11,628 research outputs found
The formation of spiral arms and rings in barred galaxies
We propose a theory to explain the formation of both spirals and rings in
barred galaxies using a common dynamical framework. It is based on the orbital
motion driven by the unstable equilibrium points of the rotating bar potential.
Thus, spirals, rings and pseudo-rings are related to the invariant manifolds
associated to the periodic orbits around these equilibrium points. We examine
the parameter space of three barred galaxy models and discuss the formation of
the different morphological structures according to the properties of the bar
model. We also study the influence of the shape of the rotation curve in the
outer parts, by making families of models with rising, flat or falling rotation
curves in the outer parts. The differences between spiral and ringed structures
arise from differences in the dynamical parameters of the host galaxies.Comment: 6 pages, 3 figures, conference proceedings of "Semaine de
l'Astrophysique Francaise", Grenoble 2007, eds. J. Bouvier, A. Chalabaev, C.
Charbonne
The formation of spiral arms and rings in barred galaxies
In this and in a previous paper (Romero-Gomez et al. 2006) we propose a
theory to explain the formation of both spirals and rings in barred galaxies
using a common dynamical framework. It is based on the orbital motion driven by
the unstable equilibrium points of the rotating bar potential. Thus, spirals,
rings and pseudo-rings are related to the invariant manifolds associated to the
periodic orbits around these equilibrium points. We examine the parameter space
of three barred galaxy models and discuss the formation of the different
morphological structures according to the properties of the bar model. We also
study the influence of the shape of the rotation curve in the outer parts, by
making families of models with rising, flat, or falling rotation curves in the
outer parts. The differences between spiral and ringed structures arise from
differences in the dynamical parameters of the host galaxies. The results
presented here will be discussed and compared with observations in a
forthcoming paper.Comment: 16 pages, 13 figures, accepted in A&A. High resolution version
available at http://www.oamp.fr/dynamique/pap/merce.htm
Pattern-induced anchoring transitions in nematic liquid crystals
In this paper we revisit the problem of a nematic liquid crystal in contact
with patterned substrates. The substrate is modelled as a periodic array of
parallel infinite grooves of well-defined cross section sculpted on a
chemically homogeneous substrate which favors local homeotropic anchoring of
the nematic. We consider three cases: a sawtooth, a crenellated and a
sinusoidal substrate. We analyse this problem within the modified Frank-Oseen
formalism. We argue that, for substrate periodicities much larger than the
extrapolation length, the existence of different nematic textures with distinct
far-field orientations, as well as the anchoring transitions between them, are
associated with the presence of topological defects either on or close to the
substrate. For the sawtooth and sinusoidal case, we observe a homeotropic to
planar anchoring transition as the substrate roughness is increased. On the
other hand, a homeotropic to oblique anchoring transition is observed for
crenellated substrates. In this case, the anchoring phase diagram shows a
complex dependence on the substrate roughness and substrate anchoring strength.Comment: 36 pages, 15 figures, revised version submitted to Journal of
Physics: Condensed Matte
On the origin of rR_1 ring structures in barred galaxies
We propose a new theory for the formation of rR_1 ring structures, i.e. for
ring structures with both an inner and an outer ring, the latter having the
form of ``8''. We propose that these rings are formed by material from the
stable and unstable invariant manifolds associated with the Lyapunov orbits
around the equilibrium points of a barred galaxy. We discuss the shape and
velocity structure of the rings thus formed and argue that they are in
agreement with the observed properties of rR_1 structures.Comment: 8 pages, 9 figures, accepted in Astronomy and Astrophysics. High
quality figures are available upon reques
Invariant manifolds as building blocks for the formation of spiral arms and rings in barred galaxies
We propose a theory to explain the formation of spiral arms and of all types
of outer rings in barred galaxies, extending and applying the technique used in
celestial mechanics to compute transfer orbits. Thus, our theory is based on
the chaotic orbital motion driven by the invariant manifolds associated to the
periodic orbits around the hyperbolic equilibrium points. In particular, spiral
arms and outer rings are related to the presence of heteroclinic or homoclinic
orbits. Thus, R1 rings are associated to the presence of heteroclinic orbits,
while R1R2 rings are associated to the presence of homoclinic orbits. Spiral
arms and R2 rings, however, appear when there exist neither heteroclinic nor
homoclinic orbits. We examine the parameter space of three realistic, yet
simple, barred galaxy models and discuss the formation of the different
morphologies according to the properties of the galaxy model. The different
morphologies arise from differences in the dynamical parameters of the galaxy.Comment: 8 pages, 4 figures, in the proceedings of the conference: "Chaos in
Astronomy", Athens, September 2007, G. Contopoulos and P.A. Patsis (eds), to
be published by Springe
On the embedding of spacetime in five-dimensional Weyl spaces
We revisit Weyl geometry in the context of recent higher-dimensional theories
of spacetime. After introducing the Weyl theory in a modern geometrical
language we present some results that represent extensions of Riemannian
theorems. We consider the theory of local embeddings and submanifolds in the
context of Weyl geometries and show how a Riemannian spacetime may be locally
and isometrically embedded in a Weyl bulk. We discuss the problem of classical
confinement and the stability of motion of particles and photons in the
neighbourhood of branes for the case when the Weyl bulk has the geometry of a
warped product space. We show how the confinement and stability properties of
geodesics near the brane may be affected by the Weyl field. We construct a
classical analogue of quantum confinement inspired in theoretical-field models
by considering a Weyl scalar field which depends only on the extra coordinate.Comment: 16 pages, new title and references adde
Discovery of Infection Associated Metabolic Markers in Human African Trypanosomiasis
Peer reviewedPublisher PD
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
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