12,668 research outputs found
Existence of periodic orbits near heteroclinic connections
We consider a potential with two different global minima
and, under a symmetry assumption, we use a variational approach to
show that the Hamiltonian system \begin{equation} \ddot{u}=W_u(u), \hskip 2cm
(1) \end{equation} has a family of -periodic solutions which, along a
sequence , converges locally to a heteroclinic solution
that connects to . We then focus on the elliptic system
\begin{equation} \Delta u=W_u(u),\;\; u:R^2\rightarrow R^m, \hskip 2cm (2)
\end{equation} that we interpret as an infinite dimensional analogous of (1),
where plays the role of time and is replaced by the action functional
We assume that
has two different global minimizers in the set of maps that connect to . We work in a symmetric
context and prove, via a minimization procedure, that (2) has a family of
solutions , which is -periodic in , converges to
as and, along a sequence
, converges locally to a heteroclinic solution that
connects to .Comment: 36 pages, 4 figure
Geometry of flux attachment in anisotropic fractional quantum Hall states
Fractional quantum Hall (FQH) states are known to possess an internal metric
degree of freedom that allows them to minimize their energy when contrasting
geometries are present in the problem (e.g., electron band mass and dielectric
tensor). We investigate the internal metric of several incompressible FQH
states by probing its response to band mass anisotropy using infinite DMRG
simulations on a cylinder geometry. We test and apply a method to extract the
internal metric of a FQH state from its guiding center structure factor. We
find that the response to band mass anisotropy is approximately the same for
states in the same Jain sequence, but changes substantially between different
sequences. We provide a theoretical explanation of the observed behavior of
primary states at filling in terms of a minimal microscopic model
of flux attachment.Comment: 12 pages including references, 14 figure
Quantifying stellar radial migration in an N-body simulation: blurring, churning, and the outer regions of galaxy discs
Radial stellar migration in galactic discs has received much attention in
studies of galactic dynamics and chemical evolution, but remains a dynamical
phenomenon that needs to be fully quantified. In this work, using a Tree-SPH
simulation of an Sb-type disc galaxy, we quantify the effects of blurring
(epicyclic excursions) and churning (change of guiding radius). We quantify
migration (either blurring or churning) both in terms of flux (the number of
migrators passing at a given radius), and by estimating the population of
migrators at a given radius at the end of the simulation compared to
non-migrators, but also by giving the distance over which the migration is
effective at all radii. We confirm that the corotation of the bar is the main
source of migrators by churning in a bar-dominated galaxy, its intensity being
directly linked to the episode of a strong bar, in the first 1-3 Gyr of the
simulation. We show that within the outer Lindblad resonance (OLR), migration
is strongly dominated by churning, while blurring gains progressively more
importance towards the outer disc and at later times. Most importantly, we show
that the OLR limits the exchange of angular momentum, separating the disc in
two distinct parts with minimal or null exchange, except in the transition
zone, which is delimited by the position of the OLR at the epoch of the
formation of the bar, and at the final epoch. We discuss the consequences of
these findings for our understanding of the structure of the Milky Way disc.
Because the Sun is situated slightly outside the OLR, we suggest that the solar
vicinity may have experienced very limited churning from the inner disc.Comment: Accepted for publication in Astronomy and Astrophysics (acceptance
date: 27/04/15), 24 pages, 24 figure
On the Virtual Element Method for Topology Optimization on polygonal meshes: a numerical study
It is well known that the solution of topology optimization problems may be
affected both by the geometric properties of the computational mesh, which can
steer the minimization process towards local (and non-physical) minima, and by
the accuracy of the method employed to discretize the underlying differential
problem, which may not be able to correctly capture the physics of the problem.
In light of the above remarks, in this paper we consider polygonal meshes and
employ the virtual element method (VEM) to solve two classes of paradigmatic
topology optimization problems, one governed by nearly-incompressible and
compressible linear elasticity and the other by Stokes equations. Several
numerical results show the virtues of our polygonal VEM based approach with
respect to more standard methods
Star formation efficiency in galaxy interactions and mergers: a statistical study
We investigate the enhancement of star formation efficiency in galaxy
interactions and mergers, by numerical simulations of several hundred galaxy
collisions. All morphological types along the Hubble sequence are considered in
the initial conditions of the two colliding galaxies, with varying
bulge-to-disk ratios and gas mass fractions. Different types of orbits are
simulated, direct and retrograde, according to the initial relative energy and
impact parameter, and the resulting star formation history is compared to that
occuring in the two galaxies when they are isolated. Our principal results are:
(1) retrograde encounters have a larger star formation efficiency (SFE) than
direct encounters; (2) the amount of gas available in the galaxy is not the
main parameter governing the SFE in the burst phase; (3) there is an
anticorrelation between the amplitude of the star forming burst and the tidal
forces exerted per unit of time, which is due to the large amount of gas
dragged outside the galaxy by tidal tails in strong interactions; (4) globally,
the Kennicutt-Schmidt law is retrieved statistically for isolated galaxies,
interacting pairs and mergers; (5) the enhanced star formation is essentially
occurring in nuclear starbursts, triggered by inward gas flows driven by
non-axisymmetries in the galaxy disks. Direct encounters develop more
pronounced asymmetries than retrograde ones. Based on these statistical
results, we derive general laws for the enhancement of star formation in galaxy
interactions and mergers, as a function of the main parameters of the
encounter.Comment: 22 pages, 37 figures, 4 tables. Accepted on Astronomy & Astrophysic
Multi-asset minority games
We study analytically and numerically Minority Games in which agents may invest in different assets (or markets), considering both the canonical and the grand-canonical versions. We find that the likelihood of agents trading in a given asset depends on the relative amount of information available in that market. More specifically, in the canonical game players play preferentially in the stock with less information. The same holds in the grand canonical game when agents have positive incentives to trade, whereas when agents payoff are solely related to their speculative ability they display a larger propensity to invest in the information-rich asset. Furthermore, in this model one finds a globally predictable phase with broken ergodicity
Effects of diversification among assets in an agent-based market model
We extend to the multi-asset case the framework of a discrete time model of a
single asset financial market developed in Ghoulmie et al (2005). In
particular, we focus on adaptive agents with threshold behavior allocating
their resources among two assets. We explore numerically the effect of this
diversification as an additional source of complexity in the financial market
and we discuss its destabilizing role. We also point out the relevance of these
studies for financial decision making.Comment: 12 pages, 5 figures, accepted for publication in the Proceedings of
the Complex Systems II Conference at the Australian National University, 4-7
December 2007, Canberra, ACT Australi
Quantum Tomography
This is the draft version of a review paper which is going to appear in
"Advances in Imaging and Electron Physics"Comment: To appear in "Advances in Imaging and Electron Physics". Some figs
with low resolutio
Old stellar counter-rotating components in early-type galaxies from elliptical-spiral mergers
We investigate, by means of numerical simulations, the possibility of forming
counter-rotating old stellar components by major mergers between an elliptical
and a spiral galaxy. We show that counter-rotation can appear both in
dissipative and dissipationless retrograde mergers, and it is mostly associated
to the presence of a disk component, which preserves part of its initial spin.
In turn, the external regions of the two interacting galaxies acquire part of
the orbital angular momentum, due to the action of tidal forces exerted on each
galaxy by the companion.Comment: 6 pages, 15 figures. Accepted on Astronomy & Astrophysic
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