53,642 research outputs found
Truncation effects in superdiffusive front propagation with L\'evy flights
A numerical and analytical study of the role of exponentially truncated
L\'evy flights in the superdiffusive propagation of fronts in
reaction-diffusion systems is presented. The study is based on a variation of
the Fisher-Kolmogorov equation where the diffusion operator is replaced by a
-truncated fractional derivative of order where
is the characteristic truncation length scale. For there is no
truncation and fronts exhibit exponential acceleration and algebraic decaying
tails. It is shown that for this phenomenology prevails in the
intermediate asymptotic regime where
is the diffusion constant. Outside the intermediate asymptotic regime,
i.e. for , the tail of the front exhibits the tempered decay
, the acceleration is transient, and
the front velocity, , approaches the terminal speed as , where it is assumed that
with denoting the growth rate of the
reaction kinetics. However, the convergence of this process is algebraic, , which is very slow compared to the exponential
convergence observed in the diffusive (Gaussian) case. An over-truncated regime
in which the characteristic truncation length scale is shorter than the length
scale of the decay of the initial condition, , is also identified. In
this extreme regime, fronts exhibit exponential tails, ,
and move at the constant velocity, .Comment: Accepted for publication in Phys. Rev. E (Feb. 2009
A large scale extinction map of the Galactic Anticenter from 2MASS
We present a 127deg x 63deg extinction map of the Anticenter of the Galaxy,
based on and colour excess maps from 2MASS. This 8001 square degree
map with a resolution of 4 arcminutes is provided as online material. The
colour excess ratio / is used to determine the power law index of
the reddening law (\beta) for individual regions contained in the area (e.g.
Orion, Perseus, Taurus, Auriga, Monoceros, Camelopardalis, Cassiopeia). On
average we find a dominant value of \beta=1.8+-0.2 for the individual clouds,
in agreement with the canonical value for the interstellar medium. We also show
that there is an internal scatter of \beta values in these regions, and that in
some areas more than one dominant \beta value is present. This indicates large
scale variations in the dust properties. The analysis of the A_V values within
individual regions shows a change in the slope of the column density
distribution with distance. This can either be attributed to a change in the
governing physical processes in molecular clouds on spatial scales of about 1pc
or an A_V dilution with distance in our map.Comment: 18 pages, 29 Figures, 1 Table, Accepted for publication by MNRAS, A
version with higher resolution figures can be found at
http://astro.kent.ac.uk/~df
Improving photon-hadron discrimination based on cosmic ray surface detector data
The search for photons at EeV energies and beyond has considerable
astrophysical interest and will remain one of the key challenges for ultra-high
energy cosmic ray (UHECR) observatories in the near future. Several upper
limits to the photon flux have been established since no photon has been
unambiguously observed up to now. An improvement in the reconstruction
efficiency of the photon showers and/or better discrimination tools are needed
to improve these limits apart from an increase in statistics. Following this
direction, we analyze in this work the ability of the surface parameter Sb,
originally proposed for hadron discrimination, for photon search.
Semi-analytical and numerical studies are performed in order to optimize Sb
for the discrimination of photons from a proton background in the energy range
from 10^18.5 to 10^19.6 eV. Although not shown explicitly, the same analysis
has been performed for Fe nuclei and the corresponding results are discussed
when appropriate. The effects of different array geometries and the
underestimation of the muon component in the shower simulations are analyzed,
as well as the Sb dependence on primary energy and zenith angle.Comment: 9 pages, 19 Figures. Accepted in Astroparticle Physics on May 31th,
201
Dynamical excitonic effects in metals and semiconductors
The dynamics of an electron--hole pair induced by the time--dependent
screened Coulomb interaction is discussed. In contrast to the case where the
static electron--hole interaction is considered we demonstrate the occurrence
of important dynamical excitonic effects in the solution of the Bethe--Salpeter
equation.This is illustrated in the calculated absorption spectra of noble
metals (copper and silver) and silicon. Dynamical corrections strongly affect
the spectra, partially canceling dynamical self--energy effects and leading to
good agreement with experiment.Comment: Accepted for publication on Phys. Rev. Let
Separatrix Reconnections in Chaotic Regimes
In this paper we extend the concept of separatrix reconnection into chaotic
regimes. We show that even under chaotic conditions one can still understand
abrupt jumps of diffusive-like processes in the relevant phase-space in terms
of relatively smooth realignments of stable and unstable manifolds of unstable
fixed points.Comment: 4 pages, 5 figures, submitted do Phys. Rev. E (1998
Reduced lysosomal acid lipase activity: A new marker of liver disease severity across the clinical continuum of non-alcoholic fatty liver disease?
Lysosomal acid lipase (LAL) plays a key role in intracellular lipid metabolism. Reduced LAL activity promotes increased multi-organ lysosomal cholesterol ester storage, as observed in two recessive autosomal genetic diseases, Wolman disease and Cholesterol ester storage disease. Severe liver steatosis and accelerated liver fibrosis are common features in patients with genetic LAL deficiency. By contrast, few reliable data are available on the modulation of LAL activity in vivo and on the epigenetic and metabolic factors capable of regulating its activity in subjects without homozygous mutations of the Lipase A gene. In the last few years, a less severe and non-genetic reduction of LAL activity was reported in children and adults with non-alcoholic fatty liver disease (NAFLD), suggesting a possible role of LAL reduction in the pathogenesis and progression of the disease. Patients with NAFLD show a significant, progressive reduction of LAL activity from simple steatosis to non-alcoholic steatohepatitis and cryptogenic cirrhosis. Among cirrhosis of different etiologies, those with cryptogenic cirrhosis show the most significant reductions of LAL activity. These findings suggest that the modulation of LAL activity may become a possible new therapeutic target for patients with more advanced forms of NAFLD. Moreover, the measurement of LAL activity may represent a possible new marker of disease severity in this clinical setting
Recommended from our members
Effects of post-translational modifications catalysed by pollen transglutaminase on the functional properties of microtubules and actin filaments
TGases (transglutaminases) are a class of calcium-dependent enzymes that catalyse the interactions between acyl acceptor glutamyl residues and amine donors, potentially making crosslinks between proteins. To assess the activity of apple (Malus domestica) pollen TGase on the functional properties of actin and tubulin, TGase was prepared from apple pollen by hydrophobic interaction chromatography and assayed on actin and tubulin purified from the same cell type. The enzyme catalysed the incorporation of putrescine in the cytoskeleton monomers. When tested on actin filaments, pollen TGase induced the formation of high-molecular-mass aggregates of actin. Use of fluorescein– cadaverine showed that the labelled polyamine was incorporated into actin by pollen TGase, similar to with guinea pig liver TGase. The pollen TGase also reduced the enzyme activity and the binding of myosin to TGase-treated actin filaments. Polymerization of tubulin in the presence of pollen TGase also yielded the formation of high molecular mass aggregates. Furthermore, the pollen TGase also affected the binding of kinesin to microtubules and reduced the motility of microtubules along kinesincoated slides. These results indicate that the pollen tube TGase can control different properties of the pollen tube cytoskeleton (including the ability of actin and tubulin to assemble and their interaction with motor proteins) and consequently regulate the development of pollen tubes
- …