106 research outputs found
Prédiction du potentiel d'invasion des espèces non natives par des modèles de niche : approches méthodologiques et applications aux poissons d'eau douce sur le territoire français
Les espèces invasives sont une des perturbations les plus importantes des milieux aquatiques d'eau douce. La prévention des invasions passe par l'identification des zones dans lesquelles ces espèces sont susceptibles de s'installer. Cette thèse a mis en évidence que : - le changement de grain spatial modifie la mesure de l'aire de la distribution observée. Par contre, le grain des données utilisées pour calibrer les modèles influence peu la qualité des prédictions. - les grains les plus larges restent utilisables pour identifier les espèces changeant de niche climatique lors du processus d'invasion. - la majorité des six espèces de poissons étudiées présentent un fort risque d'établissement sous l'effet du changement climatique, même en l'absence de changement de niche de ces espèces. - la qualité des prédictions des modèles corrélatifs peut être améliorée en utilisant la méthode d'ensemble itérative que nous avons développée. Cette méthode est particulièrement adaptée pour les espèces invasives, mais aussi pour les espèces difficiles à détecter ou les espèces en danger.Freshwaters are among the most anthropogenicaly threatened ecosystems in the world. They especially face serious threats due to invasive species. Efforts are thus needed to control invasions and increase the accuracy of the models used to predict the potential distribution of invasive species. We showed that: - the area of the observed distribution, and thus the area of the distribution predicted by correlative models, increases exponentially with the spatial grain of the data. However, model quality is little affected by the grain of the data and decreases only for the largest grains. - coarse-grained occurrence data remain useful in identifying the species that experience niche shifts. - most of the six fish species that we have studied were able to establish in France under climate change, even without niche shift. - the iterative ensemble modeling method that we developed increases the accuracy of predictions as soon as the occurrence data set contained abundant non environmental absences. This new method is of interest for invasive species niche modeling but also to model the distribution of difficult to detect or endangered species
Composition Mixing during Blue Straggler Formation and Evolution
We use smoothed-particle hydrodynamics to examine differences between direct
collisions of single stars and binary star mergers in their roles as possible
blue straggler star formation mechanisms. We find in all cases that core helium
in the progenitor stars is largely retained in the core of the remnant, almost
independent of the type of interaction or the central concentration of the
progenitor stars.
We have also modelled the subsequent evolution of the hydrostatic remnants,
including mass loss and energy input from the hydrodynamical interaction. The
combination of the hydrodynamical and hydrostatic models enables us to predict
that little mixing will occur during the merger of two globular cluster stars
of equal mass. In contrast to the results of Proctor Sills, Bailyn, & Demarque
(1995), we find that neither completely mixed nor unmixed models can match the
absolute colors of observed blue stragglers in NGC 6397 at all luminosity
levels. We also find that the color distribution is probably the crucial test
for explanations of BSS formation - if stellar collisions or mergers are the
correct mechanisms, a large fraction of the lifetime of the straggler must be
spent away from the main sequence. This constraint appears to rule out the
possibility of completely mixed models. For NGC 6397, unmixed models predict
blue straggler lifetimes ranging from about 0.1 to 4 Gyr, while completely
mixed models predict a range from about 0.6 to 4 Gyr.Comment: AASTeX, 28 pg., accepted for ApJ, also available at
http://ucowww.ucsc.edu/~erics/bspaper.htm
Phase instabilities in hexagonal patterns
The general form of the amplitude equations for a hexagonal pattern including
spatial terms is discussed. At the lowest order we obtain the phase equation
for such patterns. The general expression of the diffusion coefficients is
given and the contributions of the new spatial terms are analysed in this
paper. From these coefficients the phase stability regions in a hexagonal
pattern are determined. In the case of Benard-Marangoni instability our results
agree qualitatively with numerical simulations performed recently.Comment: 6 pages, 6 figures, to appear in Europhys. Let
Cataclysmic Variables and a New Class of Faint UV Stars in the Globular Cluster NGC 6397
We present evidence that the globular cluster NGC 6397 contains two distinct
classes of centrally-concentrated UV-bright stars. Color-magnitude diagrams
constructed from U, B, V, and I data obtained with the HST/WFPC2 reveal seven
UV-bright stars fainter than the main-sequence turnoff, three of which had
previously been identified as cataclysmic variables (CVs). Lightcurves of these
stars show the characteristic ``flicker'' of CVs, as well as longer-term
variability. A fourth star is identified as a CV candidate on the basis of its
variability and UV excess. Three additional UV-bright stars show no photometric
variability and have broad-band colors characteristic of B stars. These
non-flickering UV stars are too faint to be extended horizontal branch stars.
We suggest that they could be low-mass helium white dwarfs, formed when the
evolution of a red giant is interrupted, due either to Roche-lobe overflow onto
a binary companion, or to envelope ejection following a common-envelope phase
in a tidal-capture binary. Alternatively, they could be very-low-mass
core-He-burning stars. Both the CVs and the new class of faint UV stars are
strongly concentrated toward the cluster center, to the extent that mass
segregation from 2-body relaxation alone may be unable to explain their
distribution.Comment: 11 pages plus 3 eps figures; LaTeX using aaspp4.sty; to appear in The
Astrophysical Journal Letter
The Evolution of Blue Stragglers Formed Via Stellar Collisions
We have used the results of recent smoothed particle hydrodynamic simulations
of colliding stars to create models appropriate for input into a stellar
evolution code. In evolving these models, we find that little or no surface
convection occurs, precluding angular momentum loss via a magnetically-driven
stellar wind as a viable mechanism for slowing rapidly rotating blue stragglers
which have been formed by collisions. Angular momentum transfer to either a
circumstellar disk (possibly collisional ejecta) or a nearby companion are
plausible mechanisms for explaining the observed low rotation velocities of
blue stragglers. Under the assumption that the blue stragglers seen in NGC 6397
and 47 Tuc have been created solely by collisions, we find that the majority of
these blue stragglers cannot have been highly mixed by convection or meridional
circulation currents at anytime during their evolution. Also, on the basis of
the agreement between the predictions of our non-rotating models and the
observed blue straggler distribution, the evolution of blue stragglers is
apparently not dominated by the effects of rotation.Comment: 36 pages, including 1 table and 7 postscript figures (LaTeX2e). Also
avaliable at http://astrowww.phys.uvic.ca/~ouellet/ . Accepted for
publication in A
Oscillation patterns in negative feedback loops
Organisms are equipped with regulatory systems that display a variety of
dynamical behaviours ranging from simple stable steady states, to switching and
multistability, to oscillations. Earlier work has shown that oscillations in
protein concentrations or gene expression levels are related to the presence of
at least one negative feedback loop in the regulatory network. Here we study
the dynamics of a very general class of negative feedback loops. Our main
result is that in these systems the sequence of maxima and minima of the
concentrations is uniquely determined by the topology of the loop and the
activating/repressing nature of the interaction between pairs of variables.
This allows us to devise an algorithm to reconstruct the topology of
oscillating negative feedback loops from their time series; this method applies
even when some variables are missing from the data set, or if the time series
shows transients, like damped oscillations. We illustrate the relevance and the
limits of validity of our method with three examples: p53-Mdm2 oscillations,
circadian gene expression in cyanobacteria, and cyclic binding of cofactors at
the estrogen-sensitive pS2 promoter.Comment: 10 pages, 8 figure
Active behavior of abdominal wall muscles: Experimental results and numerical model formulation
In the present study a computational finite element technique is proposed to simulate the mechanical response of muscles in the abdominal wall. This technique considers the active behavior of the tissue taking into account both collagen and muscle fiber directions. In an attempt to obtain the computational response as close as possible to real muscles, the parameters needed to adjust the mathematical formulation were determined from in vitro experimental tests. Experiments were conducted on male New Zealand White rabbits (2047. ±. 34. g) and the active properties of three different muscles: Rectus Abdominis, External Oblique and multi-layered samples formed by three muscles (External Oblique, Internal Oblique, and Transversus Abdominis) were characterized. The parameters obtained for each muscle were incorporated into a finite strain formulation to simulate active behavior of muscles incorporating the anisotropy of the tissue. The results show the potential of the model to predict the anisotropic behavior of the tissue associated to fibers and how this influences on the strain, stress and generated force during an isometric contraction
Stability of Oscillating Hexagons in Rotating Convection
Breaking the chiral symmetry, rotation induces a secondary Hopf bifurcation
in weakly nonlinear hexagon patterns which gives rise to oscillating hexagons.
We study the stability of the oscillating hexagons using three coupled
Ginzburg-Landau equations. Close to the bifurcation point we derive reduced
equations for the amplitude of the oscillation, coupled to the phase of the
underlying hexagons. Within these equation we identify two types of long-wave
instabilities and study the ensuing dynamics using numerical simulations of the
three coupled Ginzburg-Landau equations.Comment: 25 pages, 7 figure
Millisecond Pulsars as Probes of Mass Segregation in the Galactic Center
We propose a simple test for the existence of a cluster of black hole
remnants around Sgr A* that is based on a small sample of any type of Galactic
Center objects, provided they are substantially less massive than the black
holes and constitute part of an old (> 1 Gyr) population. The test relies on
the fact that, under the presence of such a cluster of heavy remnants and
because of energy equipartition, lower mass objects would be expelled from the
central regions and settle into a distribution very different than the cusp
expected to be induced by the supermassive black hole alone. We show that with
a sample of just 50 objects and using only their angular positions on the sky
relative to Sgr A* it is possible to clearly differentiate between a
distribution consistent with the presence of the cluster of black holes and a
power-law cusp distribution. We argue that millisecond pulsars might currently
be the best candidate to perform this test, because of the large uncertainties
involved in the age determination of less exotic objects. In addition, by
measuring their first and second period derivatives, millisecond pulsars offer
the rare opportunity of determining the complete phase space information of the
objects. We show that this extra information improves the detection of mass
segregation by about 30%.Comment: 16 pages, 1 Postscript figure; version accepted for publication in
Ap
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