412 research outputs found
Uncovering the topology of configuration space networks
The configuration space network (CSN) of a dynamical system is an effective
approach to represent the ensemble of configurations sampled during a
simulation and their dynamic connectivity. To elucidate the connection between
the CSN topology and the underlying free-energy landscape governing the system
dynamics and thermodynamics, an analytical soluti on is provided to explain the
heavy tail of the degree distribution, neighbor co nnectivity and clustering
coefficient. This derivation allows to understand the universal CSN network
topology observed in systems ranging from a simple quadratic well to the native
state of the beta3s peptide and a 2D lattice heteropolymer. Moreover CSN are
shown to fall in the general class of complex networks describe d by the
fitness model.Comment: 6 figure
A simple mean field model for social interactions: dynamics, fluctuations, criticality
We study the dynamics of a spin-flip model with a mean field interaction. The
system is non reversible, spacially inhomogeneous, and it is designed to model
social interactions. We obtain the limiting behavior of the empirical averages
in the limit of infinitely many interacting individuals, and show that phase
transition occurs. Then, after having obtained the dynamics of normal
fluctuations around this limit, we analize long time fluctuations for critical
values of the parameters. We show that random inhomogeneities produce critical
fluctuations at a shorter time scale compared to the homogeneous system.Comment: 37 pages, 2 figure
The Discovery of the Most Metal-Rich White Dwarf: Composition of a Tidally Disrupted Extrasolar Dwarf Planet
Cool white dwarf stars are usually found to have an outer atmosphere that is
practically pure in hydrogen or helium. However, a small fraction have traces
of heavy elements that must originate from the accretion of extrinsic material,
most probably circumstellar matter. Upon examining thousands of Sloan Digital
Sky Survey spectra, we discovered that the helium-atmosphere white dwarf SDSS
J073842.56+183509.6 shows the most severe metal pollution ever seen in the
outermost layers of such stars. We present here a quantitative analysis of this
exciting star by combining high S/N follow-up spectroscopic and photometric
observations with model atmospheres and evolutionary models. We determine the
global structural properties of our target star, as well as the abundances of
the most significant pollutants in its atmosphere, i.e., H, O, Na, Mg, Si, Ca,
and Fe. The relative abundances of these elements imply that the source of the
accreted material has a composition similar to that of Bulk Earth. We also
report the signature of a circumstellar disk revealed through a large infrared
excess in JHK photometry. Combined with our inferred estimate of the mass of
the accreted material, this strongly suggests that we are witnessing the
remains of a tidally disrupted extrasolar body that was as large as Ceres.Comment: 7 pages in emulateapj, 5 figures, accepted for publication in Ap
Neighbourhood, Route and Workplace-Related Environmental Characteristics Predict Adults' Mode of Travel to Work
Commuting provides opportunities for regular physical activity which can reduce the risk of chronic disease. Commuters' mode of travel may be shaped by their environment, but understanding of which specific environmental characteristics are most important and might form targets for intervention is limited. This study investigated associations between mode choice and a range of objectively assessed environmental characteristics.Participants in the Commuting and Health in Cambridge study reported where they lived and worked, their usual mode of travel to work and a variety of socio-demographic characteristics. Using geographic information system (GIS) software, 30 exposure variables were produced capturing characteristics of areas around participants' homes and workplaces and their shortest modelled routes to work. Associations between usual mode of travel to work and personal and environmental characteristics were investigated using multinomial logistic regression.Of the 1124 respondents, 50% reported cycling or walking as their usual mode of travel to work. In adjusted analyses, home-work distance was strongly associated with mode choice, particularly for walking. Lower odds of walking or cycling rather than driving were associated with a less frequent bus service (highest versus lowest tertile: walking OR 0.61 [95% CI 0.20–1.85]; cycling OR 0.43 [95% CI 0.23–0.83]), low street connectivity (OR 0.22, [0.07–0.67]; OR 0.48 [0.26–0.90]) and free car parking at work (OR 0.24 [0.10–0.59]; OR 0.55 [0.32–0.95]). Participants were less likely to cycle if they had access to fewer destinations (leisure facilities, shops and schools) close to work (OR 0.36 [0.21–0.62]) and a railway station further from home (OR 0.53 [0.30–0.93]). Covariates strongly predicted travel mode (pseudo r-squared 0.74).Potentially modifiable environmental characteristics, including workplace car parking, street connectivity and access to public transport, are associated with travel mode choice, and could be addressed as part of transport policy and infrastructural interventions to promote active commuting
Analysis of scalar perturbations in cosmological models with a non-local scalar field
We develop the cosmological perturbations formalism in models with a single
non-local scalar field originating from the string field theory description of
the rolling tachyon dynamics. We construct the equation for the energy density
perturbations of the non-local scalar field in the presence of the arbitrary
potential and formulate the local system of equations for perturbations in the
linearized model when both simple and double roots of the characteristic
equation are present. We carry out the general analysis related to the
curvature and entropy perturbations and consider the most specific example of
perturbations when important quantities in the model become complex.Comment: LaTeX, 25 pages, 1 figure, v2: Subsection 3.2 and Section 5 added,
references added, accepted for publication in Class. Quant. Grav. arXiv admin
note: text overlap with arXiv:0903.517
Dpp Signaling Activity Requires Pentagone to Scale with Tissue Size in the Growing Drosophila Wing Imaginal Disc
The activity of the Dpp morphogen adapts to tissue size in the growing Drosophila wing imaginal disc, and Pentagone, an important secreted feedback regulator of the Dpp pathway, is required for this adaptation
Mifepristone Increases the Cytotoxicity of Uterine Natural Killer Cells by Acting as a Glucocorticoid Antagonist via ERK Activation
Background: Mifepristone (RU486), a potent antagonist of progesterone and glucocorticoids, is involved in immune regulation. Our previous studies demonstrated that mifepristone directly augments the cytotoxicity of human uterine natural killer (uNK) cells. However, the mechanism responsible for this increase in cytotoxicity is not known. Here, we explored whether the increased cytotoxicity in uNK cells produced by mifepristone is due to either anti-progesterone or anti-glucocorticoid activity, and also investigated relevant changes in the mitogen-activated protein kinase (MAPK) pathway. Methodology/Principal Findings: Uterine NK cells were isolated from decidual samples and incubated with different concentrations of progesterone, cortisol, or mifepristone. The cytotoxicity and perforin expression of uNK cells were detected by mitochondrial lactate dehydrogenase-based MTS staining and flow cytometry assays, respectively. Phosphorylation of components of the MAPK signaling pathway was detected by Western blot. Cortisol attenuated uNK cell-mediated cytotoxicity in a concentration-dependent manner whereas progesterone had no effect. Mifepristone alone increased the cytotoxicity and perforin expression of uNK cells; these effects were blocked by cortisol. Furthermore, mifepristone increased the phosphorylation of ERK1/2 in a cortisol-reversible manner. Specific ERK1/2 inhibitor PD98059 or U0126 blocked cortisol- and mifepristone-induced responses in uNK cells
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