Astronomy Theses and Dissertations
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Item Electrons and Spin Waves in Itinerant Ferromagnets(1976) Murray, Joanne; Korenman, Victor; Physics and Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)Though it is accepted that the 3-d magnetic electrons of transition metals such as nickel are itinerant, at high temperature these itinerant ferromagnets act as if the electrons were localized at lattice sites. In particular, three experimental results conflict with the Stoner itinerant model: 1) The spin band gap does not decrease with temperature as the average magnetization, but much more slowly. 2) Spin waves of short wavelength propagate above the Curie temperature. 3) Magnetic degrees of freedom play a role in determining thermodynamic properties n ear and above TC. The source of these discrepancies is the failure of Stoner theory to take into account magnetization fluctuations. In this paper, I do calculations of single particle and spin wave properties in a generalization of Stoner theory devised by R. E. Prange and V. Korenman to take account of fluctuations. In Stoner theory, electrons interact with an effective magnetic field proportional to the average magnetization, which becomes zero at the phase transition. The basic idea of the generalization of Stoner theory is that electrons are sensitive to their local environment and therefore that electronic and spin wave properties should be calculated in the presence of a local slowly fluctuating magnetization configuration. Only after calculating these properties should the fluctuations be thermally averaged. As a result, electrons interact with an effective magnetic field which is basically proportional to the magnitude of the local magnetization vector and which need not become zero at TC. Single particle properties are calculated by making a transformation to the spatially varying frame of reference of the local magnetization and doing perturbation theory with the magnetization gradients as the small perturbation parameter. We find that the spin eigenstates are approximately in or opposite to the direction of the local magnetization. Even when there is no longer a macroscopic magnetization, an energy gap is maintained between spin-split bands, the bands now being defined in terms of the local magnetization direction. The change in the energy gap from its zero temperature value is proportional only to the average square o f a magnetization gradient, a quantity which may be small even above TC. Thus we can understand that the gap changes only slowly with temperature and that the spin wave does not decay into Stoner single particle excitations even at high temperature. A free energy is found which is very similar in form to the free energy used to compute thermodynamic properties in localized models; thus we find that magnetic degrees of freedom are still important in computing thermodynamic properties above TC. It is the existence of a population difference and energy gap, rather than a macroscopic average magnetization that permits the existence of a spin flip collective excitation. We find a secular equation for the spin wave frequency in the presence of fluctuations which is very similar to the usua1 RPA secular equation, except for small perturbations proportional to the square of magnetization gradients. The corrections to the spin wave frequency and lifetime include the effect of the perturbation of single electron energies by the background, and also of the scattering of the spin wave from single particle spin-conserving excitations and from other spin waves. These corrections are quite small and allow for propagation even above TC. Thus it is a prediction of our theory that one see spin waves even above the critical temperature, so long as an appropriate Population difference maintains a locally ordered magnetization.Item Gravitational Radiation Detection(1976) Rydbeck, Gustaf H. B.; Weber, Joseph; Physics and Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)This dissertation studies resonant gravitational wave detectors and related data analysis. Different forms (strain amplitude) of the equation of motion for a medium responding to a gravitational wave are discussed in relation to the detection of such waves. Utilizing "Bayesian techniques" an optimal method for data analysis is developed. Noise and filter theory is reviewed. It is seen that the “Bayesian techniques" integrates filter theory and data analysis, providing both filter properties and optimal methods for integrating the data.(In particular the method leads to a non threshold type of analysis, and "looks for" correlation between two detectors without the use of time delay). Expressions for optimal sensitivity (and filters) of detector systems are given, including the limit of perfect sensors and electronics. The signal to noise ratio in terms of the spectral power of the gravitational radiation is derived. Long baseline interferometry is discussed. A computer program simulating a pair of Weber type detectors is developed to study different approaches to data analysis.Item Topics in Nonlinear Wave Theory With Applications(1984) Tracy, Eugene Raymond; Chen, Hsing Hen; Physics; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)Selected topics in nonlinear wave theory are discussed and applications to the study of modulational instabilities are presented. A historical survey is given of topics relating to solitons and modulational problems. A method is then presented for generating exact periodic and quasiperiodic solutions to several nonlinear wave equations which have important physical applications. The method is then specialized for the purposes of studying the modulational instability of a plane wave solution of the nonlinear Schrodinger equation, an equation with general applicability in one dimensional modulational problems. Some numerical results obtained in conjunction with the analytic study are presented. The analytic approach explains the recurrence phenomena seen in our numerical studies, and the numerical work of other authors. The method of solution (related to the Inverse Scattering Method) is then analyzed within te context of Hamiltonian dynamics where we show that the method can be viewed as simply a pair of canonical transformations. The Abel Transformation which appears here and in the work of other authors is shown to be a special form of Liouville's Transformation to action-angle variables. The construction of closed form solutions of these nonlinear wave equations, via the solution of Jacobi's Inversion Problem, is surveyed briefly.Item Elemental Abundances via X-ray Observations of Galaxy Clusters and the InFOCuS Hard X-ray Telescope(2004-04-30) Baumgartner, Wayne; Mushotzky, Richard F; Leventhal, Marvin; AstronomyThe first part of this dissertation deals with the oxygen abundance of the Milky Way interstellar medium. Previous measurements had shown that oxygen in the ISM was depleted compared to its abundance in the sun. This dissertation presents new measurements of the ISM oxygen abundance taken in the X-ray band by observing the oxygen 0.6 keV photoionization K-edge in absorption towards 10 galaxy clusters. These measurements show that the ISM oxygen abundance is 0.9 solar, much greater than earlier depleted values. The oxygen abundance is found to be uniform across our 10 lines of sight, showing that it is not dependent on the depth of the hydrogen column. This implies that the galactic oxygen abundance does not depend on density, and that it is the same in dense clouds and in the more diffuse ISM. The next part of the dissertation measures elemental abundances in the galaxy clusters themselves. The abundances of the elements iron, silicon, sulfur, calcium, argon, and nickel are measured using the strong resonance K-shell emission lines in the X-ray band. Over 300 clusters from the ASCA archives are analyzed with a joint fitting procedure to improve the S/N ratio and provide the first average abundance results for clusters as a function of mass. The alpha elements silicon, sulfur, argon and calcium are not found to have similar abundances as expected from their supposed common origin. Also, no combination of SN Ia and SN II yields can account for the cluster abundance ratios, perhaps necessitating a contribution from a cosmologically early generation of massive population III stars. The last part of this dissertation details the development of the Cadmium Zinc Telluride (CZT) detectors on the InFOCuS hard X-ray telescope. InFOCuS is a balloon-borne imaging spectrometer that incorporates multi-layer coated grazing-incidence optics and CZT detectors. These detectors are well suited for hard X-ray astronomy because their large bandgap and high atomic number allow for efficient room temperature detection of photons in the 20-150 keV band. The InFOCuS CZT detectors achieve an energy resolution of 4.0 keV. A 2000 flight to measure the inflight background is discussed, as well as the results of a 2001 flight to observe Cyg X-1.Item A Large Survey for Very Low Surface Brightness Galaxies(2004-12-14) Marshall, James J.; McGaugh, Stacy S.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation presents the results of a survey for very low surface brightness galaxies in the field population. These galaxies have such low brightness per unit area on the night sky that they are difficult to detect and have been overlooked by many previous surveys. By covering a large area, approximately 91 deg^2, and reaching a low limiting surface brightness, approximately 25.4 Rmag arcsec^-2, we are able to detect these galaxies down to a surface brightness level that has not been well explored, allowing us to check and extend previous results. We describe the observations, data reduction procedure, astrometric and photometric calibrations which are performed using IRAF. The data analysis procedure involves detecting objects using the SExtractor program, performing a star/galaxy separation, and fitting the detected objects with a galaxy model using the GIM2D package for IRAF in order to measure their parameters. We perform a series of cuts on the objects to eliminate detections with potential problems, using the results of the data analysis steps to remove objects that are stellar-like, saturated, unresolved, have poor chi^2 values for the model fit, have very large disk scale lengths, or reached model fitting limits of 85 deg inclination and a bulge/total ratio of 0 or 1. We then selected objects that were brighter than 18.25 mag in R, had a bulge/total ratio B/T < 0.3 (disk dominated), and inclination i < 35 deg (relatively face-on) as our sample to study, a total of 757 objects. The results of our study indicate that the observed surface brightness distribution is peaked at a disk central surface brightness of 20.5 R mag arcsec^-2 and the intrinsic surface brightness distribution, obtained by applying a volume correction to the observed distribution, is consistent with a flat distribution out to approximately 24.25 R mag arcsec^-2. We also examine the number--magnitude relation for our detections, the assumption that galaxy disks are transparent by using the inclination as a measure of the transparency, and the relation between the disk and bulge scale lengths. The results are summarized and some possibilities for future study are presented.Item Turbulence in Star Formation: Tracing the Velocity Fields of Dense Cores(2004-12-15) Volgenau, Nikolaus Herman; Mundy, Lee G; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The theory of star formation that has developed over the past several decades supposes that dense cores are quiescent and isolated from energetic events. However, observations of star-forming regions show that cores develop in active environments. Thus, although the "standard" theory is quantitatively rigorous, it can only explain a fraction of real star-forming events. The point of departure for this work is the hypothesis that turbulence is a fundamental component of the star formation process. In a turbulent star formation theory, the effects of random gas motions extend from molecular cloud scales down to scales of thousands or hundreds of AU. Dense cores form rapidly at the collision interfaces of turbulent flows and evolve according to the specific physical conditions at those interfaces. Star formation is dynamic and interactive, rather than quasi-static and isolated. This work presents evidence for turbulent motions in dense cores. The evidence comes from observations of cores in the Perseus cloud made with the BIMA interferometer and the FCRAO 14 m antenna. The cores were mapped in C18O J=1-0 emission with resolutions of ~44, 10, 5, and 3 arc-seconds. The higher angular resolutions correspond to physical scales within the characteristic core radius (~0.1 pc) identified in previous studies. In general, the range of velocities traced by the C18O, as well as the complexity of the fields, increases with resolution. No core resembles a quiescent condensation undergoing simple systematic rotation. The cores are analyzed by applying a gridding technique developed by Ostriker, Stone, & Gammie (2001) to quantify the properties of model clouds. Spectra taken through the datacubes over a wide range of sizes are used to construct correlations between line widths and spatial scale, which show a broad range of line widths even at the smallest measurable scales. The narrowest lines must have a turbulent component at least as great as the thermal component, and for nearly all lines, the turbulent component makes the dominant contribution. A statistical analysis of the variations in line properties as a function of spatial separation across a core shows that the means and variances of the central velocity and line width difference distributions exhibit properties characteristic of a hierarchy of turbulent gas motions (Miesch & Bally 1994). The high resolution BIMA data reveal that these turbulent motions persist on sub-core scales.Item Planetesimal Evolution and the Formation of Terrestrial Planets(2005-03-11) Leinhardt, Zoe Malka; Richardson, Derek C; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An accurate numerical model of solar system formation requires understanding how planetesimals grow into larger bodies. Traditionally, numerical simulations of planet formation have used extrapolations of impact experiments in the strength regime to model the effects of fragmentation in planetesimal collisions. However, planetesimals, which are large enough to decouple from the gaseous nebula, are dominated by self-gravity not material strength. As a result, such extrapolations may give misleading results since much more energy is needed to disperse than to disrupt a planetesimal in the gravity regime. In order to determine the effects of collision parameters, I have completed parameter-space studies of collisions between kilometer-sized planetesimals. The planetesimals are modeled as ``rubble piles"---gravitational aggregates of particles bound together by gravity. I find that as the mass ratio departs from unity the impact angle has less effect on the collision outcome. At the same time, the probability of planetesimal growth increases. Conversely, for a fixed impact energy, collisions between impactors with mass ratio near unity are more dispersive than those with mass ratio far from unity. For an average mass ratio of 1:5, the accretion probability is ~ 60% over all impact parameters. Results are presented from a dozen direct N-body simulations of terrestrial planet formation with various initial conditions. To increase the realism of the simulations, a self-consistent planetesimal collision model was developed based on the planetesimal model developed and investigated in the parameter space studies summarized above. The results are compared to the best numerical simulations of planet formation in the literature (Kokubo and Ida 2002) in which no fragmentation is allowed---perfect merging is the only collision outcome. After 400,000 years of integration our results are virtually indistinguishable from those of (Kokubo and Ida 2002). We find that the number and masses of protoplanets, and time required to grow a protoplanet, depends strongly on the initial conditions of the disk and is consistent with oligarchic theory. In contrast to the suggestion by (Goldreich et al. 2004), there is negligible debris remaining at the end of oligarchic growth, where ``debris" is defined to be those particles smaller than our resolution limit.Item Light Scattering Properties of Asteroids and Cometary Nuclei(2005-04-20) Li, Jian-Yang; A'Hearn, Michael F.; McFadden, Lucy A.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The photometric properties of asteroids and cometary nuclei, bodies important for understanding the origin of the Solar System, are controlled by the physical properties of their surfaces. Hapke's theory is the most widely used theoretical model to describe the reflectance of particulate surfaces, and has been applied to the disk-resolved photometric analyses of asteroid 433 Eros, comet 19P/Borrelly, and asteroid 1 Ceres, in this dissertation. Near Earth Asteroid Rendezvous returned disk-resolved images of Eros at seven wavelengths from 450nm to 1050nm. The bidirectional reflectance of Eros's surface was measured from those images with its shape model and geometric data. Its single-scattering albedo, w, was found to mimic its spectrum, with a value of 0.33+/-0.03 at 550nm. The asymmetry factor of the single-particle phase function, g, is -0.25+/-0.02, and the roughness parameter, theta_bar, is 28+/-3 deg, both of which are independent of wavelength. The V-band geometric albedo of Eros is 0.23, typical for an S-type asteroid. From the disk-resolved images of Borrelly obtained by Deep Space 1 (DS1), the maps of its w, g, and theta_bar were constructed by modeling the reflectance of Borrelly terrain by terrain. w varies by a factor of 2.5, with an average of 0.057+/-0.009. g changes from -0.1 to -0.7, averaging -0.43+/-0.07. theta_bar is <=35 deg for most of the surface, but up to 55 deg for some areas, with an average of 22+/-5 deg. The 1-D temperature measurement from DS1 can be well described by the standard thermal model assuming a dry surface, except for one area, where the discrepancy can be explained by a sublimation rate that is consistent with the observed water production rate. HST images through three filters, covering more than one rotation of Ceres, were acquired. Its V-band lightcurve agrees with earlier observations very well. A strong absorption band centered at about 280nm is noticed, but cannot be identified. w of Ceres was modeled to be 0.073+/-0.002, 0.046+/-0.002, and 0.032+/-0.003 at 555nm, 330nm, and 220nm, respectively. The maps of w for Ceres at three wavelengths were constructed, with eleven albedo features identified. Ceres' surface was found to be very uniform.Item MARS OZONE ABUNDANCES FROM INFRARED HETERODYNE SPECTRA AND THEIR APPLICATION TO THE STUDY OF THE STABILITY OF THE MARTIAN ATMOSPHERE(2005-04-21) Fast, Kelly Elizabeth; A'Hearn, Michael F; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The stability of the carbon dioxide atmosphere of Mars is thought to be maintained through catalytic chemistry involving "odd hydrogen" products of water vapor photolysis. Ozone is destroyed by odd hydrogen and therefore its abundance is an excellent tracer of those species that play a vital role in reforming carbon dioxide. Photochemical models of the atmosphere of Mars rely on observables such as ozone to test their predictions. Infrared heterodyne spectroscopy with a spectral resolution >=106 is the only technique that can directly measure ozone in the Martian atmosphere from the surface of the Earth. Observations were made using the Goddard Infrared Heterodyne Spectrometer and Heterodyne Instrument for Planetary Wind and Composition at the NASA Infrared Telescope Facility. Ozone abundances from seven data sets taken between 1988 and 2003 are presented, along with observation and analysis techniques. Measured spatial, seasonal, and orbital variability of total ozone column abundance is compared to that predicted by the first three-dimensional gas phase photochemical model of Mars. Overall agreement in the behavior of ozone across aphelion and perihelion periods supports the theory that odd hydrogen chemistry is responsible for maintaining the stability of the carbon dioxide atmosphere. Underestimation of modeled low latitude ozone around aphelion may indicate the suppression of odd hydrogen abundance through heterogeneous processes involving water ice clouds. The weak but not strict anticorrelation of the observed total column densities of ozone and water supports the role that the altitude distribution of water vapor is thought to play in regulating ozone abundance. Ozone abundances from this work are compared with those retrieved using ultraviolet techniques, showing generally good agreement. Techniques for extracting ozone altitude distribution are investigated by incorporating O2(1D) dayglow observations which indirectly probe ozone above ~20 km altitude. The abundance and altitude distribution of ozone in the Earth's atmosphere retrieved from calibration spectra are compared to nearby contemporaneous measurements using Dobson, lidar, and ozonesonde techniques. Excellent agreement with altitude distribution measured by lidar and ozonesonde is achieved when total ozone column densities from Dobson spectrophotometry are used as a constraint in the radiative transfer analysis of the spectra.Item A DEEP X-RAY SURVEY OF THE LOCKMAN HOLE NORTHWEST(2005-08-26) Yang, Yuxuan; Mushotzky, Richard F; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)I present the X-ray analysis of the Chandra Large Area Synoptic X-ray Survey (CLASXS) of the Lockman Hole Northwest field. The contiguous solid angle of the survey is about 0.4 sqr degree and the flux limits are 5x10^-16 erg/cm^2/s in the 0.4-2 keV band and 3x10^-15 erg/cm^2/s in the $2-8$~keV band. The survey bridges the gap between deep pencil beam surveys, and shallower, larger area surveys, allowing a better probe of the X-ray sources that contribute most of the 2--10 keV cosmic X-ray background. A total of 525 X-ray point sources and 4 extended sources have been found. The number counts, X-ray spectra evolution, X-ray variability of the X-ray sources are presented. We show 3 of the 4 extended sources are likely galaxy clusters or galaxy groups. We report the discovery of a gravitational lensing arc associated with one of these sources. I present the spatial correlation function analysis of non-stellar X-ray point sources in the CLASXS and Chandra Deep Field North (CDFN). I calculate both redshift-space and projected correlation functions in comoving coordinates.The correlation function for the CLASXS field over scales of 3 Mpc < s < 200 Mpc can be modeled as a power-law of the form xi(s)=(s/s_0)^{-gamma}, with gamma = 1.6^{+0.4}_{-0.3} and s_0 = 8.05^{+1.4}_{-1.5} Mpc. The redshift-space correlation function for CDFN on scales of 1 Mpc <s < 100 Mpc is found to have a similar correlation length, but a shallower slope. The real-space correlation functions are derived from the projected correlation functions. By comparing the real- and redshift-space correlation functions, we are able to estimate the redshift distortion parameter beta = 0.4 +/- 0.2 at an effective redshift z = 0.94. We found the clustering does not dependence significantly on X-ray color or luminosity. A mild evolution in the clustering amplitude is found, indicating a rapid increase of bias with redshift. The typical mass of the dark matter halo derived from the bias estimates show little change with redshift. The average halo mass is found to be log(M_{halo}/M_sun}) ~ 12.4.Item Thermal and Magnetorotational Instability in the Interstellar Medium(2005-11-09) Piontek, Robert Andrew; Ostriker, Eve C; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We have performed three sets of numerical experiments designed to study turbulence in the interstellar medium (ISM) driven by the magnetorotational instability. Our models are local, account for galactic rotation and shear, include magnetic fields, and a cooling function which permits two stable phases of gas in pressure equilibrium. The first set of simulations was performed in two dimensions, in the radial-vertical plane. These simulations laid the groundwork for the future 3D models to come. The numerical method for including the cooling function, as well as conduction, was developed and implemented. These simulations gave us a glimpse into the workings of the MRI in the presence of a two-phase medium. In our second set of simulations we extend our models to three dimensions. This allowed us to study the saturated state of the MRI in the presence of a two-phase medium. The scaling of velocity dispersion with density was found to be steeper than that of single phase models, so that at low densities larger turbulent amplitudes were found. The interaction between MRI driven turbulence and the phase structure of the gas was examined in detail. We concluded that turbulence can drive gas into a thermally unstable state, but a two phase model of the ISM was still a fairly good approximation. Finally, we added vertical gravity to our third set of models. Now, rather than specify the mean density, the vertical distribution of gas in the simulation domain is determined self-consistently. In these models cold dense clouds form due to TI and sink to the mid-plane. Turbulence driven by the MRI thickens the disk compared to non-turbulent models by as much as 100%. Turbulent amplitudes in the cold medium are relatively low, however, as the increased concentration of cold clouds near the mid-plane keep them relatively isolated from the more turbulent warm medium. Whether or not the MRI is a significant source of turbulence in the ISM is still a question without a definitive answer, but this thesis has made significant progress in furthering our understanding of the behavior of the MRI in a two-phase medium.Item Formation and Destruction of Carbon Monoxide in Cometary Comae(2006-04-18) Pierce, Donna; A'Hearn, Michael F.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation examines the potential impact of chemical reactions and outflow behavior on the chemical development of the coma within one of the most challenging problems in cometary chemistry - identifying the source of extended CO as observed in the comae of some comets. Could chemical reactions and/or outflow behavior contribute significantly to the CO abundance? In these studies, the impact of multiple photochemical processes and two-body chemical reactions are examined within a variety of cases designed to examine the effects of nuclear chemical composition and coma physics, including the dynamics of gas emanating from a region of large-scale negative relief topography. The results show that two-body chemical reactions can contribute as much as 30% to the formation of CO and as much as 60% of the loss of CO within the inner coma, depending on the production rate. Furthermore, the fractional contribution of chemical reactions to CO formation and destruction depends on the outflow behavior. However, the overall result suggests that chemical reactions only contribute a small net gain of CO (~5%) over the CO produced solely by photochemistry. The effects of chemical reactions and outflow behavior are more important, however, to secondary species formed exclusively in the coma. The results also indicate that H2CO is the primary contributor to the extended CO source in comet Halley. Observations of H2CO in comet Halley suggest that it most likely comes from an extended source as well. The extended source problems of CO and H2CO may be linked, in which case the precursor to H2CO would gradually produce H2CO after its release from the nucleus for it to form CO on the observed spatial scale. Such precursors could be large organic molecules, or grains rich in formaldehyde polymers that thermally dissociate upon heating. More difficult to explain, however, is the CO abundance in comet Hale-Bopp, for which the observed H2CO abundance is insufficient to explain the observed CO abundance.Item Growing Intermediate-Mass Black Holes with Gravitational Waves(2006-06-05) Gultekin, Kayhan; Miller, M. Coleman; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We present results of numerical simulations of sequences of binary-single scattering events of black holes in dense stellar environments. The simulations cover a wide range of mass ratios from equal mass objects to 1000:10:10 solar masses and compare purely Newtonian simulations with a relativistic endpoint, simulations in which Newtonian encounters are interspersed with gravitational wave emission from the binary, and simulations that include the effects of gravitational radiation reaction by using equations of motion that include the 2.5-order post-Newtonian force terms, which are the leading-order terms of energy loss from gravitational waves. In all cases, the sequence is terminated when the binary's merger time due to gravitational radiation is less than the arrival time of the next interloper. We also examine the role of gravitational waves during an encounter and show that close approach cross-sections for three 1-solar-mass objects are unchanged from the purely Newtonian dynamics except for close approaches smaller than 0.00001 times the initial semimajor axis of the binary. We also present cross-sections for mergers resulting from gravitational radiation during three-body encounters for a range of binary semimajor axes and mass ratios including those of interest for intermediate-mass black holes (IMBHs). We find that black hole binaries typically merge with a very high eccentricity --- extremely high when gravitational waves are included during the encounter such that when the gravitational waves are detectable by LISA, most of the binaries will have eccentricities e > 0.9 though all will have circularized by the time they are detectable by LIGO. We also investigate the implications for the formation and growth of IMBHs and find that the inclusion of gravitational waves during the encounter results in roughly half as many black holes ejected from the host cluster for each black hole accreted onto the growing IMBH. The simulations show that the Miller & Hamilton model of IMBH formation is a viable method if it is modified to start with a larger seed mass.Item Morphology, Star Formation, and Kinematics of Nuclear Rings(2006-07-23) mazzuca, lisa marie; Veilleux, Sylvain; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis presents a detailed optical study with the goal of better understanding the elusive physical nature of nuclear rings. We use H-alpha imaging of the central kilo-parsec region of a large sample of spiral galaxies to verify that nuclear rings occur primarily in spiral types Sa-Sbc. Late-type galaxies have a patchy and more diffuse circumnuclear appearance in H-alpha. From the parent sample, we identify 22 nuclear rings and analyze the HII regions that comprise them. We derive the ages throughout each nuclear ring, and find that the stellar content is consistently very young, with ages ranging from 1 Myr to 10 Myrs. Approximately half of the rings contain azimuthal age gradients that encompass at least 25% of the ring. Two-thirds of the galaxies containing a nuclear ring and a bar show a link between the youngest HII region(s) and the location along the ring where the bar dust lanes merge. We show that regions of enhanced star formation, as seen in nuclear rings, correspond to regions with (1) the strongest H-alpha emission, (2) high luminosities of order 10^40 erg/s - 10^42 erg/s, (3) low residual velocities of order 10 km/s, and (4) low velocity dispersions ranging from 20 km/s- 50 km/s. The lack of strong non-circular motions in the rings, coupled with a direct relationship between the position angles and ellipticities of the rings and those of their host galaxies, indicate the rings are in the same plane as the disk and are circular. We find a correlation between the largest velocity deviations (just outside of the rings) and the location of the bar minor axis, where the dominant family of stellar orbits transitions from the x1 family to the x2 family. Lastly, we apply two-dimensional line ratio diagnostics to separate the physical environments of the nuclear ring and an AGN present in NGC 7742. Results indicate very low gas densities in the nuclear ring, and show the transition from star formation in the ring to excitation by high-velocity shocks or by a central AGN towards the center. Comparison to starburst population models reveals that the HII regions comprising the ring are of roughly solar metallicity.Item Forming Binary Near-Earth Asteroids From Tidal Disruptions(2006-11-28) Walsh, Kevin John; Richardson, Derek C; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We present simulations and observations as part of a model of the binary near-Earth asteroid population. The study of binary asteroid formation includes a series of simulations of near-Earth asteroid (NEA) tidal disruption, analyzed for bound, mutually orbiting systems. Discrete and solid particles held together only by self-gravity are employed to model a ``rubble pile'' asteroid passing Earth on a hyperbolic encounter. This is accomplished via N-body simulations, with multiple encounter and body parameters varied. We examine the relative binary production rates and the physical and orbital properties of the binaries created as a function of the parameters. We also present the overall relative likelihoods for possible physical and dynamical properties of created binaries. In order to constrain the shape and spin properties of the bodies that feed the NEA population, an observing campaign was undertaken to observe lightcurves of small Main Belt asteroids (D < 5 km, SMBAs). Observations of 28 asteroids increases the overall number of SMBAs studied via lightcurves to 86. These observations allow direct comparison between NEAs and MBAs of a similar size. The shape and spin for the SMBAs are incorporated into a Monte Carlo model of a steady-state NEA population, along with the binaries created by tidal disruption simulations. Effects from tidal evolution and binary disruption from close planetary encounters are included as a means of altering or disrupting binaries. We find that with the best known progenitor (small Main Belt asteroids) shape and spin distributions, and current estimates of NEA lifetime and encounter probabilities, that tidal disruption should account for approximately 1-2% of NEAs being binaries. Given the observed estimate of an ~15% binary NEA fraction, we conclude that there are other formation mechanisms that contribute significantly to this population.Item Two dimensional mapping of the sodium emission in Europa's exosphere(2007-01-19) Fraga-Encinas, Raquel; Killen, Rosemary M.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We present results of the mapping of Europa's sodium exosphere in two dimensions and through a period of time. We base this research on several observations taken in 2001 and 2002 with the McMath-Pierce Solar Telescope at Kitt Peak, Arizona. We have created sodium emission intensity maps and line of sight column abundance maps of Europa's exosphere in all directions. We include calculations of vertical and tangential column abundances as well as surface densities of sodium relative to the position of Europa and to the Io plasma torus. We find that the sodium exosphere is weakly bound or escaping along the East-West direction, but not to be escaping along the North-South direction. Additionally, we find that the sodium exosphere is energetic enough to extend to a distance of roughly 40,000 km away from the surface of Europa.Item High Resolution Optical Velocity Fields of Low Surface Brightness Galaxies and the Density Profiles of Dark Matter Halos(2007-07-13) de Naray, Rachel Kuzio; McGaugh, Stacy S.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation investigates the behavior of cold dark matter (CDM) on galaxy scales. We present well-resolved Halpha velocity fields of the central regions of 17 dark matter-dominated low surface brightness (LSB) and dwarf galaxies observed with the DensePak Integrated Field Unit. We derive rotation curves from the two-dimensional data and compare them to published long-slit and HI rotation curves. We find broad consistency between the independent data sets. Under several assumptions about the velocity contribution from the baryons, we fit the dark matter component with cuspy NFW and cored pseudoisothermal halos. We find the data to be better described by cored dark matter halos. For the majority of galaxies, NFW halo fits either cannot be made or the implied concentrations are too low for LCDM. The shapes of the NFW rotation curves are also inconsistent with the galaxy rotation curves. We find that CDM predicts a substantial cusp mass excess near the centers of the galaxies and that the ratio of predicted to observed dark matter increases as baryons become more important. We investigate claims that systematic effects including beam smearing, slit misplacement and noncircular motions are responsible for slowly rising long-slit and HI rotation curves. We find the DensePak rotation curves to also be slowly rising, supporting the idea that this is an intrinsic feature of LSB rotation curves. We also model the two-dimensional NFW halo and test several modifications to the potential in an attempt to simultaneously reconcile both the NFW velocity field and rotation curve with observed galaxy data. We present mock DensePak velocity fields and rotation curves of axisymmetric and non-axisymmetric potentials. We find that a non-axisymmetric NFW potential with a constant axis ratio can reduce the cusp mass excess in the observed galaxy data, but the observer's line-of-sight must be along the minor axis of the potential, and the NFW pinch is not erased from the velocity field. We find that a non-axisymmetric NFW potential with a radially varying axis ratio tends to wash out the NFW pinch but introduces a twist to the velocity field.Item Spatio-Temporal Dynamics of the Magnetosphere during Geospace Storms(2007-07-25) Chen, Jian; Sharma, A. Surjalal; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nonlinear dynamical models have became powerful tools for studying and forecasting magnetospheric dynamics driven by solar wind inputs. In this thesis, the techniques of phase space reconstruction from time series data are used to develop new methods for modeling and predicting the spatio-temporal dynamics of the magnetosphere. For these studies, new databases covering the solar maximum period were compiled to enable accurate modeling of the magnetosphere during intense geospace storms. The main contributions of the thesis are: Weighted Mean Field Model and Its Application to the Intense Storms. The nonlinear dynamical models of the coupled solar wind-magnetosphere system derived from observational data yield efficient forecasts of space weather. An improved version of the mean field model, derived from a set of nearest neighbors in the phase space reconstructed from the data, was developed by assigning weights to the nearest neighbors. A new correlated database was compiled and used to model and forecast the geospace storms of October-November 2003 and April 2002, and resulted in improved forecasts of the intense storms. Mutual Information Analysis of Spatio-Temporal Dynamics. The mutual information functions enable studies of the nonlinear correlations of dynamical systems. A high resolution database for a six month period of solar wind and ground-based magnetometer data from 12 high latitude stations was used to compute the mutual information functions representing the correlations inherent in the system. Using two different window lengths of 6 and 24 hr, the spatio-temporal dynamics was analyzed using these functions for the different stations. The spreads in the average mutual information show strong correlations with the solar wind changes and the time evolution of mutual information yields a westward expansion of the disturbed region, starting from the near midnight sectors. Modeling and Predictions of Spatio-Temporal Dynamics of the Magnetosphere. The spatial structure of the magnetospheric dynamics is crucial to space weather forecasting. The database of the magnetic field perturbations at 39 magnetometers belonging to the IMAGE and CANOPUS during year 2002 was used to study the spatio-temporal structure. A longitudinal sampling process utilizing the daily rotation of Earth was used to construct a two-dimensional representation of the high latitude magnetic perturbations. The nonlinear model was used to predict the spatial structure of geomagnetic disturbances during geospace storms. Results presented in this dissetation provide a comprehensive study of the magnetosphere using nonlinear data derived models. The new weighted mean field model, mutual information analysis and spatio-temporal dynamics advance our understanding of the solar wind-magnetosphere coupling. These results can be used to develop new and more detailed space weather forecasting tools.Item Resonant and Secular Orbital Interations(2007-08-01) Zhang, Ke; Hamilton, Douglas P; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In stable solar systems, planets remain in nearly elliptical orbits around their stars. Over longer timescales, however, their orbital shapes and sizes change due to mutual gravitational perturbations. Orbits of satellites around a planet vary for the same reason. Because of their interactions, the orbits of planets and satellites today are different from what they were earlier. In order to determine their original orbits, which are critical constraints on formation theories, it is crucial to understand how orbits evolve over the age of the Solar System. Depending on their timescale, we classify orbital interactions as either short-term (orbital resonances) or long-term (secular evolution). My work involves examples of both interaction types. Resonant history of the small Neptunian satellites In satellite systems, tidal migration brings satellite orbits in and out of resonances. During a resonance passage, satellite orbits change dramatically in a very short period of time. We investigate the resonant history of the six small Neptunian moons. In this unique system, the exotic orbit of the large captured Triton (with a circular, retrograde, and highly tilted orbit) influences the resonances among the small satellites very strongly. We derive an analytical framework which can be applied to Neptune's satellites and to similar systems. Our numerical simulations explain the current orbital tilts of the small satellites as well as constrain key physical parameters of both Neptune and its moons. Secular orbital interactions during eccentricity damping Long-term periodic changes of orbital shape and orientation occur when two or more planets orbit the same star. The variations of orbital elements are superpositions of the same number of fundamental modes as the number of planets in the system. We investigate how this effect interacts with other perturbations imposed by external disturbances, such as the tides and relativistic effects. Through analytical studies of a system consisting of two planets, we find that an external perturbation exerted on one planet affects the other indirectly. We formulate a general theory for how both orbits evolve in response to an arbitrary externally-imposed slow change in eccentricity.Item A Spectral Survey of Black Hole Spin in Active Galactic Nuclei(2007-09-20) Brenneman, Laura; Reynolds, Christopher S.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation explores the question of whether broad iron lines from the accretion disk can be used as viable diagnostic tools for constraining black hole spin. We begin by giving an overview of the importance of black hole angular momentum as a signature of General Relativity and as a means of testing this theory in the strong-field limit. We discuss the anatomy of the typical black hole/accretion disk system, focusing on the complex environments of active galactic nuclei, and in particular Seyfert-1 systems which we pursue in this work. After developing a robust technique for fitting the continuum and absorption parameters through a rigorous analysis of the XMM-Newton spectrum of the Sy-1 galaxy NGC 4593, we then discuss a new model we have developed that fits broad emission lines from the inner accretion disk. This model, kerrdisk, is fully relativistic and allows the black hole spin to be a free parameter in the fit. Using this model, we carefully analyze the 350 ks XMM-Newton spectrum of the Sy-1 source MCG--6-30-15, which has the broadest and best-studied iron line observed to date. Fitting for the black hole spin in this source, we conclude that a > 0.987 to 90% confidence. We then extend our source list to analyze the XMM-Newton spectra of nine other radio-quiet Sy-1 AGN that have previously been observed to harbor broad iron lines. We find that, given enough photons and a broad line indicative of an origin in the inner disk where relativistic effects are important, our new model enables us to place robust constraints on black hole spin. Four of our sampled AGN meet the criteria necessary to constrain spin. Those constraints are given, along with the full spectral fit to each source. Interestingly, the spins of these sources range from moderate (a ~ 0.5−0.7) to very high (a > 0.95), and we do not find any AGN consistent with non-rotating black holes. For those objects that had marginal spin constraints or none at all, we discuss the spectral fits and the probable reasons for the lack of robustness of our results. This is the first ever survey of black hole spin in type-1 AGN.