607 research outputs found
How do binary clusters form?
Approximately 10 per cent of star clusters are found in pairs, known as binary clusters. We propose a mechanism for binary cluster formation; we use N-body simulations to show that velocity substructure in a single (even fairly smooth) region can cause binary clusters to form. This process is highly stochastic and it is not obvious from a region's initial conditions whether a binary will form and, if it does, which stars will end up in which cluster. We find the probability that a region will divide is mainly determined by its virial ratio, and a virial ratio above 'equilibrium' is generally necessary for binary formation. We also find that the mass ratio of the two clusters is strongly influenced by the initial degree of spatial substructure in the region
Superfluid toroidal currents in atomic condensates
The dynamics of toroidal condensates in the presence of condensate flow and
dipole perturbation have been investigated. The Bogoliubov spectrum of
condensate is calculated for an oblate torus using a discrete-variable
representation and a spectral method to high accuracy. The transition from
spheroidal to toroidal geometry of the trap displaces the energy levels into
narrow bands. The lowest-order acoustic modes are quantized with the dispersion
relation with . A condensate
with toroidal current splits the co-rotating and
counter-rotating pair by the amount: . Radial dipole excitations are the lowest energy dissipation modes.
For highly occupied condensates the nonlinearity creates an asymmetric mix of
dipole circulation and nonlinear shifts in the spectrum of excitations so that
the center of mass circulates around the axis of symmetry of the trap. We
outline an experimental method to study these excitations.Comment: 8 pages, 8 figure
Nonlinear Stability in the Generalised Photogravitational Restricted Three Body Problem with Poynting-Robertson Drag
The Nonlinear stability of triangular equilibrium points has been discussed
in the generalised photogravitational restricted three body problem with
Poynting-Robertson drag. The problem is generalised in the sense that smaller
primary is supposed to be an oblate spheroid. The bigger primary is considered
as radiating. We have performed first and second order normalization of the
Hamiltonian of the problem. We have applied KAM theorem to examine the
condition of non-linear stability. We have found three critical mass ratios.
Finally we conclude that triangular points are stable in the nonlinear sense
except three critical mass ratios at which KAM theorem fails.Comment: Including Poynting-Robertson Drag the triangular equilibrium points
are stable in the nonlinear sense except three critical mass ratios at which
KAM theorem fail
Poliovirus RNA-dependent RNA polymerase (3D(pol)): Structural, biochemical, and biological analysis of conserved structural motifs A and B
We have constructed a structural model for poliovirus RNA-dependent RNA polymerase (3D(pol)) in complex with a primer-template (sym/sub) and ATP. Residues found in conserved structural motifs A (Asp-238) and B (Asn-297) are involved in nucleotide selection. Asp-238 appears to couple binding of nucleotides with the correct sugar configuration to catalytic efficiency at the active site of the enzyme. Asn-297 is involved in selection of ribonucleoside triphosphates over 2'-dNTPs, a role mediated most likely via a hydrogen bond between the side chain of this residue and the 2'-OH of the ribonucleoside triphosphate. Substitutions at position 238 or 297 of 3D(pol) produced derivatives exhibiting a range of catalytic efficiencies when assayed in vitro for poly(rU) polymerase activity or sym/sub elongation activity. A direct correlation existed between activity on sym/sub and biological phenotypes; a 2.5-fold reduction in polymerase elongation rate produced virus with a temperature-sensitive growth phenotype. These data permit us to propose a detailed, structural model for nucleotide selection by 3D(pol), confirm the biological relevance of the sym/sub system, and provide additional evidence for kinetic coupling between RNA synthesis and subsequent steps in the virus life cycle
Problem detection in legislative oversight:An analysis of legislative committee agendas in the U.K. and U.S.
This paper outlines a dynamic problem-detection model of legislative oversight where legislative committees engage in information-gathering to identify emerging policy problems. It is argued that activities of legislative committees are responsive to indicators of problem status across a range of policy domains. This enables committees to react to problems before, or at least simultaneously to, citizens. Our analyses use a new dataset on the policy agenda of UK Parliamentary Select Committees in combination with directly comparable data on US Congressional hearings. Aggregate measures of problem status (e.g. GDP, crime rates) and public opinion on the �most important problem� facing the country are used as independent variables. The comparison between a well-established and developing committee system offers insights into common dynamics across institutional contexts. The findings show that committee agendas in both the UK and US are responsive to problem status for the majority of issues
Out-of-equilibrium electromagnetic radiation
We derive general formulas for photon and dilepton production rates from an
arbitrary non-equilibrated medium from first principles in quantum field
theory. At lowest order in the electromagnetic coupling constant, these relate
the rates to the unequal-time in-medium photon polarization tensor and
generalize the corresponding expressions for a system in thermodynamic
equilibrium. We formulate the question of electromagnetic radiation in real
time as an initial value problem and consistently describe the virtual
electromagnetic dressing of the initial state. In the limit of slowly evolving
systems, we recover known expressions for the emission rates and work out the
first correction to the static formulas in a systematic gradient expansion.
Finally, we discuss the possible application of recently developed techniques
in non-equilibrium quantum field theory to the problem of electromagnetic
radiation. We argue, in particular, that the two-particle-irreducible (2PI)
effective action formalism provides a powerful resummation scheme for the
description of multiple scattering effects, such as the
Landau-Pomeranchuk-Migdal suppression recently discussed in the context of
equilibrium QCD.Comment: 34 pages, 9 figures, uses JHEP3.cl
New Fits for the Non-Perturbative Parameters in the CSS Resummation Formalism
We update the non-perturbative function of the Collins-Soper- Sterman
resummation formalism in hadron collisions. Two functional forms in impact
parameter space are considered, one with a pure Gaussian form with two
parameters and the other with an additional linear term. The results for the
two parameter fit are found to be g1=0.24+0.08-0.07 GeV^2, g2=0.34+0.07-0.08
GeV^2. The results for the three parameter fit are g1=0.15+004-0.03 GeV^2,
g2=0.48+0.07-0.05 GeV^2, and g3=-0.58+0.26-0.20 GeV^-1. We discuss the
potential for the full Tevatron Run I Z boson data for further testing of the
universality of the non-perturbative function.Comment: 22 pages, 12 figures, LaTe
Cosmic-ray strangelets in the Earth's atmosphere
If strange quark matter is stable in small lumps, we expect to find such
lumps, called ``strangelets'', on Earth due to a steady flux in cosmic rays.
Following recent astrophysical models, we predict the strangelet flux at the
top of the atmosphere, and trace the strangelets' behavior in atmospheric
chemistry and circulation. We show that several strangelet species may have
large abundances in the atmosphere; that they should respond favorably to
laboratory-scale preconcentration techniques; and that they present promising
targets for mass spectroscopy experiments.Comment: 28 pages, 4 figures, revtex
Structure-function relationships of the RNA-dependent RNA polymerase from poliovirus (3Dpol): A surface of the primary oligomerization domain functions in capsid precursor processing and VPg uridylylation
The primary oligomerization domain of poliovirus polymerase, 3Dpol, is stabilized by the interaction of the back of the thumb subdomain of one molecule with the back of the palm subdomain of a second molecule, thus permitting the head-to-tail assembly of 3Dpol monomers into long fibers. The interaction of Arg-455 and Arg-456 of the thumb with Asp-339, Ser-341, and Asp-349 of the palm is key to the stability of this interface. We show that mutations predicted to completely disrupt this interface do not produce equivalent growth phenotypes. Virus encoding a polymerase with changes of both residues of the thumb to alanine is not viable; however, virus encoding a polymerase with changes of all three residues of the palm to alanine is viable. Biochemical analysis of 3Dpol derivatives containing the thumb or palm substitutions revealed that these derivatives are both incapable of forming long fibers, suggesting that polymerase fibers are not essential for virus viability. The RNA binding activity, polymerase activity, and thermal stability of these derivatives were equivalent to that of the wild-type enzyme. The two significant differences observed for the thumb mutant were a modest reduction in the ability of the altered 3CD proteinase to process the VP0/VP3 capsid precursor and a substantial reduction in the ability of the altered 3Dpol to catalyze oriI-templated uridylylation of VPg. The defect to uridylylation was a result of the inability of 3CD to stimulate this reaction. Because 3C alone can substitute for 3CD in this reaction, we conclude that the lethal replication phenotype associated with the thumb mutant is caused, in part, by the disruption of an interaction between the back of the thumb of 3Dpol and some undefined domain of 3C. We speculate that this interaction may also be critical for assembly of other complexes required for poliovirus genome replication
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