Astrophysics and Astronomy Commons^™

Upper Limits To The Nightside Ionosphere Of Mars, Jane L. Fox

Physics Faculty Publications

The nightside ionosphere of Mars could be produced by electron precipitation or by plasma transport from the dayside, by analogy to the Venus, but few measurements are available. We report here model calculations of upper limits to the nightside ion densities on Mars that would be produced by both mechanisms. For the auroral model, we have adopted the downward traveling portions of the electron spectra measured by the HARP instrument on the Soviet Phobos spacecraft in the Martian plasma sheet and in the magnetotail lobes. For the plasma transport case, we have imposed on a model of the nightside thermosphere, …

The Narrow-Line Region Of High-Luminosity Active Galactic Nuclei, Beverley J. Wills, H. Netzer, M. S. Brotherton, Mingsheng Han, D. Wills, J. A. Baldwin, Gary J. Ferland, I. W. A. Browne

Physics and Astronomy Faculty Publications

We have made high signal-to-noise spectroscopic observations of seven radio-loud quasars with the Faint Object Spectrograph on the Hubble Space Telescope and from the ground at McDonald Observatory and at Kitt Peak National Observatory. The resolution is 300-400 km s^-1 over the wavelength range 1000-8500 Å, enabling us to separate the broad and narrow components of the emission lines. This is the first study of the optical and UV narrow lines in such high-luminosity active galactic nuclei (AGNs).

The most important and striking observational result is the relative weakness of the narrow ultraviolet lines, assuming that they have …

The Dynamics Of Stellar Outflows Dominated By Interaction Of Dust And Radiation, Nathan Netzer, Moshe Elitzur

Physics and Astronomy Faculty Publications

Wind outflow around a late-type star driven by radiation pressure on dust grains is investigated in detail. The equation of motion for the outflow coupled with the equation of radiative transfer is solved treating the circumstellar envelope, which consists of gas and dust, as a two-component fluid. Because of the drift of the dust particles through the gas, the dust-to-gas ratio varies with distance even if grain formation is a prompt process. The coupling between dust and gas weakens as the mass-loss rate decreases until finally the rate of momentum transfer to the gas is insufficient to overcome gravity when …

Large Amplitude L=1 Coherent Structures In Non-Neutral Plasmas Confined In A Cylindrical Trap, Ross L. Spencer, Grant W. Mason

Faculty Publications

The computation of l= 1 coherent structures in non-neutral plasmas with arbitrary density profiles and for large displacements of the plasma from the symmetry axis of a confining cylindrical trap is described. As the structures are displaced from the axis, they revolve about the symmetry axis with a frequency that typically increases with displacement. The plasma also is distorted into an approximately elliptical shape. The frequency shifts and the eccentricities as a function of displacement, plasma size, and the shape of the density profile are both computed numerically and calculated analytically. The results are shown to be consistent with data …

Photoluminescence And Absorption Studies Of Defects In Cdte And Znxcd1-Xte Crystals, Cheryl Barnett Davis, David D. Allred, A. Reyes-Mena, Jesus González-Hernández, Ovidio González, Bret C. Hess, Worth P. Allred

Faculty Publications

We have studied at cryogenic temperatures photoluminescence features which lie more than 0.15 eV below the band edge in ZnxCd1-xTe (0≤x≤0.09) crystals. The same features, namely a defect band which lies at about 0.13-0.20 eV below the band-gap energy and a peak at 1.1 eV, that are observed in pure CdTe samples are observed in these alloy materials. In annealed samples we observe that the 1.1 eV feature, which has been attributed to tellurium vacancies, increases with fast cooling. Increased concentrations of tellurium vacancies can be understood in terms of the phase diagram of CdTe which indicates that higher concentrations …

Distinguishing A Charged Higgs Signal From A Heavy Wr Signal, David I. Kaiser

Dartmouth Scholarship

It is shown that non-Standard Model bosons should obey an observable asymmetry in their decays to taus. This asymmetry enables a distinction to be made between charged Higgsboson signalsand heavy right-handed Wboson signals,by reconstructing the orientation of the z with respect to the beam axis.

The Measurement Of Astronomical Parallaxes With Ccd Imaging Cameras On Small Telescopes, Stephen J. Ratcliff, Thomas J. Balonek, Laurence A. Marschall, David L. Dupuy, Carlton R. Pennypacker, Ritu Verma, Anastasia Alexov, Vivian Bonney

Physics and Astronomy Faculty Publications

Small telescopes equipped with charge-coupled device (CCD) imaging cameras are well suited to introductory laboratory exercises in positional astronomy (astrometry). An elegant example is the determination of the parallax of extraterrestrial objects, such as asteroids. For laboratory exercises suitable for introductory students, the astronomical hardware needs are relatively modest, and under the best circumstances, the analysis requires little more than arithmetic and a microcomputer with image display capabilities. Results from the first such coordinated parallax observations of asteroids ever made are presented. In addition, procedures for several related experiments, involving single-site observations and/or parallaxes of earth-orbiting artificial satellites, are outlined.

Exponential Growth Of An Unstable L=1 Diocotron Mode For A Hollow Electron Column In A Warm-Fluid Model, S. Neil Rasband, Ross L. Spencer, Richard R. Vanfleet

Faculty Publications

Numerical investigations of a warm-fluid model with an isothermal equation of state for the perpendicular dynamics of an axisymmetric, magnetically confined pure electron plasma predict an exponentially unstable, l=1, diocotron mode for hollow density profiles. The unstable mode can be identified with a stable, nonsmooth mode that exists in cold drift models but which is destabilized by finite temperature effects. The unstable mode has many properties similar to the experimental results reported by Driscoll [Phys. Rev. Lett. 64, 645 (1990)].

Low-Frequency Feature In The First-Order Raman Spectrum Of Amorphous Carbon, David D. Allred, Qi Wang, Jesus González-Hernández

Faculty Publications

In the first-order Raman spectrum of amorphous carbon (a-C) there is a low-frequency feature in the 200-900-cm-1 region. This feature is characteristic of the highly disordered amorphous-carbon materials. We note that the intensity of this feature is very sensitive to the thermal history of samples, thus suggesting that it is an important measure of the degree of disorder of the a-C materials. We also discuss the relationship between this feature and the phonon density of states of graphite.

Quantam Wave Turbulence, Alejandro Garcia, M. Haeri, S. Putterman, P. Roberts

Faculty Publications

The nonlinear quantum kinetic equation for the interaction of sound waves is solved via analytic and numerical techniques. In the classical regime energy cascades to higher frequency (ω) according to the steady-state power law ω-3/2. In the quantum limit, the system prefers a reverse cascade of energy which follows the power law ω-6. Above a critical flux, a new type of spectrum appears which is neither self-similar nor close to equilibrium. This state of nonlinear quantum wave turbulence represents a flow of energy directly from the classical source to the quantum degrees of freedom.

Time Projection Chamber's Efficiency, Obtained Using Cern's Geant Code, Christine A. Byrd, Charles M. Byrd, Wilfred J. Braithwaite

Journal of the Arkansas Academy of Science

Geometrical acceptance and reconstruction of tracks have been carried out for a Time Projection Chamber (TPC) used in Experiment NA35: the 35th experiment in the North Area of the Super Proton Synchrotron (SPS), located at the European Organization for Nuclear Research (CERN). NA35 used the SPS at CERN to produce 6.4 TeV beams of 32S for central collisions with Au nuclei. The TPC modeling effort used a modified version of CERN's Monte Carlo program GEANT, which simulates the response of the NA35 TPC to output from CERN's primary event generators. GEANT was used to simulate three-dimensional pixel data in the …

The Production And Escape Of Nitrogen Atoms On Mars, Jane L. Fox

Physics Faculty Publications

We have computed the production rates and densities of odd nitrogen species in the Martian atmosphere using updated rate coefficients and a revised ionosphere-thermosphere model. We find that the computed densities of NO are somewhat smaller than those measured by Viking 1, but reasonable agreement can be obtained by assuming that the rate coefficient for loss of odd nitrogen in the reaction of N with NO is smaller at temperatures that prevail in the lower Martian thermosphere (about 130–160 K) than the standard value, which applies to temperatures of 200–400 K. We have also modeled the escape fluxes of N …

On The System Of Diffuse Interstellar Bands At 5844 And 5850 Å, J Krelowski, Theodore P. Snow, J Papaj, C G. Seab, B Wszolek

Physics Faculty Publications

Two neighboring diffuse interstellar bands (DIBs) at 5844 and 5850 Å are shown to be of different origin. The presence or absence of the 5844 DIB is related to the ratio of the two prominent DIBs at 5780 and 5797 Å as well as to the nature of the ultraviolet extinction curve. The 5844 Å DIB is very sensitive to the extinction, being completely absent in cases where the far-UV extinction is low. This suggests that the carrier is destroyed by photons having energies above 11 or 12 eV. The 5850 Å DIB correlates very well with the well-studied DIB …

Monte Carlo Director Modeling And Display, Using The Cern Laboratory, Christine A. Byrd, Charles M. Byrd, Wilfred J. Braithwaite

Journal of the Arkansas Academy of Science

Detectors for high energy nuclear physics experiments are being modeled using programs developed and maintained at CERN, the European Organization for Nuclear Research. These programs include data handling and display routines, as well as those using random-sampling Monte Carlo techniques to calculate energy depositions for high energy particles as they pass through the various parts of the detector system. The complete CERN library has been imported for use with our Workstation computers in a multiple user environment. The enormous CERN Monte Carlo program GEANT(French for GIANT) tracks the progress of a particle through a detector on a simulated event-by-event basis. …

Modeling Sio Maser Emission From Late-Type Stars, Philip B. Lockett, Moshe Elitzur

Physics and Astronomy Faculty Publications

We have performed a thorough study of both radiative and collisional pumping of the SiO masers around late-type stars, carefully considering the combined and separate actions of each type of pump in order to gauge its effectiveness. We find that collisional pumping is severely underestimated when the model calculations use a small number (less than about 18) of rotational levels in each vibrational state. We have developed a procedure that corrects this problem and gives results that are nearly independent of the number of levels utilized in the calculations. We recognize, but do not solve, an important problem that afflicts …

Linear Theory Of Non-Neutral Plasma Equilibrium In A Tilted Magnetic Field, Ross L. Spencer, Grant W. Hart

Faculty Publications

A linear perturbation expansion has been found that allows the rapid and accurate calculation of the response of a non-neutral plasma to a tilted magnetic field. The results of the calculation have been found to agree with previous three-dimensional equilibrium calculations, and also to agree with Keinigs' [Phys. Fluids 24, 860 (1981)] calculation of zero-frequency resonances caused by magnetic field errors. This expansion also allows the perturbed velocity to be calculated. It is speculated that this perturbed flow may be related to the enhanced radial transport in a non-neutral plasma with a tilted magnetic field.

Use Of Raman Spectroscopy In Characterizing Soft X-Ray Multilayers: Tools In Understanding Structure And Interfaces, Ming Cai, Qi Wang, David D. Allred, Larry V. Knight, Dorian M. Hatch, A. Reyes-Mena, Guizhong Zhang

Faculty Publications

Our group is studying the structure and interfaces of soft x-ray multilayers by various techniques including x-ray diffraction and Raman spectroscopy. Raman spectroscopy is particularly useful since it is sensitive to the identity of individual bonds and thus can potentially characterize the abruptness of interfaces in multilayers. Blocking interfacial mixing is very important in achieving and maintaining high reflectivity. We report our studies of the as-deposited and postannealed structure of Mo/Si and W/C multilayers. The Mo/Si system is probably the most widely studied multilayer currently because of its potential applications for soft x-ray projection lithography for the range of 13 …

Planar H2O Masers In Star-Forming Regions, Moshe Elitzur, David J. Hollenbach, Christopher F. Mckee

Physics and Astronomy Faculty Publications

The planar geometry of shocked material is the key property in enabling the high brightness temperatures of H20 masars in star-forming regions. We solve for the brightness temperature, the beaming angle, and the maser spot size for thin, saturated planar masers under the assumption that the velocity change across the maser due to ordered motions is small compared with the thermal or microturbulent line width. For a given set of physical parameters, the brightness temperature is essentially fully determined by the length of the velocity-coherent region in the shocked plane along the line of sight. The geometry in the transverse …

Water Masers In W49n - The Youngest Stellar Jet?, Mordecai-Mark Mac Low, Moshe Elitzur

Physics and Astronomy Faculty Publications

Observations by Gwinn, Moran, and Reid of the proper motions of water masers in W49N show that they have an elongated distribution expanding from a common center. Features with high space velocity only occur far from the center, while low-velocity features occur at all distances. It is proposed that water masers in star-forming regions occur in expanding shells swept up by high-velocity winds from young, massive stars during the early phases of the expansion. In W49N, confinement of the bubble by a density distribution with an axial cavity can explain both the velocity field and the shape of the maser …

Pressure Dependence Of The Thermal Conductivity Of Pyrophyllite To 40 Kbar, Wei Chen, Daniel L. Decker

Faculty Publications

A mathematical model for calculating the temperature distribution as a function of power delivered to a line source and the thermal conductivity of the surrounding medium in the pressure cell of a cubic-anvil press was derived. The model will handle anisotropic thermal conductivities. A simple sample assembly consisting of a line source and two or three thermocouple junctions is described. A comparison of measured to calculated temperatures yields the thermal conductivity. Thermal conductivity measurements were made on natural pyrophyllite and baked pyrophyllite to 40 kbar. For the natural pyrophyllite the thermal conductivity parallel to the bedding plane at room temperature …

Slip Length In A Dilute Gas, Alejandro Garcia, D. Morris, L. Hannon

Faculty Publications

We study the phenomenon of slip length using molecular dynamics and direct simulation Monte Carlo simulations of a dilute gas. Our work extends the range of Knudsen numbers that have been previously studied. In a recent paper, Bhattacharya and Lie [Phys. Rev. 43, 761 (1991)] suggest a logarithmic dependence of slip length on Knudsen number. By a simple redefinition of the mean free path, we obtain good agreement between simulation results and Maxwell theory for slip length. The anomalies seen by Bhattacharya and Lie appear to be due to their definition of the mean free path.

Science On The Great Plains: The History Of Physics And Astronomy At The University Of Nebraska-Lincoln, M. Eugene Rudd

M. Eugene Rudd Publications

The year 1988 marked the 100th anniversary of the founding of the Department of Physics at the University of Nebraska. While the year passed without fanfare in the department, DuaneJaecks, one of the members of the faculty, suggested to the author that it would be a good time to write a history of what is now the Department of Physics and Astronomy. He not only provided the initial impetus for the project, but has also given it his continual help and encouragement. While the initial plan was to write only a brief, informal departmental history, the project, like Topsy, grew. …

Spatial Correlation Between Ch, Cn And The Diffuse Interstellar Band Carriers, J Krelowski, T P. Snow, C G. Seab, J Papaj

Physics Faculty Publications

Observations are presented of the diffuse interstellar bands at 5780 and 5797 A and of the ultraviolet lines of CH and CN in the light of six bright stars which lie behind isolated interstellar clouds. It is found that CN and CH are only present when the 5797 band is deeper than the 5780 one. Comparisons with satellite measurements of the UV extinction show that the shape of the extinction curve is linked to the same band ratio in these stars. The results support a previous suggestion that the various components of the absorption spectrum of an isolated cloud (the …

Manufacturing Of Atomically Sharp Silicon Tips And Their Use As Photocathodes, W. I. Karian, Larry V. Knight, David D. Allred, A. Reyes-Mena

Faculty Publications

The discovery and understanding of the photoelectric effect led to the study of photoemissive materials fall into two major categories: classical photoemitters and negative-electron-affinity (NEA) materials. Classical photoemitters usually involve an alkali metal, a group-V element such as phosphorus, silver, and/or oxygen. An example is the Ag-O-Cs (S1) photoemitter. NEA photocathodes consist of a photoconductive single crystal semiconductor covered with a thin layer of cesium and oxygen. This layer lowers the work function of the photocathode. A dipole layer is formed at the surface, and band bending occurs. This lowers the effective work function. An example is the GaAs(CsO) photocathode …

Characterization Of As-Prepared And Annealed W/C Multilayer Thin Films, David D. Allred, Qi Wang, Jesus González-Hernández, B. S. Chao, D. A. Pawlik

Faculty Publications

Tungsten/carbon (W/C) multilayer thin films were prepared by dc magnetron sputtering. All samples consisted of 30 layer pairs with a nominal d-spacing varying from 2.5 to 14 nm, the W layer thickness was kept at 2 nm in all samples. The W/C multilayers were subjected to isochronal anneals in a quartz tube furnace at the temperature range from 500 to 950 °C under a flow of high purity Ar gas. X-ray diffraction, Raman scattering, and Auger depth profile were used to characterize the structure of the as-prepared and annealed multilayer films. Both the W and C layers appear to be …

X-Ray Diode Using A Silicon Field Emission Photocathode, W. I. Karian, Larry V. Knight, David D. Allred, A. Reyes-Mena

Faculty Publications

We have produced arrays of 10,000 sharp p-type silicon points using an etch plus oxidation method. The points were used as electron emitters. No high vacuum cesiation or high temperature cleaning was needed to observe the electron emission. These are seen to be photosensitive sources of electrons at 200 K and 300 K. They were also used to produce AlKα x-rays. This constitutes the first use of etched, point arrays for generating electrons for x-ray sources.

Second-Order Parametrized-Post-Newtonian Lagrangian, Matthew J. Benacquista

Physics & Astronomy Faculty Publications

A many-body Lagrangian to second post-Newtonian order using an extension of the parametrized-post-Newtonian (PPN) formalism is introduced and the properties of new parameters are explored. A parametrized gauge transformation is developed to permit comparison with theories of gravity in a variety of different coordinate systems. A procedure to impose Lorentz invariance on a general second-order post-Newtonian Lagrangian is developed. The Lagrangian is then constrained to possess Lorentz invariance and a "Lorentz-invariant" gauge is introduced. The constrained Lagrangian is found to be described by ten new second-order PPN parameters. When the Lagrangian is further constrained to describe theories of gravity for …

The Effect Of A Tilted Magnetic Field On The Equilibrium Of A Pure Electron Plasma, Grant W. Hart

Faculty Publications

If the magnetic field in a pure electron plasma containment device is not aligned with the axis of the conducting walls, the electrons in the device will accumulate at the ends of the plasma where the magnetic field lines come closest to the walls and the electrons bound to the field lines can be closest to their image charges. If the plasma is also offset radially from the center (as with an l=1 diocotron mode), then more density will accumulate at one end than the other. As the plasma revolves around the center, the electrons will slosh from one end …

Fluctuations In Astronomical Masers, Moshe Elitzur

Physics and Astronomy Faculty Publications

The radiation of astronomical masers fluctuates on the time scale 1/Gamma, where Gamma is the levels' loss rate. These intensity fluctuations reflect fluctuations of the level poulations around their mean, steady state values over the length scale λ_c = c/Γ. In saturated masers, the intensity fluctuations are dominated by passage through the unsaturated core. The effects of the saturated zones and of the seed radiation that the masers amplify can be neglected.

These fluctuations may have been detected in recent observations by Clegg and Cordes (1991) of Galactic H II/OH masers, providing a possible direct determination of the …

Polarization Of Astronomical Maser Radiation, Moshe Elitzur

Physics and Astronomy Faculty Publications

The polarization of maser radiation when the source is permeated by an aligned magnetic field is derived for arbitrary angular momenta of the transition states. This generalization is made possible by an analysis of the structure of the propagating waves in a frame aligned with the magnetic axis. The key elements in determining the polarization properties are the assumption of independent and incoherent pump and loss processes for all magnetic sublevels, and the beaming of maser radiation. The radiation propagating in the direction of maximal intensity growth is polarized according to the solutions derived by Goldreich, Keeley, and Kwan for …