840 research outputs found
Swarm behavior of self-propelled rods and swimming flagella
Systems of self-propelled particles are known for their tendency to aggregate
and to display swarm behavior. We investigate two model systems, self-propelled
rods interacting via volume exclusion, and sinusoidally-beating flagella
embedded in a fluid with hydrodynamic interactions. In the flagella system,
beating frequencies are Gaussian distributed with a non-zero average. These
systems are studied by Brownian-dynamics simulations and by mesoscale
hydrodynamics simulations, respectively. The clustering behavior is analyzed as
the particle density and the environmental or internal noise are varied. By
distinguishing three types of cluster-size probability density functions, we
obtain a phase diagram of different swarm behaviors. The properties of
clusters, such as their configuration, lifetime and average size are analyzed.
We find that the swarm behavior of the two systems, characterized by several
effective power laws, is very similar. However, a more careful analysis reveals
several differences. Clusters of self-propelled rods form due to partially
blocked forward motion, and are therefore typically wedge-shaped. At higher rod
density and low noise, a giant mobile cluster appears, in which most rods are
mostly oriented towards the center. In contrast, flagella become
hydrodynamically synchronized and attract each other; their clusters are
therefore more elongated. Furthermore, the lifetime of flagella clusters decays
more quickly with cluster size than of rod clusters
An Interactive Method to Dynamically Create Transition Rules in a Land-use Cellular Automata Model
Propagation dynamics on networks featuring complex topologies
Analytical description of propagation phenomena on random networks has
flourished in recent years, yet more complex systems have mainly been studied
through numerical means. In this paper, a mean-field description is used to
coherently couple the dynamics of the network elements (nodes, vertices,
individuals...) on the one hand and their recurrent topological patterns
(subgraphs, groups...) on the other hand. In a SIS model of epidemic spread on
social networks with community structure, this approach yields a set of ODEs
for the time evolution of the system, as well as analytical solutions for the
epidemic threshold and equilibria. The results obtained are in good agreement
with numerical simulations and reproduce random networks behavior in the
appropriate limits which highlights the influence of topology on the processes.
Finally, it is demonstrated that our model predicts higher epidemic thresholds
for clustered structures than for equivalent random topologies in the case of
networks with zero degree correlation.Comment: 10 pages, 5 figures, 1 Appendix. Published in Phys. Rev. E (mistakes
in the PRE version are corrected here
MS 2053.7-0449: Confirmation of a bimodal mass distribution from strong gravitational lensing
We present the first strong lensing study of the mass distribution in the
cluster MS 2053-04 based on HST archive data. This massive, X-ray luminous
cluster has a redshift z=0.583, and it is composed of two structures that are
gravitationally bound to each other. The cluster has one multiply imaged system
constituted by a double gravitational arc.
We have performed a parametric strong lensing mass reconstruction using NFW
density profiles to model the cluster potential. We also included perturbations
from 23 galaxies, modeled like elliptical singular isothermal sphere, that are
approximately within 1'x1' around the cluster center. These galaxies were
constrained in both the geometric and dynamical parameters with observational
data. Our analysis predicts a third image which is slightly demagnified. We
found a candidate for this counter-image near the expected position and with
the same F702W-F814W colors as the gravitational arcs in the cluster. The
results from the strong lensing model shows the complex structure in this
cluster, the asymmetry and the elongation in the mass distribution, and are
consistent with previous spectrophotometric results that indicate that the
cluster has a bimodal mass distribution. Finally, the derived mass profile was
used to estimate the mass within the arcs and for comparison with X-ray
estimates.Comment: To be published in ApJ (accepted
Is Gravitational Lensing by Intercluster Filaments Always Negligible?
Intercluster filaments negligibly contribute to the weak lensing signal in
general relativity (GR), . In the context of
relativistic modified Newtonian dynamics (MOND) introduced by Bekenstein,
however, a single filament inclined by from the line of
sight can cause substantial distortion of background sources pointing towards
the filament's axis (); this is rigorous
for infinitely long uniform filaments, but also qualitatively true for short
filaments (Mpc), and even in regions where the projected matter
density of the filament is equal to zero. Since galaxies and galaxy clusters
are generally embedded in filaments or are projected on such structures, this
contribution complicates the interpretation of the weak lensing shear map in
the context of MOND. While our analysis is of mainly theoretical interest
providing order-of-magnitude estimates only, it seems safe to conclude that
when modeling systems with anomalous weak lensing signals, e.g. the "bullet
cluster" of Clowe et al., the "cosmic train wreck" of Abell 520 from Mahdavi et
al., and the "dark clusters" of Erben et al., filamentary structures might
contribute in a significant and likely complex fashion. On the other hand, our
predictions of a (conceptual) difference in the weak lensing signal could, in
principle, be used to falsify MOND/TeVeS and its variations.Comment: 11 pages, 6 figures, published versio
Analysis of electron-positron momentum spectra of metallic alloys as supported by first-principles calculations
Electron-positron momentum distributions measured by the coincidence Doppler
broadening method can be used in the chemical analysis of the annihilation
environment, typically a vacancy-impurity complex in a solid. In the present
work, we study possibilities for a quantitative analysis, i.e., for
distinguishing the average numbers of different atomic species around the
defect. First-principles electronic structure calculations self-consistently
determining electron and positron densities and ion positions are performed for
vacancy-solute complexes in Al-Cu, Al-Mg-Cu, and Al-Mg-Cu-Ag alloys. The
ensuing simulated coincidence Doppler broadening spectra are compared with
measured ones for defect identification. A linear fitting procedure, which uses
the spectra for positrons trapped at vacancies in pure constituent metals as
components, has previously been employed to find the relative percentages of
different atomic species around the vacancy [A. Somoza et al. Phys. Rev. B 65,
094107 (2002)]. We test the reliability of the procedure by the help of
first-principles results for vacancy-solute complexes and vacancies in
constituent metals.Comment: Submitted to Physical Review B on September 19 2006. Revised version
submitted on November 8 2006. Published on February 14 200
Improved beam waist formula for ultrashort, tightly focused linearly, radially, and azimuthally polarized laser pulses in free space
We derive an asymptotically accurate formula for the beam waist of ultrashort, tightly focused fundamental linearly polarized, radially polarized, and azimuthally polarized modes in free space. We compute the exact beam waist via numerical cubature to ascertain the accuracy with which our formula approximates the exact beam waist over a broad range of parameters of practical interest. Based on this, we describe a method of choosing parameters in the model given the beam waist and pulse duration of a laser pulse
Differential methylation of inflammatory and insulinotropic genes after metabolic surgery in women
Context: Biliopancreatic diversion with duodenal switch (BPD-DS), a metabolic bariatric operation, induces durable loss of excess weight and reduced cardiometabolic risk. Altered epigenetic marks are mechanistically associated with environment-driven phenotypic variations. Objective: The current study aimed to compare gene methylation levels before and after BPD-DS to identify epigenetic marks potentially linked to metabolic improvements induced by BPD-DS. Design and patients: Metabolic risk factors and gene methylation levels of 20 women studied mean 12 years (range 4-22) after BPD-DS were compared to those of 20 severely obese surgical candidates as controls, matched for pre-surgical age, body mass index and dyslipidemia and hypertension prevalences. Whole-genome blood DNA methylation analysis enabled between-group differential methylation analyses. We calculated correlations between methylation levels of the most differentially methylated CpG sites and plasma glucose and insulin levels and HOMA-IR. Results: Differential methylation analysis identified 15,343 genes demonstrating at least one differentially methylated CpG site (p<1.43x10-7). Diabetic and inflammation/immune functions were among the most overrepresented from the 200 genes exhibiting the largest group differences in methylation levels. CpG sites methylation levels of genes related to insulin action correlated significantly with fasting insulin levels and homeostatic model of insulin resistance (p≤0.002 for all). Conclusion: These findings suggest that differential methylation levels in obese controls versus treated women may partially explain the durable metabolic improvements after BPD-DS
The Prenatal Environment in Twin Studies: A Review on Chorionicity
A literature search was conducted to identify articles examining the association of chorionicity (e.g., whether twins share a single chorion and thus placenta or have separate chorions/placentas) and genetics, psychiatry/behavior, and neurological manifestations in humans twins and higher-order multiples. The main aim was to assess how frequently chorionicity has been examined in relation to heritability estimates, and to assess which phenotypes may be most sensitive to, or affected by, bias in heritability estimates because of chorionicity. Consistent with the theory that some chorionicity effects could lead to overestimation and others to underestimation of heritability, there were instances of each across the many phenotypes reviewed. However, firm conclusions should not be drawn since some of the outcomes were only examined in one or few studies and often sample sizes were small. While the evidence for bias due to chorionicity was mixed or null for many outcomes, results do, however, consistently suggest that heritability estimates are underestimated for measures of birth weight and early growth when chorionicity is not taken into account
Oscillating epidemics in a dynamic network model: stochastic and mean-field analysis
An adaptive network model using SIS epidemic propagation with link-type-dependent link activation and deletion is considered. Bifurcation analysis of the pairwise ODE approximation and the network-based stochastic simulation is carried out, showing that three typical behaviours may occur; namely, oscillations can be observed besides disease-free or endemic steady states. The oscillatory behaviour in the stochastic simulations is studied using Fourier analysis, as well as through analysing the exact master equations of the stochastic model. By going beyond simply comparing simulation results to mean-field models, our approach yields deeper insights into the observed phenomena and help better understand and map out the limitations of mean-field models
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