1,625 research outputs found
Moment Equations for a Spatially Extended System of Two Competing Species
The dynamics of a spatially extended system of two competing species in the
presence of two noise sources is studied. A correlated dichotomous noise acts
on the interaction parameter and a multiplicative white noise affects directly
the dynamics of the two species. To describe the spatial distribution of the
species we use a model based on Lotka-Volterra (LV) equations. By writing them
in a mean field form, the corresponding moment equations for the species
concentrations are obtained in Gaussian approximation. In this formalism the
system dynamics is analyzed for different values of the multiplicative noise
intensity. Finally by comparing these results with those obtained by direct
simulations of the time discrete version of LV equations, that is coupled map
lattice (CML) model, we conclude that the anticorrelated oscillations of the
species densities are strictly related to non-overlapping spatial patterns.Comment: 10 pages, 3 figure
Moment equations in a Lotka-Volterra extended system with time correlated noise
A spatially extended Lotka-Volterra system of two competing species in the
presence of two correlated noise sources is analyzed: (i) an external
multiplicative time correlated noise, which mimics the interaction between the
system and the environment; (ii) a dichotomous stochastic process, whose jump
rate is a periodic function, which represents the interaction parameter between
the species. The moment equations for the species densities are derived in
Gaussian approximation, using a mean field approach. Within this formalism we
study the effect of the external time correlated noise on the ecosystem
dynamics. We find that the time behavior of the order moments are
independent on the multiplicative noise source. However the behavior of the
order moments is strongly affected both by the intensity and the
correlation time of the multiplicative noise. Finally we compare our results
with those obtained studying the system dynamics by a coupled map lattice
model.Comment: 12 pages, 7 figures, to appear in Acta Phys. Pol.
Candidate hypervelocity stars of spectral type G and K revisited
Hypervelocity stars (HVS) move so fast that they are unbound to the Galaxy.
When they were first discovered in 2005, dynamical ejection from the
supermassive black hole (SMBH) in the Galactic Centre (GC) was suggested as
their origin. The two dozen HVSs known today are young massive B stars, mostly
of 3-4 solar masses. Recently, 20 HVS candidates of low mass were discovered in
the Segue G and K dwarf sample, but none of them originates from the GC. We
embarked on a kinematic analysis of the Segue HVS candidate sample using the
full 6D phase space information based on new proper motion measurements. Their
orbital properties can then be derived by tracing back their trajectories in
different mass models of our Galaxy. We present the results for 14 candidate
HVSs, for which proper motion measurements were possible. Significantly lower
proper motions than found in the previous study were derived. Considering three
different Galactic mass models we find that all stars are bound to the Galaxy.
We confirm that the stars do not originate from the GC. The distribution of
their proper motions and radial velocities is consistent with predictions for
runaway stars ejected from the Galactic disk by the binary supernova mechanism.
However, their kinematics are also consistent with old disk membership.
Moreover, most stars have rather low metallicities and strong -element
enrichment as typical for thick disk and halo stars, whereas the metallicity of
the three most metal-rich stars could possibly indicate that they are runaway
stars from the thin disk. One star shows halo kinematics.Comment: A&A letter accepte
Dacron Patch Infection After Carotid Angioplasty. A Report of 6 Cases
ObjectiveWe describe our experience with Dacron patch infections after carotid endarterectomy (CEA).ReportFrom 633 patients undergoing carotid endarterectomy with Dacron patching, six re-presented with prosthetic infections. In 3 of the 6 cases a neck haematoma had necessitated surgical revision after the original carotid surgery. Five patients underwent interposition vein grafting and 1 vein patch angioplasty. Postoperatively, 2 patients developed a repeat infection including the 1 patient with patch angioplasty. All patients were free of infection and neurological symptoms after a maximum follow-up of 56.5 months.ConclusionFollowing the development of haemorrhage or wound complications careful clinical surveillance should be carried out after carotid reconstruction
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