3,137 research outputs found
A deep dive: Chandra observations of the NGC 4839 group falling into the Coma cluster
Cosmological simulations of structure formation predict that galaxy clusters
continue to grow and evolve through ongoing mergers with group-scale systems.
During these merging events, the ram pressure applied by the intracluster
medium acts to strip the gas from the infalling groups, forming large tails of
stripped gas, which eventually become part of the main cluster. In this work,
we present a detailed analysis of our new deep Chandra observations of the NGC
4839 group falling into the nearby Coma cluster, providing a unique opportunity
to explore the way galaxy clusters in the local universe continue to grow. Our
analysis reveals a cold front feature at the leading head of the group,
preceded by a bow shock of hot gas in front with a Mach number of .
The power spectrum of surface brightness fluctuations in the tail shows that
the slope gets less steep as the distance from the leading head increases,
changing from at the inner part of the tail to
at the outermost part of the tail. These values are
shallower than the slope of the Kolmogorov 2D power spectrum, indicating that
thermal conduction is being suppressed throughout the tail, enabling long-lived
small-scale turbulence, which would typically be washed out if thermal
conduction was not inhibited. The characteristic amplitude of surface
brightness fluctuations in the tail suggests a mild level of turbulence with a
Mach number in the range of 0.1-0.5, agreeing with that found for the infalling
group in Abell 2142.Comment: 10 pages, 10 figures, 3 tables. Accepted for publication in MNRA
Optical Investigations on Plasma Temperature Estimation in a Model Spark Gap for Surge Currents
In this experimental investigation optical emission spectroscopy is used to characterize the radiation of the plasma in a spark gap during surge. Different approaches are used, compared and discussed in order to estimate plasma temperatures. The measurements were carried out in a narrow gap arrangement based on spark gap technology. This model is tested using 8/20 µs surge currents according to the IEC 62475 with amplitudes of 5 kA and 11 kA
Quantum simulations of the superfluid-insulator transition for two-dimensional, disordered, hard-core bosons
We introduce two novel quantum Monte Carlo methods and employ them to study
the superfluid-insulator transition in a two-dimensional system of hard-core
bosons. One of the methods is appropriate for zero temperature and is based
upon Green's function Monte Carlo; the other is a finite-temperature world-line
cluster algorithm. In each case we find that the dynamical exponent is
consistent with the theoretical prediction of by Fisher and co-workers.Comment: Revtex, 10 pages, 3 figures (postscript files attached at end,
separated by %%%%%% Fig # %%%%%, where # is 1-3). LA-UR-94-270
Anisotropic two-dimensional Heisenberg model by Schwinger-boson Gutzwiller projected method
Two-dimensional Heisenberg model with anisotropic couplings in the and
directions () is considered. The model is first solved in the
Schwinger-boson mean-field approximation. Then the solution is Gutzwiller
projected to satisfy the local constraint that there is only one boson at each
site. The energy and spin-spin correlation of the obtained wavefunction are
calculated for systems with up to sites by means of the
variational Monte Carlo simulation. It is shown that the antiferromagnetic
long-range order remains down to the one-dimensional limit.Comment: 15 pages RevTex3.0, 4 figures, available upon request, GWRVB8-9
Non-circular semiconductor nanorings of type I and II: Emission kinetics in the exciton Aharonov-Bohm effect
Transition energies and oscillator strengths of excitons in dependence on
magnetic field are investigated in type I and II semiconductor nanorings. A
slight deviation from circular (concentric) shape of the type II nanoring gives
a better observability of the Aharonov-Bohm oscillations since the ground state
is always optically active. Kinetic equations for the exciton occupation are
solved with acoustic phonon scattering as the major relaxation process, and
absorption and luminescence spectra are calculated showing deviations from
equilibrium. The presence of a non-radiative exciton decay leads to a quenching
of the integrated photoluminescence with magnetic field.Comment: The first version submitted to Phys. Rev. B on April 16, 2007.
Revised (this) version on July 31, 200
Dispersal Modeling of Fish Early Life Stages: Sensitivity with Application to Atlantic Cod in the Western Gulf of Maine
As an initial step in establishing mechanistic relationships between environmental variability and recruitment in Atlantic cod Gadhus morhua along the coast of the western Gulf of Maine, we assessed transport success of larvae from major spawning grounds to nursery areas with particle tracking using the unstructured grid model FVCOM (finite volume coastal ocean model). In coastal areas, dispersal of early planktonic life stages of fish and invertebrate species is highly dependent on the regional dynamics and its variability, which has to be captured by our models. With state-of-the-art forcing for the year 1995, we evaluate the sensitivity of particle dispersal to the timing and location of spawning, the spatial and temporal resolution of the model, and the vertical mixing scheme. A 3 d frequency for the release of particles is necessary to capture the effect of the circulation variability into an averaged dispersal pattern of the spawning season. The analysis of sensitivity to model setup showed that a higher resolution mesh, tidal forcing, and current variability do not change the general pattern of connectivity, but do tend to increase within-site retention. Our results indicate strong downstream connectivity among spawning grounds and higher chances for successful transport from spawning areas closer to the coast. The model run for January egg release indicates 1 to 19 % within-spawning ground retention of initial particles, which may be sufficient to sustain local populations. A systematic sensitivity analysis still needs to be conducted to determine the minimum mesh and forcing resolution that adequately resolves the complex dynamics of the western Gulf of Maine. Other sources of variability, i.e. large-scale upstream forcing and the biological environment, also need to be considered in future studies of the interannual variability in transport and survival of the early life stages of cod
Allowable Take of Black Vultures in the Eastern United States
Black vultures (Coragyps atratus) have been increasing in density and expanding their range in the eastern United States since at least the 1960s. In many areas, their densities have increased to the level where they are causing damage to property and livestock and the number of requests for allowable take permits has increased throughout these areas. The United States Fish and Wildlife Service (USFWS) requires updated information to help inform the number of take permits that could reduce conflicts while meeting obligations under the Migratory Bird Treaty Act.We expanded analyses used to estimate allowable take in Virginia to cover the range of black vultures in the eastern United States. We used the prescribed take level approach, which integrates demographic rates, population size estimates, and management objectives into an estimate of allowable take. We provide estimates of allowable take at 4 different scales: individual states, Bird Conservation Regions, USFWS administrative regions, and flyways. Our updated population time series provides evidence of rapidly increasing black vulture populations in many regions of the eastern United States, with an overall population estimate of approximately 4.26 million in 2015 in the Atlantic and Mississippi Flyways. Estimated allowable take ranged from a few hundred individuals per year in states at the northern end of the species range to approximately 287,000/year over the entire eastern United States. The USFWS has no legal mandate regarding the spatial scale at which take should be managed and we found little biological evidence of subpopulation structure for black vultures in the eastern United States.We suggest that allowable take for the species be implemented at a scale that meets stakeholder objectives (e.g., reducing conflict, and ensuring that black vultures are not extirpated from local areas) and is efficient for administrative and monitoring purposes. Published 2018. This article is a U.S. Government work and is in the public domain in the USA
The Effect of Embedding Formative Assessment on Pupil Attainment
Evidence suggests that adapting teaching responsively to pupil assessment can be effective in improving students’ learning. However, existing studies tend to be small-scale, leaving unanswered the question of how such formative assessment can operate when embedded as standard practice. In this study, we present the results of a randomized trial conducted in 140 English secondary schools. The intervention uses light-touch training and support, with most of the work done by teacher-led teaching and learning communities within schools. It is, therefore, well-suited to widespread adoption. In our pre-registered primary analysis, we estimate an effect size of 0.09 on general academic attainment in national, externally assessed examinations. Sensitivity analysis, excluding schools participating in a similar program at baseline, and complier analysis both suggest a larger effect size of 0.11. These results are encouraging for this approach to improving the implementation of formative assessment and, hence, academic attainment. Our findings also suggest that the intervention may help to narrow the gap between high and low prior attainment pupils, although not the gap between those from disadvantaged backgrounds and the rest of the cohort
Correction to: Evaluation of different deployment strategies for larviciding to control malaria: a simulation study
Center of mass and relative motion in time dependent density functional theory
It is shown that the exchange-correlation part of the action functional
in time-dependent density functional theory , where
is the time-dependent density, is invariant under the
transformation to an accelerated frame of reference , where is an arbitrary
function of time. This invariance implies that the exchange-correlation
potential in the Kohn-Sham equation transforms in the following manner:
. Some of the
approximate formulas that have been proposed for satisfy this exact
transformation property, others do not. Those which transform in the correct
manner automatically satisfy the ``harmonic potential theorem", i.e. the
separation of the center of mass motion for a system of interacting particles
in the presence of a harmonic external potential. A general method to generate
functionals which possess the correct symmetry is proposed
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