98 research outputs found
Instabilities and stickiness in a 3D rotating galactic potential
We study the dynamics in the neighborhood of simple and double unstable
periodic orbits in a rotating 3D autonomous Hamiltonian system of galactic
type. In order to visualize the four dimensional spaces of section we use the
method of color and rotation. We investigate the structure of the invariant
manifolds that we found in the neighborhood of simple and double unstable
periodic orbits in the 4D spaces of section. We consider orbits in the
neighborhood of the families x1v2, belonging to the x1 tree, and the z-axis
(the rotational axis of our system). Close to the transition points from
stability to simple instability, in the neighborhood of the bifurcated simple
unstable x1v2 periodic orbits we encounter the phenomenon of stickiness as the
asymptotic curves of the unstable manifold surround regions of the phase space
occupied by rotational tori existing in the region. For larger energies, away
from the bifurcating point, the consequents of the chaotic orbits form clouds
of points with mixing of color in their 4D representations. In the case of
double instability, close to x1v2 orbits, we find clouds of points in the four
dimensional spaces of section. However, in some cases of double unstable
periodic orbits belonging to the z-axis family we can visualize the associated
unstable eigensurface. Chaotic orbits close to the periodic orbit remain sticky
to this surface for long times (of the order of a Hubble time or more). Among
the orbits we studied we found those close to the double unstable orbits of the
x1v2 family having the largest diffusion speed.Comment: 29pages, 25 figures, accepted for publication in the International
Journal of Bifurcation and Chao
Structures induced by companions in galactic discs
Using N-body simulations we study the structures induced on a galactic disc
by repeated flybys of a companion in decaying eccentric orbit around the disc.
Our system is composed by a stellar disc, bulge and live dark matter halo, and
we study the system's dynamical response to a sequence of a companion's flybys,
when we vary i) the disc's temperature (parameterized by Toomre's Q-parameter)
and ii) the companion's mass and initial orbit. We use a new 3D Cartesian grid
code: MAIN (Mesh-adaptive Approximate Inverse N-body solver). The main features
of MAIN are reviewed, with emphasis on the use of a new Symmetric Factored
Approximate Sparse Inverse (SFASI) matrix in conjunction with the multigrid
method that allows the efficient solution of Poisson's equation in three space
variables. We find that: i) companions need to be assigned initial masses in a
rather narrow window of values in order to produce significant and more
long-standing non-axisymmetric structures (bars and spirals) in the main
galaxy's disc by the repeated flyby mechanism. ii) a crucial phenomenon is the
antagonism between companion-excited and self-excited modes on the disc. Values
of are needed in order to allow for the growth of the
companion-excited modes to prevail over the the growth of the disc's
self-excited modes. iii) We give evidence that the companion-induced spiral
structure is best represented by a density wave with pattern speed nearly
constant in a region extending from the ILR to a radius close to, but inside,
corotation.Comment: Published in MNRA
Quantitative measures of competitiveness: theoretical derivations and application on Greek exports of agricultural produce
Determinants of cheese consumption in European Union: a meta-analysis of consumer behaviour bibliography
The structure of invariant tori in a 3D galactic potential
We study in detail the structure of phase space in the neighborhood of stable
periodic orbits in a rotating 3D potential of galactic type. We have used the
color and rotation method to investigate the properties of the invariant tori
in the 4D spaces of section. We compare our results with those of previous
works and we describe the morphology of the rotational, as well as of the tube
tori in the 4D space. We find sticky chaotic orbits in the immediate
neighborhood of sets of invariant tori surrounding 3D stable periodic orbits.
Particularly useful for galactic dynamics is the behavior of chaotic orbits
trapped for long time between 4D invariant tori. We find that they support
during this time the same structure as the quasi-periodic orbits around the
stable periodic orbits, contributing however to a local increase of the
dispersion of velocities. Finally we find that the tube tori do not appear in
the 3D projections of the spaces of section in the axisymmetric Hamiltonian we
examined.Comment: 26 pages, 34 figures, accepted for publication in the International
Journal of Bifurcation and Chao
Asymptotic Orbits in Barred Spiral Galaxies
We study the formation of the spiral structure of barred spiral galaxies,
using an -body model. The evolution of this -body model in the adiabatic
approximation maintains a strong spiral pattern for more than 10 bar rotations.
We find that this longevity of the spiral arms is mainly due to the phenomenon
of stickiness of chaotic orbits close to the unstable asymptotic manifolds
originated from the main unstable periodic orbits, both inside and outside
corotation. The stickiness along the manifolds corresponding to different
energy levels supports parts of the spiral structure. The loci of the disc
velocity minima (where the particles spend most of their time, in the
configuration space) reveal the density maxima and therefore the main
morphological structures of the system. We study the relation of these loci
with those of the apocentres and pericentres at different energy levels. The
diffusion of the sticky chaotic orbits outwards is slow and depends on the
initial conditions and the corresponding Jacobi constant.Comment: 17 pages, 24 figure
Invariant manifolds and the response of spiral arms in barred galaxies
The unstable invariant manifolds of the short-period family of periodic
orbits around the unstable Lagrangian points and of a barred galaxy
define loci in the configuration space which take the form of a trailing spiral
pattern. In the present paper we investigate this association in the case of
the self-consistent models of Kaufmann & Contopoulos (1996) which provide an
approximation of real barred-spiral galaxies. We also examine the relation of
`response' models of barred-spiral galaxies with the theory of the invariant
manifolds. Our main results are the following: The invariant manifolds yield
the correct form of the imposed spiral pattern provided that their calculation
is done with the spiral potential term turned on. We provide a theoretical
model explaining the form of the invariant manifolds that supports the spiral
structure. The azimuthal displacement of the Lagrangian points with respect to
the bar's major axis is a crucial parameter in this modeling. When this is
taken into account, the manifolds necessarily develop in a spiral-like domain
of the configuration space, delimited from below by the boundary of a
banana-like non-permitted domain, and from above either by rotational KAM tori
or by cantori forming a stickiness zone. We construct `spiral response' models
on the basis of the theory of the invariant manifolds and examine the
connection of the latter to the `response' models (Patsis 2006) used to fit
real barred-spiral galaxies, explaining how are the manifolds related to a
number of morphological features seen in such models.Comment: 16 Page
The structure and evolution of confined tori near a Hamiltonian Hopf Bifurcation
We study the orbital behavior at the neighborhood of complex unstable
periodic orbits in a 3D autonomous Hamiltonian system of galactic type. At a
transition of a family of periodic orbits from stability to complex instability
(also known as Hamiltonian Hopf Bifurcation) the four eigenvalues of the stable
periodic orbits move out of the unit circle. Then the periodic orbits become
complex unstable. In this paper we first integrate initial conditions close to
the ones of a complex unstable periodic orbit, which is close to the transition
point. Then, we plot the consequents of the corresponding orbit in a 4D surface
of section. To visualize this surface of section we use the method of color and
rotation [Patsis and Zachilas 1994]. We find that the consequents are contained
in 2D "confined tori". Then, we investigate the structure of the phase space in
the neighborhood of complex unstable periodic orbits, which are further away
from the transition point. In these cases we observe clouds of points in the 4D
surfaces of section. The transition between the two types of orbital behavior
is abrupt.Comment: 10 pages, 14 figures, accepted for publication in the International
Journal of Bifurcation and Chao
Outer Regions of the Milky Way
With the start of the Gaia era, the time has come to address the major
challenge of deriving the star formation history and evolution of the disk of
our MilkyWay. Here we review our present knowledge of the outer regions of the
Milky Way disk population. Its stellar content, its structure and its dynamical
and chemical evolution are summarized, focussing on our lack of understanding
both from an observational and a theoretical viewpoint. We describe the
unprecedented data that Gaia and the upcoming ground-based spectroscopic
surveys will provide in the next decade. More in detail, we quantify the expect
accuracy in position, velocity and astrophysical parameters of some of the key
tracers of the stellar populations in the outer Galactic disk. Some insights on
the future capability of these surveys to answer crucial and fundamental issues
are discussed, such as the mechanisms driving the spiral arms and the warp
formation. Our Galaxy, theMilkyWay, is our cosmological laboratory for
understanding the process of formation and evolution of disk galaxies. What we
learn in the next decades will be naturally transferred to the extragalactic
domain.Comment: 22 pages, 10 figures, Invited review, Book chapter in "Outskirts of
Galaxies", Eds. J. H. Knapen, J. C. Lee and A. Gil de Paz, Astrophysics and
Space Science Library, Springer, in pres
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