203 research outputs found
Génie végétal contre l'érosion des berges de cours d'eau dans un contexte de changement climatique : quelles nouvelles espÚces utiliser ?
National audienceL'érosion des berges est un phénomÚne naturel participant activement aux processus de dynamique fluviale. Toutefois, afin de protéger des enjeux riverains (habitations, infrastructures de transport,) il est parfois nécessaire de fixer les berges avec des ouvrages de protection. Pour se faire, le génie végétal est aujourd'hui de plus en plus employé. Il s'agit de mettre en place des matériaux végétaux vivants qui vont stabiliser les berges. Ces ouvrages comprennent notamment des boutures de saules, dont les racines vont fixer la zone érodée. Les saules, sont des espÚces qui apprécient l'eau et restent inféodées à la nappe phréatique. Or la sécheresse est une des causes majeures d'échec des ouvrages de génie végétal, et ces derniÚres devraient s'aggraver avec le changement climatique. Il apparaßt donc particuliÚrement intéressant de tester de nouvelles espÚces présentant à la fois de bonnes capacités en termes de génie végétal et une meilleure résistance à la sécheresse. Myricaria germanica et Tamarix gallica, deux espÚces pionniÚres des bords de cours d'eau, présentent à priori de telles caractéristiques, mais leurs patrons de développement ainsi que leur capacité de résistance à la sécheresse sont peu connues. Les comportements de boutures de ces deux espÚces ont été comparés avec celle d'un saule dans trois expérimentations en laboratoire et sur le terrain. On a ainsi pu déterminer les dynamiques de croissance des parties aériennes et souterraines en fonction de différentes modalités de sécheresse. Myricaria germanica et Tamarix gallica montrent généralement un taux de reprise au bouturage élevé et proche de celui du saule, et un investissement dans les racines supérieur en cas de sécheresse, Cependant la croissance de Myricaria germanica apparaßt nettement plus faible que celles des deux autres espÚces Ces deux espÚces présentent un fort potentiel pour le génie végétal, notamment dans un contexte de changement climatique. Elles s'adaptent bien aux conditions de sÚcheresse et disposent de bonnes capacités de stabilisation. Leur utilisation dans ce domaine devrait contribuer à augmenter la biodiversité des ouvrages
NGC 4138 - A Case Study in Counterrotating Disk Formation
The Sa(r) galaxy NGC 4138 has been recently found to contain an extensive
counterrotating disk which appears to be still forming. Up to a third of the
stars in the disk system may be on retrograde orbits. A counterrotating ring of
H II regions, along with extended counterrotating H I gas, suggests that the
retrograde material has been recently acquired in the gas phase and is still
trickling in. Using numerical simulations, we have attempted to model the
process by which the counterrotating mass has been accreted by this galaxy. We
investigate two possibilities: continuous retrograde infall of gas, and a
retrograde merger with a gas-rich dwarf galaxy. Both processes are successful
in producing a counterrotating disk of the observed mass and dimensions without
heating up the primary significantly. Contrary to our experience with a
fiducial cold, thin primary disk, the gas-rich merger works well for the
massive, compact primary disk of NGC 4138 even though the mass of the dwarf
galaxy is a significant fraction of the mass of the primary disk. Although we
have restricted ourselves mainly to coplanar infall and mergers, we report on
one inclined infall simulation as well. We also explore the possibility that
the H-alpha ring seen in the inner half of the disk is a consequence of
counterrotating gas clouds colliding with corotating gas already present in the
disk and forming stars in the process.Comment: To appear in ApJ, 21 pages, LaTeX (aaspp4) format, 17 figs (gzipped
tar file) also available at ftp://bessel.mps.ohio-state.edu/pub/thakar/cr2/
or at http://www-astronomy.mps.ohio-state.edu/~thakar
Hyperacceleration in a stochastic Fermi-Ulam model
Fermi acceleration in a Fermi-Ulam model, consisting of an ensemble of
particles bouncing between two, infinitely heavy, stochastically oscillating
hard walls, is investigated. It is shown that the widely used approximation,
neglecting the displacement of the walls (static wall approximation), leads to
a systematic underestimation of particle acceleration. An improved
approximative map is introduced, which takes into account the effect of the
wall displacement, and in addition allows the analytical estimation of the long
term behavior of the particle mean velocity as well as the corresponding
probability distribution, in complete agreement with the numerical results of
the exact dynamics. This effect accounting for the increased particle
acceleration -Fermi hyperacceleration- is also present in higher dimensional
systems, such as the driven Lorentz gas.Comment: 4 pages, 3 figures. To be published in Phys. Rev. Let
Detection of arcs in Saturn's F ring during the 1995 Sun ring-plane crossing
Observations of the November 1995 Sun crossing of the Saturn's ring-plane
made with the 3.6m CFH telescope, using the UHAO adaptive optics system, are
presented here. We report the detection of four arcs located in the vicinity of
the F ring. They can be seen one day later in HST images. The combination of
both data sets gives accurate determinations of their orbits. Semi-major axes
range from 140020 km to 140080 km, with a mean of 140060 +- 60 km. This is
about 150 km smaller than previous estimates of the F ring radius from Voyager
1 and 2 data, but close to the orbit of another arc observed at the same epoch
in HST images.Comment: 8 pages, 3 figures, 1 table, To appear in A&A, for comments :
[email protected]
Integration of Lie 2-algebras and their morphisms
Given a strict Lie 2-algebra, we can integrate it to a strict Lie 2-group by
integrating the corresponding Lie algebra crossed module. On the other hand,
the integration procedure of Getzler and Henriques will also produce a 2-group.
In this paper, we show that these two integration results are Morita
equivalent. As an application, we integrate a non-strict morphism between Lie
algebra crossed modules to a generalized morphism between their corresponding
Lie group crossed modules.Comment: 19 pages, Lett. Math. Phys. 102 (2), (2012.11), 223-24
Dynamics of some piecewise smooth Fermi-Ulam Models
We find a normal form which describes the high energy dynamics of a class of
piecewise smooth Fermi-Ulam ping pong models; depending on the value of a
single real parameter, the dynamics can be either hyperbolic or elliptic. In
the first case we prove that the set of orbits undergoing Fermi acceleration
has zero measure but full Hausdorff dimension. We also show that for almost
every orbit the energy eventually falls below a fixed threshold. In the second
case we prove that, generically, we have stable periodic orbits for arbitrarily
high energies, and that the set of Fermi accelerating orbits may have infinite
measure.Comment: 22 pages, 4 figure
Free particle scattering off two oscillating disks
We investigate the two-dimensional classical dynamics of the scattering of
point particles by two periodically oscillating disks. The dynamics exhibits
regular and chaotic scattering properties, as a function of the initial
conditions and parameter values of the system. The energy is not conserved
since the particles can gain and loose energy from the collisions with the
disks. We find that for incident particles whose velocity is on the order of
the oscillating disk velocity, the energy of the exiting particles displays
non-monotonic gaps of allowed energies, and the distribution of exiting
particle velocities shows significant fluctuations in the low energy regime. We
also considered the case when the initial velocity distribution is Gaussian,
and found that for high energies the exit velocity distribution is Gaussian
with the same mean and variance. When the initial particle velocities are in
the irregular regime the exit velocity distribution is Gaussian but with a
smaller mean and variance. The latter result can be understood as an example of
stochastic cooling. In the intermediate regime the exit velocity distribution
differs significantly from Gaussian. A comparison of the results presented in
this paper to previous chaotic static scattering problems is also discussed.Comment: 9 doble sided pages 13 Postscript figures, REVTEX style. To appear in
Phys. Rev.
-body Simulation of Planetesimal Formation Through Gravitational Instability of a Dust Layer
We performed N-body simulations of a dust layer without a gas component and
examined the formation process of planetesimals. We found that the formation
process of planetesimals can be divided into three stages: the formation of
non-axisymmetric wake-like structures, the creation of aggregates, and the
collisional growth of the aggregates. Finally, a few large aggregates and many
small aggregates are formed. The mass of the largest aggregate is larger than
the mass predicted by the linear perturbation theory. We examined the
dependence of system parameters on the planetesimal formation. We found that
the mass of the largest aggregates increase as the size of the computational
domain increases. However the ratio of the aggregate mass to the total mass
is almost constant . The mass of
the largest aggregate increases with the optical depth and the Hill radius of
particles.Comment: 34 pages, 11 figures. Accepted for publication in Ap
Valoriser et conserver le pin de Salzmann en France -
Cet article fait le point sur l'Ă©tat des connaissances disponibles sur le pin de Salzmann, dans les domaines Ă©cologique, sylvicole et de diversitĂ© gĂ©nĂ©tique. Il fait aussi Ă©tat des risques qui menacent cette ressource française originale, caractĂ©ristique de la forĂȘt mĂ©diterranĂ©enne et indique quelles sont les stratĂ©gies actuelles employĂ©es pour sauvegarder et utiliser durablement les populations naturelles
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