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At least 1,081 records · Page 60
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NASA NTRS1985

Giant molecular clouds as regions of particle acceleration

One of the most interesting results of investigations carried out on the satellites SAS-II and COS-B is the discovery of unidentified discrete gamma sources. Possibly a considerable part of them may well be giant molecular clouds. Gamma emission from clouds is caused by the processes with participation of cosmic rays. The estimation of the cosmic ray density in clouds has shown that for the energy E approx. = I GeV their density can 10 to 1000 times exceed the one in intercloud space. We have made an attempt to determine the mechanism which could lead to the increase in the cosmic ray density in clouds.

Dogiel, V. A.↗
NASA NTRS1985

Analysis of holographic interferograms of aerodynamic models in a wind tunnel

Holographic interferometry provides a non-invasive technique for estimating variations in the air density distribution around aerodynamic models in wind tunnels. The testing of this technique has been underway for some time and has been reported previously for a two dimensional aerodynamic model. Results obtained from tests using three dimensional aerodynamic models are summarized. Holograms were made of aerodynamic models in a wind tunnel. Interferograms were made from these holograms. The interference fringes in these holographic interferograms were digitized and this information was entered into the HOLOFT program. The HOLOFT program successfully calculated the known stagnation air density at the nose of a model and the known air density distribution across the cross section passing through the stagnation point for the axisymmetrical case of this model at a Mach number of 0.8. Thus the technique of holographic interferometry does work.The HOLOFT program stands for HOLOgraphic Inversion by 2-D Fourier Transform.

Perry, R. L.↗
NASA NTRS1986

Activities of the Department of Astronomy and McDonald Observatory of the University of Texas at Austin

McDonald Observatory of the University of Texas at Austin reports on its activities during the period 1 Jan. 1986 to 30 June 1986. Extensive observations of Halley's Comet were obtained. The comet exhibited large variability; moreover, its variability was much more rapid than can be accounted for by water vaporization as the sole controller of activity. Jupiter satellite Io's atmosphere was found to be distended by more than the equilibrium scale height but less than for unimpeded streaming into space. The atmosphere is at least temporarily bound to IO. Uranus' (3-0) H2 quadrupole line shapes require a modification of Baines and Bergstralh's standard model which incorporates at high altitude absorbing haze in addition to the lower haze layer. A fraction of normal H2 equal to 0.25 + or 0.10 is derived, in good agreement with the standard model. This result is unchanged when the preliminary temperature structure derived by the Voyager Radio Occultation Experiment is used instead of Appleby's model c. Out of the six Pluto-Charon mutual events observed this year, data were obtained on four. Preliminary analysis is yielding improved estimates for the diameters, masses, densities, and albedos of these objects.

Smith, H. J.↗
NASA NTRS1985

Thunderstorm currents and lightning charges at the NASA Kennedy Space Center

The NASA Kennedy Space Center, (KSC) and the U.S. Air Force are currently operating an extensive network of ground-based electric-field mills in order to detect atmospheric electrical hazards to ground operations, launches, and landings of spacecraft. Over the past 10 years, the University of Arizona has utilized data provided by this network, and other instrumentation, to investigate the electrical structure of thunderstorms and lightning in Florida. Recent analyses have included estimates of the Maxwell current density that thunderstorms produce at the ground and computations of the locations and magnitudes of lightning-caused changes in the cloud charge distribution. This work is reviewed, and an example of how thunderstorm current patterns and lightning charges develop in both space and time is presented.

Krider, E. P.↗
NASA NTRS1987

Models relating the radio emission and ionised gas in Seyfert nuclei

Possible models are discussed in which the radio emitting components in Seyfert II nuclei can compress and accelerate the ambient nuclear medium to produce the characteristics of the narrow line region. A first order model, which considers only the expansion of the radio components, is briefly described. However, in many Seyfert nuclei it appears that the linear motion of the radio components is also important. This can result in shock heating of the ambient medium, and if the cooling time is long enough, can lead to a displacement between the radio component and the associated emission lines. This effect may be present in NGC 1068 and NGC 5929 and by considering ram pressure balance and the cooling length it is possible to estimate lobe velocities and ambient densities.

Pedlar, A.↗
NASA NTRS1988

Invertible canopy reflectance modeling of vegetation structure in semiarid woodland

The Li-Strahler canopy reflectance model, driven by Landsat Thematic Mapper (TM) data, provided regional estimates of tree size and density in two bioclimatic zones in West Africa. This model exploits tree geometry in an inversion technique to predict average tree size and density from reflectance data using a few simple parameters measured in the field (spatial pattern, shape, and size distribution of trees) and in the imagery (spectral signatures of scene components). The model was tested in sparse woodland and wooded grassland in the Sahelian and Sudanian bioclimatic zones in West Africa.

Franklin, Janet↗
NASA NTRS1988

Wide binary stars at the Galactic poles

Results are reported from a search for high-Galactic-latitude wide binaries down to visual magnitude 14 on five digitized Schmidt-telescope plates. The data-reduction and analysis techniques are described in detail, and the results are presented in graphs and briefly characterized. The number of wide binaries and the estimated wide-binary number density are both three times smaller than those found by Bahcall and Soneira (1981), but still too large to be consistent with the predictions of physical models. Possible explanations for this discrepancy are explored.

Garnavich, Peter M.↗
NASA NTRS1989

Photochemical stability of CO2 in the Martian atmosphere - Reevaluation of the eddy diffusion coefficient and the role of water vapor

Theoretical models of CO2 photodissociation in the atmosphere of Mars are developed and analyzed, with a focus on the conditions prevailing during the Mariner 6/7 experiments. The aeronomy of Martian CO2 is reviewed; the atomic O distribution in the upper atmosphere is characterized; the construction of the basic model is described; and simulation results are presented in extensive tables and graphs and discussed in detail. The vertical eddy-diffusion coefficient corresponding to the estimated upper-atmosphere O density (1-2 percent at 135 km) is found to be about 5 x 10 to the 7th sq cm/sec, and HO(x) is found to play a significant role in catalyzing the recombination of CO and O to form CO2. The reasons why the model overpredicts the production of HO(x) and hence of CO2 are explored.

Shimazaki, Tatsuo↗
NASA NTRS1989

Dust-enshrouded asymptotic giant branch stars in the solar neighborhood

Using available infrared catalogs, an inventory is taken of the AGB star losing large amounts of mass within about 1 kpc of the sun. A surface density of these stars is estimated of about 25/sq kpc projected onto the plane of the Galaxy. Of these stars, about one-half are oxygen-rich while the other half are carbon-rich. The total mass-loss rate from AGB stars into the interstellar medium is probably between 3 and 6 x 10 to the -4th solar mass/sq kpc/yr. Within the uncertainties, this is in reasonable agreement with an estimated net loss rate of about 8 x 10 to the -4th solar mass/sq kpc/yr for main-sequence stars with initial masses between 1 and 5 solar masses as they evolve to white dwarfs. However, it is possible that there are important sources of mass loss which have not yet been identified. In the solar neighborhood, about one-half of all about 1.2 solar mass main-sequence stars spend greater than 30,000 yr in a carbon-star phase where they lose 1-2 x 10 to the -5th solar mass/yr and then become white dwarfs with about 0.7 solar mass.

Jura, M.↗
NASA NTRS1989

Rocket observations of the precipitation of electrons by ground VLF transmitters

Recent results obtained with electric and magnetic receivers aboard a NASA sounding rocket launched on July 31, 1987 are presented which relate multiple electron spectral peaks observed in the bounce loss cone fluxes to the resonant interaction of electrons with VLF waves from ground transmitters. The correlation of transmitter signals passing through the ionosphere with the precipitated electrons was investigated. The analysis of these in situ wave and particle data addresses the propagation of waves through the ionosphere, and, through an application of the resonant theory, enables an estimation of the cold plasma density in the interaction region.

Arnoldy, Roger L.↗
NASA NTRS1990

Radiative magnetized thermal conduction fronts

The evolution of plane-parallel magnetized thermal conduction fronts in the interstellar medium (ISM) was studied. Separating the coronal ISM phase and interstellar clouds, these fronts have been thought to be the site of the intermediate-temperature regions whose presence was inferred from O VI absorption-line studies. The front evolution was followed numerically, starting from the initial discontinuous temperature distribution between the hot and cold medium, and ending in the final cooling stage of the hot medium. It was found that, for the typical ISM pressure of 4000 K/cu cm and the hot medium temperature of 10 to the 6th K, the transition from evaporation to condensation in a nonmagnetized front occurs when the front thickness is 15 pc. This thickness is a factor of 5 smaller than previously estimated. The O VI column densities in both evaporative and condensation stages agree with observations if the initial hot medium temperature Th exceeds 750,000 K. Condensing conduction fronts give better agreement with observed O VI line profiles because of lower gas temperatures.

Borkowski, Kazimierz J.↗
NASA NTRS1990

IUE observations of PG 1115 + 080 - The He I Gunn-Peterson test and a search for the lensing galaxy

Five observations of PG 1115 + 080 taken with the IUE SWP camera have been combined in order to carry out the He I Gunn-Peterson test and to search for a Lyman limit which could determine the redshift of the lens candidate reported by Christian et al. (1987). No Lyman-limit discontinuities are found, implying that the lensing galaxy does not intercept the line of sight or does not contain enough neutral hydrogen to be detected as a Lyman-limit edge. It is estimated that the lens column density for neutral hydrogen is 3 x 10 to the 16th/sq cm or less if it intercepts the line of sight.

Tripp, Todd M.↗
NASA NTRS1992

Light element nucleosynthesis - A false clue?

It is proposed that the dynamically estimated value for the cosmological density parameter, Omega(dy) = 0.15 x 10 exp +/- 0.20, reflects the baryon density at decoupling, resulting in lower initial, primordial values of D and the He-3 than are observed. An early generation of massive stars, forming somewhat after decoupling, collapses to black holes with masses of about 10 exp 6.5 solar masses. If they later accrete gas and emit a quasarlike (X, gamma)-ray spectrum, then (gamma, (He-4)) photodisintegration reactions will increase D and He-3 to the observed range, leaving a high-energy background radiation field similar to that observed. The massive black holes become the dynamically observed dark matter galactic halos. This scenario obviates the need for nonbaryonic dark matter and provides a specific form for the requisite baryonic dark matter; it thus reduces the number of density parameters: Omega(0) - Omega(dy) = Omega(BBN) is approximately equal to 0.15.

Gnedin, N. IU.↗
NASA NTRS1992

Cosmic string with a light massive neutrino

We have estimated the power spectra of density fluctuations produced by cosmic strings with neutrino hot dark matter (HDM). Normalizing at 8/h Mpc, we find that the spectrum has more power on small scales than HDM + inflation, less than cold dark matter (CDM) + inflation, and significantly less the CDM + strings. With HDM, large wakes give significant contribution to the power on the galaxy scale and may give rise to large sheets of galaxies.

Albrecht, Andreas↗
NASA NTRS1992

Hubble Space Telescope Faint Object Spectrograph and ground-based observations of the broad absorption line quasar 0226-1024

Faint Object Spectrograph data from the Hubble Space Telescope of the broad absorption line quasar 0226-1024 have revealed the presence of 8-10 absorbing ions between 680 and 1000 A (restframe): C III, N III, N IV, O III, O IV, O VI, S V, S VI, possibly Ne VIII, and possibly O V* arising from a metastable excited state. We also present ground-based optical observations of the broad line troughs for the following ions: H I, C IV, N V, Si IV, and possibly Fe III, S IV, P V, and C III* (also arising from a metastable excited state). The results of this fit are used to estimate the absorbing ionic column densities. There is evidence that the broad absorption line clouds are optically thick and either do not completely cover the continuum source or narrow unresolved lines are present.

Korista, Kirk T.↗
NASA NTRS1994

The solar neighborhood, 1: Standard spectral types (K5-M8) for northern dwarfs within eight parsecs

Spectral types on a standard system are presented for late-type dwarfs within 8 pc. All known main-sequence stars north of -25 deg and with M(sub V) greater than or = 8.00 have been observed, resulting in 92 spectra. Based upon the stellar system density to 5 pc, we estimate that approximately 35 systems in the 8 pc sample are 'missing.' In an effort to reveal these systems, we use an empirical spectral type-M(sub V) relation to estimate distances to additional stars that may lie within 8 pc.

Henry, Todd J.↗
NASA NTRS1995

Correlations between critical current density, j(sub c), critical temperature, T(sub c),and structural quality of Y1B2Cu3O(7-x) thin superconducting films

Correlations between critical current density (j(sub c)) critical temperature (T(sub c)) and the density of edge dislocations and nonuniform strain have been observed in YBCO thin films deposited by pulsed laser ablation on (001) LaAlO3 single crystals. Distinct maxima in j(sub c) as a function of the linewidths of the (00 l) Bragg reflections and as a function of the mosaic spread have been found in the epitaxial films. These maxima in j(sub c) indicate that the magnetic flux lines, in films of structural quality approachingthat of single crystals, are insufficiently pinned which results in a decreased critical current density. T(sub c) increased monotonically with improving crystalline quality and approached a value characteristic of a pure single crystal. A strong correlation between j(sub c) and the density of edge dislocations ND was found. At the maximum of the critical current density the density of edge dislocations was estimated to be N(sub D) approximately 1-2 x 10(exp 9)/sq cm.

Chrzanowski, J.↗
NASA NTRS1996

Improved Closed-Loop Carrier-Phase Synchronization In BPSK

Scheme motivated by maximum-likelihood approach offers advantages over other well-known schemes. Proposed signal-processing system generates error signal proportional to derivative of conditional probability density function with respect to estimated carrier phase. Signal serves as feedback signal in carrier-phase-tracking loop.

Simon, Marvin K.↗