8,978 research outputs found
Dark-Halo Cusp: Asymptotic Convergence
We propose a model for how the buildup of dark halos by merging satellites
produces a characteristic inner cusp, of a density profile \rho \prop r^-a with
a -> a_as > 1, as seen in cosmological N-body simulations of hierarchical
clustering scenarios. Dekel, Devor & Hetzroni (2003) argue that a flat core of
a<1 exerts tidal compression which prevents local deposit of satellite
material; the satellite sinks intact into the halo center thus causing a rapid
steepening to a>1. Using merger N-body simulations, we learn that this cusp is
stable under a sequence of mergers, and derive a practical tidal mass-transfer
recipe in regions where the local slope of the halo profile is a>1. According
to this recipe, the ratio of mean densities of halo and initial satellite
within the tidal radius equals a given function psi(a), which is significantly
smaller than unity (compared to being 1 according to crude resonance criteria)
and is a decreasing function of a. This decrease makes the tidal mass transfer
relatively more efficient at larger a, which means steepening when a is small
and flattening when a is large, thus causing converges to a stable solution.
Given this mass-transfer recipe, linear perturbation analysis, supported by toy
simulations, shows that a sequence of cosmological mergers with homologous
satellites slowly leads to a fixed-point cusp with an asymptotic slope a_as>1.
The slope depends only weakly on the fluctuation power spectrum, in agreement
with cosmological simulations. During a long interim period the profile has an
NFW-like shape, with a cusp of 1<a<a_as. Thus, a cusp is enforced if enough
compact satellite remnants make it intact into the inner halo. In order to
maintain a flat core, satellites must be disrupted outside the core, possibly
as a result of a modest puffing up due to baryonic feedback.Comment: 37 pages, Latex, aastex.cls, revised, ApJ, 588, in pres
Effect of In Ovo Exposure to PCBs and Hg on Clapper Rail Bone Mineral Chemistry from a Contaminated Salt Marsh in Coastal Georgia
The effect of Hg and PCBs (Aroclor 1268) on bone characteristics was investigated in a population of Clapper Rails (Rallus longirostris) inhabiting contaminated and unimpacted estuarine marsh systems in coastal Georgia. Exposure to contaminants did not affect the length or weight of leg bones, but it significantly altered the chemical composition of the bone. Specifically, bone in the contaminated site had a higher Ca to P, and lower carbonate and acid phosphate content. These characteristics are typical of more mature bone mineral and indicate that toxicants have accelerated bone maturation. FTIR spectroscopy data revealed a dose dependent change in the crystallinity of bone mineral, and the relative proportion of specific PO4 groups in different molecular environments in the bone, with toxicants loads. These changes are most probably related to a hormonal alteration of the rate of bone remodelation induced by exposure to toxicant loads
The Effect of a Single Supernova Explosion on the Cuspy Density Profile of a Small-Mass Dark Matter Halo
Some observations of galaxies, and in particular dwarf galaxies, indicate a
presence of cored density profiles in apparent contradiction with cusp profiles
predicted by dark matter N-body simulations. We constructed an analytical
model, using particle distribution functions (DFs), to show how a supernova
(SN) explosion can transform a cusp density profile in a small-mass dark matter
halo into a cored one. Considering the fact that a SN efficiently removes
matter from the centre of the first haloes, we study the effect of mass removal
through a SN perturbation in the DFs. We found that the transformation from a
cusp into a cored profile is present even for changes as small as 0.5% of the
total energy of the halo, that can be produced by the expulsion of matter
caused by a single SN explosion.Comment: 6 pages, 4 figures, accepted for publication in MNRA
Evaluating the surface circulation in the Ebro delta (northeastern Spain) with quality-controlled high-frequency radar measurements
The Ebro River delta is a relevant marine protected area in the western
Mediterranean. In order to promote the conservation of its ecosystem and
support operational decision making in this sensitive area, a three-site
standard-range (13.5 MHz) CODAR SeaSonde high-frequency (HF) radar was
deployed in December 2013. The main goal of this work is to explore basic
features of the sea surface circulation in the Ebro deltaic region as derived
from reliable HF radar surface current measurements. For this aim, a combined
quality control methodology was applied: firstly, 1-year long (2014)
real-time web monitoring of nonvelocity-based diagnostic parameters was
conducted to infer both radar site status and HF radar system performance.
The signal-to-noise ratio at the monopole exhibited a consistent monthly
evolution, although some abrupt decreases (below 10 dB), occasionally
detected in June for one of the radar sites, impacted negatively on the
spatiotemporal coverage of total current vectors. It seemed to be sporadic
episodes since radar site overall performance was found to be robust during
2014. Secondly, a validation of HF radar data with independent in situ
observations from a moored current meter was attempted for May–October 2014.
The accuracy assessment of radial and total vectors revealed a consistently
high agreement. The directional accuracy of the HF radar was rated at better
than 8°. The correlation coefficient and root mean square error
(RMSE) values emerged in the ranges [0.58–0.83] and
[4.02–18.31] cm s−1, respectively. The analysis of the monthly
averaged current maps for 2014 showed that the HF radar properly represented
basic oceanographic features previously reported, namely, the predominant
southwestward flow, the coastal clockwise eddy confined south of the Ebro
delta mouth, or the Ebro River impulsive-type freshwater discharge. The EOF
analysis related the flow response to local wind forcing and confirmed that
the surface current field evolved in space and time according to three
significantly dominant modes of variability
Universal stability of coherently diffusive 1D systems with respect to decoherence
Static disorder in a 3D crystal degrades the ideal ballistic dynamics until
it produces a localized regime. This Metal-Insulator Transition is often
preceded by coherent diffusion. By studying three different paradigmatic 1D
models, the Harper-Hofstadter-Aubry-Andr\'e and the Fibonacci tight-binding
chains, and the power-banded random matrix model, we show that whenever
coherent diffusion is present, transport is exceptionally stable against
decoherent noise. This is completely at odds with what happens for ballistic
and localized dynamics, where the diffusion coefficient strongly depends on the
environmental decoherence. A universal dependence of the diffusion coefficient
on the decoherence strength is analytically derived: the diffusion coefficient
remains almost decoherence-independent until the coherence time becomes
comparable with the mean elastic scattering time. Thus, systems with a quantum
diffusive regime could be used to design stable quantum wires and may explain
the functionality of many biological systems, which often operate at the border
between the ballistic and localized regimes.Comment: Main: 8 pages, 3 figures. Supplementary: 17 pages, 10 figure
The First Galaxies: Chemical Enrichment, Mixing, and Star Formation
Using three-dimensional cosmological simulations, we study the assembly
process of one of the first galaxies, with a total mass of 10^8 M_sun,
collapsing at z = 10. Our main goal is to trace the transport of the heavy
chemical elements produced and dispersed by a pair-instability supernova
exploding in one of the minihalo progenitors. To this extent, we incorporate an
efficient algorithm into our smoothed particle hydrodynamics code which
approximately models turbulent mixing as a diffusion process. We study this
mixing with and without the radiative feedback from Population III stars that
subsequently form in neighboring minihalos. Our simulations allow us to
constrain the initial conditions for second-generation star formation, within
the first galaxy itself, and inside of minihalos that virialize after the
supernova explosion. We find that most minihalos remain unscathed by ionizing
radiation or the supernova remnant, while some are substantially photoheated
and enriched to supercritical levels, likely resulting in the formation of
low-mass Population III or even Population II stars. At the center of the newly
formed galaxy, 10^5 M_sun of cold, dense gas uniformly enriched to 10^-3 Z_sun
are in a state of collapse, suggesting that a cluster of Population II stars
will form. The first galaxies, as may be detected by the James Webb Space
Telescope, would therefore already contain stellar populations familiar from
lower redshifts.Comment: 13 pages, 9 figures, published in Ap
Nature of the spin-glass phase at experimental length scales
We present a massive equilibrium simulation of the three-dimensional Ising
spin glass at low temperatures. The Janus special-purpose computer has allowed
us to equilibrate, using parallel tempering, L=32 lattices down to T=0.64 Tc.
We demonstrate the relevance of equilibrium finite-size simulations to
understand experimental non-equilibrium spin glasses in the thermodynamical
limit by establishing a time-length dictionary. We conclude that
non-equilibrium experiments performed on a time scale of one hour can be
matched with equilibrium results on L=110 lattices. A detailed investigation of
the probability distribution functions of the spin and link overlap, as well as
of their correlation functions, shows that Replica Symmetry Breaking is the
appropriate theoretical framework for the physically relevant length scales.
Besides, we improve over existing methodologies to ensure equilibration in
parallel tempering simulations.Comment: 48 pages, 19 postscript figures, 9 tables. Version accepted for
publication in the Journal of Statistical Mechanic
Photon, Neutrino and Charged Particle Spectra from R-violating Gravitino Decays
We study photonic, neutrino and charged particle signatures from slow decays
of gravitino dark matter in supersymmetric theories where R-parity is
explicitly broken by trilinear operators. Photons and (anti-)fermions from loop
and tree-level processes give rise to spectra with distinct features, which, if
observed, can give crucial input on the possible mass of the gravitino and the
magnitude and flavour structure of R-violating operators. Within this
framework, we make detailed comparisons of the theoretical predictions to the
recent experimental data from PAMELA, ATIC and Fermi LAT.Comment: Version published in Phys. Lett.
Thermodynamic glass transition in a spin glass without time-reversal symmetry
Spin glasses are a longstanding model for the sluggish dynamics that appears
at the glass transition. However, spin glasses differ from structural glasses
for a crucial feature: they enjoy a time reversal symmetry. This symmetry can
be broken by applying an external magnetic field, but embarrassingly little is
known about the critical behaviour of a spin glass in a field. In this context,
the space dimension is crucial. Simulations are easier to interpret in a large
number of dimensions, but one must work below the upper critical dimension
(i.e., in d<6) in order for results to have relevance for experiments. Here we
show conclusive evidence for the presence of a phase transition in a
four-dimensional spin glass in a field. Two ingredients were crucial for this
achievement: massive numerical simulations were carried out on the Janus
special-purpose computer, and a new and powerful finite-size scaling method.Comment: 10 pages, 6 figure
Factors predicting cessation of status epilepticus in clinical practice: Data from a prospective observational registry (SENSE).
To investigate the initial termination rate of status epilepticus (SE) in a large observational study and explore associated variables.
Data of adults treated for SE were collected prospectively in centers in Germany, Austria, and Switzerland, during 4.5 years. Incident episodes of 1,049 patients were analyzed using uni- and multivariate statistics to determine factors predicting cessation of SE within 1 hour (for generalized convulsive SE [GCSE]) and 12 hours (for non-GCSE) of initiating treatment.
Median age at SE onset was 70 years; most frequent etiologies were remote (32%) and acute (31%). GCSE was documented in 43%. Median latency between SE onset and first treatment was 30 minutes in GCSE and 150 minutes in non-GCSE. The first intravenous compound was a benzodiazepine in 86% in GCSE and 73% in non-GCSE. Bolus doses of the first treatment step were lower than recommended by current guidelines in 76% of GCSE patients and 78% of non-GCSE patients. In 319 GCSE patients (70%), SE was ongoing 1 hour after initiating treatment and in 342 non-GCSE patients (58%) 12 hours after initiating treatment. Multivariate Cox regression demonstrated that use of benzodiazepines as first treatment step and a higher cumulative dose of anticonvulsants within the first period of treatment were associated with shorter time to cessation of SE for both groups.
In clinical practice, treatment guidelines were not followed in a substantial proportion of patients. This underdosing correlated with lack of cessation of SE. Our data suggest that sufficiently dosed benzodiazepines should be used as a first treatment step. ANN NEUROL 2019;85:421-432
- …