12,575 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
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
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
QCD corrections to stoponium production at hadron colliders
If the lighter top squark has no kinematically allowed two-body decays that
conserve flavor, then it will live long enough to form hadronic bound states.
The observation of the diphoton decays of stoponium could then provide a
uniquely precise measurement of the top squark mass. In this paper, we
calculate the cross section for the production of stoponium in a hadron
collider at next-to-leading order (NLO) in QCD. We present numerical results
for the cross section for production of stoponium at the LHC and study the
dependence on beam energy, stoponium mass, and the renormalization and
factorization scale. The cross-section is substantially increased by the NLO
corrections, counteracting a corresponding decrease found earlier in the NLO
diphoton branching ratio.Comment: 24 page
Shift in the LHC Higgs diphoton mass peak from interference with background
The Higgs diphoton amplitude from gluon fusion at the LHC interferes with the
continuum background induced by quark loops. I investigate the effect of this
interference on the position of the diphoton invariant mass peak used to help
determine the Higgs mass. At leading order, the interference shifts the peak
towards lower mass by an amount of order 150 MeV or more, with the precise
value dependent on the methods used to analyze and fit the data.Comment: 10 pages. v2: comments on scale variation added, reference adde
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
Altering the stability of the Cdc8 overlap region modulates the ability of this tropomyosin to bind cooperatively to actin and regulate myosin.
Tropomyosin (Tm) is an evolutionarily conserved ?-helical coiled-coil protein, dimers of which form end-to-end polymers capable of associating with and stabilising actin-filaments and regulate myosin function. The fission yeast, Schizosaccharomyces pombe, possesses a single essential Tm, Cdc8, which can be acetylated on its amino terminal methionine to increase its affinity for actin and enhance its ability to regulate myosin function. We have designed and generated a number of novel Cdc8 mutant proteins with amino terminal substitutions to explore how stability of the Cdc8-polymer overlap region affects the regulatory function of this Tm. By correlating the stability of each protein, its propensity to form stable polymers, its ability to associate with actin and to regulate myosin, we have shown the stability of the amino terminal of the Cdc8 ?-helix is crucial for Tm function. In addition we have identified a novel Cdc8 mutant with increased amino-terminal stability, dimers of which are capable of forming Tm-polymers significantly longer than the wild-type protein. This protein had a reduced affinity for actin with respect to wild type, and was unable to regulate actomyosin interactions. The data presented here are consistent with acetylation providing a mechanism for modulating the formation and stability of Cdc8 polymers within the fission yeast cell. The data also provide evidence for a mechanism in which Tm dimers form end-to-end polymers on the actin-filament, consistent with a cooperative model for Tm binding to actin
Structure of Mandelate Racemase with Bound Intermediate Analogues Benzohydroxamate and Cupferron
Mandelate racemase (MR, EC 5.1.2.2) from Pseudomonas putida catalyzes the Mg2+-dependent interconversion of the enantiomers of mandelate, stabilizing the altered substrate in the transition state by 26 kcal/mol relative to the substrate in the ground state. To understand the origins of this binding discrimination, we determined the X-ray crystal structures of wild-type MR complexed with two analogues of the putative aci-carboxylate intermediate, benzohydroxamate and Cupferron, to 2.2-Å resolution. Benzohydroxamate is shown to be a reasonable mimic of the transition state and/or intermediate because its binding affinity for 21 MR variants correlates well with changes in the free energy of transition state stabilization afforded by these variants. Both benzohydroxamate and Cupferron chelate the active site divalent metal ion and are bound in a conformation with the phenyl ring coplanar with the hydroxamate and diazeniumdiolate moieties, respectively. Structural overlays of MR complexed with benzohydroxamate, Cupferron, and the ground state analogue (S)-atrolactate reveal that the para carbon of the substrate phenyl ring moves by 0.8−1.2 Å between the ground state and intermediate state, consistent with the proposal that the phenyl ring moves during MR catalysis while the polar groups remain relatively fixed. Although the overall protein structure of MR with bound intermediate analogues is very similar to that of MR with bound (S)-atrolactate, the intermediate−Mg2+ distance becomes shorter, suggesting a tighter complex with the catalytic Mg2+. In addition, Tyr 54 moves closer to the phenyl ring of the bound intermediate analogues, contributing to an overall constriction of the active site cavity. However, site-directed mutagenesis experiments revealed that the role of Tyr 54 in MR catalysis is relatively minor, suggesting that alterations in enzyme structure that contribute to discrimination between the altered substrate in the transition state and the ground state by this proficient enzyme are extremely subtle
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