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  • ASTROPHYSICS  (1.236)
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  • 1
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    Science, citation, and funding (1991)
    American Association for the Advancement of Science (AAAS)
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    Publikationsdatum: 1991-03-22
    Beschreibung: 〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉New York, N.Y. -- Science. 1991 Mar 22;251(5000):1408-10.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/2006414" target="_blank"〉PubMed〈/a〉
    Schlagwort(e): Astronomical Phenomena ; Astronomy ; Physical Phenomena ; Physics ; Publishing/*trends ; Research Support as Topic ; Science/*methods ; Sociology
    Print ISSN: 0036-8075
    Digitale ISSN: 1095-9203
    Thema: Biologie , Chemie und Pharmazie , Informatik , Medizin , Allgemeine Naturwissenschaft , Physik
    Publiziert von American Association for the Advancement of Science (AAAS)
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  • 2
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    The best-fit universe (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: Inflation provides very strong motivation for a flat Universe, Harrison-Zel'dovich (constant-curvature) perturbations, and cold dark matter. However, there are a number of cosmological observations that conflict with the predictions of the simplest such model: one with zero cosmological constant. They include the age of the Universe, dynamical determinations of Omega, galaxy-number counts, and the apparent abundance of large-scale structure in the Universe. While the discrepancies are not yet serious enough to rule out the simplest and most well motivated model, the current data point to a best-fit model with the following parameters: Omega(sub B) approximately equal to 0.03, Omega(sub CDM) approximately equal to 0.17, Omega(sub Lambda) approximately equal to 0.8, and H(sub 0) approximately equal to 70 km/(sec x Mpc) which improves significantly the concordance with observations. While there is no good reason to expect such a value for the cosmological constant, there is no physical principle that would rule out such.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Primordial nucleosynthesis and evolution of early universe; Proceedings of the International Conference, Tokyo, Japan, Sept. 4-8, 1990 (A93-17626 05-90); p. 337-350.
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  • 3
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    Pulsar polarization measurements and the nonsymmetric gravitational theory (1991)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Because of the breakdown of the Einstein equivalence principle in the nonsymmetric gravitational theory (NGT) of Moffat, orthogonally polarized electromagnetic waves can propagate at different velocities in a gravitational field. Moffat has proposed that galactic dark matter, in the form of cosmions, may act as a significant source of gravity in the NGT. We discuss how observations of the highly polarized radiation from distant pulsars could provide significant limits on the strength of the coupling of cosmions in the NGT.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Physical Review D - Particles and Fields, 3rd Series (ISSN 0556-2821); 44; 8; p. R2211-R2213.
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  • 4
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    The hypersonic double ellipse in rarefied flow (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: The problem of the hypersonic double ellipse in rarefied flow is treated by a particle method using the collision model first described by McDonald (1988). In the approach used here, the computational overhead is reduced by using simple cubic cells. The problem of the definition of complex geometries is addressed by developing an algorithm to define the relation of a body surface to the network of cells.
    Schlagwort(e): AERODYNAMICS
    Materialart: In: Hypersonic flows for reentry problems. Vol. 2 (A93-42576 17-02); p. 912-923.
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  • 5
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    Application of program LAURA to thermochemical nonequilibrium flow through a nozzle (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: Program LAURA (Langley Aerothermodynamic Upwind Relaxation Algorithm) is an upwind-biased, point-implicit relaxation algorithm for obtaining the numerical solution to the governing equations for 3D viscous hypersonic flows in chemical and thermal nonequilibrium. The algorithm is derived using a finite-volume formulation in which the inviscid components of flux across cell walls are described with a modified Roe's averaging and with second-order corrections based on Yee's Symmetric Total Variation Diminishing scheme. The code has been applied to Problem 8.2 of this workshop for the case of thermochemical nonequilibrium flow through a nozzle. Chemical reaction rates are defined with the model of Park (1987). Thermal nonequilibrium is modeled using a two-temperature approximation in which the vibrational energies of all molecules are assumed to be in equilibrium at a single temperature which is generally different from the translational-rotational temperature. Two grids were used to define the flow for the original problem, with a stagnation temperature of 6500 K. A third case with a stagnation temperature of 10,000 K is also presented. The solution domain includes the converging nozzle, subsonic flow domain in which the gas is substantially in thermochemical equilibrium and the diverging nozzle, hypersonic flow domain in which the gas is substantially in thermochemical nonequilibrium.
    Schlagwort(e): AERODYNAMICS
    Materialart: In: Hypersonic flows for reentry problems. Vol. 2 (A93-42576 17-02); p. 1145-1158.
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  • 6
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    Hypersonic cone flow predictions using an implicit upwind space-marching code (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: Solutions have been computed and results are presented for Problem 1, the case of Mach 9 transitional flow past a 7 deg half-angle cone at zero incidence. The solutions were computed using a code developed for the integration of the parabolized Navier-Stokes equations. The algorithm employed in the code is based on a Roe-type flux-difference-splitting scheme applied following a finite-volume approach. The basic algorithm has been modified to make it implicit and second-order accurate in the crossflow directions. Results are presented in terms of surface pressure and heat transfer as well as boundary layer profiles of pitot pressure, Mach number, and tangential velocity. The case was recalculated several times in an effort to determine sensitivities to such parameters as grid density, wall temperature, turbulence model parameters, as well as freestream expansion. Comparisons with the experimental data are presented and discussed.
    Schlagwort(e): AERODYNAMICS
    Materialart: In: Hypersonic flows for reentry problems. Vol. 2 (A93-42576 17-02); p. 75-91.
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  • 7
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    Correlation of airloads on a two-bladed helicopter rotor (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: Airloads measured on a two-bladed helicopter rotor in flight, from the Tip Aerodynamic and Acoustic Test, are compared with calculations from a comprehensive helicopter analysis (CAMRAD/JA), and the pressures compared with calculations from a full-potential rotor code (FPR). The flight test results cover an advance ratio range from 0.19 to 0.38. The lowest speed case is characterized by the presence of significant blade-vortex interactions. Good correlation of peak-to-peak vortex-induced loads and the corresponding pressures is obtained. The results of the correlation for this two-bladed rotor are substantially similar to the results for three- and four-bladed rotors, concerning the tip vortex core size for best correlation, calculation of the peak-to-peak loads on the retreating side, and calculation of vortex-induced loads on inboard radial stations.
    Schlagwort(e): AERODYNAMICS
    Materialart: In: AHS and Royal Aeronautical Society, Technical Specialists' Meeting on Rotorcraft Acoustics(Fluid Dynamics, Philadelphia, PA, Oct. 15-17, 1991, Proceedings (A93-29401 10-71); 38 p.
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  • 8
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    The development of a CFD potential method for the analysis of tilt-rotors (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: A new CFD potential code, FPX (eXtended Full-Potential), has been developed for application to both helicopters and tilt-rotors. The code solves the unsteady, three-dimensional full potential equation and is an extension of the rotor code, FPR. Both entropy and viscosity corrections are included to enhance the physical modeling capabilities. A number of efficiency related modifications have yielded a factor of two speed-up in the code. An axial flow capability has been added to treat tilt-rotor in forward flight (cruise mode). In order to employ streamwise periodicity and accurately solve for the propagation of acoustic signals in the tip region, an H-H topology has been added to the basic O-H grid system. Computations are performed for the XV-15 Standard and ATB blades at high-speed conditions. Comparisons are made for the blade aerodynamics and the induced fuselage cabin pressure for a range of Mach numbers. Grid generation, wake treatment, and far-field wall treatment are identified as problem areas with recommendations for future research.
    Schlagwort(e): AERODYNAMICS
    Materialart: In: AHS and Royal Aeronautical Society, Technical Specialists' Meeting on Rotorcraft Acoustics(Fluid Dynamics, Philadelphia, PA, Oct. 15-17, 1991, Proceedings (A93-29401 10-71); 15 p.
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  • 9
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    Recent developments in rotor wake modeling for helicopter noise prediction (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: A preliminary test/theory correlation evaluation is conducted for wake measurement test results obtained by LDV for a B360 helicopter rotor, at conditions critical to the understanding of wake-rollup and blade-vortex interaction phenomena. The LDV data were complemented by acoustic, blade pressure, rotor performance, and blade/control load measurements.
    Schlagwort(e): AERODYNAMICS
    Materialart: In: AHS and Royal Aeronautical Society, Technical Specialists' Meeting on Rotorcraft Acoustics(Fluid Dynamics, Philadelphia, PA, Oct. 15-17, 1991, Proceedings (A93-29401 10-71); 16 p.
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  • 10
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    Cosmic evolution of Quasar radio structure (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: We discuss the results of a survey of Quasar radio structures over redshifts from 0.6 to 3.7. There are clear evolutionary trends in size and luminosity, which suggest that the duty cycle of individual Quasars has increased over cosmic time. This affects source count statistics and gives clues on the evolution of Quasar environments.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: The space distribution of quasars; Proceedings of the Workshop, Victoria, Canada, June 3-5, 1991 (A93-28776 10-90); p. 331, 332; Discuss
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  • 11
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    Are there two disk star cluster systems in the LMC? (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: The surface distribution of star clusters in the LMC has revealed the existence of two elliptical systems superimposed. A small surface density outer ellipse outlining the large system and a higher density elliptical inner system. The major axis of the two subsystems form an angle of 60 deg. The central subsystem contains all youngest populous globular star clusters from both the stellar and dynamical point of view. The large outer system seems to contain all old globular clusters, showing a dumpy distribution and outlining two arms at the northeast and southwest. The small LMC clusters were found to have masses less than 1000 solar masses and are distributed all over the large elliptical system.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: The formation and evolution of star clusters (A93-48676 20-90); p. 407-409.
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  • 12
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    Early phases of LMC star clusters? (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: On the basis of optical and IUE observations we have investigated the possibility that some of the very young stellar systems of the LMC, such as SL360, may be regarded as globular clusters at the very early phases of their dynamical evolution after the gas cloud expulsion.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: The formation and evolution of star clusters (A93-48676 20-90); p. 404-406.
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  • 13
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    Grid-refinement study of hypersonic laminar flow over a 2-D ramp (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: Computations were made for those test cases of Problem 3 which were designated as laminar flows, viz., test cases 3.1, 3.2, 3.4, and 3.5. These test cases corresponded to flows over a flat plate and a compression ramp at high Mach number and at high Reynolds number. The computations over the compression ramps indicate a substantial streamwise extent of separation. Based on previous experience with separated laminar flows at high Mach numbers which indicated a substantial effect with spatial grid refinement, a series of computations with different grid sizes were performed. Also, for the flat plate, comparisons of the results for two different algorithms were made.
    Schlagwort(e): AERODYNAMICS
    Materialart: In: Hypersonic flows for reentry problems. Vol. 2 (A93-42576 17-02); p. 244-254.
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  • 14
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    Computational fluid dynamics for hypersonic airbreathing aircraft (1991)
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    Publikationsdatum: 2011-08-24
    Beschreibung: A development status evaluation is presented for CFD methods applicable to fuselage-integrated scramjet powerplant incorporating hypersonic vehicles; these methods are critically important due to the unavailability of experimental facilities for such elevated Mach number/high-enthalphy conditions. Advancements are required in algorithm robustness and speed, geometric flexibility, and the inclusion of more complete flow physics. The most serious deficiencies lie in turbulence modeling, the lack of complete transition-prediction methods, and combustion modeling.
    Schlagwort(e): AERODYNAMICS
    Materialart: In: Hypersonic flows for reentry problems. Vol. 1 (A93-42576 17-02); p. 55-71.
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  • 15
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    CFD for hypersonic propulsion (1991)
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    Publikationsdatum: 2011-08-24
    Beschreibung: An overview is given of research activity on the application of computational fluid dynamics (CDF) for hypersonic propulsion systems. After the initial consideration of the highly integrated nature of air-breathing hypersonic engines and airframe, attention is directed toward computations carried out for the components of the engine. A generic inlet configuration is considered in order to demonstrate the highly three dimensional viscous flow behavior occurring within rectangular inlets. Reacting flow computations for simple jet injection as well as for more complex combustion chambers are then discussed in order to show the capability of viscous finite rate chemical reaction computer simulations. Finally, the nozzle flow fields are demonstrated, showing the existence of complex shear layers and shock structure in the exhaust plume. The general issues associated with code validation as well as the specific issue associated with the use of CFD for design are discussed. A prognosis for the success of CFD in the design of future propulsion systems is offered.
    Schlagwort(e): AERODYNAMICS
    Materialart: In: Hypersonic flows for reentry problems. Vol. 1 (A93-42576 17-02); p. 170-186.
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  • 16
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    Concept for the subresolution measurement of earthquake strain fields using SPOT panchromatic imagery (1991)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The use of satellite-derived imagery for measuring subresolution horizontal terrain displacments associated with present-day earthquakes is discussed with reference to data from the French SPOT satellite whose sensor array provides 10-m panchromatic imagery. The measured terrain displacements can be up to several meters, but usually no more than 6-8 m even for major earthquakes. The general approach is to spatially match the after image to the before image at each point on a half-kilometer grid by iteratively interpolating one and testing its correlation with the other. The discussion covers the basic algorithm, results of initial tests, error types and limitations, and future work.
    Schlagwort(e): GEOPHYSICS
    Materialart: In: Earth and atmospheric remote sensing; Proceedings of the Meeting, Orlando, FL, Apr. 2-4, 1991 (A93-24176 08-42); p. 370-377.
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  • 17
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    Spectral stratigraphy (1991)
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    Publikationsdatum: 2011-08-24
    Beschreibung: A new approach to stratigraphic analysis is described which uses photogeologic and spectral interpretation of multispectral remote sensing data combined with topographic information to determine the attitude, thickness, and lithology of strata exposed at the surface. The new stratigraphic procedure is illustrated by examples in the literature. The published results demonstrate the potential of spectral stratigraphy for mapping strata, determining dip and strike, measuring and correlating stratigraphic sequences, defining lithofacies, mapping biofacies, and interpreting geological structures.
    Schlagwort(e): GEOPHYSICS
    Materialart: In: Earth and atmospheric remote sensing; Proceedings of the Meeting, Orlando, FL, Apr. 2-4, 1991 (A93-24176 08-42); p. 351-357.
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  • 18
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    Mid- and high-latitude total ozone observations from over three earth radii measured from Dynamics Explorer I (1991)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Column ozone measurements obtained from a Spin-scan Ozone Imager (SOI) aboard the Dynamics Explorer I (DE-I) satellite are discussed. The orbital geometry has many features similar to the Molniya satellite orbits. The ozone imagery obtained from DE-I are unique in being synoptic views of a region of continental scale for a period of a few hours. A brief description of the instrument, data, and orbit is followed by a discussion of some applications of DE-I data and the potential usefulness of an ozone imager aboard a satellite in a Molniya-type orbit.
    Schlagwort(e): GEOPHYSICS
    Materialart: In: Conference on the Meteorological and Oceanographic Uses of Satellites in Molniya Orbits, Univ. of Alabama, Huntsville, May 3, 1991, Proceedings (A93-44542 18-47); p. 62-72.
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  • 19
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    Monitoring Colima Volcano, Mexico, using satellite data (1991)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The Colima Volcanic Complex at the western end of the Mexican Volcanic Belt is the most active andesitic volcano in Mexico. Short-wavelength infrared data from the Landsat Thematic Mapper satellite were used to determine the temperature and fractional area of radiant picture elements for two January data acquisitions in 1985 and 1986. The 1986 data showed four 28.5 m by 28.5 m pixels (picture elements) whose hot subpixel components had temperatures ranging from 511-774 C and areas of 1.8-13 sq m. The 1985 data had no radiating areas above background temperatures. Ground observations and measurements in November 1985 and February 1986 reported the presence of hot fumaroles at the summit with temperatures of 135-895 C. This study demonstrates the utility of satellite data for monitoring volcanic activity.
    Schlagwort(e): GEOPHYSICS
    Materialart: Bulletin of Volcanology (ISSN 0258-8900); p. 571-574.
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  • 20
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    Why did Halley hiccup? (1991)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The outburst of Halley comet that occurred in February 1991 when the comet was over 14 AU from the sun, between the orbits of Saturn and Uranus, is described. This region of the solar system was known to be so cold that a comet nucleus would be totally inert there. On February 12th, 1991 Halley brightened by a factor of about 300, and a dust coma was observed around the nucleus. Different explanations of the comet's strange behavior are suggested.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Nature (ISSN 0028-0836); p. 793, 794.
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  • 21
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    Horizontal-branch evolution with time-dependent convective overshooting (1991)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Horizontal-branch (HB) sequences have been computed for different rates of convective overshooting at the edge of the convective core. We find that the convective core undergoes a series of 'breathing pulses' during most of the HB phase. The average behavior of these sequences closely mimics the behavior of sequences computed with canonical semiconvection.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: The formation and evolution of star clusters (A93-48676 20-90); p. 299-301.
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  • 22
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    Earth, atmosphere (1991)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Present understanding of the earth's atmosphere is briefly reviewed. The structure and composition of the atmosphere are described. The origin of the atmosphere and the factors involved in global atmospheric change are addressed.
    Schlagwort(e): GEOPHYSICS
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  • 23
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    Optical properties of aerosol emissions from biomass burning in the tropics, BASE-A (1991)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Ground-based and airborne measurements of biomass-burning smoke particle optical properties, obtained with a view to aerosol-absorption properties, are presented as a function of time and atmospheric height. The wavelength dependence of the optical thickness can be explained by a log-normal size distribution, with particles' effective radius varying between 0.1 and 0.2 microns. The strong correlation noted between aerosol particle profile and CO profile indicates that smoke particulates constitute a good tracer for emission trace gases from tropical biomass burning.
    Schlagwort(e): GEOPHYSICS
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  • 24
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    Aerosol characterization in smoke plumes from a wetlands fire (1991)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Results are presented from airborne measurements of aerosol mass loading, size distribution, and elemental composition obtained in a smoke plume from a fire at a Florida wildlife refuge. It is found that there was a high concentration of carbon-containing aerosols and salt crystals in the 0.1-0.2 micron size range; the composition and morphology of the aerosols differed with size. Aerosol mass concentrations are used in conjunction with those obtained for CO2 in order to calculate ratios of aerosol and CO2. The ratios are noted to be higher for the smoldering phase of the fire than for its flaming phase.
    Schlagwort(e): GEOPHYSICS
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  • 25
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    Computational methods for shock waves in three-dimensional supersonic flow (1991)
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    Publikationsdatum: 2011-08-24
    Beschreibung: A central artificial-viscosity and an upwind-biased difference method are contrived to solve the Euler equations for flowfields over typical spacecrafts. The spatial discretization is based on either nodal or cell-vertex formulation in the domain extending from free stream to the end of the vehicle. The outer boundary is treated as a bow shock in the first method but is placed in the free stream in the second, which captures both bow and internal shocks using an approximate Riemann solver based on high-order extrapolation to the cell face. These methods were tested for the Shuttle and Hermes orbiters at wind-tunnel conditions and angles of attack ranging from 0 to 60 deg. The artificial-viscosity method incorporated with a shock-fitting procedure shows smeared crossflow and wing-shock positions and required 15 percent more CPU per node than the upwind method. Greater flexibility and robustness is demonstrated by the latter on a fixed grid for all cases considered.
    Schlagwort(e): AERODYNAMICS
    Materialart: Computer Methods in Applied Mechanics and Engineering (ISSN 0045-7825); 87; 2-3,
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  • 26
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    Vertical column abundances and seasonal cycle of acetylene, C2H2, above the Jungfraujoch station, derived from IR solar observations (1991)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Results derived from a spectroscopic analysis of the nu5 band R19 transition of C2H2 observed in solar spectra recorded at the Jungfraujoch station, Switzerland, between June 1986 and April 1991 are reported. A least-squares sine fit to the data reveals a seasonal variability with an amplitude of about +/-40 percent of the mean; the maximum occurs during mid-winter and the minimum in summer. In general, the observed seasonal observations are comparable with those derived from airborne in-situ measurements and those reported from ground-based stations.
    Schlagwort(e): GEOPHYSICS
    Materialart: Journal of Atmospheric Chemistry (ISSN 0167-7764); 13; 4, No; 359-372
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  • 27
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    Molecular hydrogen emission in L1448 associated with a highly collimated molecular outflow (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: A near-infrared camera was used to search for jets around low-luminosity embedded infrared sources in nearby molecular clouds. The near-infrared offers the advantage that the extinction is very low compared with the optical. A jet is detected in molecular hydrogen at 2.12 microns toward a source in L1448. Given the sample size this indicates a detection rate of no more than a couple percent. The average visual extinction in L1448 is roughly 5 mag. The properties of the molecular hydrogen emission are similar to those measured for known Herbig-Haro objects, suggesting the jet is a buried Herbig-Haro object/jet that would be visible in the optical if the extinction were lower. The L1448 jet coincides with the unusual CO outflow that is highly collimated and contains high-velocity CO 'bullets'. The properties of the L1448 source suggest it defines a transition case between molecular outflows and Herbig-Haro jets, combining the characteristics of both.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Societa Astronomica Italiana, Memorie (ISSN 0037-8720); 62; 4, 19
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  • 28
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    Ultraviolet flares from RS CVn-type stars and related systems (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: Properties and parameters of UV flares have been observed from RS CVn systems approximately once per few days observing time. The peak flare luminosities are five or six orders of magnitude more intense than the most energetic solar flares. The nonradiative energy losses, e.g., turbulence and flows, are likely to be at least comparable to the radiative loss.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Societa Astronomica Italiana, Memorie (ISSN 0037-8720); 62; 291-305
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  • 29
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    The cosmic neutrino background (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: The cosmic neutrino background is expected to consist of relic neutrinos from the big bang, of neutrinos produced during nuclear burning in stars, of neutrinos released by gravitational stellar collapse, and of neutrinos produced by cosmic ray interactions with matter and radiation in the interstellar and intergalactic medium. Formation of baryonic dark matter in the early universe, matter-antimatter annihilation in a baryonic symmetric universe, and dark matter annihilation could have also contributed significantly to the cosmic neutrino background. The purpose of this paper is to review the properties of these cosmic neutrino backgrounds, the indirect evidence for their existence, and the prospects for their detection.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: After the first three minutes; Proceedings of the 1st Astrophysics Workshop, Univ. of Maryland, College Park, Oct. 15-17, 1990 (A93-23605 07-90); p. 497-511.
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  • 30
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    The X-ray flux dipole of AGNs as an indicator of the local gravitational field (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: We have investigated the dipole anisotropy of the sky X-ray flux from nearby AGNs to compare with the peculiar motion of the Local Group (LG) using a more extensive sample than that of Miyaji and Boldt (1990). We have sampled 56 low redshift emission-line AGNs (z less than 0.06) from the HEAO 1 A-2 experiment with a flux limit lower than that of Piccinotti et al. (1982) and with a lower galactic latitude cutoff. Our sample shows a significant dipole anisotropy whose apex is only about 30 deg away from the direction of the Local Group's peculiar motion for the objects with z less than 0.015. The dipole growth vs. redshift shows a sharp rise between z = 0.006 and z = 0.015; the amplitude of the dipole is 40 +/- 10 percent of the corresponding monopole at z = 0.015. The outer redshift cutoff is consistent with the results obtained from analyses using optical and IRAS galaxies. The present sample strengthens our previous conclusion that X-ray emission from AGNs traces the underlying mass distribution as strongly as optical and IR emission from galaxies.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: After the first three minutes; Proceedings of the 1st Astrophysics Workshop, Univ. of Maryland, College Park, Oct. 15-17, 1990 (A93-23605 07-90); p. 431-434.
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  • 31
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    The missing mass in clusters of galaxies and elliptical galaxies (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: We review the available data for the existence of dark matter in clusters of galaxies and elliptical galaxies. While the amount of dark matter in clusters is not well determined, both the X-ray and optical data show that more than 50 percent of the total mass must be dark. There is in general fair agreement in the binding mass estimates between the X-ray and optical techniques, but there is not detailed agreement on the form of the potential or the distribution of dark matter. The X-ray spectral and spatial observations of elliptical galaxies demonstrate that dark matter is also required in these objects and that it must be considerably more extended than the stellar distribution.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: After the first three minutes; Proceedings of the 1st Astrophysics Workshop, Univ. of Maryland, College Park, Oct. 15-17, 1990 (A93-23605 07-90); p. 394-404.
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  • 32
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    Evidence for evolution of the luminosity function of clusters of galaxies (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: From an all sky, X-ray flux limited sample of clusters of galaxies evidence for a significant deficit in the number of high luminosity clusters is found in the redshift range z approximately 0.1 to 0.2 compared with numbers of nearby clusters. This indicates that the X-ray luminous clusters are undergoing strong evolution. The strength of the effect is consistent with hierarchical merging models. The implications of such strong evolution for clusters are discussed.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: After the first three minutes; Proceedings of the 1st Astrophysics Workshop, Univ. of Maryland, College Park, Oct. 15-17, 1990 (A93-23605 07-90); p. 343-346.
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  • 33
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    Spectral-luminosity evolution of active galactic nuclei and the cosmic X-ray background (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: Black hole accretion disk dynamo processes are generally regarded as the central power source for AGN (Rees, 1984). If the precursor active galaxies for such AGN are formed at redshift z greater than about 4 and contain initial central seed black holes about 10 exp 6 solar masses, then the Eddington limited X-ray radiation emitted during their lifetime will undergo the phenomenon of 'spectral-luminosity evolution'. When accretion disks are first formed at the onset of galaxy formation, the accretion rate occurs at high values of luminosity/size compactness parameter L/R greater than 10 exp 30 erg/cm-sec. Such high values of L/R generate dynamic constraints which suppress nonthermal black hole accretion disk dynamo processes in favor of thermal processes. This causes the spectrum of X-radiation emitted by early AGN to be predominantly thermal. A superposition of such thermal, comptonized PAG sources can account for the residual cosmic X-ray background and can act as a source of X-ray heating of the intergalactic medium for z greater than about 4.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: After the first three minutes; Proceedings of the 1st Astrophysics Workshop, Univ. of Maryland, College Park, Oct. 15-17, 1990 (A93-23605 07-90); p. 339-342.
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  • 34
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    Regular variability of the shape of the primary minimum of the orbital light curve of SS 433 with the phase of the precessional period (1991)
    In:  CASI
    Details
    Publikationsdatum: 2009-11-16
    Beschreibung: The regular variations of the shape of the primary minimum of the orbital 13.082-day light curve of SS 433 with the phase of the 162.5-day processional period were discovered by analysis of the photometrical databank. The regular variability of the shape of the primary minimum of the discovered light curve of SS 433 reflects displacement with the 26-day double orbital period of at least two hot bright spots on the surface of the processional accretional disk and their eclipse by a normal star. Other aspects of the investigation are further discussed.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 65-67
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  • 35
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    Geophysical validation of TOPEX/Poseidon altimetry products (1991)
    In:  CASI
    Details
    Publikationsdatum: 2006-06-13
    Beschreibung: The objectives of this study are the following: (1) to assess the quality of the sea level observed by altimetry on a global scale, especially in tropical regions where the atmospheric effects are the most critical, and to prepare the altimeter data for their assimilation into tropical Oceanic General Circulation Models; and (2) to validate and calibrate the altimeter-derived sea state parameters, to assimilate these parameters into numerical models, and to estimate the altimeter's sea state and electromagnetic biases.
    Schlagwort(e): GEOPHYSICS
    Materialart: JPL, TOPEX(Poseidon Science Investigations Plan; p 82-88
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  • 36
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    Geophysical investigations with TOPEX/Poseidon altimetry data (1991)
    In:  CASI
    Details
    Publikationsdatum: 2006-06-13
    Beschreibung: In the proposed research, TOPEX/POSEIDON altimeter data will be used with Geosat and European Remote Sensing satellite (ERS-1) data to compute global, region, and local oceanic geoid surfaces. These observations will then be analyzed to conduct geophysical studies relative to the structure of the oceanic lithosphere and mantle.
    Schlagwort(e): GEOPHYSICS
    Materialart: JPL, TOPEX(Poseidon Science Investigations Plan; p 34-35
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  • 37
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    Hydrodynamic study of Supernova 1987A: The phase of a wave of cooling and recombination (1991)
    In:  CASI
    Details
    Publikationsdatum: 2006-06-13
    Beschreibung: The dependence of the bolometric light curve and of the effective temperature on the density distribution in a progenitor, its chemical composition, mass, radius, and explosion energy is studied. It is shown that, just before the supernova 1987 A outburst, the outside layers of the blue supergiant Sk-69.202 deg had a density distribution similar to that of the polytropic model with an index of n equals 3, a chemical composition with a mass fraction of hydrogen of the order of 0.1, and a relative helium abundance of about 0.9.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 403-409
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  • 38
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    Basic physics and cosmology from pulsar timing data (1991)
    In:  CASI
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    Publikationsdatum: 2006-06-13
    Beschreibung: Radio pulsars provide unparalleled opportunities for making measurements of astrophysically interesting phenomena. The author concentrates on two particular applications of high precision timing observations of pulsars: tests of relativistic gravitation theory using the binary pulsar 1913+16, and tests of cosmological models using timing data from millisecond pulsars. New upper limits are presented for the energy density of a cosmic background of low frequency gravitational radiation.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 385-393
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  • 39
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    Gas flow and generation of x ray emission in WR+OB binaries (1991)
    In:  CASI
    Details
    Publikationsdatum: 2006-06-13
    Beschreibung: The supersonic flow of the ionized gas in WR+OB binaries and X-ray generation are considered. X-ray emission is caused by gas heating up to temperatures of 10(exp 7) to 10(exp 8) K behind the front of shock waves. These are found in the collision of gas flowing out from the WR star with either the OB star's surface or the gas of the OB star's wind. The distribution of temperature and concentration behind the shock front are obtained. Using these distributions, the spectral power of bremsstrahlung X-ray emission of hot gas is calculated. Possible reasons that lead to a considerable difference between the observed parameters of X-ray emission of the WR binary of V 444 Cygni and the theoretically expected are discussed.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 394-402
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  • 40
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    X ray radiation from Supernova 1987A. The results of the Kvant module in 1987-1989 (1991)
    In:  CASI
    Details
    Publikationsdatum: 2006-06-13
    Beschreibung: The results of two years of SN1987A hard X-ray observations by the HEXE instrument aboard the Kvant module are summarized. By May to June 1989, the hard x-ray flux had declined more than 8.5 times in comparison with the maximum of the x-ray light curve. The upper limit of the ratio of Co-57/Co-56 abundances at the level of ratio of Fe-57/Fe-56 abundances at the Earth is a factor of 1.5.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 368-384
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  • 41
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    The peculiar velocity field predicted from the distribution of IRAS galaxies (1991)
    In:  CASI
    Details
    Publikationsdatum: 2006-06-13
    Beschreibung: It is noted, in the wake of the recent completion of a redshift survey of about 2300 IRAS galaxies with a characteristic depth of about 4000 km/sec, that the redshift distribution obtained is entirely consistent with that of the observed optical galaxies in the direction of the Great Attractor. The IRAS velocity field also qualitatively reproduces recent observations of the spiral and elliptical galaxies. Although it is not possible to rule out the existence of excess mass in the galactic plane in the direction of the Great Attractor, this is not needed to explain the observed peculiar velocities.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 356-367
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  • 42
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    Extragalactic x ray source counts (1991)
    In:  CASI
    Details
    Publikationsdatum: 2006-06-13
    Beschreibung: Extragalactic x-ray source counts carry information about the luminosity function and cosmic evolution of galaxies, BL Lac objects, Seyfort galaxies, and quasars. We discuss two available x-ray source samples with complete optical identifications and redshifts. We find evidence for instrumental bias in the detection of clusters for cosmic evolution of quasars, and of absorption effects in low luminosity Seyfert galaxies. Modest spectral and density evolution of Seyfort galaxies would allow the soft x-ray background to be made up entirely of discrete sources. We present a source count prognosis for the Advanced X-ray Astrophysics Facility (AXAF) energy range 0.5 to 10 keV.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 336-343
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  • 43
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    Ultraluminous infrared galaxies (1991)
    In:  CASI
    Details
    Publikationsdatum: 2006-06-13
    Beschreibung: One of the major discoveries of the the IRAS all-sky survey was a population of sources having the bolometric luminosities of quasars, but where more than 90 percent of the luminosity emerges in the infrared. These objects, more numerous than quasars, are found exclusivly in interacting/merging galaxies that are extremely rich in interstellar gas. We have accumulated evidence that suggests that these systems are indeed quasars, obscured by many tens of magnitudes of extinction. We suggest that these Ultraluminous Infrared Galaxies are the formation stage of quasars, and that colliding galaxies, ultraluminous infrared galaxies, and quasars might be linked through an evolutionary sequence where the infrared bright phase is one in which the quasar is formed in the nucleus of a merger system, and is enshrouded in gas and dust, while the UV excess quasars are at the end state of quasar evolution where most of the enveloping dust cloud has been dissipated, and the quasar is visible directly.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 344-355
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  • 44
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    The PSR 2127+12 as an indicator of a massive blac k hole in the core of globular cluster M 15 (1991)
    In:  CASI
    Details
    Publikationsdatum: 2006-06-13
    Beschreibung: The 110-millisecond pulsar PSR 2127+12 in the core of the globular cluster M15 is distinguished by having a negative period derivative P approximately equals -2 x 10(exp -17). This value cannot be provided by acceleration in the mean gravitational potential of the core. A flyby of a star approximately equal to 300 AU away could explain the P observed, but the probability of such an event is small, approximately equal to 10(exp -13). We suggest that the pulsar motion is governed by the presence of a moderately massive (approximately equals 2 divided by 3 x 10(exp 4) solar mass) black hole in the clusters center. The idea is further supported by an observed post-collapse morphology of the M15 core.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 316-321
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  • 45
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    On the observational appearances of a freely precessing neutron star in Hercules X-1 (1991)
    In:  CASI
    Details
    Publikationsdatum: 2006-06-13
    Beschreibung: Evidence for neutron star free precession is discussed, which is often presumed to be responsible for the observed 35-day cycle in Hercules X-1. The precise formula for the period derivative due to free precession is obtained under assumption that the precession period is much longer than that of the neutron star rotation. The optical light curves to be seen from the binary, with freely precessing accretion neutron star are simulated numerically. This simulation takes into account the reflection effect on the surface of the secondary component and on the accretion disk itself for different diagrams of x-ray emission.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 307-315
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  • 46
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    The large-scale surface brightness distribution of the x ray background (1991)
    In:  CASI
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    Publikationsdatum: 2006-06-13
    Beschreibung: The x-ray background (XRB) and the microwave background are the dominant isotropic radiation fields available for measurement. There was extensive work on trying to determine the physical origin of the background. That is, whether it is due to a superposition of numerous faint well-known sources, such as active galaxies, an early unidentified population of AGN at high redshift, a new population of objects, or to truly diffuse processes or to a superposition of these. However, while of great intrinsic interest, these studies were not aimed at using the XRB to provide the cosmological information that was gleaned from the microwave background. An alternate approach is presented, which uses the available information on the large, greater than 5 deg, scale distribution of the sky flux to see if the XRB can provide such constraints.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 285-296
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  • 47
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    The evolution of the gravitational radiation from stellar components of galaxies (1991)
    In:  CASI
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    Publikationsdatum: 2006-06-13
    Beschreibung: The evolution is discussed of gravitational waves spectra produced by binary stars, supernova explosions, and coalescences of binary compact stars in outer galaxies. These spectra are integrated over a simple model of the universe to give an estimate of the stochastic gravitational wave background due to astrophysical sources.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 261-269
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  • 48
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    Quasi-periodic oscillations in low-mass x ray binaries (1991)
    In:  CASI
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    Publikationsdatum: 2006-06-13
    Beschreibung: Variability on short time scales in the x-ray flux is a very general property of binary x-ray sources. Not until after the discovery of intensity-dependent quasi-periodic oscillations (QPO) and associated red noise form luminous low-mass x-ray binaries were systematic studies of the shape of the power-spectral components made. A brief account is given of the main developments since this discovery which have led to a new picture of the properties of low-mass x-ray binaries.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 251-260
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  • 49
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    Gravitational lenses: The current sample, recent results, and continuing searches (1991)
    In:  CASI
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    Publikationsdatum: 2006-06-13
    Beschreibung: Gravitational lensing is one of the topics in astrophysics that was quite extensively discussed over time before it was actually discovered. Ten years after the discovery of the first one, it is interesting to note how the field has developed. After an initial slow rate of discovery, the last few years have seen an explosion in the number of reported cases. Attention was drawn to the first few cases because quasars at the same red shift, with similar optical spectra, were observed with angular separations of only a few arc seconds. Most observational effort has been devoted to searching for new candidate lens systems and carefully measuring their properties, both to test whether they are indeed lensed and to provide constraints for modeling. A classification of the lenses is into rings, arcs, multiples, and doubles, where the progression is from sources close to the optical axis to far from the optical axis. The known candidate systems are listed. The searches for gravitational lenses are proving to be successful, and more lenses continue to be discovered serendipitously. Many searches are under way, along with instruments that will routinely increase the resolution of astronomical imaging.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 192-203
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  • 50
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    On two-dimensional relativistic stellar winds (1991)
    In:  CASI
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    Publikationsdatum: 2006-06-13
    Beschreibung: Stellar wind is of great interest, because many of the astrophysical systems possess wind like structures. For a long time only nonrelativistic winds have been studied. However, recently it was proposed by Kennel et al. that the relativistic pulsar wind with the plasma, consisting of electrons and positrons, can be responsible for the observed features of the Crab nebula. The study of Kennel et al. revealed the inconsistency of the assumption of the wind zero temperature with the observational data. It has been shown that only for high relativistic temperatures can high Mach numbers be reached, which allows the possibility of a shock formation. The analysis of Kennel et al. was extended into the 2-D case by using conventional magnetohydrodynamic equations for a relativistic plasma with an isotropic relativistic temperature. The state equation is assumed polytropic.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 108-111
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  • 51
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    The cyclotron absorption line and eclipse transition phenomena of 4U 1538-52 (1991)
    In:  CASI
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    Publikationsdatum: 2006-06-13
    Beschreibung: Observations of the eclipsing binary X ray pulsar, 4U 1538-52 have revealed a cyclotron absorption feature at 20 keV in the x ray spectrum. The pulse phase dependence of the intensity and spectrum can be mimicked by a model of x ray emission from thin accretion heated slabs at the magnetic poles of a rotating neutron star with its magnetic dipole axis inclined at 45 deg from the rotation axis. The observations also yielded data on the eclipse transitions which show that the radial density function at the base of the supersonic wind of the O-type supergiant primary has the form of an exponential like that which characterizes the density run in the similar region of the O-type supergiant primary of Cen X-3. As in the Cen X-3 system, the scale height of the exponential implies a temperature in the base region much greater than that of the supersonic wind.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 77-86
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  • 52
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    Chaotic inflationary universe and the anisotropy of the large-scale structure (1991)
    In:  CASI
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    Publikationsdatum: 2006-06-13
    Beschreibung: It has been realized that the inflationary universe is in fact chaotic, that globally it is strongly inhomogeneous, and that the inflation in the universe as a whole is eternal. In such a picture the region available to modern observations is just a tiny part of the universe, in which inflation finished about 10(exp 10) years ago. In spite of the great popularity of the chaotic inflationary universe models, it is usually taken for granted that their specific features (such as strong global inhomogeneity of the universe) can hardly lead to any observable consequences. The argument is that all that is seen is just a tiny part of the universe, a region about 10(exp 28) cm, and the typical scales of considerable inhomogeneities are much greater than this size. In contrast to this opinion, an attempt is made to show that such observable consequences can really exist. The phenomenon closely connected with the origin of structure (galaxies, clusters, etc.) in the observable region is discussed. The main idea considered is the vacuum fluctuations evolution on the inhomogeneous background.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 68-76
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  • 53
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    Turbulization of shear flows in astrophysics (1991)
    In:  CASI
    Details
    Publikationsdatum: 2006-06-13
    Beschreibung: The problem of instability in shear flows is treated in a number of works which present a global analysis of irrotational (potential) modes in a 2-D compressible shear flow, implying that, in the presence of reflecting boundaries, those modes can increase and that the characteristic time of the increase is several orders as high as the dynamic time in the medium. Due to the slowness of the increase of irrotational modes and to the problematic character of the theory's applicability for thin accretion disks, an attempt was made to develop an alternative theory of turbulization shear flows. It is quite contrary to the above theory. Instead of the global analysis, a local one is made far from the flow boundaries. Instead of irrotational perturbations, vortical ones are examined. Lastly, the medium is considered to be incompressible. The flow in the accretion disk can be modeled by a plane Couette flow, without regard for boundary conditions: it can be modeled by a plane shear flow in infinite incompressible fluid. According to this scenario, there is a critical perturbation level in the free shear flow. If the level of initial perturbations exceeds the critical one, the flow is turbulized.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 55-64
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    Manifestations of dynamo driven large-scale magnetic field in accretion disks of compact objects (1991)
    In:  CASI
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    Publikationsdatum: 2006-06-13
    Beschreibung: A turbulent dynamo nonlinear theory of turbulence was developed that shows that in the compact objects of accretion disks, the generated large-scale magnetic field (when the generation takes place) has a practically toroidal configuration. Its energy density can be much higher than turbulent pulsations energy density, and it becomes comparable with the thermal energy density of the medium. On this basis, the manifestations to which the large-scale magnetic field can lead at the accretion onto black holes and gravimagnetic rotators, respectively, are presented.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 46-54
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    The shock breakout in SN1987A modelled with the time-dependent radiative transfer (1991)
    In:  CASI
    Details
    Publikationsdatum: 2006-06-13
    Beschreibung: A fully implicit high-order scheme was developed for the independent multi-group radiative transfer coupled with implicit hydrodynamics. The application of this scheme to the SN1987A explosion shows that shortly after the shock breakout a dense shell forms. A gasdynamic code describing the time dependent radiation transport in the multi-group approximation with variable Eddington factors was developed. Results and other aspects of the project are presented.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 39-45
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  • 56
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    Axially symmetrical supernova remnants (1991)
    In:  CASI
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    Publikationsdatum: 2006-06-13
    Beschreibung: The origin of cylindrically symmetric supernova remnants (SNR's) is discussed. The results of numerical simulations of two most distinguished barrel-like SNR's SN1006 and G296.5+10.0 are presented.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 19-27
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    On the evolution of pulsars (1991)
    In:  CASI
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    Publikationsdatum: 2006-06-13
    Beschreibung: Data from a previous investigation on the angle chi between the axis of rotation and the magnetic dipole axis, determined from polarization observations, provides a complete catalog which makes it possible to carry out a detailed comparison of the theoretical results of this present investigation with the observed distribution of radio pulsars over the angel chi. Before such a comparison is made, the main features of a theory for pulsar evolution is described.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 9-13
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  • 58
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    Instabilities in SN1987A and other supernovae (1991)
    In:  CASI
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    Publikationsdatum: 2006-06-13
    Beschreibung: While giving a remarkably good description of many aspects of SN1987A, calculations assuming spherical symmetry have a number of flaws. Many of these problems naturally disappear in the development of a 2-D calculation, as nonspherical instabilities grow. This nonspherical behavior, made evident in SN1987A, has implications for other types of supernovae.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NAS-NRC, High-Energy Astrophysics. American and Societ Perspectives; p 1-8
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    The spin down of the radio pulsars: Braking index (1991)
    In:  CASI
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    Publikationsdatum: 2006-06-13
    Beschreibung: Presently, the value of the retardation dP/dt is well known for most radio pulsars. It is negative for all cases except one and is of the order of 10(exp -15). That single case is when the pulsar, which is located in the star globular system, can have a considerable acceleration leading to the opposite sign of P'= dP/dt due to the Doppler effect. Careful measurements of the period, P, also allow one to determine the variation of this retardation with the course of time- P'' = d(exp 2)P/dt(exp 2). The results of these measurements are usually represented in the form of the dimensionless retardation index n = omega'' omega/omega(exp 2)= 2 - P''P/P(exp 2) (omega is the angular velocity). The data for 21 pulsars are given. The parameter, n, is strongly undetermined both in value and sign in all cases except for four pulsars. Changes of the rotation period, P, and the inclination angle, chi, the angle between the axes of rotation and the magnetic moment are caused by two processes: the regular retardation and nutation due to deviation from the strict spherical shape of the neutron star. Losses which are caused by the currents flowing in the magnetosphere of the neutron star and by being closed on the star surface are considered. Such losses are critical for the neutron star magnetosphere which is full of dense plasma. Since the radio emission is generated in the dense plasma of the polar magnetosphere, then practically all radio pulsars are retarded by the current mechanism. The formula for the braking index is presented along with other aspects of the investigation.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 14-18
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    A TOPEX/Poseidon study of oceanic effects on observations of the Earth's interior (1991)
    In:  CASI
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    Publikationsdatum: 2006-06-13
    Beschreibung: The primary objective of our research program is the application of TOPEX/POSEIDON oceanic data to the study of various geophysical observations, thereby improving our knowledge of the Earth's interior. Observations of the Earth's solid tides, rotation, time-dependent gravity, and crustal deformation provide potentially useful constraints on properties of the solid Earth and its liquid core, but all these parameters are also sensitive to oceanic effects. Depending on the type of observation, those effects are usually either removed by modeling (often using particularly simple assumptions about the dynamics of the ocean) or ignored entirely. Useful oceanic data are rarely available. In many cases, the uncertainty in the oceanic correction is the main source of error in the geophysical interpretation of the results. We describe, below several of these geophysical observations, how their interpretation in terms of the Earth's interior is affected by the oceans, and how data from the TOPEX/POSEIDON mission can be used to improve this situation.
    Schlagwort(e): GEOPHYSICS
    Materialart: JPL, TOPEX(Poseidon Science Investigations Plan; p 159-160
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    Present-day plate motions: Retrieval from the TOPEX/Poseidon orbitography network (DORIS system) (1991)
    In:  CASI
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    Publikationsdatum: 2006-06-13
    Beschreibung: The goal of the proposal is to determine the present motion of the main tectonic plates from the Doppler data of the Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) orbitography system, which includes in its final configuration about 50 tracking stations with a world-wide distribution.
    Schlagwort(e): GEOPHYSICS
    Materialart: JPL, TOPEX(Poseidon Science Investigations Plan; p 131-136
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  • 62
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    The 10 January 1986 disconnection event in Comet Halley (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: The disconnection event (DE) in the plasma tail of Comet Halley on January 9-12, 1986 is examined. The distances between the comet head and the disconnected tail are measured for a series of images for that time period and then extrapolated to the nucleus to determine the disconnection time: January 9.60 +/- 0.2 days. The approximate solar-wind conditions at the time of the DE were obtained by corotation of IMP-8 satellite data in earth orbit to Comet Halley. At the time of the DE, Comet Halley is inferred to have been close to a magnetic-sector boundary and a high-speed stream-compression region. However, a heliographic latitude separation of 22 degrees (between the comet and IMP-8) and gaps in the IMP-8 data render a more definitive statement about the linkage of the DE to external conditions quite difficult. It is not possible to resolve the effects of magnetic changes associated with the sector boundary and plasma pressure in the compression region.
    Schlagwort(e): ASTROPHYSICS
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    The future of cometary plasma research (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: Advances in the acquisition and analysis of cometary data are reviewed with attention given to current research and projects under development. The need for supplementing the present cometary data is underscored by discussing observational missions from the ground and earth orbit such as the Giotto Extended Mission and the Comet Rendezvous Asteroid Flyby. The interpretation of Comet Halley data is characterized as advanced with respect to both complete observational data and sophisticated modeling.
    Schlagwort(e): ASTROPHYSICS
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    High-Reynolds-number test requirements in low-speed aerodynamics (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: Research requirements to an ultra-high-Reynolds-number liquid helium facility are reviewed. Aerodynamic research areas under consideration include wave vortex hazard reduction, vortex control and diagnostics for maneuvering fighter aircraft, and performance of high-lift devices.
    Schlagwort(e): AERODYNAMICS
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    Engineering calculations of three-dimensional inviscid hypersonic flowfields (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Schlagwort(e): AERODYNAMICS
    Materialart: Journal of Spacecraft and Rockets (ISSN 0022-4650); 28; 628-635
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    Recent trends in stratospheric total ozone - Implications of dynamical and El Chichon perturbations (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: The recently reprocessed Nimbus-7 TOMS total ozone data from 1979 to 1989 are analyzed to assess the global impact of the El Chichon volcanic eruption on stratospheric total ozone. An apparent decrease in total ozone of 5 to 6 percent is present during the winter of 1982-1983 following the eruption of El Chichon. A regression analysis of total ozone with the equatorial zonal wind at 30 mb indicates that response to the quasi-biennial oscillation can explain much of the observed ozone anomaly, and that the total ozone decrease which can be attributed to El Chichon is at most 2 to 4 percent. This study also suggests that the interannual variability caused by the quasi-biennial oscillation and planetary wave activity may introduce apparent seasonal trends in total ozone and temperature in the lower stratosphere, particularly if the length of the data record is not very long. Such trends may affect the assessment of total ozone changes caused by chemical perturbations.
    Schlagwort(e): GEOPHYSICS
    Materialart: Geophysical Research Letters (ISSN 0094-8276); 18; 2277-228
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    The BLISS measurements of NO2 - Some new insights (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: The present study compares the diurnal variation of NO2 measured near 30 km by the BLISS in situ laser spectrometer with calculations from a photochemical model that includes a detailed description of multiple scattering. Even better agreement is found between the data and the model, both at sunset and during the day. The conclusions of an earlier study that the high-resolution in situ measurements of NO2 facilitated validation of the understanding of the diurnal chemistry of NO2 are confirmed.
    Schlagwort(e): GEOPHYSICS
    Materialart: Geophysical Research Letters (ISSN 0094-8276); 18; 2261-226
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    A nonradial pulsation model for the rapidly rotating Delta Scuti star Kappa(2) Bootis (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: A sectorial nonradial pulsation model is used to construct theoretical line profiles which mimic the variations for Kappa(2) Boo. Synthetic spectra generated with the appropriate Teff and log g are used as input. It is found that the data can be reproduced by the combination of a high-degree l is approximately equal to 12 mode with P(osc) aproximately equal to 0.071 d, and a low-degree mode, l is approximately equal to 0-2 with P(osc) approximately equal to 0.071-0.079 d. The projected rotational velocity (v sin i - 115 +/-5 km/s) was determined by fitting synthetic line profiles to the observed spectra. The velocity amplitude of the high-degree oscillations is estimated to be about 3.5 km/s. It is found that the ratio of the horizontal and radial pulsation amplitudes is small (about 0.02) and consistent with p-mode oscillations. Comparisons are made with models invoking starspots, and it is impossible to fit the observations of Kappa(2) Boo by a starspot model without assuming unrealistic values of radius or equatorial velocity.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomical Society of the Pacific, Publications (ISSN 0004-6280); 103; 1250-125
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    New insights from a statistical analysis of IUE spectra of dwarf novae and nova-like stars. I - Inclination effects in lines and continua (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: IUE observations of dwarf novae at maximum at quiescence and novalike objects at the high brightness state are analyzed for effects of the inclination angle on the emitted continuum and line radiation. A clear pattern in the continuum flux distribution is exhibited only by dwarf novae at maximum where some 80 percent of the non-double-eclipsing systems show essentially identical distributions. This result is not in disagreement with theoretical expectations. All classes of objects exhibit a clear, but in each case different, dependence of the line radiation on the inclination angle.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomy and Astrophysics (ISSN 0004-6361); 252; 1, De
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    Crustal magnetization and temperature at depth beneath the Yilgarn block, Western Australia inferred from Magsat data (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: Variations in crustal magnetization along a seismic section across the Archean Yilgarn block of Western Australia inferred from Magsat data are interpreted as a subtle thermal effect arising from variations in depth to the Curie isotherm. The isotherm lies deep within the mantle of the eastern part of the province, but transects the crust-mantle transition and rises well into the crust on the western side. The model is consistent with heat flow variations along the section line. The mean crustal magnetization implied by the model is approximately 2 A/m. The temperature variation implied by the model is consistent with the hypothesis that the crust-mantle transition seen seismically corresponds to the mafic granulite-eclogite phase transition within a zone of igneous crustal underplating.
    Schlagwort(e): GEOPHYSICS
    Materialart: Earth and Planetary Science Letters (ISSN 0012-821X); 107; 3-4,; 515-522
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    Static aeroelastic analysis for generic configuration wing (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: A static aeroelastic analysis capability that calculates flexible air loads for generic configuration wings was developed. It was made possible by integrating a finite element structural analysis code (MSC/NASTRAN) and a panel code of aerodynamic analysis based on linear potential flow theory. The framework already built in MSC/NASTRAN was used, and the aerodynamic influence coefficient matrix was computed externally and inserted in the NASTRAN by means of a DMAP program. It was shown that deformation and flexible air loads of an oblique wing configuration including asymmetric wings can be calculated reliably by this code both in subsonic and supersonic speeds.
    Schlagwort(e): AERODYNAMICS
    Materialart: Journal of Aircraft (ISSN 0021-8669); 28; 801
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    Numerical prediction of subsonic turbulent flows over slender bodies at high incidence (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Schlagwort(e): AERODYNAMICS
    Materialart: AIAA Journal (ISSN 0001-1452); 29; 2054-206
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    Intercomparisons of the solar irradiance measurements from the Nimbus-7 SBUV, the NOAA-9 and NOAA-11 SBUV/2, and the STS-34 SSBUV instruments - A preliminary study (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: Results are presented of solar irradiance measurements in the spectral range 160-400 nm at approximately 0.15-0.20-nm intervals and at 1-nm resolution performed continually since November 1978. Solar irradiance data from the Nimbus-7 SBUV satellite instrument, the SBUV/2 instruments on the NOAA-9 and NOAA-11 satellites, and the October 1989 flight of the Shuttle SBUV instrument are presented and compared. Uncertainties in the instruments' absolute and long-term radiometric calibrations, which vary among the four instruments, are discussed. Comparisons of the initial solar spectra from the four instruments show agreement to within approximately 10 percent, with spectral biases on the order of +/-4 percent. Irradiances measured by the two NOAA instruments and SSBUV agree to within about 5 percent overall from 270 to 360 nm, with spectral biases on the order of about +/-2 percent. The Nimbus-7 SBUV irradiances are an additional 5-10 percent lower in this region than those measured by the other three instruments.
    Schlagwort(e): GEOPHYSICS
    Materialart: Journal of Atmospheric and Terrestrial Physics (ISSN 0021-9169); 53; 993-997
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    Spectroscopic measurement of HO2, H2O2, and OH in the stratosphere (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: Stratospheric concentrations of HO2, H2O2, and OH have been retrieved simultaneously from the far-infrared emission spectra obtained with a balloon-borne Fourier transform spectrometer in June 1983 at 32 deg N latitude. Retrieved concentrations of HO2 and H2O2 are reported, along with vertical distributions of OH which were reported in an earlier paper for the afternoon, sunset, and nighttime periods for altitudes from 26 to 38 km. HO2 distributions are obtained with uncertainties that are about the same as OH for the same vertical range and for the afternoon and sunset periods. H2O2 concentration is obtained at an altitude of 30 km for the period that covers afternoon and sunset hours. The retrieved concentrations of these HO(x) species agree well with other individually measured results and the steady state photochemical predictions. The ratio HO2/OH at around 32 km seems to increase from the afternoon period to the sunset period.
    Schlagwort(e): GEOPHYSICS
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 96; 22
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    A diagnostic stratospheric aerosol size distribution inferred from SAGE II measurements (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: An aerosol size distribution model for the stratosphere is inferred based on 5 years of Stratospheric Aerosol and Gas Experiment (SAGE) II measurements of multispectral aerosol and water vapor extinction. The SAGE II aerosol and water vapor extinction data strongly suggest that there is a critical particle radius below which there is a relatively weak dependence of particle number density with size and above which there are few, if any, particles. A segmented power law model, as a simple representation of this dependence, is used in theoretical calculations and intercomparisons with a variety of aerosol measurements including dustsondes, longwave lidar, and wire impactors and shows a consistently good agreement.
    Schlagwort(e): GEOPHYSICS
    Materialart: Journal of Geophysical Research (ISSN 0148-0227); 96; 22
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    Inhomogeneous structure in the chromospheres of dwarf M stars (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: Linear combinations of observed spectra of the H-alpha and Ca-II resonance and IR lines from the chromospheres of a quiet (Gl 1) and an active (Gl 735) dwarf-M star are compared with the corresponding spectra from a star of intermediate activity (Gl 887). It is shown that the intermediate spectra cannot be explained as a simple juxtaposition of the extreme chromospheric states. It is concluded that the range of observed strengths of chromospheric activity indicators in dwarf-M stars is due, at least in part, to changes in the radial structure of the chromospheric heating function and not to changes in the area filling factor.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Royal Astronomical Society, Monthly Notices (ISSN 0035-8711); 253; 575-578
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    Cometary apparitions - 1990-2010 (1991)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Osculating orbital elements are listed (chronologically by perihelion passage time) for all periodic comets expected to arrive at perihelion during the 1990-2010 interval. Plots which make it possible to readily determine the earth-based viewing conditions of a particular cometary apparition are presented. These plots are drawn in a rotating coordinate system so that the sun-earth line is fixed for each apparition.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Comets in the post-Halley era. Vol. 2 (A93-13551 02-90); p. 1281-1308.
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    Cometary plasma waves and instabilities (1991)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Various plasma waves and instabilities that have been observed near comets are discussed. Observational results are ordered by plasma parameters and compared with specific instabilities. The variation in the cometary ion and electron beam densities and velocities are studied by examining regions far from and near the comet nucleus. Spacecraft observations relevant to nonlinear wave evolution, wave cascading, and the development of turbulence are reviewed. The implications of the results for wave-particle interactions - linear, nonlinear, resonant, and nonresonant - and their effects on stochastic particle acceleration are addressed. Higher-frequency ELF/VLF wave observations are also reviewed, and comparisons between the various measurements are made.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Comets in the post-Halley era. Vol. 2 (A93-13551 02-90); p. 1171-1210.
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    Cometary activity, discrete outgassing areas, and dust-jet formation (1991)
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    Publikationsdatum: 2011-08-24
    Beschreibung: Conceptual models for various types of features observed in cometary comae (jets, spirals, halos, fans, etc.), their computer simulation, and the hydrodynamic models for jet formation are critically reviewed, and evidence for anisotropic, strongly collimated flows of ejecta emanating from discrete active regions (vents) on the rotating cometary nuclei is presented. Techniques employed to generate synthetic comet images that simulate the features observed are described, and their relevance to the primary objects of coma-morphology studies is discussed. Modeling of temporal variations in the water emission from discrete active regions suggests that production curves asymmetric with respect to perihelion should be commonplace. Critical comparisons with the activity profiles of Enke's comet and with light curves of disappearing comets and comets that undergo outbursts are presented. Recent developments in the understanding of the processes that cause the nongravitational perturbations of cometary motions are reviewed, and the observed discontinuities are identified with the birth of new sources and/or deactivation of old vents.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Comets in the post-Halley era. Vol. 2 (A93-13551 02-90); p. 769-823.
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    Dynamical history of the Oort cloud (1991)
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    Publikationsdatum: 2011-08-24
    Beschreibung: An overview of recent dynamical studies on the Oort cloud of comets surrounding the solar system is presented. Cometary orbits in the cloud evolve under the complex interaction of stellar, galactic, and giant molecular cloud perturbations, as well as planetary and nongravitational perturbations when the orbits reenter the planetary region. There is mounting evidence for a dense, inner Oort cloud of comets which acts as a reservoir to replenish the outer cloud as comets there are stripped away. A ring of comets beyond the orbit of Neptune, which may be the source of the short-period comets, is also likely. Temporal variations in the flux of comets from the Oort cloud into the planetary region by a factor of 50 percent are typical, and by factors of 20 to 200 are possible. Comets in the Oort cloud are processed by galactic cosmic rays, heated by nearby supernovae, eroded by interstellar dust impacts, and disrupted by mutual collisions.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Comets in the post-Halley era. Vol. 1 (A93-13551 02-90); p. 463-486.
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    The use of transition heights for the representation of ion composition (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: Several characteristic transition heights can be found in ionospheric ion-composition profiles. These are the boundaries between the regions dominated by light ions, atomic oxygen ions, molecular ions, and cluster ions, respectively. Ion-composition modeling can benefit from the use of these transition heights. Special emphasis is given to the 'upper' transition height H sub T (light ions to atomic oxygen) and to the 'lower' transition height h sub t (atomic oxygen to molecular ions). Transition-height models deduced from rocket and satellite measurements are compared with the heights predicted by the International Reference Ionosphere. Considerable discrepancies are found between the different models as well as between the models and independent measurements.
    Schlagwort(e): GEOPHYSICS
    Materialart: Advances in Space Research (ISSN 0273-1177); 11; 10, 1
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    The temperature of the circumstellar envelope of Alpha Orionis (1991)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The processes that heat and cool the circumstellar envelope of Alpha Ori are analyzed and the temperature calculated as a function of distance from the star. Cooling by fine structure lines is included for the first time in a general way in the theory of the thermal balance of a circumstellar envelope. Adiabatic cooling, dust-drag heating, and various line coolants contribute in the outer envelope, whereas the fine structure lines of O I and Si II and adiabatic cooling are dominant in the inner envelope. The rotational levels of CO cool the gas only at very small and at very large distances; elsewhere they provide a small amount of heating. The temperature declines very rapidly at first and then varies roughly as r exp -1/2 in the outer envelope. At very large distances, the temperature distribution flattens out due to photoelectron heating and reaches a minimum of about 4 K at about 250 arcsec.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 382; 606-616
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  • 83
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    10 micron spectra of protostars and the solid methanol abundance (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: A search has been made for the strong C-O stretching absorption of solid methanol near 9.8 microns toward the heavily obscured protostars AFGL 961, AFGL 2591, the BN object and Mon R2 IRS 3. There is no clear evidence for this feature in the spectra, resulting in very conservative upper limits to the methanol abundance of 6 percent to 17 percent relative to solid H2O toward these objects. This is well below previous estimates of 50-80 percent obtained toward W33 A, NGC 7538 IRS 9, AFGL 2136, and W3 IRS 5, which were based on the assignment of the interstellar 6.85 microns absorption feature to the methanol C-H bending mode. This study shows that such high methanol abundances are not a characteristic of all interstellar ices.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 382; 523-529
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    On the evolution of gravitationally unstable protostellar disks (1991)
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    Publikationsdatum: 2011-08-24
    Beschreibung: 2D, N-body simulations of the nonlinear development of long-wavelength instabilities predicted to occur early in the formation stage of protostellar disks forming single stars are presented under conditions of controlled energy loss. It is found that the thermodynamics and long-term evolution of gravitationally unstable disks are intimately coupled: the global evolution rate is linearly proportional to the rate at which dissipated energy reaches and escapes from the surfaces of the disk. An upper limit is obtained to the rate at which energy is lost if it is assumed that the disk maintains vertical isothermality.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 382; 530-543
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  • 85
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    Probe systems for static pressure and cross-stream turbulence intensity (1991)
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    Publikationsdatum: 2011-08-24
    Beschreibung: A recent study of total-pressure probes for use in highly turbulent streams is extended herein by developing probe systems that measure time-averaged static or ambient pressure and turbulence intensity. Arrangements of tubular probes of circular and elliptical cross section are described that measure the pressure at orifices on the sides of the probes to obtain different responses to the cross-stream velocity fluctuations. When the measured data are combined to remove the effect of the presence of the probes on the local pressure, the time-averaged static pressure and the cross-stream components of turbulence intensity can be determined. If a system of total pressure tubes, as described in an accompanying paper, is added to the static pressure group to form a single cluster, redundant measurements are obtained that permit accuracy and consistency checks.
    Schlagwort(e): AERODYNAMICS
    Materialart: Journal of Aircraft (ISSN 0021-8669); 28; 750-755
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  • 86
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    The accumulation and structure of comets (1991)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The paper reviews evidence for the accumulation of the terrestrial planets and comets from solid grains, with emphasis on the various proposals for the formation of cometary nuclei. With three exceptions, all hypotheses conclude or imply that a single compact object forms. Several hypotheses start with Goldreich-Ward-type gravitational instabilities. The collapse for this case also occurs at low velocities in the cm/s to m/s range. Experiment and theory show that under these conditions, low-density, filamentary clusters form that are fractal aggregates with a fractal dimension approximately equal to 2. In order to form cometary nuclei, the initial temperature must be about 50 K and not undergo a significant temperature rise during the accumulation process. The calculations show that accumulation will occur at low temperatures. Models of cometary nuclei are reviewed, and a simple model of the structure that results fom the accumulation of fluffy aggregates is described.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Comets in the post-Halley era. Vol. 1 (A93-13551 02-90); p. 335-359.
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  • 87
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    Cometary orbital dynamics and astrometry (1991)
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    Publikationsdatum: 2011-08-24
    Beschreibung: An overview of research on cometary orbital dynamics and astrometry is presented, with emphasis on the nongravitational acceleration model which provides accurate orbits and ephemerides, and on new asymmetric models that represent the cometary outgassing phenomena more closely. Using a combination of long-focus telescopes, CCD detectors, microdensitometer reductions, and modern star catalogs, cometary astrometric data that are accurate to the subarcsecond level can be generated.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Comets in the post-Halley era. Vol. 1 (A93-13551 02-90); p. 3-17.
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  • 88
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    Synchrotron emission as a tracer of the outflow in M82 (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: The discovery of radio synchrotron emission from the outflow in M82 is reported. It is found that the outflow in M82 is visible in synchrotron radiation. IC losses are evident in the steepening of the radio spectrum, though this process is not a major contributor to the observed halo X-ray emission. Particle acceleration and/or diffusion in the wind may be important. Outflow speeds of 1000-3000 km/s are inferred from the spectral index profiles, which significantly exceeds the optically determined values. The fraction of the wind energy in the form of relativistic particles is consistent with a supernova origin. A termination shock produced by the intergalactic medium may be shaping the observed halo of M82.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: The interstellar disk-halo connection in galaxies (A93-18998 05-90); p. 281-285.
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    Implicit flux-split Euler schemes for unsteady aerodynamic analysis involving unstructured dynamic meshes (1991)
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    Publikationsdatum: 2011-08-24
    Schlagwort(e): AERODYNAMICS
    Materialart: AIAA Journal (ISSN 0001-1452); 29; 1836-184
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  • 90
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    Viscous flow solutions for slender bodies of revolution at incidence (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: Flow over slender prolate spheroids at incidence is examined. The incidence angle is chosen high enough to cause streamwise separation of the flow in addition to crossflow separation generally observed at lower incidence angles. The freestream Mach number for the cases investigated here is subsonic, thus precluding the use of parabolized procedures. Laminar, transitional and turbulent flow cases are investigated.
    Schlagwort(e): AERODYNAMICS
    Materialart: Computers and Fluids (ISSN 0045-7930); 20; 3, 19
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  • 91
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    The effect of the changing polarity and neutral sheet of the IMF on the cosmic ray diurnal anisotropy at neutron monitor energies (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: A drift with a simulated wavy neutral sheet have been used to study the effects of the reversal of the solar magnetic field every 11 years and the changes in the waviness of the heliospheric neutral sheet, corresponding to changes in solar activity, on the diurnal anisotropy at an energy of 20 GeV. The results indicate that the long-term behavior of the diurnal anisotropy, especially the phase shift from one solar minimum period to another, which seems to depend on the polarity of the IMF, has a theoretical explanation in the drift picture of the modulation of cosmic rays in the heliosphere.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Planetary and Space Science (ISSN 0032-0633); 39; 1233-123
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  • 92
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    Application of a monotonic upstream-biased transport scheme to three-dimensional constituent transport calculations (1991)
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    Publikationsdatum: 2011-08-24
    Beschreibung: The application of van Leer's scheme, a monotonic, upstream-biased differencing scheme, to three-dimensional constituent transport calculations is shown. The major disadvantage of the scheme is shown to be a self-limiting diffusion. A major advantage of the scheme is shown to be its ability to maintain constituent correlations. The scheme is adapted for a spherical coordinate system with a hybrid sigma-pressure coordinate in the vertical. Special consideration is given to cross-polar flow. The vertical wind calculation is shown to be extremely sensitive to the method of calculating the divergence. This sensitivity implies that a vertical wind formulation consistent with the transport scheme is essential for accurate transport calculations. The computational savings of the time-splitting method used to solve this equation are shown. Finally, the capabilities of this scheme are illustrated by an ozone transport and chemistry model simulation.
    Schlagwort(e): GEOPHYSICS
    Materialart: Monthly Weather Review (ISSN 0027-0644); 119; 2456-246
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  • 93
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    Thermochemical nonequilibrium issues for earth re-entry of Mars mission vehicles (1991)
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    Publikationsdatum: 2011-08-24
    Schlagwort(e): AERODYNAMICS
    Materialart: Journal of Spacecraft and Rockets (ISSN 0022-4650); 28; 552-559
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  • 94
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    Aeroassist flight experiment heating-rate sensitivity study (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Schlagwort(e): AERODYNAMICS
    Materialart: Journal of Thermophysics and Heat Transfer (ISSN 0887-8722); 5; 456-462
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  • 95
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    Energetic water-group ions at Comet Giacobini-Zinner - An overview of observations by the EPAS instrument (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: An overview is presented of the observations of energetic (45-480 keV) cometary water group ions made by the EPAS instrument on the ICE spacecraft during the fly-by of Comet Giacobini-Zinner in September 1985. The discussion covers ion observations in the large-scale ion pick-up region surrounding the comet on about 10 exp 6 km spatial scales, observations of the cometary shock at about 100,000 km and the region of slowed mass-loaded flows which occur downstream, and the finite gyroradius effects which occur near the closest approach.
    Schlagwort(e): ASTROPHYSICS
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    Synergic inversion technique for active and passive microwave remote sensing of the ocean (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: A unified approach for combining active and passive microwave measurements for remote sensing applications is described. A synergic inversion technique has been developed and applied to the retrieval of geophysical parameters of the ocean surface and of the atmosphere. It is based on the combination of radiometric and radar measurements at the electromagnetic and cell level and not only on the correction of radar measurements by radiometric measurements, or conversely. Such a combination is performed through a common quantity: the bistatic scattering coefficient of the observed surface. This is used in a direct model to simulate combined measurements from active and passive sensors. It requires a rather complete and accurate calculation of the scattering of microwaves by the rough sea surface. Comparisons with experimental models derived for scatterometers and radar altimeters or with data derived from radiometric measurements show that the results of this unified approach are consistent with those from separate microwave sensors, at least for a foam-free sea surface.
    Schlagwort(e): GEOPHYSICS
    Materialart: IEEE Transactions on Geoscience and Remote Sensing (ISSN 0196-2892); 29; 391-406
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  • 97
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    Noise-driven radiative shocks - A new model for the optical quasi-periodic oscillations of the AM Herculis objects (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: A model for the 0.3-1.2 Hz optical quasi-periodic oscillations (QPOs) observed in a number of AM Her-type binary systems has been developed. It is suggested that the observed optical modulation is the result of shock oscillations induced by nonsteady accretion flows. It is shown that time-dependent models of radiative shock waves in nonsteady accretion flows onto magnetic white dwarfs with mass 0.6 solar mass and magnetic field strength of 30 MG can produce optical QPOs similar to those observed in the AM Her objects. Theoretical calculations have shown that oscillations cannot be sustained for these white dwarf parameters when the accretion rate is constant.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 375; L31-L34
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    GRIS observations of positron annihilation radiation from the Galactic center (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: In the present Gamma-Ray Imaging Spectrometer (GRIS) observations, the 511 keV line has been spectrally resolved and the Galactic plane and center components have been independently measured. These data, in combination with those from other narrow-field observations in the 1980s, support the two-component model of the positron annihilation source. A strong hard-X-ray continuum was detected in the Galactic plane observation; this 'diffuse' continuum component solves the mystery as to why wide-field instruments have detected such high continuum emission from the Galactic center, in virtue of its source contributions.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 375; L13-L16
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    Depletion of Arctic ozone in the winter 1990 (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: Ozone mixing ratios were measured by ozonesondes on board balloons launched from Esrange, near Kiruna, Sweden (68 deg N, 20 deg E) from January 11 to February 9, 1990. The data obtained prior to a sudden warming on February 7, 1990 show that at potential temperatures between 460 and 640 K, the ozone mixing ratio just inside the polar vortex was systematically smaller than that outside, the largest difference being 29 percent at around 525 K. The ozone mixing ratio at 525 K inside the vortex decreased at a rate of about 1.5 percent per day between January 26 and February 4. The temperatures simultaneously observed were quite often low enough to allow for formation of nitric acid trihydrate particles around this altitude. Depletion of ozone due to highly perturbed chemical conditions in late January and early February is strongly suggested.
    Schlagwort(e): GEOPHYSICS
    Materialart: Geophysical Research Letters (ISSN 0094-8276); 18; 791-794
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    Oxygen isotope fractionation in stratospheric CO2 (1991)
    In:  Other Sources
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    Publikationsdatum: 2011-08-19
    Beschreibung: A new cryogenic collection system has been flown on board a balloon gondola to obtain separate samples of ozone and carbon dioxide without entrapping major atmospheric gases. Precision laboratory isotopic analysis of CO2 samples collected between 26 and 35.5 km show a mass-independent enrichment in both O-17 and O-18 of about 11 per mil above tropospheric values. Ozone enrichment in its heavy isotopes was 9 to 16 percent in O3-50 and 8 to 11 percent in O3-49, respectively (Schueler et al., 1990). A mechanism to explain the isotope enrichment in CO2 has been recently proposed by Yung et al. (1991). The model is based on the isotope exchange between CO2 and O3 via O(1D), resulting in a transfer of the ozone isotope enrichment to carbon dioxide. Predicted enrichment and measured values agree well.
    Schlagwort(e): GEOPHYSICS
    Materialart: Geophysical Research Letters (ISSN 0094-8276); 18; 669-672
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