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The concept of an N-path bandpass Delta Sigma A/D convertor is introduced. A multibit sixth-order SC implementation is described. The new scheme appears to be very effective for the realization of the narrowband bandpass delta-sigma modulators needed for communication applications.
A list of galaxies characterized by low surface brightness (LSB) is presented which facilitates the recognition of galaxies with brightnesses close to that of the sky. A total of 198 objects and 140 objects are listed in the primary and secondary catalogs respectively, and LSB galaxies are examined by means of H I redshift distributions. LSB disk galaxies are shown to have similar sizes and masses as the high-surface-brightness counterparts, and ellipticals and SOs are rarely encountered. Many LSB spirals have stellarlike nuclei, and most of the galaxies in the present catalog are late-type galaxies in the Sc, Sm, and Im classes. The LSB region of observational parameter space is shown to encompass a spectrum of types as full as that of the Hubble sequence. It is suggested that studies of LSB galaxies can provide important data regarding the formation and star-formation history of all galaxies.
Observations of NGC 4565 at 100, 160, and 200 microns with the University of Chicago Far-Infrared Camera and the NASA-Kuiper Airborne Observatory are reported. In order to examine the dependence of FIR emission on spiral structure and star formation activity, these observations of NGC 4565, a quiescent Sb galaxy, are compared with observations of NGC 6946, an active Sc galaxy, made by Engargiola (1991) using the same instruments. Warm dust (30 K) in a bisymmetric spiral pattern superposed on an exponential disk of cool dust (20 K) can account for the FIR morphology of NGC 4565. Optical and IR data suggest that there are more embedded sources heating dust locally in the southeast arm region and more UV radiation from unobscured young stellar associations heating the cool, neutral medium in the northeast arm region.
Ab initio calculations were performed to study the bonding of the first-row transition-metal ions with ethylene. While Sc(+) and Ti(+) insert into the pi bond of ethylene to form a three-membered ring, the ions V(+) through Cu(+) form an electrostatic complex with ethylene. The binding energies are compared with those from experiment and with those of comparable calculations performed previously for the metal-acetylene ion systems.
We present measurements of the spallation cross sections of carbon, oxygen, and iron in helium and hydrogen, at beam energies from 540 to 1600 MeV/nucleon, performed by exposing liquid helium, CH2, and C targets. Charge changing cross sections are reported for fragments down to Ne for Fe + alpha and Fe + p reactions, and down to B for O + alpha, O + p, C + alpha, and C + p reactions. Alpha- to p-induced cross section ratios (sigma(sub alpha)/sigma(sub p)) are determined at the same energy per nucleon. From these measurements an empirical formula for the (sigma(sub alpha)/sigma(sub p)) ratios is derived and is found in good agreement with available isotopic cross sections data from radioactivity and radiochemical techniques. These results are applied to the propagation of heavy charged cosmic rays in an interstellar medium with a helium to hydrogen abundance ratio of 0.10. It is shown that the Sc-Mn/Fe ratio prediction is decreased relative to the B/C ratio when compared to propagation calculations in a pure hydrogen interstellar medium.
LKFM basaltic impact melts are abundant among Apollo lunar samples, especially those from Apollo 15, 16, and 17. They are generally basaltic in composition, but are found exclusively as impact melts. They seem to be related to basins and so could represent the composition of the lower lunar crust. They contain lithic clasts that cannot be mixed in any proportion to produce the composition of the melt matrix; components rich in transition elements (Ti, Cr, Sc) and REE are not considered. To search for the mysterious cryptic component, we previously investigated the mineral clast population in two Apollo 14 LKFM basaltic impact melts, 15445 and 15455. The cryptic component was not present in the mineral clast assemblage of these breccias either, but some olivine and pyroxene grains appeared to be from lithologies not represented among identified igneous rocks from the lunar highlands. In addition, none of the mineral clasts could be unambiguously assigned to a ferroan anorthosite source. We have now extended this study to Apollo 17, starting with two LKFM impact melt breccias (76295 and 76315) from the Apollo 17 station 6 boulder. The results from the study are presented.
In a time-dependent three dimensional MHD simulation for cometary plasmas, Schmidt-Voigt (1989) could observe the formation of condensations in the plasma tail after a 90 degree change in the interplanetary magnetic field (IMF) sweeping over the comet. We investigated the IMF measurements of the Vega SC in the vicinity of the comet Halley for 90 degree changes in the clock angle and studied the relation between them and optical observations of condensations in the plasma tail. For the time interval 24 Feb. 86 to 14 Mar. 86, we could not find a correlation between such changes and the release of condensations from the cometary head.
An instance-based learning system is presented. SC-net is a fuzzy hybrid connectionist, symbolic learning system. It remembers some examples and makes groups of examples into exemplars. All real-valued attributes are represented as fuzzy sets. The network representation and learning method is described. To illustrate this approach to learning in fuzzy domains, an example of segmenting magnetic resonance images of the brain is discussed. Clearly, the boundaries between human tissues are ill-defined or fuzzy. Example fuzzy rules for recognition are generated. Segmentations are presented that provide results that radiologists find useful.
While continuing the application of the inverse dynamics approach in obtaining the optimal numerical solutions, the research during the past six months has been focused on the formulation and derivation of closed-form solutions for constrained hypersonic flight trajectories. Since it was found in the research of the first year that a dominant portion of the optimal ascent trajectory of the aerospace plane is constrained by dynamic pressure and heating constraints, the application of the analytical solutions significantly enhances the efficiency in trajectory optimization, provides a better insight to understanding of the trajectory and conceivably has great potential in guidance of the vehicle. Work of this period has been reported in four technical papers. Two of the papers were presented in the AIAA Guidance, Navigation, and Control Conference (Hilton Head, SC, August, 1992) and Fourth International Aerospace Planes Conference (Orlando, FL, December, 1992). The other two papers have been accepted for publication by Journal of Guidance, Control, and Dynamics, and will appear in 1993. This report briefly summarizes the work done in the past six months and work currently underway.
Primitive achondrites are meteorites that have mineral and bulk chemical compositions similar to the most primitive meteorites (chondrites) but have textures similar to more evolved meteorites (achondrites). The unique geochemistry and texture of the primitive achondrites suggest these meteorites may be genetic intermediates between chondrites and achondrites and may preserve evidence of processes occurring in the early solar system. Five primitive achondrites LEW 88280, MAC 88177, ALHA 81187, EET 84302, and LEW 88663 were examined in this study in order to classify the meteorites and to determine processes that have affected them. Bulk chemical analyses of Nap, K2O, CaO, FeO, Cr, Co, Ni, Sc, Ir, Au, As, Sb, Se, Br, Cs, Ba, La, Ce, Nd, Sm, Eu, Tb, Yb, and Lu were determined for each meteorite by Instrumental Neutron Activation Analysis (INAA). Concentrations of Hf, U, and Th were determined for some meteorites. Polished thin sections of the five meteorites were examined in transmitted and reflected light microscopy to identify minerals and examine petrographic relationships. Minerals found in the meteorites include olivine, orthopyroxene, clinopyroxene, plagioclase, Cr-spinel, phosphates, troilite, kamecite, and taenite along with other minor phases. Mineral compositions were determined with an electron microprobe. The initial study suggests that the meteorites have been altered by metamorphic processes although igneous processes may also have played a role in the evolution of these rocks. Further studies of isotope and bulk chemistry are planned for these meteorites.
We present ultraviolet spectra of 143 star-forming galaxies of different morphological types and activity classes including S0, Sa, Sb, Sc, Sd, irregular, starburst, blue compact, blue compact dwarf, Liner, and Seyfert 2 galaxies. These IUE spectra cover the wavelength range from 1200 to 3200 A and are taken in a large aperture (10 x 20 inch). The ultraviolet spectral energy distributions are shown for a subset of the galaxies, ordered by spectral index, and separated by type for normal galaxies, Liners, starburst galaxies, blue compact (BCG) and blue compact dwarf (BCDG) galaxies, and Seyfert 2 galaxies. The ultraviolet spectra of Liners are, for the most part, indistinguishable from the spectra of normal galaxies. Starburst galaxies have a large range of ultraviolet slope, from blue to red. The star-forming galaxies which are the bluest in the optical (BCG and BCDG), also have the 'bluest' average ultraviolet slope of beta = -1.75 +/- 0.63. Seyfert 2 galaxies are the only galaxies in the sample that consistently have detectable UV emission lines.
We are studying the disk-halo interface in several edge-on spiral galaxies through extensive imagery in H(alpha) and other emission lines from Diffuse Ionized Gas (DIG), also referred to as the Warm Ionized Medium (WIM). In addition, for the nearby Sc galaxy NGC4631 we have obtained x-ray observations with ROSAT, to map the distribution of hot (10(exp 6) - 10(exp 7)) gas in the disk and halo. Here we present initial results for two late-type spirals, NGC4244 and NGC4631.
We present the discovery of a 'synchrotron halo' in NGC 253. NGC 253 is an inclined, dusty, barred Sc galaxy in the Sculptor Group. It is also one of the prototype nuclear star burst galaxies. Very Large Array observations at 327 MHz and 1.4 GHz have revealed a synchrotron emitting 'halo' extending 9 kpc above and below the plane of NGC 253 (assuming a distance of 4 Mpc). The spectral index for the radio emission steepens from -0.7 in the disk to -1.0 in the halo. The fractional polarization at 1.4 GHz increases from less than or equal to 2 percent in the disk up to 20 percent in the halo, and the magnetic field vectors project parallel to the plane over most of the halo.
Radioactivities induced in several Long Duration Exposure Facilities (LDEF) samples were measured by low-level counting at Los Alamos and elsewhere. These radionuclides have activities similar to those observed in meteorites and lunar samples. Some trends were observed in these measurements in terms of profiles in trunnion layers and as a function of radionuclide half-life. Several existing computer codes were used to model the production by the protons trapped in the Earth's radiation belts and by the galactic cosmic rays of some of these radionuclides, Mn-54 and Co-57 in steel, Sc-46 in titanium, and Na-22 in alloys of titanium and aluminum. Production rates were also calculated for radionuclides possibly implanted in LDEF, Be-7, Be-10, and C-14. Enhanced concentrations of induced isotopes in the surfaces of trunnion sections relative to their concentrations in the center are caused by the lower-energy protons in the trapped radiation. Secondary neutrons made by high-energy trapped protons and by galactic cosmic rays produce much of the observed radioactivities, especially deep in an object. Comparisons of the observed to calculated activities of several radionuclides with different half-lives indicate that the flux of trapped protons at LDEF decreased significantly at the end of the mission.
It was indicated that Si3N4 doped with Sc2O3 may exhibit high temperature mechanical properties superior to Si3N4 systems with various other oxide sintered additives. High temperature deformation of samples was studied by characterizing the microstructures before and after deformation. It was found that elements of the additive, such as Sc and O, exist in small amounts at very thin grain boundary layers and most of them stay in secondary phases at tripple and multiple grain boundary junctions. These secondary phases are devitrified as crystalline Sc2Si2O7. Deformation of the samples was dominated by cavitational processes rather than movements of dislocations. Thus the excellent deformation resistance of the samples at high temperature can be attributed to the very small thickness of the grain boundary layers and the crystalline secondary phase.
Measurements of elemental abundances by neutron activation methods across the Cenomanian-Turonian extinction interval in samples collected from 16 sites in the Western Interior Basin of North America from 12 widely separated locations around the globe, including six ODP/DSDP sites, are reported. In most Western Interior Basin sites, in Colombia, and in Western Europe, two closely spaced elemental abundance peaks occur in the upper Cenomanian (about 92 m.y.), spanning the ammonite zones of Sciponoceras gracile through Neocardioceras juddii. Elements with anomalously high concentrations include Sc, Ti, V, Cr, Mn, Co, Ni, Ir, Pt, and Au. The lower peak coincides with the disappearance (extinction) of the foraminifer Rotalipora cushmani. In North American sections R. greenhornensis also disappears at or just below this horizon, but in Europe it disappears considerably earlier than R. cushmani. Although the weak geochemical signal from comet impact(s) could be masked by the strong terrestrial-like overprint, these anomalies more likely resulted in the large late Cenomanian through early Turonian eustatic rise and deep-water opening of the South Atlantic.
The interaction of Fe(+) with benzyne is studied using the self-consistent-field (SCF), complete active space SCF, and modified-coupled-pair functional levels of theory. The most stable structure is planar, where the Fe(+) has inserted into the in-plane pi bond, although the C-C bond distance suggests that some in-plane pi bonding remains. This system is compared with Sc(+) bonding to benzyne and other ligands.
Yamato-86032 is a shock-lithified anorthositic fragmental breccia. It consists mainly of highly feldspathic meta-breccias and meta-meltrocks and possibly contains a small contribution from mare lithologies, but there is no indication of a KREEP component. In many respects Y-86032 is similar to the previously described lunar meteorites Y-82192/3, but there are some notable differences. We have analyzed about 40 major and trace elements in bulk matrix, impact melt, and clast samples from two chips of Y-86032. The abundances of most lithophile and incompatible elements are lower in Y-86032 than in Y-82192 (which contains very low abundances compared to normal lunar highland rocks). The REE abundances are comparable to those of Y-82192. The elements Sc, Cr, Mn, Fe and Co have significantly lower abundances than in Y-82192, and the siderophile element pattern is also different. Since cosmic ray exposure data indicate pairing of Y-86032 with Y-82192/3, the source region for these meteorites on the moon must have been fairly heterogeneous.