7,628 research outputs found
Advanced expander test bed program
The Advanced Expander Test Bed (AETB) is a key element in NASA's Space Chemical Engine Technology Program for development and demonstration of expander cycle oxygen/hydrogen engine and advanced component technologies applicable to space engines as well as launch vehicle upper stage engines. The AETB will be used to validate the high-pressure expander cycle concept, investigate system interactions, and conduct investigations of advanced mission focused components and new health monitoring techniques in an engine system environment. The split expander cycle AETB will operate at combustion chamber pressures up to 1200 psia with propellant flow rates equivalent to 20,000 lbf vacuum thrust. Contract work began 27 Apr. 1990. During 1992, a major milestone was achieved with the review of the final design of the oxidizer turbopump in Sep. 1992
A (p,q)-deformed Landau problem in a spherical harmonic well: spectrum and noncommuting coordinates
A (p,q)-deformation of the Landau problem in a spherically symmetric harmonic
potential is considered. The quantum spectrum as well as space noncommutativity
are established, whether for the full Landau problem or its quantum Hall
projections. The well known noncommutative geometry in each Landau level is
recovered in the appropriate limit p,q=1. However, a novel noncommutative
algebra for space coordinates is obtained in the (p,q)-deformed case, which
could also be of interest to collective phenomena in condensed matter systems.Comment: 9 pages, no figures; updated reference
Fluctuations of company yearly profits versus scaled revenue: Fat tail distribution of Levy type
We analyze annual revenues and earnings data for the 500 largest-revenue U.S.
companies during the period 1954-2007. We find that mean year profits are
proportional to mean year revenues, exception made for few anomalous years,
from which we postulate a linear relation between company expected mean profit
and revenue. Mean annual revenues are used to scale both company profits and
revenues. Annual profit fluctuations are obtained as difference between actual
annual profit and its expected mean value, scaled by a power of the revenue to
get a stationary behavior as a function of revenue. We find that profit
fluctuations are broadly distributed having approximate power-law tails with a
Levy-type exponent , from which we derive the associated
break-even probability distribution. The predictions are compared with
empirical data.Comment: 6 pages, 6 figure
Magnetic fields in noncommutative quantum mechanics
We discuss various descriptions of a quantum particle on noncommutative space
in a (possibly non-constant) magnetic field. We have tried to present the basic
facts in a unified and synthetic manner, and to clarify the relationship
between various approaches and results that are scattered in the literature.Comment: Dedicated to the memory of Julius Wess. Work presented by F. Gieres
at the conference `Non-commutative Geometry and Physics' (Orsay, April 2007
Palaeoenvironmental significance of middle Oxfordian deep marine deposits from La Manga Formation, Neuquén Basin, Argentina
[EN] The Callovian-Oxfordian of the Neuquén Basin (Argentina) is characterized by an extensive marine carbonate system (La Manga Formation)
with a predominance of shallow and middle ramp deposits, although locally in tectonically controlled settings, deeper deposits
also formed. These middle Oxfordian deeper deposits consist of mudstone-wackestone carbonates alternating with black shales and show
marked centimetre-scale rhythmicity, probably controlled by the Earth’s orbit parameters, mainly precession and eccentricity. The present
study was designed to examine these deposits in terms of their sedimentology, geochemistry, and small-scale stratigraphy in Arroyo Los
Blancos, southern Mendoza Province, where they are exceptionally well exposed and preserved. Results indicate that the sediments were
deposited in an anoxic to dysoxic, relatively deep, sea-floor environment as revealed by the presence of: a) organic-rich shales, b) undisrupted
lamination in most facies, c) pyrite framboids, and d) absence or scarcity of trace fossils and benthic fauna. Thin beds of graded
wackestone-packstone and accumulations of thin shells of Bositra bivalves indicate sporadic reworking of the sea floor by weak currents.
Organic petrology, Rock-Eval pyrolysis and thermal maturity indicators were used to characterize TOC and different types of organic matter.
Low pyrolysis S yields along with low hydrogen indices suggest poor kerogen convertibility. Vitrinite reflectance (%Ro) ranged from
2.39 to 2.97 with an average of 2.70. The thermal alteration index (TAI) was 4+(5), indicating overmaturity. A tectono-sedimentary model
is proposed for these deposits. According to this model, the relatively deep, organic-matter rich facies of La Manga Formation would have
been deposited in the deepest zones of topographic lows controlled by tilting and differential subsidence of tectonic blocks bounded by
normal faults.These faults were probably inherited from previous extensional tectonics of Late Triassic - Early Jurassic age.[ES] El Calloviense-Oxfordiense en el norte de la Cuenca Neuquina (Argentina) estuvo caracterizado por el desarrollo de un amplio sistema
de rampa carbonatada (Formación La Manga) en el que predominaron los depósitos de rampa media y somera aunque localmente, y debido
a control tectónico, pudieron acumularse depósitos más profundos. Estos depósitos, de edad Oxfordiense medio, están expuestos y preservados
excepcionalmente en la zona de Arroyo Los Blancos, al sur de la provincia de Mendoza, y su estudio sedimentológico, geoquímico y
estratigráfico representan el objetivo de este trabajo. Consisten en carbonatos (mudstone-wackestone) con acumulaciones de finas conchas
de bivalvos de tipo Bositra que indican retrabajamientos esporádicos del sustrato marino debido a corrientes de baja intensidad. Se han
caracterizado diferentes tipos de materia orgánica y TOC mediante petrología orgánica, Rock-Eval Pyrolisis e indicadores de madurez térmica.
Los bajos valores en el campo del S unido al también bajo índice de H sugieren una baja convertibilidad de Kerógeno. La reflectancia
de la vitrinita (%Ro) tiene un rango que oscila entre 2,39 y 2,97, con una media de 2,70, mientras que el índice de alteración térmico (TAI)
es de 4+(5) indicando sobremaduración. Se propone también un modelo tectono-sedimentario para estos depósitos. En este modelo, estas
facies relativamente profundas y ricas en materia orgánica de la Formación La Manga estarían depositadas en la parte más deprimida de bajos
topográficos controlados por una subsidencia diferencial y de pulsos en bloques marcados por fallas normales heredadas probablemente
de una tectónica extensional durante el Triásico Superior-Jurásico Inferior.Peer reviewe
Charge-transfer interactions stabilize g-quadruplex-forming thrombin binding aptamers and can improve their anticoagulant activity
In the search for optimized thrombin binding aptamers (TBAs), we herein describe the synthesis of a library of TBA analogues obtained by end-functionalization with the electron-rich 1,5-dialkoxy naphthalene (DAN) and the electron-deficient 1,8,4,5-naphthalenetetra-carboxylic diimide (NDI) moieties. Indeed, when these G-rich oligonucleotides were folded into the peculiar TBA G-quadruplex (G4) structure, effective donor–acceptor charge transfer interactions between the DAN and NDI residues attached to the extremities of the sequence were induced, providing pseudo-cyclic structures. Alternatively, insertion of NDI groups at both extremities produced TBA analogues stabilized by π–π stacking interactions. All the doubly-modified TBAs were characterized by different biophysical techniques and compared with the analogues carrying only the DAN or NDI residue and unmodified TBA. These modified TBAs exhibited higher nuclease resistance, and their G4 structures were markedly stabilized, as evidenced by increased Tm values compared to TBA. These favorable properties were also associated with improved anticoagulant activity for one DAN/NDI-modified TBA, and for one NDI/NDI-modified TBA. Our results indicated that TBA pseudo-cyclic structuring by ad hoc designed end-functionalization represents an efficient approach to improve the aptamer features, while pre-organizing and stabilizing the G4 structure but allowing sufficient flexibility to the aptamer folding, which is necessary for optimal thrombin recognition
The absolute age of the globular cluster M15 using near-infrared adaptive optics images from PISCES/LBT
We present deep near-infrared (NIR) J, Ks photometry of the old, metal-poor
Galactic globular cluster M\,15 obtained with images collected with the LUCI1
and PISCES cameras available at the Large Binocular Telescope (LBT). We show
how the use of First Light Adaptive Optics system coupled with the (FLAO)
PISCES camera allows us to improve the limiting magnitude by ~2 mag in Ks. By
analyzing archival HST data, we demonstrate that the quality of the LBT/PISCES
color magnitude diagram is fully comparable with analogous space-based data.
The smaller field of view is balanced by the shorter exposure time required to
reach a similar photometric limit. We investigated the absolute age of M\,15 by
means of two methods: i) by determining the age from the position of the main
sequence turn-off; and ii) by the magnitude difference between the MSTO and the
well-defined knee detected along the faint portion of the MS. We derive
consistent values of the absolute age of M15, that is 12.9+-2.6 Gyr and
13.3+-1.1 Gyr, respectively.Comment: 13 pages, 8 figures, ApJ accepte
Numerical renormalization group of vortex aggregation in 2D decaying turbulence: the role of three-body interactions
In this paper, we introduce a numerical renormalization group procedure which
permits long-time simulations of vortex dynamics and coalescence in a 2D
turbulent decaying fluid. The number of vortices decreases as ,
with instead of the value predicted by a na\"{\i}ve
kinetic theory. For short time, we find an effective exponent
consistent with previous simulations and experiments. We show that the mean
square displacement of surviving vortices grows as .
Introducing effective dynamics for two-body and three-body collisions, we
justify that only the latter become relevant at small vortex area coverage. A
kinetic theory consistent with this mechanism leads to . We find that
the theoretical relations between kinetic parameters are all in good agreement
with experiments.Comment: 23 RevTex pages including 7 EPS figures. Submitted to Phys. Rev. E
(Some typos corrected; see also cond-mat/9911032
On the Morphology and Chemical Composition of the HR 4796A Debris Disk
[abridged] We present resolved images of the HR 4796A debris disk using the
Magellan adaptive optics system paired with Clio-2 and VisAO. We detect the
disk at 0.77 \microns, 0.91 \microns, 0.99 \microns, 2.15 \microns, 3.1
\microns, 3.3 \microns, and 3.8 \microns. We find that the deprojected center
of the ring is offset from the star by 4.761.6 AU and that the deprojected
eccentricity is 0.060.02, in general agreement with previous studies. We
find that the average width of the ring is 14, also comparable to
previous measurements. Such a narrow ring precludes the existence of
shepherding planets more massive than \about 4 \mj, comparable to hot-start
planets we could have detected beyond \about 60 AU in projected separation.
Combining our new scattered light data with archival HST/STIS and HST/NICMOS
data at \about 0.5-2 \microns, along with previously unpublished Spitzer/MIPS
thermal emission data and all other literature thermal data, we set out to
constrain the chemical composition of the dust grains. After testing 19
individual root compositions and more than 8,400 unique mixtures of these
compositions, we find that good fits to the scattered light alone and thermal
emission alone are discrepant, suggesting that caution should be exercised if
fitting to only one or the other. When we fit to both the scattered light and
thermal emission simultaneously, we find mediocre fits (reduced chi-square
\about 2). In general, however, we find that silicates and organics are the
most favored, and that water ice is usually not favored. These results suggest
that the common constituents of both interstellar dust and solar system comets
also may reside around HR 4796A, though improved modeling is necessary to place
better constraints on the exact chemical composition of the dust.Comment: Accepted to ApJ on October 27, 2014. 21 pages, 12 figures, 4 table
Duality and Braiding in Twisted Quantum Field Theory
We re-examine various issues surrounding the definition of twisted quantum
field theories on flat noncommutative spaces. We propose an interpretation
based on nonlocal commutative field redefinitions which clarifies previously
observed properties such as the formal equivalence of Green's functions in the
noncommutative and commutative theories, causality, and the absence of UV/IR
mixing. We use these fields to define the functional integral formulation of
twisted quantum field theory. We exploit techniques from braided tensor algebra
to argue that the twisted Fock space states of these free fields obey
conventional statistics. We support our claims with a detailed analysis of the
modifications induced in the presence of background magnetic fields, which
induces additional twists by magnetic translation operators and alters the
effective noncommutative geometry seen by the twisted quantum fields. When two
such field theories are dual to one another, we demonstrate that only our
braided physical states are covariant under the duality.Comment: 35 pages; v2: Typos correcte
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