3,880 research outputs found
Photometric observations of selected, optically bright quasars for Space Interferometry Mission and other future celestial reference frames
Photometric observations of 235 extragalactic objects that are potential
targets for the Space Interferometry Mission (SIM) are presented. Mean B, V, R,
I magnitudes at the 5% level are obtained at 1 - 4 epochs between 2005 and 2007
using the 1-m telescopes at Cerro Tololo Inter-American Observatory and Naval
Observatory Flagstaff Station. Of the 134 sources which have V magnitudes in
the Veron & Veron-Cetty catalog a difference of over 1.0 mag is found for the
observed-catalog magnitudes for about 36% of the common sources, and 10 sources
show over 3 mag difference. Our first set of observations presented here form
the basis of a long-term photometric variability study of the selected
reference frame sources to assist in mission target selection and to support in
general QSO multi-color photometric variability studies.Comment: 40 pages, 13 figures, 4 table
Local Lagrangian Approximations for the Evolution of the Density Distribution Function in Large-Scale Structure
We examine local Lagrangian approximations for the gravitational evolution of
the density distribution function. In these approximations, the final density
at a Lagrangian point q at a time t is taken to be a function only of t and of
the initial density at the same Lagrangian point. A general expression is given
for the evolved density distribution function for such approximations, and we
show that the vertex generating function for a local Lagrangian mapping applied
to an initially Gaussian density field bears a simple relation to the mapping
itself. Using this result, we design a local Lagrangian mapping which
reproduces nearly exactly the hierarchical amplitudes given by perturbation
theory for gravitational evolution. When extended to smoothed density fields
and applied to Gaussian initial conditions, this mapping produces a final
density distribution function in excellent agreement with full numerical
simulations of gravitational clustering. We also examine the application of
these local Lagrangian approximations to non-Gaussian initial conditions.Comment: LaTeX, 22 pages, and 11 postscript figure
URAT: astrometric requirements and design history
The U.S. Naval Observatory Robotic Astrometric Telescope (URAT) project aims
at a highly accurate (5 mas), ground-based, all-sky survey. Requirements are
presented for the optics and telescope for this 0.85 m aperture, 4.5 degree
diameter field-of-view, specialized instrument, which are close to the
capability of the industry. The history of the design process is presented as
well as astrometric performance evaluations of the toleranced, optical design,
with expected wavefront errors included.Comment: 12 pages, 7 figures, SPIE 2006 Orlando conf. proc. Vol. 626
Study of fault tolerant software technology for dynamic systems
The major aim of this study is to investigate the feasibility of using systems-based failure detection isolation and compensation (FDIC) techniques in building fault-tolerant software and extending them, whenever possible, to the domain of software fault tolerance. First, it is shown that systems-based FDIC methods can be extended to develop software error detection techniques by using system models for software modules. In particular, it is demonstrated that systems-based FDIC techniques can yield consistency checks that are easier to implement than acceptance tests based on software specifications. Next, it is shown that systems-based failure compensation techniques can be generalized to the domain of software fault tolerance in developing software error recovery procedures. Finally, the feasibility of using fault-tolerant software in flight software is investigated. In particular, possible system and version instabilities, and functional performance degradation that may occur in N-Version programming applications to flight software are illustrated. Finally, a comparative analysis of N-Version and recovery block techniques in the context of generic blocks in flight software is presented
Computational study of the thermal conductivity in defective carbon nanostructures
We use non-equilibrium molecular dynamics simulations to study the adverse
role of defects including isotopic impurities on the thermal conductivity of
carbon nanotubes, graphene and graphene nanoribbons. We find that even in
structurally perfect nanotubes and graphene, isotopic impurities reduce thermal
conductivity by up to one half by decreasing the phonon mean free path. An even
larger thermal conductivity reduction, with the same physical origin, occurs in
presence of structural defects including vacancies and edges in narrow graphene
nanoribbons. Our calculations reconcile results of former studies, which
differed by up to an order of magnitude, by identifying limitations of various
computational approaches
Mott metal-insulator transition on compressible lattices
The critical properties of the finite temperature Mott endpoint are
drastically altered by a coupling to crystal elasticity, i.e., whenever it is
amenable to pressure tuning. Similar as for critical piezoelectric
ferroelectrics, the Ising criticality of the electronic system is preempted by
an isostructural instability, and long-range shear forces suppress microscopic
fluctuations. As a result, the endpoint is governed by Landau criticality. Its
hallmark is thus a breakdown of Hooke's law of elasticity with a non-linear
strain-stress relation characterized by a mean-field exponent. Based on a
quantitative estimate, we predict critical elasticity to dominate the
temperature range DeltaT/Tc ~ 8% close to the Mott endpoint of
kappa-(BEDT-TTF)2X.Comment: 4 pages, 6 figure
Ammonia scavenging agents for people with cirrhosis and hepatic encephalopathy
This is the protocol for a review and there is no abstract. The objectives are as follows: To evaluate the benefits and harms of ammonia scavenging agents versus placebo, no intervention, or other active interventions for people with cirrhosis and hepatic encephalopathy
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