous hot{ lm measurements employing the same experimental facility, the data of the current study have been significantly improved by means of introducing a new calibration technique. The new experimental data discussed here confirm recent theoretical results of our research group in that they corroborate that an increase in the roughness level can reduce the number of stationary vortices and also stabilize the Type{I (cross{ ow) instability mode. However, the detailed analysis of the experimental data, in comparison to the theoretically predicted magnitude of the roughness{induced and the geometry{induced effects, reveal that future studies would greatly bene t from the availability of a new air{based rotating{disk apparatus.
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On, Sai Tung. "Efficient transaction recovery on flash disks." HKBU Institutional Repository, 2010. http://repository.hkbu.edu.hk/etd_ra/1170.
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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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Gorlova, Nadiya Igorivna. "Debris Disks in Open Stellar Clusters." Diss., The University of Arizona, 2006. http://hdl.handle.net/10150/195908.
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Indirect searches for planets (such as radial velocity studies)show that their formation may be quite common. The planets are however too small and faint to be seen against the glare of their host stars; therefore, their direct detectionis limited to the nearest systems. Alternatively one can study planets by studying their "by-product" -- dust. We see raw material available for planets around young stars, anddebris dust around old stars betraying planet-induced activity. Dust has a larger surface area per unit mass compared with a large body; it can be spread over a largersolid angle, intercepting more starlight and emitting much more lightvia reprocessing. By studying dusty disks we can infer the presence of planets at larger distances.Here we present results of a survey conducted with the SpitzerSpace Telescope of debrisdisks in three open clusters. With ages of 30--100 Myrs, these clusters are old enough that the primordialdust should have accreted into planetesimals, fallen onto the star, or been blown away due to a numberof physical processes. The dust we observe must come from collisions or sublimation of larger bodies.The purpose of this study is to investigate the dustevolution in the terrestrial planet zone, analogous to the Zodiacal cloud in our Solar system. We are most sensitive to this zone becausethe peak of a 125 K black body radiation falls into the primary pass-band of our survey -- 24 micron. We investigate the fraction and amount of the infra-red excesses around intermediate- to solar-mass stars in open stellar clusterswith well defined ages. The results are analyzed in the context of disk studies at other wavelengths and ages, providing an understanding of the time-scale for diskdissipation and ultimately planet building and frequency.
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Herbert-Fort, Stephane. "The Outer Disks of Nearby Galaxies." Diss., The University of Arizona, 2011. http://hdl.handle.net/10150/202752.
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This dissertation presents three observational projects designed to characterize the outer disks of nearby galaxies (beyond the optical radius R₂₅). Until very recently, outer disks remained an elusive and poorly-understood component of disk galaxies. We first present a Large Binocular Telescope (LBT) optical imaging survey of nearby outer disks to examine the basic properties of this component. Our LBT observations indicate that most nearby galaxies host an outer disk with star formation occurring at a very low level. We detect hundreds of outer disk star clusters and show that they typically have masses ∼ 10² − 10⁴M⊙ and ages up to a Gyr. The clusters are born in groups that can remain clustered for a Gyr or more, while the clusters slowly evaporate stars into a diffuse stellar component. The clusters appear to form from localized overdensities in the gas distribution primarily associated with spiral structure. The clusters extend to 2R₂₅ in our sample. We find that some clusters may also reside well outside of their host galaxy’s gas disk. Our second project is a kinematic study of Hɑ knots in the outer disk of the large, isolated, face-on galaxy NGC 628, using Inamori Magellan Areal Camera and Spectrograph (IMACS) observations from the Magellan telescope. This galaxy shows a kinematically cold outer disk (velocity dispersion < 11 km s⁻¹) with a mass density ∑ = 7.5 M⊙ pc⁻². Our observations cannot exclude uniform star formation lasting a Hubble time in this outer disk and confirm that this component is an extension of the kinematically-cold inner disk. Our third project is a search for molecular emission in the outer disk of NGC 628, using the sensitive Atacama LargeMillimeter Array (ALMA) receiver on the Submillimeter Telescope (SMT). We did not detect emission from our outer disk pointings, though we are able to provide useful estimates for future ALMA observations of outer disk knots. Our SMT observations indicate that the H₂ / H I ratio is ∼ 100× lower in the outer disk than in the inner disk, which likely explains, at least in part, the trend towards smaller clusters and lower star formation rates at larger radii.
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Fedele, Davide. "Structure and Evolution of Protoplanetary Disks." Doctoral thesis, Università degli studi di Padova, 2008. http://hdl.handle.net/11577/3427197.
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The scope of this thesis is to investigate the structure and evolution of protoplanetary disk. High resolution observational techniques such as high resolution optical/infrared spectroscopy and infrared interferometry are well suited for this purpose. High resolution spectroscopy allow to resolve the velocity profile of disk emission lines and determine some important parameters such as the disk geometry and the physical conditions of the line emitting region. Infrared interferometry allows to spatially resolve and constraint the disk geometry within the planet forming region.
The work presented here aims at contributing to the comprehension of the disk structure and evolution at three different evolutionary stages: 1) the early phase when the system is still (partially) embedded in a remnant of the molecular cloud; 2) the so-called Class II phase (from the classification of Lada 1987). At this stage gas and dust evolve rapidly leading to drastic changes of the disk structure; 3) the transition phase from Class II to Class III when gas and dust are dissipated leaving, eventually, a planetary system.
During the early phases of disk evolution the star-disk- envelope system experience powerful instability which are related to rapid enhancement of the mass accretion rate on a timescale of few months. These events are recognizable as so-called FU Orionis outbursts, in which the optical brightness of the system can increase by 4 or more magnitudes. The mass accretion rate increases from 10-7-- 10-8 Myr to 10-3 --10-4 Myr. Statistical studies suggest that young low-mass stars experience several FU Orionis outburst.
In late 2003, the young star V1647 Orionis in the L1630 Ori cloud within the Orion B molecular cloud went into outburst. The outburst shares some properties of the FU Orionis outburst. Following spectro-photometric observations confirmed the nature of the outburst as a disk-instability event. We also find, for the first time, probe of a direct link between an accretion event and the ejection of an Herbig-Haro object (HH).
During the Class II phase dust coagulation and grain growth occur. This is the first step of planet formation. We applied high resolution optical spectroscopy and infrared interferometry to direct compare gas and dust emission from the disk surface of three protoplanetary disks. This study gives some insight on the relative distribution of gas and dust in disk and on the temporal evolution of the two components. A physical decoupling of gas and dust may occur leading to changes in the relative structure of the two (different scale height) and to rapid settling of dust on the disk midplane. This may increase the dust-to-gas mass ratio in the disk interior and, according to recent simulation, may trigger the formation of planetesimals via gravitational instability.
The transition phase from a Class II to a Class III system is characterized by various processes which dissipate the disk material. In particular, viscous accretion and photo-evaporation are very efficient in removing disk material and planet formation is likely in competition with disk dispersion. For this reason, a fundamental quantity is the mass accretion timescale, i.e. the time at which the disk accretion phase ceases. In turn, the time at which the disk accretion phase ceases is a strong constraint on the gas dissipation timescale, relevant for the formation of giant planets. We have observed a number of young stellar clusters of different age aimed at tracing the
evolution viscous accretion with time. The preliminary results show that the accretion seems to cease at similar age of the dust dissipation, i.e. within 5 -- 10 Myr.
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O'Sullivan, Mark George. "Warping, dust settling and dynamics of protoplanetary disks." Thesis, St Andrews, 2008. http://hdl.handle.net/10023/782.
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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.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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de, Val Borro Miguel. "Studies of Gas Disks in Binary Systems." Doctoral thesis, Stockholms universitet, Institutionen för astronomi, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-8337.
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There are over 300 exoplanets detected through radial velocity surveys and photometric studies showing a tremendous variety of masses, compositions and orbital parameters. Understanding the way these planets formed and evolved within the circumstellar disks they were initially embedded in is a crucial issue. In the first part of this thesis we study the physical interaction between a gaseous protoplanetary disk and an embedded planet using numerical simulations. In order to trust the results from simulations it is important to compare different methods. However, the standard test problems for hydrodynamic codes differ considerably from the case of a protoplanetary disk interacting with an embedded planet. We have carried out a code comparison in which the problem of a massive planet in a protoplanetary disk was studied with various numerical schemes. We compare the surface density, potential vorticity and azimuthally averaged density profiles at several times. There is overall good agreement between our codes for Neptune and Jupiter-sized planets. We performed simulations for each planet in an inviscid disk and including physical viscosity. The surface density profiles agree within about 5% for the grid-based schemes while the particle codes have less resolution in the low density regions and weaker spiral wakes. In Paper II, we study hydrodynamical instabilities in disks with planets. Vortices are generated close to the gap in our numerical models in agreement with the linear modal analysis. The vortices exert strong perturbations on the planet as they move along the gap and can change its migration rate. In addition, disk viscosity can be modified by the presence of vortices. The last part of this thesis studies the mass transfer in symbiotic binaries and close T Tauri binary systems. Our simulations of gravitationally focused wind accretion in binary systems show the formation of stream flows and enhanced accretion rates onto the compact component.
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Takahashi, Sanemichi. "Formation and evolution of the protoplanetary disks." 京都大学 (Kyoto University), 2015. http://hdl.handle.net/2433/199099.
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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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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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Sohan, Ripduman Singh. "Operating system support for application managed disks." Thesis, University of Cambridge, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.611297.
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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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Faes, Daniel Moser. "An interferometric view of hot star disks." Universidade de São Paulo, 2015. http://www.teses.usp.br/teses/disponiveis/14/14131/tde-16122015-201029/.
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Optical long baseline interferometry was recently established as a technique capable of resolving stars and their circumstellar environments at the milliarcsecond (mas) resolution level. This high-resolution opens an entire new window to the study of astrophysical systems, providing information inaccessible by other techniques. Astrophysical disks are observed in a wide variety of systems, from galaxies up to planetary rings, commonly sharing similar physical processes. Two particular disk like systems are studied in the thesis: (i) B He-rich stars that exhibits magnetic fields in order of kG and that trap their winds in structures called magnetospheres; and (ii) Be stars, fast rotating stars that create circumstellar viscous disks. This study uses the interferometric technique to investigate both the photosphere proper and the circumstellar environment of these stars. The objective is to combine interferometry with other observational techniques (such as spectroscopy and polarimetry) to perform a complete and well-constrained physical description of these systems. This description is accompanied by radiative transfer models performed by the HDUST code.Interferometria óptica de longa linha de base recentemente estabeleceu-se como uma técnica capaz de resolver estrelas e seus ambientes circunstelares no nível de mili segundos de arcos (\\textit). Esta alta resolução abre uma janela inteiramente nova para o estudo de sistemas astrofísicos, fornecendo informações inacessíveis por outras técnicas. Discos astrofísicos são observados numa ampla variedade de sistemas, de galáxias à discos planetários, em geral compartilhando de processos físicos similares. Dois sistemas de discos foram estudados nesta tese: (i) o estrelas B ricas em He e que possuem campos magnéticos da ordem de kG e que confinam seus ventos em estruturas chamadas magnetosferas; e (ii) estrelas Be, estrelas de rotação rápida que criam um disco circumstelar viscoso. Este estudo usa a técnica interferométrica para investigar ambas a própria fotosfera e o ambiente circunstelar destas estrelas. O objetivo é combinar a interferometria com outras técnicas observacionais (tal como espectroscopia e polarimetria) para realizar uma descrição física completa e precisa destes sistemas. Esta descrição é acompanhada por modelos de transferência radiativa executados pelo código HDUST.
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Close, Jacob Louis. "The hydrodynamic stripping of spheroids and disks." Thesis, University of Leeds, 2016. http://etheses.whiterose.ac.uk/16131/.
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This thesis presents numerical simulations of the ablation of galaxies and stellar disks. A sub-grid turbulence model is used to improve the accuracy of the results. Firstly simulations of galactic ram pressure stripping are presented. A spherically symmetric galactic halo is subjected to a wind of a number of different Mach numbers, both with and without the turbulence model. The initial, instantaneous stripping is unaffected by the use of the turbulence model. The turbulence model leads to significantly greater levels of long term ablation, which is more consistent with what simple analytic calculations indicate, showing that the continual Kelvin-Helmholtz stripping plays a significant role in the ram pressure stripping and this is hidden by the finite resolution of simulations without turbulence models. Secondly the turbulence model is applied to galactic disk simulations. In high Mach number (∼ 15) rotating disks the turbulence generated from the internal stresses in the disk causes it to become unstable and erroneously expand in the z direction. This is not the case for lower Mach number disks, such as dwarf disk galaxies. Further developments are needed in turbulence modelling if disk galaxies are to be simulated in this way. Finally inviscid simulations of the interaction of a supernova remnant on a stellar disk are presented. The supernova remnant is simulated hydrodynamically as opposed to using analytical approximations which represents an improvement on previous models. A number of inclination angles of the disk are considered which is not possible with previous two dimensional simulations. Mass loss rates are calculated and compared to other competing processes. Similarly to disk galaxies, the inclination angle only has a large effect on the evolution when the disk is close to edge on. Edge on disks develop an asymmetry when the ablation period is less than the rotation period of the disk. Contamination of the disk with supernova material is also investigate and is found to be low (about one part in 200, 000 in the highest case), consistent with previous results.
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Moeckel, Nickolas Barry. "Massive stars, disks, and clustered star formation." Connect to online resource, 2008. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3303877.
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Rusu, Mara Camelia. "Preparation and properties of isolated Z-disks." Thesis, University of Leeds, 2014. http://etheses.whiterose.ac.uk/8829/.
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Z-disks form the boundaries of the sarcomeres, the basic contractile units of muscle cells. Within the Z-line thin filaments containing mainly actin interdigitate and are crosslinked by α-actinin. Ends of the giant proteins titin and nebulin are also anchored in the Z-disk. The Z-line was originally thought to have the purely mechanical function of transmitting contractile force along the myofibrils. However, more recently, the Z-disk has emerged as a highly dynamic structure involved in stress sensing and important signaling pathways that govern muscle homeostasis. In order to fully understand how the Z-disk functions a detailed description of its molecular organization is essential. Even though the structure the structure of the Z-disk has been studied by electron microscopy techniques its molecular organization is known only in outline to a resolution of about 5 nm, whereas at least 3 nm is required to begin distinguishing protein shapes and to accurately dock crystal structure. Reports describing the isolation of intact Z-disks from insect indirect flight muscle date from 30-40 years ago, but these preparations have not been subjected to modern electron microscopy techniques. We improved the existing methods for the isolation of the Z-disk from honeybee flight muscle and investigated its structure using cryo-electron tomography and subtomogram averaging. The preliminary data indicate that the resolution was improved when compared with past studies of plastic sectioned muscle. We have also investigated the protein composition of the preparations to monitor the components that are washed away during preparation. Methods for the isolation of intact Z-disks from vertebrate muscle are not available. We explored strategies for isolating Z-disks from skeletal and cardiac muscle. Even though such a preparation has not been achieved we present promising approaches that, with optimization, should enable isolation of Z-disks from vertebrate muscle.
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Nelson, Andrew Frederick. "Dynamics and observational appearance of circumstellar disks." Diss., The University of Arizona, 1999. http://hdl.handle.net/10150/288969.
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In my thesis I present a study of the dynamics and observational characteristics of massive circumstellar disks in two dimensions (r, φ) using two complimentary hydro-dynamic codes: a 'Smoothed Particle Hydrodynamic' (SPH) code and a 'Piecewise Parabolic Method' (PPM) code. I also study the detection limits available to radial velocity searches for low mass companions to main sequence stars. This thesis is organized as a series of published or submitted papers, connected by introductory and concluding material. I strongly recommend that readers of this abstract obtain the published versions of each of these papers. I first outline the progress which has been made in the modeling of the structure and origins of the solar system, then in chapter 2 (The Astrophysical Journal v502, p342, with W. Benz, F. Adams and D. Arnett), I proceed with numerical simulations of circumstellar disks using both hydrodynamic codes assuming a 'locally isothermal' equation of state. The disks studied range in mass from 0.05M* to 1.0 M* and in initial minimum Toomre Q value from 1.1 to 3.0. Massive disks (M(D) > 0.2 M*) tend to form grand design spiral structure with 1-3 arms, while low mass disks (M(D) ≤ 0.2M*) tend to form filamentary, > 4 armed spiral structures. In chapter 4 (submitted to The Astrophysical Journal with W. Benz and T. Ruzmaikina), I relax the assumption the locally isothermal evolution assumption and instead include simple heating and cooling prescriptions for the system. Under these physical conditions, the spiral arm growth is suppressed in the inner 1/3 of the disks relative to the isothermal evolution and in the remainder, changes character to more diffuse spiral structures. I synthesize spectral energy distributions (SEDs) from the simulations and compare them to fiducial SEDs derived from observed systems. The size distribution of grains in the inner disk can have marked consequences on the near infrared portion of the SED. After being vaporized in a hot midplane region, the grains do not reform quickly into the size distribution on which most opacity calculations are based. In chapter 6 (The Astrophysical Journal v500, p940 with Roger Angel), I examine the limits which may be placed upon the detection of planets, brown dwarfs and low mass stellar companions using radial velocity measurements. I derive an analytic expression describing the amplitude limits for periodic signals which may be obtained from a set of data of known duration, number of measurements and precision. In chapter 7, I outline several problems which may be profitably addressed by building on this work. (Abstract shortened by UMI.)
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Upton, Robert Stewart. "Detector patterns and crosstalk from optical disks." Diss., The University of Arizona, 2001. http://hdl.handle.net/10150/289967.
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Crosstalk in the readout channel of a data storage system is obtained by a linear decomposition of the reflected field from the optical disk according to Babinet's principle. Crosstalk is the jitter introduced into the readout signal. Also, the formation of the exit pupil irradiance is described heuristically as the combination of fields in a three beam interferometer resulting from the Babinet decomposition of the reflected disk field. Three detector patterns result. The properties of the detector patterns are affected by media parameters. The media parameters are the reflectivity function of the disk and the depths of the tracking grooves. The detector patterns are also affected by the aberrations introduced into the read laser beam. This dissertation provides a complete description of crosstalk and the detection process in optical data storage systems.
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Moro-Martin, Maria A. "Signatures of planets in circumstellar debris disks." Diss., The University of Arizona, 2004. http://hdl.handle.net/10150/290124.
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Main sequence stars are commonly surrounded by debris disks, composed of cold dust continuously replenished by a reservoir of undetected dust-producing planetesimals. In the outer Solar System, Kuiper Belt (KB) objects produce dust by mutual or interstellar grain collisions. The orbital evolution of KB dust has been numerically modeled. Its equilibrium radial density distribution can be accurately estimated even though there are inherent uncertainties in the prediction of structure, owing to the chaotic dynamics of dust orbital evolution imposed by resonant gravitational perturbations of the planets. The particle size distribution of dust is greatly changed from the distribution at production, as a result of radiation forces and the perturbations of the planets. The contribution of KB dust to the population of interplanetary dust particles collected at Earth may be as low as a few percent. Gravitational scattering by giant planets creates an outflow of large grains. We quantify the characteristics of this large-particle outflow in different planetary architectures, discuss its implications for exo-planetary debris disks, and for the interpretation of in-situ dust detection experiments in space probes traveling in the outer Solar System. These outflows may contribute to the clearing of circumstellar debris in planetary systems, affecting the particle size distribution of their local ISM. In anticipation of future observations of unresolved debris disks with Spitzer, we are interested in studying how the structure carved by planets affects the shape of the disk's spectral energy distribution (SED), and consequently if the SED can be used to infer the presence of planets. We numerically calculate the equilibrium spatial density distributions and SEDs of dust disks originated by an outer belt of planetesimals (35-50 AU) in the presence of different planetary configurations, and for a representative sample of chemical compositions. The dynamical models are needed to estimate the enhancement of particles near the mean motion resonances with the planets, and to determine how many particles drift inside the planet's orbit. Based on the SEDs and predicted Spitzer colors we discuss what types of planetary systems can be distinguishable from one another.