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Articles 8191 - 8220 of 8894

Full-Text Articles in Astrophysics and Astronomy

Simultaneous Observations Of The Continuum Emission Of The Quasar 3c 273 From Radio To Γ‐Ray Energies, G G. Lichti, C Von Montigny, T Balonek, T Courvoisier, N Johnson, Mark L. Mcconnell, W Paciesas, E I. Robson, A Sadun, C Schalinski, H Steppe, A G. Smith, R Staubert, B Swanenburg, M Turner, M Ulrich, O R. Williams Jan 1994

Simultaneous Observations Of The Continuum Emission Of The Quasar 3c 273 From Radio To Γ‐Ray Energies, G G. Lichti, C Von Montigny, T Balonek, T Courvoisier, N Johnson, Mark L. Mcconnell, W Paciesas, E I. Robson, A Sadun, C Schalinski, H Steppe, A G. Smith, R Staubert, B Swanenburg, M Turner, M Ulrich, O R. Williams

Space Science Center

From June 15–28, 1991 the Compton Gamma‐Ray Observatory (CGRO) observed the radio‐loud quasar 3C 273. All four CGRO instruments detected radiation from this quasar in their relevant energy range (from 20 keV to 5 GeV). Simultaneous and quasi‐simultaneous observations by instruments sensitive at other wavelengths have also been obtained. The data from all these observations, spanning the frequency range from ∼109 Hz–∼102 6 Hz, were collected and analyzed. Details of the observations and an overall energy‐density spectrum are presented. This spectrum shows two maxima of nearly equal strength. One is in the UV, while the other one is …

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Comptel Observations Of The Quasar Pks 0528+134, W Collmar, R Diehl, G G. Lichti, V Schonfelder, H Steinle, A W. Strong, H Bloemen, W Hermsen, Mark L. Mcconnell, James M. Ryan, G Stacy, K Bennett, O R. Williams, C Winkler Jan 1994

Comptel Observations Of The Quasar Pks 0528+134, W Collmar, R Diehl, G G. Lichti, V Schonfelder, H Steinle, A W. Strong, H Bloemen, W Hermsen, Mark L. Mcconnell, James M. Ryan, G Stacy, K Bennett, O R. Williams, C Winkler

Space Science Center

During Phase I and Phase II of the CGRO‐mission, the quasar PKS 0528+134 was in the field of view of the COMPTEL instrument during several viewing periods. The quasar was detected by COMPTEL mainly at energies above 10 MeV. Below 10 MeV there is evidence for the source during some CGRO viewing periods, while below 3 MeV no signal is detected. The detections and non‐detections during different viewing periods follow the trend seen by EGRET, thereby indicating a time‐variable MEV‐flux of the quasar. The COMPTEL spectral results together with the simultaneously measured EGRET spectrum, indicate a spectral break in the …

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Mev Emission From The Black‐Hole Candidate Gro J0422+32 Measured With Comptel, R Vandijk, H Bloemen, W Hermsen, W Collmar, R Diehl, J Greiner, G G. Lichti, V Schonfelder, A W. Strong, K Bennett, L O. Hanlon, C Winkler, Mark L. Mcconnell, James M. Ryan Jan 1994

Mev Emission From The Black‐Hole Candidate Gro J0422+32 Measured With Comptel, R Vandijk, H Bloemen, W Hermsen, W Collmar, R Diehl, J Greiner, G G. Lichti, V Schonfelder, A W. Strong, K Bennett, L O. Hanlon, C Winkler, Mark L. Mcconnell, James M. Ryan

Space Science Center

On Aug. 5, 1992 BATSE discovered the bright X‐ray transient GRO J0422+32, also known as Nova Per 1992. The COMPTEL instrument (0.75–30 MeV) aboard CGRO observed this black‐hole candidate twice. During the first observation, which started when the X‐ray flux was at its maximum, GRO J0422+32 was detected up to 2 MeV. The fluxes in the 0.75–1 and 1–2 MeV range were signficantly higher than expected based on an extrapolation of the contemporaneous OSSE data. The COMPTEL spectral points can be interpreted as evidence for a Wien‐type spectral component with temperature k T≊300 keV. During the second observation, 3 …

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Spectral Properties Of Gamma‐Ray Bursts Observed By Comptel, L O. Hanlon, K Bennett, O R. Williams, C Winkler, W Collmar, R Diehl, J Greiner, V Schonfelder, H Steinle, A W. Strong, M Varendorff, R Vandijk, J W. Den Herder, W Hermsen, L Kuiper, A Connors, R M. Kippen, Mark L. Mcconnell, James M. Ryan Jan 1994

Spectral Properties Of Gamma‐Ray Bursts Observed By Comptel, L O. Hanlon, K Bennett, O R. Williams, C Winkler, W Collmar, R Diehl, J Greiner, V Schonfelder, H Steinle, A W. Strong, M Varendorff, R Vandijk, J W. Den Herder, W Hermsen, L Kuiper, A Connors, R M. Kippen, Mark L. Mcconnell, James M. Ryan

Space Science Center

During the first year of operation, the COMPTEL instrument on board the Compton Gamma Ray Observatory detected 22 γ‐ray bursts within its field of view. Spectra and time histories for the strongest 7 of these bursts have been obtained from both the main instrument (0.75–30 MeV) and the burst modules (0.1–10 MeV). The deconvolved photon spectra for the majority of bursts are fit by a single power law model with spectral index between −1.6 and −2.8. One strong burst, GRB 910814, exhibited significant curvature and could not be fit by a single power law model. A broken power law model …

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Recent Results From Comptel Observations Of Cygnus X‐1, Mark L. Mcconnell, D J. Forrest, James M. Ryan, W Collmar, V Schonfelder, H Steinle, A W. Strong, R Vandijk, W Hermsen, K Bennett, R Much Jan 1994

Recent Results From Comptel Observations Of Cygnus X‐1, Mark L. Mcconnell, D J. Forrest, James M. Ryan, W Collmar, V Schonfelder, H Steinle, A W. Strong, R Vandijk, W Hermsen, K Bennett, R Much

Space Science Center

The COMPTEL experiment on the Compton Gamma‐Ray Observatory (CGRO) has now observed Cyg X‐1 on four separate occasions during phase 1 and phase 2 of its orbital mission (April, 1991 to August, 1993). Here we report on the results of the latest analysis of these data, which provide a spectrum extending to energies greater than 2 MeV. A spectral analysis of these data, in the context of a classical Comptonization model, indicates an electron temperature much higher than previous hard X‐ray measurements would suggest (200 keV vs 60–80 keV). This implies either some limitations in the standard Comptonization model and/or …

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Dilaton Black Holes With Electric Charge, Malik Rakhmanov Jan 1994

Dilaton Black Holes With Electric Charge, Malik Rakhmanov

Physics & Astronomy Faculty Publications

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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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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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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Oscillator Strengths For Fe Ii Transitions At 224.918 And 226.008 Nanometers, Scott D. Bergeson, K. L. Mullman, J. E. Lawler Jan 1994

Oscillator Strengths For Fe Ii Transitions At 224.918 And 226.008 Nanometers, Scott D. Bergeson, K. L. Mullman, J. E. Lawler

Faculty Publications

We report accurate experimental absorption oscillator strengths (f-values) for transitions out of the ground level of Fe II to the z4Do7/2 and z4Do9/2 levels at 224.918 and 226.008 nm (air wavelengths) to be 0.00182(14) and 0.00244(19), respectively. The number in parenthesis is the uncertainty in the last digits. These two lines are important for studying Fe abundances and grain depletions in the interstellar medium. These f-values are determined by combining emission branching fractions with radiative lifetimes. Branching fractions are measured using classical spectroradiometry on an optically thin source. Radiative lifetimes are from …

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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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Evidence Signalling The Start Of Enhanced Counterjet Flow In The Symbiotic System R Aquarii, A. G. Michalitsianos, M. Perez, Menas Kafatos Jan 1994

Evidence Signalling The Start Of Enhanced Counterjet Flow In The Symbiotic System R Aquarii, A. G. Michalitsianos, M. Perez, Menas Kafatos

Mathematics, Physics, and Computer Science Faculty Articles and Research

The velocity structure of strong far-UV emission lines observed in the symbiotic variable R Aqr suggests the start of new jet activity which will probably culminate in the appearance of a series of intense nebular emission knots within a decade. This is indicated by a systematic redward wavelength drift of emission lines, which we have followed with the International Ultraviolet Explorer (IV E) since the discovery of the brilliant northeast jet emission knots more than 10 years ago. The C IV λλ1548, 1550 resonance lines, which previously showed a prominent blue asymmetric wing that extended to velocities in excess -200 …

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A Far-Ultraviolet Atlas Of Symbiotic Stars Observed With Iue. I. The Swp Range, S. R. Meier, Menas Kafatos, R. P. Fahey, A. G. Michalitsianos Jan 1994

A Far-Ultraviolet Atlas Of Symbiotic Stars Observed With Iue. I. The Swp Range, S. R. Meier, Menas Kafatos, R. P. Fahey, A. G. Michalitsianos

Mathematics, Physics, and Computer Science Faculty Articles and Research

This atlas contains sample spectra from the far-ultraviolet observations of32 symbiotic stars obtained with the lnternational Ultraviolet Explorer (IUE) satellite. In all, 394 low-resolution spectra from the short-wavelength pnmary (SWP) camera covering the range 1200-2000 A have been extracted from the IUE archive, calibrated, and measured. Absolute line fluxes and wavelengths for the prominent emission lines have been tabulated. Tables of both the general properties of these symbiotics and of features specific to the spectrum of each are included. The spectra shown are representative of the different classes of symbiotic stars that are currently in the IUE archive. These include …

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Relativistic Particle Transport In Hot Accretion Disks, P. A. Becker, Menas Kafatos, M. Maisack Jan 1994

Relativistic Particle Transport In Hot Accretion Disks, P. A. Becker, Menas Kafatos, M. Maisack

Mathematics, Physics, and Computer Science Faculty Articles and Research

Accretion disks around rapidly rotating black holes provide one of the few plausible models for the production of intense radiation in AGNs above energies of several hundred MeV. The rapid rotation of the hole increases the binding energy per nucleon in the last stable orbit relative to the Schwarzschild case, and naturally leads to ion temperatures in the range 10^12-10^13 K for sub-Eddington accretion rates. The protons in the hot inner region of a steady, two-temperature disk form a reservoir of energy that is sufficient to power the observed EGRET outbursts if the black hole mass is 10^10 M0 • …

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Transonic Inviscid Disc Flows In The Schwarzschild Metric – I, Menas Kafatos, Ruixin Yang Jan 1994

Transonic Inviscid Disc Flows In The Schwarzschild Metric – I, Menas Kafatos, Ruixin Yang

Mathematics, Physics, and Computer Science Faculty Articles and Research

The coupled hydrodynamic equations governing equatorial flows applicable to inviscid disc accretion in the Schwarzschild metric are solved analytically and numerically. Here, we concentrate on the transonic solutions, that represent physically allowed accretion on to black holes. A polytropic equation linking gas pressure and density is assumed, and solutions are obtained for different conditions, such as isothermal and adiabatic gas flows. The dependence of those solutions on the angular momentum is explored. Under certain conditions, when there exist multiple possible sonic points, the numerical simulation automatically zeros in to the unique transonic solution passing through one of the sonic points.

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Monte Carlo Simulation Of The Scintillating Optical Fiber Calorimeter (Sofcal), Zibin Yang, Russell Gillum, Donald C. Wold Jan 1994

Monte Carlo Simulation Of The Scintillating Optical Fiber Calorimeter (Sofcal), Zibin Yang, Russell Gillum, Donald C. Wold

Journal of the Arkansas Academy of Science

A scintillating optical fiber calorimeter (SOFCAL) is being developed by NASA/Marshall Space Flight Center for use in balloon-borne emulsion chambers to study the spectrum of high-energy cosmic rays and gamma rays. SOFCAL will not saturate for long exposures, and the detector will be helpful for the study of primary cosmic-ray nuclei energies from 100 GeV to 1,000 TeV. For a given incident particle and energy, computer simulations of electromagnetic cascades allow computation of energy deposited in different regions of the calorimeter. For these initial simulations, a 5-cm x 5-cm x 7-cm calorimeter was used. Each subsection contained a 0.4-cm thick …

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Automated Classification Of Stellar Spectra: Where Are We Now?, Ted Von Hippel, L.J. Storrie-Lombardi, M.C. Storrie-Lombardi, M.J. Irwin Jan 1994

Automated Classification Of Stellar Spectra: Where Are We Now?, Ted Von Hippel, L.J. Storrie-Lombardi, M.C. Storrie-Lombardi, M.J. Irwin

Publications

We briefly review the work of the past decade on automated classification of stellar spectra and discuss techniques which show par­ticular promise. Emphasis is placed on Artificial Neural Network and Principle Component Analysis based techniques, due both to our greater familiarity with these and to their rising popularity. As an example of the abilities of current techniques we report on our automated classification work based on the visual classifications of N. Houk (Michigan Spectral Catalogue, Vol. 1 - 4, 1975, 1978, 1982, 1988).

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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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Evidence For Day-To-Night Ion Transport At Low Solar Activity In The Venus Pre-Dawn Ionosphere, J. F. Brannon, Jane L. Fox, H. S. Porter Dec 1993

Evidence For Day-To-Night Ion Transport At Low Solar Activity In The Venus Pre-Dawn Ionosphere, J. F. Brannon, Jane L. Fox, H. S. Porter

Physics Faculty Publications

Periapsis of the Pioneer Venus spacecraft dropped below 180 km on August 28, 1992 near midnight, and 42 orbits of low altitude data at moderately low solar activity in the pre-dawn sector were obtained before contact was lost to the spacecraft in October, 1992. Through a combination of analysis of data from the PV orbiter ion mass spectrometer (OIMS) and modeling, we consider here what can be learned about the relative importance of plasma transport from the dayside and electron precipitation in maintaining the nightside ionosphere during the re-entry period. In particular, we examine here the atomic ion density profiles. …

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Chromospherically Active Stars. Xi. Giants With Compact Hot Companions And The Barium Star Scenario, Francis C. Fekel, Gregory W. Henry, Michael R. Busby, Joseph J. Eitter Dec 1993

Chromospherically Active Stars. Xi. Giants With Compact Hot Companions And The Barium Star Scenario, Francis C. Fekel, Gregory W. Henry, Michael R. Busby, Joseph J. Eitter

Information Systems and Engineering Management Research Publications

We have determined spectroscopic orbits for three chromsopherically active giants that have hot compact companions. They are HD 160538 (KO III + wd, P = 904 days), HD 165141 (G8 III + wd, P approximately 5200 days), and HD 185510 (KO III + sdB, P = 20.6619 days). By fitting an IUE spectrum with theoretical models, we find the white dwarf companion of HD 165141 has a temperature of about 35,000 K. Spectral types and rotational velocities have been determined for the three giants and distances have been estimated. These three systems and 39 Ceti are compared with the barium …

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Extremely Active Long-Period Rs Cvn Binary Hd 12545, Bernard W. Bopp, Francis C. Fekel, Jason P. Aufdenberg, Robert C. Dempsey, Virgiljus Dadonas Dec 1993

Extremely Active Long-Period Rs Cvn Binary Hd 12545, Bernard W. Bopp, Francis C. Fekel, Jason P. Aufdenberg, Robert C. Dempsey, Virgiljus Dadonas

Information Systems and Engineering Management Research Publications

The active-chromosphere giant HD 12545 is noteworthy for its remarkable 1990 photmetric amplitude of 0.6 mag in V, which implies that nearly half the visible hemisphere of the star was covered by cool spots. We report the results of a spectroscopic study of HD 12545, showing it to be an active-chromosphere binary with an orbital period of 23.97 days. We establish the spectral type as KO III, and measure v sin i = 17 +/- 2 km/s. The various indicators of activity in the optical and ultraviolet suggest that HD 12545 is one of the most active RS CVn systems …

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Optical Spectroscopy Of The Dusty K5 V Star Hd 98800, Francis C. Fekel, Bernard W. Bopp Dec 1993

Optical Spectroscopy Of The Dusty K5 V Star Hd 98800, Francis C. Fekel, Bernard W. Bopp

Information Systems and Engineering Management Research Publications

High-resolution CCD spectra of the red region of the dusty K5 V star HD 98800, which may contain a protoplanetary disk, have been obtained at Kitt Peak National Observatory. The spectra verify the dwarf spectral type and show rotationally broadened photospheric lines, Hα emission, and a very strong Li Ι λ6707 absorption line, all of which confirm the star as a very young object. The overall spectrum of HD 98800 closely resembles that of the young, chromospherically active, single BY Dra star HD 82558 (=LQ Hya). The measured Li Ι equivalent width of 0.39 Å implies an abundance log N …

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Rotation Periods Of Open Cluster Stars, Ii, Charles F. Prosser, Matthew D. Shetrone, Ettore Marilli, Santo Catalano, Scott D. Williams, Dana E. Backman, Bentley D. Laaksonen, Vikram Adige, Laurence A. Marschall, John R. Stauffer Dec 1993

Rotation Periods Of Open Cluster Stars, Ii, Charles F. Prosser, Matthew D. Shetrone, Ettore Marilli, Santo Catalano, Scott D. Williams, Dana E. Backman, Bentley D. Laaksonen, Vikram Adige, Laurence A. Marschall, John R. Stauffer

Physics and Astronomy Faculty Publications

We present the results from a photometric monitoring program of 21 stars observed during 1992 in the Pleiades and Alpha Persei open clusters. Period determinations for 16 stars are given, 13 of which are the first periods reported for these stars. Brightness variations for an additional five cluster stars are also given. One K dwarf member of the a Per cluster is observed to have a period of rotation of only 4.39 hr, perhaps the shortest period currently known among BY Draconis variables. The individual photometric measurements have been deposited with the NSSDC. Combining current X-ray flux determinations with known …

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The Chemical Evolution Of Qsos And The Implications For Cosmology And Galaxy Formation, Fred Hamann, Gary J. Ferland Nov 1993

The Chemical Evolution Of Qsos And The Implications For Cosmology And Galaxy Formation, Fred Hamann, Gary J. Ferland

Physics and Astronomy Faculty Publications

We examine the chemical evolution of QSO broad-line gas by applying spectral synthesis and chemical enrichment models to the N V/C IV and N V/He II emission-line ratios. The models indicate that BLR metallicities are typically ~1 to perhaps ≳10 times solar. The enrichment must occur in ≲1 Gyr for sources where the redshift is ≳3 (if q0 = ½). The higher metallicity QSOs require star formation favoring massive stars (compared to the Galactic disk). These results imply that extensive evolution usually occurs before the QSOs become observable. Our models of the evolution are equivalent to models proposed for …

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Spot On Rs Cvn From Spectroscopy And Photometry, Joel A. Eaton, Gregory W. Henry, Coretta Bell, Albert Okorogu Sep 1993

Spot On Rs Cvn From Spectroscopy And Photometry, Joel A. Eaton, Gregory W. Henry, Coretta Bell, Albert Okorogu

Information Systems and Engineering Management Research Publications

We have used contemporaneous spectra and V light curves to form spot models for RS CVn in 1991 and 1992. More than two spots are needed to fit all the properties of the observations. In fact, moderately small spots (22 x 28 deg in latitude and longitude) having only a slight effect on the rotational light curve were eclipsed in both years, and we find that a collection of 6-8 such moderate spots is required to fit the line profiles in each year. These groups of spots also account naturally for a difference in level of light between the two …

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The Spectroscopic Orbit Of Gamma Geminorum And A Search For Its Secondary, Francis C. Fekel, Jocelyn Tomkin Sep 1993

The Spectroscopic Orbit Of Gamma Geminorum And A Search For Its Secondary, Francis C. Fekel, Jocelyn Tomkin

Information Systems and Engineering Management Research Publications

New spectroscopic observations of Gamma Geminorum, a bright, 'normal' A1 IV star have resulted in an improved spectroscopic orbit. The orbital period is 12.632 +/- 0.002 yr, the eccentricity is 0.893 +/-0.002, and the orbital parallax is 0.0291 +/-0.0024 arcsec, or a distance of 34 pc. We determine a v sin i of 8 +/-1 km/s for the A star, which confirms that it is a slow rotator. Near-infrared spectroscopic observations at 8806 A, obtained through the brief nodal passage, have resulted in the probable detection of the secondary. The masses are 2.8 and 1.07 solar mass and the Delta …

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Radial-Velocity And Light Variations Of Ir Cephei, Laurence A. Marschall, Michael A. Seeds, Robert J. Davis Sep 1993

Radial-Velocity And Light Variations Of Ir Cephei, Laurence A. Marschall, Michael A. Seeds, Robert J. Davis

Physics and Astronomy Faculty Publications

Radical-velocity observations of the short-period Cepheid, IR Cephei, have been used to derive a complete radial-velocity versus phase curve for the variable, to investigate the presence of a possible binary companion, and to address the question of its membership in the Cepheus OB2 association. The observations are consistent with the absence of a close binary companion and shed doubt on its membership in the association. Photoelectric observations made with the Phoenix 10 Automated Photometric Telescope confirm the single nature of the star. We present simulations to show the effects of an equiluminous companion on the light curve of a Cepheid, …

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On The Escape Of Oxygen And Hydrogen From Mars, Jane L. Fox Sep 1993

On The Escape Of Oxygen And Hydrogen From Mars, Jane L. Fox

Physics Faculty Publications

Escape rates of oxygen atoms from dissociative recombination of O2+ above the Martian exobase are computed in light of new information from ab initio calculations of the dissociative recombination process, and our recently revised understanding of the Martian dayside ionosphere. Only about 60% of the dissociative recombinations occur in channels in which the O atoms are released with energies in excess of the escape velocity. Futhermore, we find that the computed escape fluxes for O depend greatly on the nature of the ion loss process that has been found necessary to reproduce the topside ion density profiles measured …

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A Masing [Fe Xi] Line, Gary J. Ferland Sep 1993

A Masing [Fe Xi] Line, Gary J. Ferland

Physics and Astronomy Faculty Publications

I draw attention to a maser which occurs within the ground term of Fe10+. In many photoionized environments, infrared fine-structure lines and the [O I] λ6300 line become optically thick but maser amplification of ionic fine-structure lines is unusual. During the course of development of a code designed to simulate gas under radiative-collisional equilibrium, the radiative transfer of roughly 500 ionic/atomic emission lines was treated using escape probabilities. Nearly all forbidden lines can become optically thick under extreme conditions, but the 3Pj = 1, 0 [Fe XI] 6.08 μm transition is the only line …

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Star Formation Thresholds In Low Surface Brightness Galaxies, Stacy S. Mcgaugh Aug 1993

Star Formation Thresholds In Low Surface Brightness Galaxies, Stacy S. Mcgaugh

Faculty Scholarship

Low Surface Brightness (LSB) galaxies appear to have low star formation rates despite their often quite normal H I contents as judged from global H I properties such as MH I/L and MH I/MT ratios. H I imaging with the Very Large Array of the National Radio Astronomy Observatory (the NRAO is operated by Associated Universities Inc. under contract with the National Science Foundation) of eight LSB galaxies shows that the H I is extended compared with the optical size and has average surface densities which are about a factor 2 lower than in High Surface Brightness (HSB) galaxies of …

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The Small Scale Environment Of Low Surface Brightness Disk Galaxies, Stacy S. Mcgaugh Aug 1993

The Small Scale Environment Of Low Surface Brightness Disk Galaxies, Stacy S. Mcgaugh

Faculty Scholarship

We use a sample of ≈340 low surface brightness (LSB) disk galaxies with measured redshifts in combination with the Center for Astrophysics redshift survey to test the hypothesis that LSB galaxies have a deficit of nearby companion galaxies compared to high surface brightness (HSB) disk galaxies. We find a very strong statistical deficit of galaxies located within a projected radius of 0.5 Mpc and within a velocity of 500 km s-1 around LSB disk galaxies compared to HSB ones. Further, comparing LSB and HSB disk galaxies which are located in the same portion of the sky indicates that the average …

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