10,191 research outputs found
Small-body deflection techniques using spacecraft: techniques in simulating the fate of ejecta
We define a set of procedures to numerically study the fate of ejecta
produced by the impact of an artificial projectile with the aim of deflecting
an asteroid. Here we develop a simplified, idealized model of impact conditions
that can be adapted to fit the details of specific deflection-test scenarios,
such as what is being proposed for the AIDA project. Ongoing studies based upon
the methodology described here can be used to inform observational strategies
and safety conditions for an observing spacecraft. To account for ejecta
evolution, the numerical strategies we are employing are varied and include a
large N-Body component, a smoothed-particle hydrodynamics (SPH) component, and
an application of impactor scaling laws. Simulations that use SPH-derived
initial conditions show high-speed ejecta escaping at low angles of
inclination, and very slowly moving ejecta lofting off the surface at higher
inclination angles, some of which re-impacts the small-body surface. We are
currently investigating the realism of this and other models' behaviors. Next
steps will include the addition of solar perturbations to the model and
applying the protocol developed here directly to specific potential mission
concepts such as the proposed AIDA scenario.Comment: 19 pages, 11 figures, accepted for publication in Advances in Space
Research, Special Issue: Asteroids & Space Debri
Dynamics, dephasing and clustering of impurity atoms in Bose-Einstein condensates
We investigate the influence of a Bose-Einstein condensate (BEC) on the
properties of immersed impurity atoms, which are trapped in an optical lattice.
Assuming a weak coupling of the impurity atoms to the BEC, we derive a quantum
master equation for the lattice system. In the special case of fixed impurities
with two internal states the atoms represent a quantum register and the quantum
master equation reproduces the exact evolution of the qubits. We characterise
the qubit dephasing which is caused by the interspecies coupling and show that
the effect of sub- and superdecoherence is observable for realistic
experimental parameters. Furthermore, the BEC phonons mediate an attractive
interaction between the impurities, which has an important impact on their
spatial distribution. If the lattice atoms are allowed to move, there occurs a
sharp transition with the impurities aggregating in a macroscopic cluster at
experimentally achievable temperatures. We also investigate the impact of the
BEC on the transport properties of the impurity atoms and show that a crossover
from coherent to diffusive behaviour occurs with increasing interaction
strength.Comment: 22 pages, 8 figures, some typos correcte
Comparison between friction stir and submerged arc welding applied to joining DH36 and E36 shipbuilding steel
With the impending development of FSW tools for steel with useful lifetimes, attention has turned to the mechanical properties of the welds that can be made in a range of industrially significant steels. This work reports on a comparative study undertaken to examine the use of friction stir and submerged arc welding on DH36 and E36 shipbuilding steels. The study made an assessment of the distortion induced in fabricating plates by the two welding techniques, and provides initial comparative data on weld tensile strength, toughness and fatigue life. In each case, friction stir welding was shown to outperform submerged arc welding
Density Matrix Renormalization Group in the Heisenberg Picture
In some cases the state of a quantum system with a large number of subsystems
can be approximated efficiently by the density matrix renormalization group,
which makes use of redundancies in the description of the state. Here we show
that the achievable efficiency can be much better when performing density
matrix renormalization group calculations in the Heisenberg picture, as only
the observable of interest but not the entire state is considered. In some
non-trivial cases, this approach can even be exact for finite bond dimensions.Comment: version to appear in PRL, acronyms in title and abstract expanded,
new improved numerical example
Development of an Epifaunal Assemblage on an Estuarine Artificial Reef
Twelve Polyolefin™ cones were placed in Choctawhatchee Bay, Okaloosa County, Florida, on 20 Oct. 1987, as an artificial reef to examine the succession of the associated epibenthic invertebrate assemblage and its relationship to environmental factors. Ninety-five epifaunal settling plates with individual surface areas of 706.5 cm2 were removed from the reefs. The epifaunal settling plates bore 124,893 organisms from 90 taxa. The number of taxa and individuals associated with the reef gradually increased from deployment in Oct. to a maximum in June. Species diversity tended to increase through the 12-mo study. Coverage by the epifauna stabilized after 4 mo of deployment, A canonical correlation model attributed 86% of the variation in taxa abundance to variation in the independent variables equatorial moon position, salinity, temperature, reef module surface area, absolute time from noon, and vertical height of each settling plate above the substrate. Initial colonizers were typically motile organisms that were gradually succeeded by sedentary organisms. Arborescent bryozoans may have created a complex microhabitat, which facilitated the colonization of grazing species such as gastropods
Nephrogenic systemic fibrosis risk and liver disease.
Objective. Evaluate the incidence of nephrogenic systemic fibrosis (NSF) in patients with liver disease in the peritransplant period. Materials and Methods. This IRB approved study retrospectively reviewed patients requiring transplantation for cirrhosis, hepatocellular carcinoma (HCC), or both from 2003 to 2013. Records were reviewed identifying those having gadolinium enhanced MRI within 1 year of posttransplantation to document degree of liver disease, renal disease, and evidence for NSF. Results. Gadolinium-enhanced MRI was performed on 312 of 837 patients, including 23 with severe renal failure (GFR < 30 mL/min/1.73 cm(2)) and 289 with GFR > 30. Two of 23 patients with renal failure developed NSF compared to zero NSF cases in 289 patients with GFR > 30 (0/289; P < 0.003). High dose gadodiamide was used in the two NSF cases. There was no increased incidence of NSF with severe liver disease (1/71) compared to nonsevere liver disease (1/241; P = 0.412). Conclusion. Renal disease is a risk factor for NSF, but in our small sample our evidence suggests liver disease is not an additional risk factor, especially if a low-risk gadolinium agent is used. Noting that not all patients received high-risk gadolinium, a larger study focusing on patients receiving high-risk gadolinium is needed to further evaluate NSF risk in liver disease in the peritransplant period
Virtual effects of light gauginos and higgsinos: a precision electroweak analysis of split supersymmetry
We compute corrections to precision electroweak observables in supersymmetry
in the limit that scalar superpartners are very massive and decoupled. This
leaves charginos and neutralinos and a Standard Model-like Higgs boson as the
only states with unknown mass substantially affecting the analysis. We give
complete formulas for the chargino and neutralino contributions, derive simple
analytic results for the pure gaugino and higgsino cases, and study the general
case. We find that in all circumstances, the precision electroweak fit improves
when the charginos and neutralinos are near the current direct limits. Larger
higgsino and gaugino masses worsen the fit as the theory predictions
asymptotically approach those of the Standard Model. Since the Standard Model
is considered by most to be an adequate fit to the precision electroweak data,
an important corollary to our analysis is that all regions of parameter space
allowed by direct collider constraints are also allowed by precision
electroweak constraints in split supersymmetry.Comment: 22 pages, 5 figures, v2: typos fixed and note adde
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