Astronomy

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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.
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.
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 t􀀏e 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.
Cross talk in 1872 Reticon diode arrays
(University of Chicago Press, 1990) Walker, G. A. H.; Johnson, R.; Richardson, D.; Campbell, B.; Irwin, A. W.; Yang, S.
Stellar Populations in Shell Galaxies
(Copyright: American Astronomical Society, 1990-10) McGaugh, Stacy S.; Bothun, Gregory D.
A Catalog of Low Surface Brightness Galaxies: List II
(Copyright American Astronomical Society, 1992-04) SCHOMBERT, JAMES M.; BOTHUN, GREGORY D.; SCHNEIDER, STEPHEN E.; MCGAUGH, STACY S.
Star Formation Thresholds in Low Surface Brightness Galaxies
(Copyright: American Astronomical Society, 1993-08) VAN DER HULST, J. M.; SKILLMAN, E. D.; SMITH, T. R.; BOTHUN, G. D.; MCGAUGH, S. S.; DE BLOK, W. J. G.
The Tully-Fisher Relation for Low Surface Brightness Galaxies - Implications for Galaxy Evolution
(Blackwell, 1995) Zwaan, M. A.; van der Hulst, J. M.; de Blok, W. J. G.; McGaugh, S. S.
HI Observations of Low Surface Brightness Galaxies: Probing Low Density Galaxies
(Blackwell, 1996) de Blok, W.J.G.; McGaugh, S.S.; van der Hulst, J.M.
We present Very Large Array (vla) and Westerbork Synthesis Radio Telescope (wsrt) 21-cm Hi observations of 19 late-type low surface brightness (LSB) galaxies. Our main findings are that these galaxies, as well as having low surface brightnesses, have low Hi surface densities, about a factor of ~ 3 lower than in normal late-type galaxies. We show that LSB galaxies in some respects resemble the outer parts of late-type normal galaxies, but may be less evolved. LSB galaxies are more gas-rich than their high surface brightness counterparts. The rotation curves of LSB galaxies rise more slowly than those of HSB galaxies of the same luminosity, with amplitudes between 50 and 120 km s−1, and are often still increasing at the outermost measured point. The shape of the rotation curves suggests that LSB galaxies have low matter surface densities. We use the average total mass surface density of a galaxy as a measure for the evolutionary state, and show that LSB galaxies are among the least compact, least evolved galaxies. We show that both MHI/LB and Mdyn/LB depend strongly on central surface brightness, consistent with the surface brightness–mass-to-light ratio relation required by the Tully-Fisher relation. LSB galaxies are therefore slowly evolving galaxies, and may well be low surface density systems in all respects.
Orbital migration of the planetary companion of 51 Pegasi to its present location
(Nature Publishing Group, 1996) Lin, D. N. C.; Bodenheimer, P.; Richardson, D. C.
The Number, Luminosity, and Mass Density of Spiral Galaxies as a Function of Surface Brightness
(Blackwell, 1996) McGaugh, Stacy S.
The Dark and Baryonic Matter Content of Low Surface Brightness Galaxies
(Blackwell, 1997) de Blok, W.J.G.; McGaugh, S.S.
We present mass models of a sample of 19 low surface brightness (LSB) galaxies and compare the properties of their constituent mass components with those of a sample of high surface brightness (HSB) galaxies. We find that LSB galaxies are dark matter dominated. Their halo parameters are only slightly affected by assumptions on stellar mass-to-light ratios. Comparing LSB and HSB galaxies we find that mass models derived using the maximum disk hypothesis result in the disks of LSB galaxies having systematically higher stellar mass-to-light ratios than HSB galaxies of similar rotation velocity. This is inconsistent with all other available evidence on the evolution of LSB galaxies. We argue therefore that the maximum disk hypothesis does not provide a representative description of the LSB galaxies and their evolution. Mass models with stellar mass-to-light ratios determined by the colors and stellar velocity dispersions of galactic disks imply that LSB galaxies have dark matter halos that are more extended and less dense than those of HSB galaxies. Surface brightness is thus related to the halo properties. LSB galaxies are slowly evolving, low density and dark matter dominated galaxies.
From Sir Isaac Newton to the Sloan survey: calculating the structure and chaos owing to gravity in the universe
(Copyright: SIAM, 1997-01) Lake, George; Quinn, Thomas; Richardson, Derek C.
1620 GEOGRAPHOS AND 433 EROS: SHAPED BY PLANETARY TIDES?
(University of Chicago Press, 1999) BOTTKE, W. F. JR.; RICHARDSON, D. C.; MICHEL, P.; LOVE, S. G.
Until recently, most asteroids were thought to be solid bodies whose shapes were determined largely by collisions with other asteroids. Recent work by Burns and others has shown that many asteroids may be little more than rubble piles, held together by self-gravity ; this means that their shapes may be strongly distorted by tides during close encounters with planets. Here we report on numerical simulations of encounters between an ellipsoid-shaped rubble-pile asteroid and Earth. After an encounter, many of the simulated asteroids develop the same rotation rate and distinctive shape as 1620 Geographos (i.e., highly elongated with a single convex side, tapered ends, and small protuberances swept back against the rotation direction). Since our numerical studies show that these events occur with some frequency, we suggest that Geographos may be a tidally distorted object. In addition, our work shows that 433 Eros, which will be visited by the NEAR spacecraft in 1999, is much like Geographos, suggesting that it too may have been molded by tides in the past.
Planetesimal clusters in a Keplerian disk I. gravitational evolution
(EDP Sciences, 2002-08-30) Tanga, P.; Michel, P.; Richardson, D. C.
It was recently demonstrated by numerical simulations that a turbulent flow in a rotating system is capable of efficiently concentrating passively advected particles having a density larger than the fluid – inside anti-cyclonic vortices. This process has important consequences on the distribution of solid particles in protoplanetary disks, since dust surface densities 1–2 orders of magnitude higher than the background are rapidly reached in vortex cores. However, until now, the role of self-gravitation of captured solids has been neglected. In this work we study the action of mutual gravitational interactions - after the gas has dissipated - over the dynamics of planetesimals inside clusters similar to those created in vortex cores. A comparison is made between the behavior of idealized clusters of planetesimals characterized by ad-hoc velocity profiles, and more complex initial conditions such as those obtained in previous hydrodynamical simulations. We show here that, within the explored interval of parameters, mutual scattering of particles can quickly disperse the cluster. Our results are demonstrated to be not dependent on the resolution employed. It can be concluded that if large planetesimals were formed inside vortex cores, they would be ejected by mutual perturbations.
The pursuit of the whole NChilada: Virtual petaflops using multi-adaptive algorithms for gravitational systems
(IBM, 2004) Lake, G.; Quinn, T.; Richardson, D. C.; Stadel, J.
We describe the keys to meeting the challenges of N-body simulation: adaptive potential solvers, adaptive integration, and volume renormalization. With these techniques and a dedicated teraflop facility, simulation can keep pace with observations of the universe. We also describe some problems in simulating the formation and stability of planetary systems.
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; Astronomy
The 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.
Gravitational instability and clustering in a disk of planetesimals
(EDP Sciences, 2004-08-05) Tanga, P.; Weidenschilling, S. J.; Michel, P.; Richardson, D. C.
For a long time, gravitational instability in the disk of planetesimals has been suspected to be the main engine responsible for the beginning of dust growth, its advantage being that it provides for rapid growth. Its real importance in planetary formation is still debated, mainly because the potential presence of turbulence can prevent the settling of particles into a gravitationally unstable layer. However, several mechanisms could yield strongly inhomogeneous distributions of solids in the disk: radial drift, trapping in vortices, perturbations by other massive bodies, etc. In this paper we present a numerical study of a gravitationally unstable layer. This allows us to go beyond the classical analytical study of linear perturbations, exploring a highly non-linear regime. A hierarchical growth of structure in the presence of dissipation (gas drag) can yield large, virialized clusters of planetesimals, the first time such clusters have been observed in the context of planetesimal disks.
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.
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.

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