1,882 research outputs found
Scaling and localization lengths of a topologically disordered system
We consider a noninteracting disordered system designed to model particle
diffusion, relaxation in glasses, and impurity bands of semiconductors.
Disorder originates in the random spatial distribution of sites. We find strong
numerical evidence that this model displays the same universal behavior as the
standard Anderson model. We use finite-size-scaling to find the localization
length as a function of energy and density, including localized states away
from the delocalization transition. Results at many energies all fit onto the
same universal scaling curve.Comment: 5+ page
The Connection Between Pulsation, Mass Loss and Circumstellar Shells in Classical Cepheids
Recent observations of Cepheids using infrared interferometry and Spitzer
photometry have detected the presence of circumstellar envelopes (CSE) of dust
and it has been hypothesized that the CSE's are due to dust forming in a
Cepheid wind. Here we use a modified Castor, Abbott & Klein formalism to
produce a Cepheid wind, and this is used to estimate the contribution of mass
loss to the Cepheid mass discrepancy Furthermore, we test the OGLE-III
Classical Cepheids using the IR fluxes from the SAGE survey to determine if
Large Magellanic Cloud Cepheids have CSE's. It is found that IR excess is a
common phenomenon for LMC Cepheids and that the resulting mass-loss rates can
explain at least a fraction of the Cepheid mass discrepancy, depending on the
assumed dust-to-gas ratio in the wind.Comment: 5 pages, 3 figures, proceeding for "Stellar Pulsation: Challenges for
Theory and Observation", Santa Fe 200
The LHCb Timing and Fast Control system
In this paper we describe the LHCb Timing and Fast Control (TFC) system. It is different from that of the other LHC experiments in that it has to support two levels of high-rate triggers. Furthermore, emphasis has been put on partitioning and on locating the TFC mastership in one type of module: the Readout Supervisor. The Readout Supervisor handles all timing, trigger, and control command distribution. It generates auto-triggers as well as controls the trigger rates. Partitioning is handled by a programmable patch panel/switch introduced in the TTC distribution network between a pool of Readout Supervisors and the Front-End electronics. I
Nitric oxide modulates expression of extracellular matrix genes linked to fibrosis in kidney mesangial cells
Mesangial cells are thought to be important mediators of glomerular inflammation and fibrosis. Studies have established a direct role for nitric oxide (NO) in the regulation of gene expression in mesangial cells. Representational difference analysis was used to investigate changes in gene expression elicited by the treatment of S-nitroso-L-glutathione in rat mesangial cells. Seven upregulated and 11 downregulated genes were identified. Four out of 11 downregulated genes (connective tissue growth factor, thrombospondin-1, collagen type I all and collagen type I alpha 2) are known to be linked to inflammation and fibrosis. Results were verified across species in mesangial cells treated with a series of NO donors using Northern blot analysis, quantitative real-time PCR and protein analysis methods. Induction of endogenous NO production by cytokine stimulation also triggered regulation of the genes. One example gene, connective tissue growth factor, was studied at the promoter level. Promoter-reporter gene studies in mesangial cells demonstrated that NO acts at the transcriptional level to suppress gene expression. Our results reveal a complex role of NO in regulating gene expression in mesangial cells and suggest an antifibrotic potential for NO
Discrete single-photon quantum walks with tunable decoherence
Quantum walks have a host of applications, ranging from quantum computing to
the simulation of biological systems. We present an intrinsically stable,
deterministic implementation of discrete quantum walks with single photons in
space. The number of optical elements required scales linearly with the number
of steps. We measure walks with up to 6 steps and explore the
quantum-to-classical transition by introducing tunable decoherence. Finally, we
also investigate the effect of absorbing boundaries and show that decoherence
significantly affects the probability of absorption.Comment: Published version, 5 pages, 4 figure
Quantum Algorithm for Molecular Properties and Geometry Optimization
It is known that quantum computers, if available, would allow an exponential
decrease in the computational cost of quantum simulations. We extend this
result to show that the computation of molecular properties (energy
derivatives) could also be sped up using quantum computers. We provide a
quantum algorithm for the numerical evaluation of molecular properties, whose
time cost is a constant multiple of the time needed to compute the molecular
energy, regardless of the size of the system. Molecular properties computed
with the proposed approach could also be used for the optimization of molecular
geometries or other properties. For that purpose, we discuss the benefits of
quantum techniques for Newton's method and Householder methods. Finally, global
minima for the proposed optimizations can be found using the quantum basin
hopper algorithm, which offers an additional quadratic reduction in cost over
classical multi-start techniques.Comment: 6 page
Vibrational Instability of Metal-Poor Low-Mass Main-Sequence Stars
We find that low-degree low-order g-modes become unstable in metal-poor
low-mass stars due to the -mechanism of the pp-chain. Since the
outer convection zone of these stars is limited only to the very outer layers,
the uncertainty in the treatment of convection does not affect the result
significantly. The decrease in metallicity leads to decrease in opacity and
hence increase in luminosity of a star. This makes the star compact and results
in decrease in the density contrast, which is favorable to the
-mechanism instability. We find also instability for high order
g-modes of metal-poor low-mass stars by the convective blocking mechanism.
Since the effective temperature and the luminosity of metal-poor stars are
significantly higher than those of Pop I stars, the stars showing
Dor-type pulsation are substantially less massive than in the case of Pop I
stars. We demonstrate that those modes are unstable for about
stars in the metal-poor case.Comment: 4 pages, 4 figures, To be published in Astrophysics and Space Science
Proceedings series (ASSP). Proceedings of the "20th Stellar Pulsation
Conference Series: Impact of new instrumentation and new insights in stellar
pulsations", 5-9 September 2011, Granada, Spai
Toward a New Kind of Asteroseismic Grid Fitting
Recent developments in instrumentation (e.g., in particular the Kepler and
CoRoT satellites) provide a new opportunity to improve the models of stellar
pulsations. Surface layers, rotation, and magnetic fields imprint erratic
frequency shifts, trends, and other non-random behavior in the frequency
spectra. As our observational uncertainties become smaller, these are
increasingly important and difficult to deal with using standard fitting
techniques. To improve the models, new ways to compare their predictions with
observations need to be conceived. In this paper we present a completely
probabilistic (Bayesian) approach to asteroseismic model fitting. It allows for
varying degrees of prior mode identification, corrections for the discrete
nature of the grid, and most importantly implements a treatment of systematic
errors, such as the "surface effects." It removes the need to apply semi-
empirical corrections to the observations prior to fitting them to the models
and results in a consistent set of probabilities with which the model physics
can be probed and compared. As an example, we show a detailed asteroseismic
analysis of the Sun. We find a most probable solar age, including a 35 +- 5
million year pre-main sequence phase, of 4.591 billion years, and initial
element mass fractions of X_0 = 0.72, Y_0 = 0.264, Z_0 = 0.016, consistent with
recent asteroseismic and non-asteroseismic studies.Comment: 15 pages, 5 figures, accepted for publication in The Astrophysical
Journal; v2 contains minor changes made in the proofs (updated references &
corrected typos
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