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Spectral Dependence Of Coherent Backscattering Of Light In A Narrow-Resonance Atomic System, D. V. Kupriyanov, I. M. Sokolov, N. V. Larionov, P. Kulatunga, C. I. Sukenik, S. Balik, M. D. Havey Mar 2004

Spectral Dependence Of Coherent Backscattering Of Light In A Narrow-Resonance Atomic System, D. V. Kupriyanov, I. M. Sokolov, N. V. Larionov, P. Kulatunga, C. I. Sukenik, S. Balik, M. D. Havey

Physics Faculty Publications

We report a combined theoretical and experimental study of the spectral and polarization dependence of near-resonant radiation coherently backscattered from an ultracold gas of 85Rb atoms. Measurements in a 6 MHz range about the 5s 2S1/25p 2P3/2, F=3F'=4 hyperfine transition are compared with simulations based on a realistic model of the experimental atomic density distribution. In the simulations, the influence of heating of the atoms in the vapor, magnetization of the vapor, finite spectral bandwidth, and other nonresonant hyperfine transitions are considered. Good agreement is found between the simulations and measurements.

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Relativistic Allowed And Forbidden Transition Probabilities For Fluorine-Like Fe Xviii, V. Jonauskas, F. P. Keenan, M. E. Foord, R. F. Heeter, S. J. Rose, P. A. M. Van Hoof, Gary J. Ferland, K. M. Aggarwal, R. Kisielius, P. H. Norrington Mar 2004

Relativistic Allowed And Forbidden Transition Probabilities For Fluorine-Like Fe Xviii, V. Jonauskas, F. P. Keenan, M. E. Foord, R. F. Heeter, S. J. Rose, P. A. M. Van Hoof, Gary J. Ferland, K. M. Aggarwal, R. Kisielius, P. H. Norrington

Physics and Astronomy Faculty Publications

Energy levels and the corresponding transition probabilities for allowed and forbidden transitions among the levels of the ground configuration and first 23 excited configurations of fluorine-like Fe XVIII have been calculated using the multiconfigurational Dirac-Fock GRASP code. A total of 379 lowest bound levels of Fe XVIII is presented, and the energy levels are identified in spectroscopic notations. Transition probabilities, oscillator strengths and line strengths for electric dipole (E1), electric quadrupole (E2) and magnetic dipole (M1) transitions among these 379 levels are also presented. The calculated energy levels and transition probabilities are compared with experimental data.

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A Rapidly-Converging Alternative To Source Iteration For Solving The Discrete Ordinates Radiation Transport Equations In Slab Geometry, Nicholas J. Wager Mar 2004

A Rapidly-Converging Alternative To Source Iteration For Solving The Discrete Ordinates Radiation Transport Equations In Slab Geometry, Nicholas J. Wager

Theses and Dissertations

I present a numerical technique to solve the time independent Boltzmann Transport Equation for the transport of neutrons and photons. The technique efficiently solves the discrete ordinates equations with a new iteration scheme. I call this new scheme the angle space distribution iteration method because it combines a non-linear, high angular-resolution flux approximation within individual spatial cells with a coarse angular-resolution flux approximation that couples all cells in a spatial mesh. This shown to be an efficient alternative to source iteration. The new method is implemented using the step characteristic and exponential characteristic spatial quadrature schemes. The latter was introduced …

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Analysis Of Uncertainties In Infrared Camera Measurements Of A Turbofan Engine In An Altitude Test Cell, Thomas A. Morris Mar 2004

Analysis Of Uncertainties In Infrared Camera Measurements Of A Turbofan Engine In An Altitude Test Cell, Thomas A. Morris

Theses and Dissertations

This research investigated the uncertainties in measuring the IR signature of a General Electric F110-GE-129 turbofan engine inside an altitude test cell. The requirements for the measurement system include the intensity and radiance of the engine surfaces in three bands of operation, two Medium Wave IR (MWIR) bands and one Long Wave IR (LWIR) band. The altitude test cell provides a venue for measuring the IR signature in simulated altitude operation, but the imaging process is laden with measurement uncertainty due to stray radiation from the facility structure, hot exhaust gases, and the measurement equipment itself. The atmosphere and a …

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Leptonic Cp Violation Phases Using An Ansatz For The Neutrino Mass Matrix And Application To Leptogenesis, Joseph Schechter, Salah Nasri, Sherif Moussa Feb 2004

Leptonic Cp Violation Phases Using An Ansatz For The Neutrino Mass Matrix And Application To Leptogenesis, Joseph Schechter, Salah Nasri, Sherif Moussa

Physics - All Scholarship

We further study the previously proposed Ansatz, Tr(M)=0, for a prediagonal light Majorana type neutrino mass matrix. If CP violation is neglected this enables one to use the existing data on squared mass differences to estimate (up to a discrete ambiguity)the neutrino masses themselves. If it is assumed that only the conventional CP phase is present, the ansatz enables us to estimate this phase in addition to all three masses. If it is assumed that only the two Majorana CP phases are present, the Ansatz enables us to obtain a one parameter family of solutions for the masses and phases. …

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Nondipole Effects In Photo-Double-Ionization Of He By A Vuv Photon, Andrei Y. Istomin, N. L. Manakov, A. V. Meremianin, Anthony F. Starace Feb 2004

Nondipole Effects In Photo-Double-Ionization Of He By A Vuv Photon, Andrei Y. Istomin, N. L. Manakov, A. V. Meremianin, Anthony F. Starace

Department of Physics and Astronomy: Faculty Publications

We present a dynamical model-independent, ab initio parametrization of the quadrupole transition amplitude for photo-double-ionization of He. An asymmetry of the triple differential cross section induced by the nondipole corrections is discussed and shown to be significant even for an excess energy as low as 80 eV. We provide predictions for two different kinds of experiments in which nondipole effects should be observable with current experimental capabilities.

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Bond Breaking And Temporary Anion States In Uracil And Halouracils: Implications For The Dna Bases, Adam M. Scheer, Kayvan Aflatooni, Gordon A. Gallup, Paul Burrow Feb 2004

Bond Breaking And Temporary Anion States In Uracil And Halouracils: Implications For The Dna Bases, Adam M. Scheer, Kayvan Aflatooni, Gordon A. Gallup, Paul Burrow

Paul Burrow Publications

Low energy electrons are capable of breaking bonds in gas phase DNA bases by means of the dissociative electron attachment process.With the aid of new total scattering data in the halouracils and input from quantum chemical calculations, we describe the dipole bound and valence anion states in these compounds and present assignments for the two types of structure appearing in the cross sections. A clear distinction between the two mechanisms for bond breaking is necessary for an understanding of electron induced damage to DNA.

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Reply To Comment By R. Bousquet Et Al. On “Subduction Factory: 1. Theoretical Mineralogy, Densities, Seismic Wave Speeds And H2O Contents”, Bradley R. Hacker, Geoffrey A. Abers, Simon M. Peacock, Scott Joshnston Feb 2004

Reply To Comment By R. Bousquet Et Al. On “Subduction Factory: 1. Theoretical Mineralogy, Densities, Seismic Wave Speeds And H2O Contents”, Bradley R. Hacker, Geoffrey A. Abers, Simon M. Peacock, Scott Joshnston

Physics

No abstract provided.

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High Resolution Measurements Of Kinetic Energy Release Distributions Of Neon, Argon, And Krypton Cluster Ions Using A Three Sector Field Mass Spectrometer, K. Gluch, S. Matt-Leubner, L. Michalak, Olof E. Echt, A. Stamatovic, P. Scheier, T. D. Mark Feb 2004

High Resolution Measurements Of Kinetic Energy Release Distributions Of Neon, Argon, And Krypton Cluster Ions Using A Three Sector Field Mass Spectrometer, K. Gluch, S. Matt-Leubner, L. Michalak, Olof E. Echt, A. Stamatovic, P. Scheier, T. D. Mark

Physics & Astronomy

Using a newly constructed three sector field mass spectrometer (resulting in a BE1E2 field configuration) we have measured the kinetic energy release distributions of neon, argon, and krypton cluster ions. In the present study we used the first two sectors, B and E1, constituting a high resolution mass spectrometer, to select the parent ions in terms of mass, charge, and energy, and studied the decay of those ions in the third field free region. Due to the improved mass resolution we were able to extend earlier studies carried out with a two sector field machine, where an upper size limit …

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Boron Alloying In Gan, Gus L. W. Hart, Laurian Escalanti Feb 2004

Boron Alloying In Gan, Gus L. W. Hart, Laurian Escalanti

Faculty Publications

Using first-principles calculations in the local density approximation, we studied effects of adding up to 6% boron to zinc-blende GaN. We found that the band gap increases monotonically with boron incorporation, in agreement with experiment. A composition-independent band-gap bowing parameter of 4.30 eV was determined, and proved to be large compared to bowing for other mixed cation systems. The formation enthalpy of mixing, ΔH, was determined for BxGa1-xN, BxGa1-xAs, and GaAs1-xNx. A comparison of enthalpies indicates that the production of BxGa1-xN films with boron concentrations of at least 5% may be possible.

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The Case For An Optically-Thick High-Velocity Broad-Line Region, Stephanie A. Snedden, C. Martin Gaskell Feb 2004

The Case For An Optically-Thick High-Velocity Broad-Line Region, Stephanie A. Snedden, C. Martin Gaskell

Department of Physics and Astronomy: Faculty Publications

A combined analysis of the profiles of the main broad quasar emission lines in both Hubble Space Telescope and optical spectra shows that while the profiles of the strong UV lines are quite similar, there is frequently a strong increase in the Lyα/Hα ratio in the high-velocity gas. We show that the suggestion that the high velocity gas is optically-thin presents many problems. We show that the relative strengths of the high velocity wings arise naturally in an optically-thick BLR component. An optically-thick model successfully explains the equivalent widths of the lines, the Lyα/Hα ratios and flatter Balmer decrements in …

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A Demonstration Of Phonons That Implements The Linear Theory, Dietrich Lüerßen, Nalini Easwar, Ayesha Malhotra, Libby Hutchins, Kim Schulze, Brandi Wilcox Feb 2004

A Demonstration Of Phonons That Implements The Linear Theory, Dietrich Lüerßen, Nalini Easwar, Ayesha Malhotra, Libby Hutchins, Kim Schulze, Brandi Wilcox

Physics: Faculty Publications

Beads on a vibrating wire are used to simulate the discrete structure of a solid-state material. The novel idea of the experiment is to use very small oscillation amplitudes of the wire to avoid nonlinearities in the interaction. We achieve a good signal-to-noise ratio using a lock-in technique. We find quantitative agreement between theory and experiment for not only a mono- and a diatomic chain, but also for the bare wire. The latter agreement is the crucial aspect that distinguishes our experiment from previous ones. This agreement assures that the fundamental assumption of the theory ~Hooke’s law! is satisfied. We …

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The Surface Band Structure Of W(112), Yaroslav B. Losovyj, I. N. Yakovkin, Hae-Kyung Jeong, Peter A. Dowben Feb 2004

The Surface Band Structure Of W(112), Yaroslav B. Losovyj, I. N. Yakovkin, Hae-Kyung Jeong, Peter A. Dowben

Peter Dowben Publications

Both the experimental and theoretical band structure of the W(112) surface are presented, with the theoretical band structure calculated by the film–LAPW (linearized augmented plane waves) method. The results permit one to compare the W(112) and Mo(112) band structures and illuminate the similarities. It is found that for the W(112) surface the main photoemission features combine contributions from both the surface and bulk, as has been previously noted for Mo(112). The main differences between the electronic structures of the furrowed W(112) and Mo(112) surfaces are seen in the width of occupied bands. The differences are attributed to the extent of …

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High Resolution Solar Observations In The Context Of Space Weather Prediction, Guo Yang Jan 2004

High Resolution Solar Observations In The Context Of Space Weather Prediction, Guo Yang

Dissertations

Space weather has a great impact on the Earth and human life. It is important to study and monitor active regions on the solar surface and ultimately to predict space weather based on the Sun's activity. In this study, a system that uses the full power of speckle masking imaging by parallel processing to obtain high-spatial resolution images of the solar surface in near real-time has been developed and built. The application of this system greatly improves the ability to monitor the evolution of solar active regions and to predict the adverse effects of space weather. The data obtained by …

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Statistical Study Of Solar Radio Bursts, Gelu-Marius Nita Jan 2004

Statistical Study Of Solar Radio Bursts, Gelu-Marius Nita

Dissertations

The peak flux distribution of 40 years of solar radio burst data recorded by NOAA, as a function of frequency and time over a wide range of frequencies, was investigated to quantify the potential impact of radio bursts on wireless systems. Tables of fit parameters, which can be used to find burst occurrence rates in a number of frequency ranges, are presented. The typical power-law index of number density distribution, -1.8, is similar to that found in many hard X-ray studies. Significant changes were found in power-law index with frequency. The results may be useful for designers of current and …

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Measurement Of The Average Time-Integrated Mixing Probability Of B-Flavored Hadrons Produced At The Fermilab Tevatron, Darin Acosta, Kenneth A. Bloom, Collider Detector At Fermilab Collaboration Jan 2004

Measurement Of The Average Time-Integrated Mixing Probability Of B-Flavored Hadrons Produced At The Fermilab Tevatron, Darin Acosta, Kenneth A. Bloom, Collider Detector At Fermilab Collaboration

Kenneth Bloom Publications

We have measured the number of like-sign (LS) and opposite-sign (OS) lepton pairs arising from double semileptonic decays of b and b-bar hadrons, pair produced at the Fermilab Tevatron collider. The data samples were collected with the Collider Detector at Fermilab during the 1992–1995 collider run by triggering on the existence of µµ or eµ candidates in an event. The observed ratio of LS to OS dileptons leads to a measurement of the average time-integrated mixing probability of all produced b-flavored hadrons which decay weakly, Χ̅=0.152±0.007 (stat)±0.011 (syst), that is significantly larger than the world average Χ̅=0.118±0.005.

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Quantum And Classical Coincidence Imaging, Ryan S. Bennink, Sean J. Bentley, Robert W. Boyd, John C. Howell Jan 2004

Quantum And Classical Coincidence Imaging, Ryan S. Bennink, Sean J. Bentley, Robert W. Boyd, John C. Howell

Mathematics, Physics, and Computer Science Faculty Articles and Research

Coincidence, or ghost, imaging is a technique that uses two correlated optical fields to form an image of an object. In this work we identify aspects of coincidence imaging which can be performed with classically correlated light sources and aspects which require quantum entanglement. We find that entangled photons allow high-contrast, high-resolution imaging to be performed at any distance from the light source. We demonstrate this fact by forming ghost images in the near and far fields of an entangled photon source, noting that the product of the resolutions of these images is a factor of 3 better than that …

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High-Precision Lattice Qcd Confronts Experiment, James E. Hetrick, C. T. Davies Jan 2004

High-Precision Lattice Qcd Confronts Experiment, James E. Hetrick, C. T. Davies

All Faculty Articles - School of Engineering and Computer Science

The recently developed Symanzik-improved staggered-quark discretization allows unquenched lattice-QCD simulations with much smaller (and more realistic) quark masses than previously possible. To test this formalism, we compare experiment with a variety of nonperturbative calculations in QCD drawn from a restricted set of “gold-plated” quantities. We find agreement to within statistical and systematic errors of 3% or less. We discuss the implications for phenomenology and, in particular, for heavy-quark physics.

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Are Half-Metallic Ferromagnets Half Metals? (Invited), Peter A. Dowben, Ralph Skomski Jan 2004

Are Half-Metallic Ferromagnets Half Metals? (Invited), Peter A. Dowben, Ralph Skomski

Peter Dowben Publications

Several classes of materials are currently under investigation as potential high-spin-polarization materials. Unfortunately, the proposed half-metallic materials, including the semi-Heusler alloys, the manganese perovskites, and the "simpler" oxides such as chromium dioxide and magnetite, suffer from fundamental limitations. First, the postulated half-metallic systems lose their full (T = 0) spin polarization at finite temperatures and, second, surfaces, interfaces, and structural inhomogenities destroy the complete spin polarization of half-metallic systems even at zero temperature. In a strict sense, half-metallic ferromagnetism is limited to zero temperature since magnon and phonon effects lead to reductions in polarization at finite temperatures. ©2004 American Institute …

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Hole Drift-Mobility Measurements And Multiple-Trapping In Microcrystalline Silicon, T. Dylla, F. Finger, Eric A. Schiff Jan 2004

Hole Drift-Mobility Measurements And Multiple-Trapping In Microcrystalline Silicon, T. Dylla, F. Finger, Eric A. Schiff

Physics - All Scholarship

We present photocarrier time-of-flight measurements of the hole drift-mobility in microcrystalline silicon samples with a high crystalline volume fraction; typical roomtemperature values are about 1 cm2/Vs. Temperature-dependent measurements are consistent with the model of multiple-trapping in an exponential bandtail. While this model has often been applied to amorphous silicon, its success for predominantly crystalline samples is unexpected. The valence bandtail width is 31 meV, which is about 10-20 meV smaller than values reported for a-Si:H, and presumably reflects the greater order in the microcrystalline material. The hole band-mobility is about 1 cm2/Vs – essentially the same magnitude as has been …

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Magnetic Isomers And Local Moment Distribution In Mn5o And Mn6o Clusters, N. O. Jones, Shiv N. Khanna, T. Baruah, M. R. Pederson, W.-J. Zheng, J. M. Nilles, K. H. Bowen Jan 2004

Magnetic Isomers And Local Moment Distribution In Mn5o And Mn6o Clusters, N. O. Jones, Shiv N. Khanna, T. Baruah, M. R. Pederson, W.-J. Zheng, J. M. Nilles, K. H. Bowen

Physics Publications

A synergistic approach combining the experimental photoelectron spectroscopy and theoretical electronic structure studies is used to demonstrate the existence of magnetic isomers as well as configurations having comparable binding energies, identical spins, and yet differing in the distribution of local moments (called isomags). Our studies carried out on Mn5O and Mn6O clusters show that while the O atom occupies either bridge or hollow site with a binding energy of 6.88eV in Mn5, it prefers a hollow site with a binding energy of 7.18eV in Mn6 clusters. Mn5O is shown to possess two magnetic isomers with total spins of 13 and …

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Spectroscopic Signature Of Magnetic Bistability In Mn2oà Anions And Its Implications For Piezomagnetism At The Nanoscale, Shiv N. Khanna, Puru Jena, W.-J. Zheng, J. M. Nilles, K. H. Bowen Jan 2004

Spectroscopic Signature Of Magnetic Bistability In Mn2oà Anions And Its Implications For Piezomagnetism At The Nanoscale, Shiv N. Khanna, Puru Jena, W.-J. Zheng, J. M. Nilles, K. H. Bowen

Physics Publications

A synergistic study, using gradient corrected density functional theory and photoelectron spectroscopy, shows that the presence of oxygen can fundamentally alter the interaction between two Mn atoms. The normally weak interaction between two Mn atoms exhibits strong bonding when an oxygen atom is added to form Mn2O. This allows the atomiclike magnetic moments of each Mn atom to couple either ferromagnetically or antiferromagnetically depending on whether Mn2O is anionic or neutral. Furthermore, the Mn2O− anion is found to possess an antiferromagnetic isomer that lies only 0.01 eV above its nearly degenerate ferromagnetic ground state. The possibility of exploiting this magnetic …

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Energetics And Local Spin Magnetic Moment Of Single 3,4d Impurities Encapsulated In An Icosahedral Au12 Cage, Shan-Ying Wang, Jing-Zhi Yu, Hiroshi Mizuseki, Qiang Sun, Chong-Yu Wang, Yoshiyuki Kawazoe Jan 2004

Energetics And Local Spin Magnetic Moment Of Single 3,4d Impurities Encapsulated In An Icosahedral Au12 Cage, Shan-Ying Wang, Jing-Zhi Yu, Hiroshi Mizuseki, Qiang Sun, Chong-Yu Wang, Yoshiyuki Kawazoe

Physics Publications

The energetics and local spin magnetic moment of a single 3,4d impurity (Sc-Ni, Y-Pd) encapsulated in an icosahedral Au12 cage have been studied theoretically by using a real-space first-principles cluster method with generalized gradient approximation for exchange-correlation functional. The relativistic effect is considered by scalar relativistic pseudopotentials. All doped clusters show unexpected large relative binding energies compared with icosahedral Au13cluster. The smallest and the largest values appear at Pd and Zr, 2.186 and 7.791eV per cluster, respectively, indicating doping could stabilize the icosahedral Au12 cage and promote the formation of a new binary alloy cluster. Comparatively large magnetic moments are …

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Classical Stern-Gerlach Profiles Of Mn5 And Mn6 Clusters, N. O. Jones, Shiv N. Khanna, Tunna Baruah, M. R. Pederson Jan 2004

Classical Stern-Gerlach Profiles Of Mn5 And Mn6 Clusters, N. O. Jones, Shiv N. Khanna, Tunna Baruah, M. R. Pederson

Physics Publications

Mn5 and Mn6 clusters have recently been found to exhibit Stern-Gerlach profiles marked by a central peak that broadens with the increasing field gradient. The profiles neither exhibit a reminiscence of space quantization as observed through a splitting of beams for the case of free atoms, nor a net deflection characteristic of superparamagnetic relaxations observed in other transition metal clusters. It is proposed that this new behavior results from a weak coupling of localized atomic moments. ab initio electronic structure studies are carried out to show that a Mn5cluster has isomers with spin magnetic moments of 3μB, 13μB, and 23μBwhile …

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Magnetism And Energetics Of Mn-Doped Zno (101¯0) Thin Films, Q. Wang, Q. Sun, B. K. Rao, Puru Jena Jan 2004

Magnetism And Energetics Of Mn-Doped Zno (101¯0) Thin Films, Q. Wang, Q. Sun, B. K. Rao, Puru Jena

Physics Publications

First-principles calculations based on gradient corrected density functional theory are performed on Mn-doped ZnO thin film. Magnetism and energetics are studied for two Mn concentrations and varying Mn configurations. It is found that in the dilute limit when Mn atoms are far apart, the ferro-magnetic and antiferromagnetic states are energetically nearly degenerate. The resulting fluctuation would, therefore, make the system paramagnetic as found in the experiment. But, as the concentration of Mn atoms increases, there is a tendency for Mn atoms to form nearest neighbors and cluster around oxygen. For such a configuration, the antiferromagnetic coupling between Mn atoms is …

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Geometry And Electronic Structure Of V-N(Bz)(M) Complexes, Anil K. Kandalam, B. K. Rao, P. Jena, Ravindra Pandey Jan 2004

Geometry And Electronic Structure Of V-N(Bz)(M) Complexes, Anil K. Kandalam, B. K. Rao, P. Jena, Ravindra Pandey

Physics Publications

First-principles calculations based on the generalized gradient approximation to the density functional theory are performed to explore the global geometries, ground-state spin multiplicities, relative stabilities, and energetics of neutral and anionic Vn(Bz)m (n51 – 3, m51 – 4, with n ,m) complexes. The calculated results show that the Vn(benzene)m complexes clearly prefer sandwich structures to rice-ball structures. The ground-state spin multiplicities of the Vn(benzene)n11 complexes increased linearly with the size of the system (i.e., n). In the anionic complexes, the V(benzene)2 complex is found to be unstable against the autodetachment of the extra electron. The energy difference between adiabatic and …

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Formation And Properties Of Halogenated Aluminum Clusters, D. E. Bergeron, A. W. Castleman Jr., T. Morisato, S. N. Khanna Jan 2004

Formation And Properties Of Halogenated Aluminum Clusters, D. E. Bergeron, A. W. Castleman Jr., T. Morisato, S. N. Khanna

Physics Publications

The fast-flow tube reaction apparatus was employed to study the halogenation of aluminum clusters. For reactions with HX (X=Cl, Br, and I), acid-etching pathways are evident, and we present findings for several reactions, whereby AlnX− generation is energetically favorable. Tandem reaction experiments allowed us to establish that for AlnCl−, AlnI−, and AlnI−2, species with n=6, 7, and 15 are particularly resistant to attack by oxygen. Further, trends in reactivity suggest that, in general, iodine incorporation leaves the aluminum clusters’ electronic properties largely unperturbed. Ab initio calculations were performed to better interpret reaction mechanisms and elucidate the characteristics of the products. …

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The Parsec-Scale Structure And Jet Motions Of The Tev Blazars 1es 1959+650, Pks 2155-304, And 1es 2344+514, B. Glenn Piner, Philip G. Edwards Jan 2004

The Parsec-Scale Structure And Jet Motions Of The Tev Blazars 1es 1959+650, Pks 2155-304, And 1es 2344+514, B. Glenn Piner, Philip G. Edwards

Physics

As part of our study of the VLBI properties of TeV blazar jets, we present here a series of high-resolution 15 GHz Very Long Baseline Array (VLBA) images of the parsec-scale jets of the TeV blazars 1ES 1959+650, PKS 2155-304, and 1ES 2344+514, with linear resolutions of ~0.5 pc. Each of these sources was observed with the VLBA at three or four epochs during 1999 and 2000. There is a notable lack of any strong moving components on the VLBI images (in contrast to the rapid superluminal motions seen in EGRET blazars), and the structure of the VLBI jet can …

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Nucleosynthesis, Reionization, And The Mass Function Of The First Stars, Jason Tumlinson, Aparna Venkatesan, J Shull Jan 2004

Nucleosynthesis, Reionization, And The Mass Function Of The First Stars, Jason Tumlinson, Aparna Venkatesan, J Shull

Physics and Astronomy

We critique the hypothesisthat the first starswere very massive stars(VMSs; M > 140 M).We review the twomajor lines of evidencefor the existence ofVMSs: (1) that therelative metal abundances ofextremely metal-poor Galactic halostars show evidence ofVMS enrichment and (2)that the high electron-scatteringoptical depth (e) tothe cosmic microwave backgroundfound by the Wilkinson Microwave Anisotropy Probe(WMAP) requires VMSs forreionization in a concordanceCDM cosmology. The yieldpatterns of VMSs explodingas pair-instability supernovae areincompatible with the Fe-peakand r-process abundances inhalo stars. Models includingType II supernovae and/or"hypernovae" from zero-metallicity progenitorswith M = 840 Mcan better explain theobserved trends. We usethe …

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Space Weather Effects On Mid-Latitude Hf Propagation Paths: Observations And A Data-Driven D-Region Model, J. V. Eccles, R. D. Hunsucker, D. Rice, Jan Josef Sojka Jan 2004

Space Weather Effects On Mid-Latitude Hf Propagation Paths: Observations And A Data-Driven D-Region Model, J. V. Eccles, R. D. Hunsucker, D. Rice, Jan Josef Sojka

All Physics Faculty Publications

A two-pronged study is under way to improve understanding of the D region response to space weather and its effects on HF propagation. One part, the HF Investigation of D region Ionospheric Variation Experiment (HIDIVE), is designed to obtain simultaneous, quantitative propagation and absorption data from an HF signal monitoring network along with solar X-ray flux from the NOAA GOES satellites. Observations have been made continuously since late December 2002 and include the severe disturbances of October–November 2003. GOES satellite X-ray observations and geophysical indices are assimilated into the Data-Driven D Region (DDDR) electron density model developed as the second …

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