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Series

Astrophysics and Astronomy

1994

Articles 1 - 15 of 15

Full-Text Articles in Physics

The Optical-Ultraviolet-Γ-Ray Spectrum Of 3c 279, Hagai Netzer, D. Kazanas, Beverley J. Wills, D. Wills, Han Mingsheng, M. S. Brotherton, J A. Baldwin, Gary J. Ferland, I. W. A. Browne Jul 1994

The Optical-Ultraviolet-Γ-Ray Spectrum Of 3c 279, Hagai Netzer, D. Kazanas, Beverley J. Wills, D. Wills, Han Mingsheng, M. S. Brotherton, J A. Baldwin, Gary J. Ferland, I. W. A. Browne

Physics and Astronomy Faculty Publications

We have obtained spectrosocpy of the violently variable quasar 3C 279, simultaneous with γ-ray observations, in 1992 April. Our combined optical (McDonald Observatory and Cerro Tololo InterAmerican Observatory (CTIO) and ultraviolet (HST) observations, made when the source was faint, show a very steep power-law continuum (Fν∝ν-1.95) and strong broad emission lines. This is the first time that the broad ultraviolet lines of this object have been measured, and we note several unusual properties of the spectrum. In particular, the profiles of C IV λ1549 and Mg II λ …

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Raman Spectroscopic Study Of The Formation Of T-Mosi2 From Mo/Si Multilayers, Ming Cai, David D. Allred, A. Reyes-Mena Jul 1994

Raman Spectroscopic Study Of The Formation Of T-Mosi2 From Mo/Si Multilayers, Ming Cai, David D. Allred, A. Reyes-Mena

Faculty Publications

We have used Raman spectroscopy, large- and small-angle x-ray diffraction spectroscopy of sputter-deposited, vacuum-annealed, soft x-ray Mo/Si thin-film multilayers to study the physics of silicide formation. Two sets of multilayer samples with d-spacing 8.4 and 2.0 nm have been studied. Annealing at temperatures above 800 °C causes a gradual formation of amorphous MoSi2 interfaces between the Si and Mo layers. The transition from amorphous to crystalline MoSi2 is abrupt. The experimental results indicate that nucleation is the dominant process for the early stage and crystallization is the dominant process after nucleation is well advanced. In the thicker multilayer, a portion …

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Emitted Current Instability From Silicon Field Emission Emitters Due To Sputtering By Residual Gas Ions, W.I. Karain, Larry V. Knight, David D. Allred, A. Reyes-Mena Jul 1994

Emitted Current Instability From Silicon Field Emission Emitters Due To Sputtering By Residual Gas Ions, W.I. Karain, Larry V. Knight, David D. Allred, A. Reyes-Mena

Faculty Publications

We have fabricated arrays of silicon field emitters using semiconductor lithography techniques. The density of the tips was 10^5/cm^2. The maximum current that can be extracted from each emitter is limited by resistive heating. We have investigated how the electron current emitted changes under constant applied voltage. We found that the current is very sensitive to the vacuum conditions. We attribute this to sputtering of the emitters due to ionized residual gas molecules. The poorer the vacuum, the higher the instability in the current. We studied this phenomenon at 10^6 and 10-x Torr. The model of two concentric spherical shells …

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Mesostructure Of Photoluminescent Porous Silicon, David D. Allred, F. Ruiz, C. Vázquez-López, Jesus González-Hernández, G. Romero-Paredes, R. Peña-Sierra, G. Torres-Delgado Jul 1994

Mesostructure Of Photoluminescent Porous Silicon, David D. Allred, F. Ruiz, C. Vázquez-López, Jesus González-Hernández, G. Romero-Paredes, R. Peña-Sierra, G. Torres-Delgado

Faculty Publications

Scanning electron microscopy, atomic force microscopy, and Raman spectroscopy were used to characterize the microstructure of photoluminescent porous silicon (PS) layers formed by the anodic etching (HF:H2O:ethanol), at various current densities, of p-type (100) silicon wafers possessing resitivity in the range 1-2 Ω cm. Existing models for the origin of luminescence in PS are not supported by our observations. Cross-sectional as well as surface atomic force micrographs show the material to be clumpy rather than columnar; rodlike structures are not observed down to a scale of 40 nm. A three-dimensional model of the mesostructure of porous silicon is discussed. Room-temperature …

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Experimental Stark Shift Of Several Nii And Oii Spectral Lines, Mara Scepanovic, Vladimir Milosavljevic, Stevan Djenize, Mihajlo Platisa, Jaroslav Labat Jun 1994

Experimental Stark Shift Of Several Nii And Oii Spectral Lines, Mara Scepanovic, Vladimir Milosavljevic, Stevan Djenize, Mihajlo Platisa, Jaroslav Labat

Articles

Stark shift of four NII and seven OII spectral lines have been measured for the first time in the linear low pressure pulsed arc plasma and compared with existing theoretical values.

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A Protostellar Jet Model For The Water Masers In W49n, Mordecai-Mark Mac Low, Moshe Elitzur, James M. Stone, Arieh Königl Jun 1994

A Protostellar Jet Model For The Water Masers In W49n, Mordecai-Mark Mac Low, Moshe Elitzur, James M. Stone, Arieh Königl

Physics and Astronomy Faculty Publications

Observations by Gwinn, Moran, & Reid of the proper motions of water masers in W49N show that they have 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. We propose that these observations can be interpreted in terms of a shell of shocked molecular gas that is driven by the expanding cocoon of a high-velocity protostellar jet. We present three-dimensional numerical simulations in support of this interpretation and argue that this source provides a unique oppurtunity for a detailed study of jet-driven cocoons.

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Nebular Properties And The Ionizing Radiation Field In The Galactic Center, Joseph C. Shields, Gary J. Ferland Apr 1994

Nebular Properties And The Ionizing Radiation Field In The Galactic Center, Joseph C. Shields, Gary J. Ferland

Physics and Astronomy Faculty Publications

Nebulosity in the central parsec of the Milky Way exhibits a low-ionization spectrum that has led previous analyses of this region to conclude that this material is photoionized by a relatively soft continuum. We have reanalyzed the infrared emission-line spectrum of te Galactic center in order to consider whether the data could actually be explained with photoionization by a relatively hard, yet dilute continuum, and additionally to constrain the properties of the reaiating plasma. We conclude that the composite infrared spectrum does not place strong restrictions on the nature of the ionizing continuum and that much of the ionized gas …

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Hot Gas And The Origin Of The Nebular Continuum In Novae, Pedro Saizar, Gary J. Ferland Apr 1994

Hot Gas And The Origin Of The Nebular Continuum In Novae, Pedro Saizar, Gary J. Ferland

Physics and Astronomy Faculty Publications

A featureless blue continuum (with constant fν) is a defining feature of declining classical novae. The fact that fν is constant into the infrared, and the absence of a Balmer jump, suggests that this continuum originates in hot tenous gas. The electron temperature and density of the hot gas phase of classical nova QU Vul 1984 are estimated from ground-based optical and IUE ultraviolet observations. This region has a temperature of 105 - 106, and a density in the range of 2 x 104 to 3 x 105cm-3. …

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On The Theory Of Astronomical Masers In Three Dimensions, Moshe Elitzur Feb 1994

On The Theory Of Astronomical Masers In Three Dimensions, Moshe Elitzur

Physics and Astronomy Faculty Publications

In the standard theory of three-dimensional astronomical masers, the radiation field is described as if the source were comprised of a collection of linear masers. To leading order, the standard theory is shown to provide the correct description of three-dimensional masers and its results remain intact, but only within a frequency core whose half-width is χsΔνD, where ΔνD is the Doppler width and χs is a dimensionless parameter. For any given geometry, χs is ~1θsat, where θsat is the beaming angle of a maser with that geometry that has …

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Gravity And Electromagnetism In Noncommutative Geometry, Giovanni Landi, Nguyen Ai Viet, Kameshwar C. Wali Jan 1994

Gravity And Electromagnetism In Noncommutative Geometry, Giovanni Landi, Nguyen Ai Viet, Kameshwar C. Wali

Physics - All Scholarship

We present a unified description of gravity and electromagnetism in the framework of a Z 2 non-commutative differential calculus. It can be considered as a “discrete version” of Kaluza-Klein theory, where the fifth continuous dimension is replaced by two discrete points. We derive an action which coincides with the dimensionally reduced one of the ordinary Kaluza-Klein theory.

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Interstellar Gas And Dust In The Young Cluster Ic 348, Theodore P. Snow, Margaret Murray Hanson, C Gregory Seab, Jon M. Saken Jan 1994

Interstellar Gas And Dust In The Young Cluster Ic 348, Theodore P. Snow, Margaret Murray Hanson, C Gregory Seab, Jon M. Saken

Physics Faculty Publications

We have completed a multiband absorption- and emission-line study of a star embedded in the young cluster IC 348, to determine the environmental effect of star formation on the interstellar medium (ISM) local to the region. The hottest and youngest star in IC 348 is BD+31°643, a B5 V star which samples the inner bright nebular region. The nearby star o Per, which lies only 8 to the north and is thought to lie beyond IC 348, samples the gas and dust which has not been processed by very recent star formation. We speculate that the ISM throughout the region …

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Direct Simulation Monte Carlo For Thin Film Bearings, Alejandro Garcia, B. Alder, F. J. Alexander Jan 1994

Direct Simulation Monte Carlo For Thin Film Bearings, Alejandro Garcia, B. Alder, F. J. Alexander

Faculty Publications

The direct simulation Monte Carlo (DSMC) scheme is used to study the gas flow under a read/write head positioned nanometers above a moving disk drive platter (the slider bearing problem). In most cases, impressive agreement is found between the particle-based simulation and numerical solutions of the continuum hydrodynamic Reynolds equation which has been corrected for slip. However, at very high platter speeds the gas is far from equilibrium, and the load capacity for the slider bearing cannot be accurately computed from the hydrodynamic pressure.

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Microscopic Simulation Of Dilute Gases With Adjustable Transport Coefficients, Alejandro Garcia, F. Baras, M. Malek Mansour Jan 1994

Microscopic Simulation Of Dilute Gases With Adjustable Transport Coefficients, Alejandro Garcia, F. Baras, M. Malek Mansour

Faculty Publications

The Bird algorithm is a computationally efficient method for simulating dilute gas flows. However, due to the relatively large transport coefficients at low densities, high Rayleigh or Reynolds numbers are difficult to achieve by this technique. We present a modified version of the Bird algorithm in which the relaxation processes are enhanced and the transport coefficients reduced, while preserving the correct equilibrium and nonequilibrium fluid properties. The present algorithm is found to be two to three orders of magnitude faster than molecular dynamics for simulating complex hydrodynamical flows.

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Rate Coefficient For The Reaction N + No, Jane L. Fox Jan 1994

Rate Coefficient For The Reaction N + No, Jane L. Fox

Physics Faculty Publications

Evidence has been advanced that the rate coefficient for the reaction N + NO → N2; + O has a small positive temperature dependence at the high temperatures (900-1500 K) that prevail in the terrestrial middle and upper thermosphere by Siskind and Rusch (1992), and at the low temperatures (100-200 K) of the Martian lower thermosphere by Fox (1993). Assuming that the rate coefficient recommended by the Jet Propulsion Laboratory evaluation (DeMore et al., 1992) is accurate at 300 K, we derive here the low temperature value of the activation energy for this reaction and thus the rate …

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Dilaton Black Holes With Electric Charge, Malik Rakhmanov Jan 1994

Dilaton Black Holes With Electric Charge, Malik Rakhmanov

Physics and Astronomy Faculty Publications and Presentations

New static spherically symmetric solutions of the Einstein-Maxwell gravity with the dilaton field are found. The solutions correspond to black holes and naked singularities. In addition to mass and electric charge these solutions are labeled by a new parameter, the dilaton charge of the black hole. Depending on the values of electric and dilaton charges there are different types of solutions. The solutions exhibit a new type of a symmetry. Namely one solution transforms into another when the mass and the dilaton charge are interchanged. We also found that there is a finite interval of values of electric charge for …

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