64,589 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
Child health in rural Colombia: determinants and policy interventions
In this paper we study the determinants of child anthropometrics on a sample of poor Colombian
children living in small municipalities. We focus on the influence of household consumption, and
public infrastructure. We take into account the endogeneity of household consumption using two
different sets of instruments: household assets and municipality average wage. We find that household
consumption is an important determinant of child health. The importance of the effect is confirmed by
the two different sets of instruments. We find that using ordinary least squares would lead to conclude
that the importance of household consumption is much smaller than the instrumental variable
estimates suggest. The presence of a public hospital in the municipality positively influences child
health. The extent of the piped water network positively influences the health of children if their
parents have at least some education. The number of hours of growth and development check-ups is
also an important determinant of child health. We find that some of these results only show up once
squared and interaction terms have been included in the regression. Overall, our estimates suggest that
both public and private investments are important to improve child health in poor environments
Soft masses in superstring models with anomalous U(1) symmetries
We analyze the general structure of soft scalar masses emerging in
superstring models involving anomalous U(1) symmetries, with the aim of
characterizing more systematically the circumstances under which they can
happen to be flavor universal. We consider both heterotic orbifold and
intersecting brane models, possibly with several anomalous and non-anomalous
spontaneously broken U(1) symmetries. The hidden sector is assumed to consist
of the universal dilaton, Kahler class and complex structure moduli, which are
supposed to break supersymmetry, and a minimal set of Higgs fields which
compensate the Fayet-Iliopoulos terms. We leave the superpotential that is
supposed to stabilize the hidden sector fields unspecified, but we carefully
take into account the relations implied by gauge invariance and the constraints
required for the existence of a metastable vacuum with vanishing cosmological
constant. The results are parametrized in terms of a constrained Goldstino
direction, suitably defined effective modular weights, and the U(1) charges and
shifts. We show that the effect induced by vector multiplets strongly depends
on the functional form of the Kahler potential for the Higgs fields. We find in
particular that whenever these are charged matter fields, like in heterotic
models, the effect is non-trivial, whereas when they are shifting moduli
fields, like in certain intersecting brane models, the effect may vanish.Comment: 35 pages, LaTe
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
Sizes of Confirmed Globular Clusters in NGC 5128: A Wide-Field High-Resolution Study
Using Magellan/IMACS images covering a 1.2 x 1.2 sq. degree FOV with seeing
of 0.4"-0.6", we have applied convolution techniques to analyse the light
distribution of 364 confirmed globular cluster in the field of NGC 5128 and to
obtain their structural parameters. Combining these parameters with existing
Washington photometry from Harris et al. (2004), we are able to examine the
size difference between metal-poor (blue) and metal-rich (red) globular
clusters. For the first time, this can be addressed on a sample of confirmed
clusters that extends to galactocentric distances about 8 times the effective
radius, R, of the galaxy. Within 1 R, red clusters are about
30% smaller on average than blue clusters, in agreement with the vast majority
of extragalactic globular cluster systems studied. As the galactocentric
distance increases, however, this difference becomes negligible. Thus, our
results indicate that the difference in the clusters' effective radii, r,
could be explained purely by projection effects, with red clusters being more
centrally concentrated than blue ones and an intrinsic r--R
dependence, like the one observed for the Galaxy.Comment: 4 figures, accepted for publication in ApJ
Perturbative evolution of far off-resonance driven two-level systems: Coherent population trapping, localization, and harmonic generation
The time evolution of driven two-level systems in the far off-resonance
regime is studied analytically. We obtain a general first-order perturbative
expression for the time-dependent density operator which is applicable
regardless of the coupling strength value. In the strong field regime, our
perturbative expansion remains valid even when the far off-resonance condition
is not fulfilled. We find that, in the absence of dissipation, driven two-level
systems exhibit coherent population trapping in a certain region of parameter
space, a property which, in the particular case of a symmetric double-well
potential, implies the well-known localization of the system in one of the two
wells. Finally, we show how the high-order harmonic generation that this kind
of systems display can be obtained as a straightforward application of our
formulation.Comment: 14 pages, LaTeX, 2 figures, acknowledgments adde
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