8,918 research outputs found
Burridge-Knopoff Models as Elastic Excitable Media
We construct a model of an excitable medium with elastic rather than the
usual diffusive coupling. We explore the dynamics of elastic excitable media,
which we find to be dominated by low dimensional structures, including global
oscillations, period-doubled pacemakers, and propagating fronts. We suggest
that examples of elastic excitable media are to be found in such diverse
physical systems as Burridge-Knopoff models of frictional sliding, electronic
transmission lines, and active optical waveguides
Spatial clustering of interacting bugs: Levy flights versus Gaussian jumps
A biological competition model where the individuals of the same species
perform a two-dimensional Markovian continuous-time random walk and undergo
reproduction and death is studied. The competition is introduced through the
assumption that the reproduction rate depends on the crowding in the
neighborhood. The spatial dynamics corresponds either to normal diffusion
characterized by Gaussian jumps or to superdiffusion characterized by L\'evy
flights. It is observed that in both cases periodic patterns occur for
appropriate parameters of the model, indicating that the general macroscopic
collective behavior of the system is more strongly influenced by the
competition for the resources than by the type of spatial dynamics. However,
some differences arise that are discussed.Comment: This version incorporates in the text the correction published as an
Erratum in Europhysics Letters (EPL) 95, 69902 (2011) [doi:
10.1209/0295-5075/95/69902
Birth, death and diffusion of interacting particles
Individual-based models of chemical or biological dynamics usually consider
individual entities diffusing in space and performing a birth-death type
dynamics. In this work we study the properties of a model in this class where
the birth dynamics is mediated by the local, within a given distance, density
of particles. Groups of individuals are formed in the system and in this paper
we concentrate on the study of the properties of these clusters (lifetime,
size, and collective diffusion). In particular, in the limit of the interaction
distance approaching the system size, a unique cluster appears which helps to
understand and characterize the clustering dynamics of the model.Comment: 15 pages, 6 figures, Iop style. To appear in Journal of Physics A:
Condensed matte
Diversity-induced resonance in a system of globally coupled linear oscillators
The purpose of this paper to analyze in some detail the arguably simplest
case of diversity-induced reseonance: that of a system of globally-coupled
linear oscillators subjected to a periodic forcing. Diversity appears as the
parameters characterizing each oscillator, namely its mass, internal frequency
and damping coefficient are drawn from a probability distribution. The main
ingredients for the diversity-induced-resonance phenomenon are present in this
system as the oscillators display a variability in the individual responses but
are induced, by the coupling, to synchronize their responses. A steady state
solution for this model is obtained. We also determine the conditions under
which it is possible to find a resonance effect.Comment: Reported at the XI International Workshop "Instabilities and
Nonequilibrium Structures" Vina del Mar (Chile
On the Excitation and Formation of Circumstellar Fullerenes
We compare and analyze the Spitzer mid-infrared spectrum of three
fullerene-rich planetary nebulae in the Milky Way and the Magellanic Clouds;
Tc1, SMP SMC16, and SMP LMC56. The three planetary nebulae share many
spectroscopic similarities. The strongest circumstellar emission bands
correspond to the infrared active vibrational modes of the fullerene species
C60 and little or no emission is present from Polycyclic Aromatic Hydrocarbons
(PAHs). The strength of the fullerene bands in the three planetary nebulae is
very similar, while the ratio of the [NeIII]15.5um/[NeII]12.8um fine structure
lines, an indicator of the strength of the radiation field, is markedly
different. This raises questions about their excitation mechanism and we
compare the fullerene emission to fluorescent and thermal models. In addition,
the spectra show other interesting and common features, most notably in the
6-9um region, where a broad plateau with substructure dominates the emission.
These features have previously been associated with mixtures of
aromatic/aliphatic hydrocarbon solids. We hypothesize on the origin of this
band, which is likely related to the fullerene formation mechanism, and compare
it with modeled Hydrogenated Amorphous Carbon that present emission in this
region.Comment: 13 pages, 2 tables, 7 figures, Accepted for publication in Ap
Machine learning techniques to select Be star candidates. An application in the OGLE-IV Gaia south ecliptic pole field
Statistical pattern recognition methods have provided competitive solutions
for variable star classification at a relatively low computational cost. In
order to perform supervised classification, a set of features is proposed and
used to train an automatic classification system. Quantities related to the
magnitude density of the light curves and their Fourier coefficients have been
chosen as features in previous studies. However, some of these features are not
robust to the presence of outliers and the calculation of Fourier coefficients
is computationally expensive for large data sets. We propose and evaluate the
performance of a new robust set of features using supervised classifiers in
order to look for new Be star candidates in the OGLE-IV Gaia south ecliptic
pole field. We calculated the proposed set of features on six types of variable
stars and on a set of Be star candidates reported in the literature. We
evaluated the performance of these features using classification trees and
random forests along with K-nearest neighbours, support vector machines, and
gradient boosted trees methods. We tuned the classifiers with a 10-fold
cross-validation and grid search. We validated the performance of the best
classifier on a set of OGLE-IV light curves and applied this to find new Be
star candidates. The random forest classifier outperformed the others. By using
the random forest classifier and colour criteria we found 50 Be star candidates
in the direction of the Gaia south ecliptic pole field, four of which have
infrared colours consistent with Herbig Ae/Be stars. Supervised methods are
very useful in order to obtain preliminary samples of variable stars extracted
from large databases. As usual, the stars classified as Be stars candidates
must be checked for the colours and spectroscopic characteristics expected for
them
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