285 research outputs found
Importance of Granular Structure in the Initial Conditions for the Elliptic Flow
We show effects of granular structure of the initial conditions (IC) of
hydrodynamic description of high-energy nucleus-nucleus collisions on some
observables, especially on the elliptic-flow parameter v2. Such a structure
enhances production of isotropically distributed high-pT particles, making v2
smaller there. Also, it reduces v2 in the forward and backward regions where
the global matter density is smaller, so where such effects become more
efficacious.Comment: 4 pages, 5 figure
Numerical Models of Binary Neutron Star System Mergers. I.: Numerical Methods and Equilibrium Data for Newtonian Models
The numerical modeling of binary neutron star mergers has become a subject of
much interest in recent years. While a full and accurate model of this
phenomenon would require the evolution of the equations of relativistic
hydrodynamics along with the Einstein field equations, a qualitative study of
the early stages on inspiral can be accomplished by either Newtonian or
post-Newtonian models, which are more tractable. In this paper we offer a
comparison of results from both rotating and non-rotating (inertial) frame
Newtonian calculations. We find that the rotating frame calculations offer
significantly improved accuracy as compared with the inertial frame models.
Furthermore, we show that inertial frame models exhibit significant and
erroneous angular momentum loss during the simulations that leads to an
unphysical inspiral of the two neutron stars. We also examine the dependence of
the models on initial conditions by considering initial configurations that
consist of spherical neutron stars as well as stars that are in equilibrium and
which are tidally distorted. We compare our models those of Rasio & Shapiro
(1992,1994a) and New & Tohline (1997). Finally, we investigate the use of the
isolated star approximation for the construction of initial data.Comment: 32 pages, 19 gif figures, manuscript with postscript figures
available at http://www.astro.sunysb.edu/dswesty/docs/nspap1.p
Numerical estimation of densities
[Abridged] We present a novel technique, dubbed FiEstAS, to estimate the
underlying density field from a discrete set of sample points in an arbitrary
multidimensional space. FiEstAS assigns a volume to each point by means of a
binary tree. Density is then computed by integrating over an adaptive kernel.
As a first test, we construct several Monte Carlo realizations of a Hernquist
profile and recover the particle density in both real and phase space. At a
given point, Poisson noise causes the unsmoothed estimates to fluctuate by a
factor ~2 regardless of the number of particles. This spread can be reduced to
about 1 dex (~26 per cent) by our smoothing procedure. [...] We conclude that
our algorithm accurately measure the phase-space density up to the limit where
discreteness effects render the simulation itself unreliable. Computationally,
FiEstAS is orders of magnitude faster than the method based on Delaunay
tessellation that Arad et al. employed, making it practicable to recover
smoothed density estimates for sets of 10^9 points in 6 dimensions.Comment: 12 pages, 18 figures, submitted to MNRAS. The code is available upon
reques
Current Distribution in the Three-Dimensional Random Resistor Network at the Percolation Threshold
We study the multifractal properties of the current distribution of the
three-dimensional random resistor network at the percolation threshold. For
lattices ranging in size from to we measure the second, fourth and
sixth moments of the current distribution, finding {\it e.g.\/} that
where is the conductivity exponent and is the
correlation length exponent.Comment: 10 pages, latex, 8 figures in separate uuencoded fil
Smoothed Particle Hydrodynamics for Relativistic Heavy Ion Collisions
The method of smoothed particle hydrodynamics (SPH) is developped
appropriately for the study of relativistic heavy ion collision processes. In
order to describe the flow of a high energy but low baryon number density
fluid, the entropy is taken as the SPH base. We formulate the method in terms
of the variational principle. Several examples show that the method is very
promising for the study of hadronic flow in RHIC physics.Comment: 14 pages, 8figure
NeXSPheRIO results on azimuthal anisotropy in Au-Au collisions at 200A GeV
In this work, we present the results obtained by the hydrodynamic code
NeXSPheRIO on anisotropic flows. In our calculation, we made use of
event-by-event fluctuating initial conditions, and chemical freeze-out was
explicitly implemented. We studied directed flow, elliptic flow and forth
harmonic coefficient for various hadrons at different centrality windows for
Au+Au collisions at 200 AGeV. The results are discussed and compared with
experimental data from RHIC.Comment: 6 pages and 6 figures, sqm2008 contributio
Elasticity of Gaussian and nearly-Gaussian phantom networks
We study the elastic properties of phantom networks of Gaussian and
nearly-Gaussian springs. We show that the stress tensor of a Gaussian network
coincides with the conductivity tensor of an equivalent resistor network, while
its elastic constants vanish. We use a perturbation theory to analyze the
elastic behavior of networks of slightly non-Gaussian springs. We show that the
elastic constants of phantom percolation networks of nearly-Gaussian springs
have a power low dependence on the distance of the system from the percolation
threshold, and derive bounds on the exponents.Comment: submitted to Phys. Rev. E, 10 pages, 1 figur
Adiabatic scaling relations of galaxy clusters
The aim of the present work is to show that, contrary to popular belief,
galaxy clusters are **not** expected to be self-similar, even when the only
energy sources available are gravity and shock-wave heating. In particular, we
investigate the scaling relations between mass, luminosity and temperature of
galaxy groups and clusters in the absence of radiative processes. Theoretical
expectations are derived from a polytropic model of the intracluster medium and
compared with the results of high-resolution adiabatic gasdynamical
simulations. It is shown that, in addition to the well-known relation between
the mass and concentration of the dark matter halo, the effective polytropic
index of the gas also varies systematically with cluster mass, and therefore
neither the dark matter nor the gas profiles are exactly self-similar. It is
remarkable, though, that the effects of concentration and polytropic index tend
to cancel each other, leading to scaling relations whose logarithmic slopes
roughly match the predictions of the most basic self-similar models. We provide
a phenomenological fit to the relation between polytropic index and
concentration, as well as a self-consistent scheme to derive the non-linear
scaling relations expected for any cosmology and the best-fit normalizations of
the M-T, L-T and F-T relations appropriate for a Lambda-CDM universe. The
predicted scaling relations reproduce observational data reasonably well for
massive clusters, where the effects of cooling and star formation are expected
to play a minor role.Comment: 12 pages, 5 figures, accepted by MNRA
Quantitative principles of cis-translational control by general mRNA sequence features in eukaryotes.
BackgroundGeneral translational cis-elements are present in the mRNAs of all genes and affect the recruitment, assembly, and progress of preinitiation complexes and the ribosome under many physiological states. These elements include mRNA folding, upstream open reading frames, specific nucleotides flanking the initiating AUG codon, protein coding sequence length, and codon usage. The quantitative contributions of these sequence features and how and why they coordinate to control translation rates are not well understood.ResultsHere, we show that these sequence features specify 42-81% of the variance in translation rates in Saccharomyces cerevisiae, Schizosaccharomyces pombe, Arabidopsis thaliana, Mus musculus, and Homo sapiens. We establish that control by RNA secondary structure is chiefly mediated by highly folded 25-60 nucleotide segments within mRNA 5' regions, that changes in tri-nucleotide frequencies between highly and poorly translated 5' regions are correlated between all species, and that control by distinct biochemical processes is extensively correlated as is regulation by a single process acting in different parts of the same mRNA.ConclusionsOur work shows that general features control a much larger fraction of the variance in translation rates than previously realized. We provide a more detailed and accurate understanding of the aspects of RNA structure that directs translation in diverse eukaryotes. In addition, we note that the strongly correlated regulation between and within cis-control features will cause more even densities of translational complexes along each mRNA and therefore more efficient use of the translation machinery by the cell
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