199 research outputs found
On the trapping of stars by a newborn stellar supercluster
Numerical experiments conducted by Fellhauer et al. (MNRAS, 372, 338, 2006)
suggest that a supercluster may capture up to about 40 per cent of its mass
from the galaxy where it belongs. Nevertheless, in those experiments the
cluster was created making appear its mass out of nothing, rather than from
mass already present in the galaxy. Here we use a thought experiment, plus a
few simple computations, to show that the difference between the dynamical
effects of these two scenarios (i.e., mass creation vs. mass concentration) is
actually very important. We also present the results of new numerical
experiments, simulating the formation of the cluster through mass
concentration, that show that trapping depends critically on the process of
cluster formation and that the amounts of gained mass are substantially smaller
than those obtained from mass creation.Comment: 6 pages, 3 figures. Submitted to MNRA
Models of cuspy triaxial stellar systems. III: The effect of velocity anisotropy on chaoticity
In several previous investigations we presented models of triaxial stellar
systems, both cuspy and non cuspy, that were highly stable and harboured large
fractions of chaotic orbits. All our models had been obtained through cold
collapses of initially spherical --body systems, a method that necessarily
results in models with strongly radial velocity distributions. Here we
investigate a different method that was reported to yield cuspy triaxial models
with virtually no chaos. We show that such result was probably due to the use
of an inadequate chaos detection technique and that, in fact, models with
significant fractions of chaotic orbits result also from that method. Besides,
starting with one of the models from the first paper in this series, we
obtained three different models by rendering its velocity distribution much
less radially biased (i.e., more isotropic) and by modifying its axial ratios
through adiabatic compression. All three models yielded much higher fractions
of regular orbits than most of those from our previous work. We conclude that
it is possible to obtain stable cuspy triaxial models of stellar systems whose
velocity distribution is more isotropic than that of the models obtained from
cold collapses. Those models still harbour large fractions of chaotic orbits
and, although it is difficult to compare the results from different models, we
can tentatively conclude that chaoticity is reduced by velocity isotropy.Comment: 11 pages, 14 figures. Accepted for publication in MNRA
Models of cuspy triaxial stellar systems. II. Regular orbits
In the first paper of this series we used the N--body method to build a dozen
cuspy (gamma ~ 1) triaxial models of stellar systems, and we showed that they
were highly stable over time intervals of the order of a Hubble time, even
though they had very large fractions of chaotic orbits (more than 85 per cent
in some cases). The models were grouped in four sets, each one comprising
models morphologically resembling E2, E3, E4 and E5 galaxies, respectively. The
three models within each set, although different, had the same global
properties and were statistically equivalent. In the present paper we use
frequency analysis to classify the regular orbits of those models. The bulk of
those orbits are short axis tubes (SATs), with a significant fraction of long
axis tubes (LATs) in the E2 models that decreases in the E3 and E4 models to
become negligibly small in the E5 models. Most of the LATs in the E2 and E3
models are outer LATs, but the situation reverses in the E4 and E5 models where
the few LATs are mainly inner LATs. As could be expected for cuspy models, most
of the boxes are resonant orbits, i.e., boxlets. Nevertheless, only the (x, y)
fishes of models E3 and E4 amount to about 10 per cent of the regular orbits,
with most of the fractions of the other boxlets being of the order of 1 per
cent or less.Comment: Accepted for publication in the Monthly Notices of the Royal
Astronomical Societ
On the correct computation of all Lyapunov exponents in Hamiltonian dynamical systems
The Lyapunov Characteristic Exponents are a useful indicator of chaos in
astronomical dynamical systems. They are usually computed through a standard,
very efficient and neat algorithm published in 1980. However, for Hamiltonian
systems the expected result of pairs of opposite exponents is not always
obtained with enough precision. We find here why in these cases the initial
order of the deviation vectors matters, and how to sort them in order to obtain
a correct result.Comment: 8 pages, 3 figure
Precision Measurements of Stretching and Compression in Fluid Mixing
The mixing of an impurity into a flowing fluid is an important process in
many areas of science, including geophysical processes, chemical reactors, and
microfluidic devices. In some cases, for example periodic flows, the concepts
of nonlinear dynamics provide a deep theoretical basis for understanding
mixing. Unfortunately, the building blocks of this theory, i.e. the fixed
points and invariant manifolds of the associated Poincare map, have remained
inaccessible to direct experimental study, thus limiting the insight that could
be obtained. Using precision measurements of tracer particle trajectories in a
two-dimensional fluid flow producing chaotic mixing, we directly measure the
time-dependent stretching and compression fields. These quantities, previously
available only numerically, attain local maxima along lines coinciding with the
stable and unstable manifolds, thus revealing the dynamical structures that
control mixing. Contours or level sets of a passive impurity field are found to
be aligned parallel to the lines of large compression (unstable manifolds) at
each instant. This connection appears to persist as the onset of turbulence is
approached.Comment: 5 pages, 5 figure
Enhanced antiadhesive properties of chitosan/hyaluronic acid polyelectrolyte multilayers driven by thermal annealing: Low adherence for mammalian cells and selective decrease in adhesion for Gram-positive bacteria
The development of antifouling coatings with restricted cell and bacteria adherence is fundamental for many biomedical applications. A strategy for the fabrication of antifouling coatings based on the layer-by-layer assembly and thermal annealing is presented. Polyelectrolyte multilayers (PEMs) assembled from chitosan and hyaluronic acid were thermally annealed in an oven at 37 degrees C for 72 h. The effect of annealing on the PEM properties and topography was studied by atomic force microscopy, zeta-potential, circular dichroism and contact angle measurements. Cell adherence on PEMs before and after annealing was evaluated by measuring the cell spreading area and aspect ratio for the A549 epithelial, BHK kidney fibroblast, C2C12 myoblast and MC-3T3-E1 osteoblast cell lines. Chitosan/hyaluronic acid PEMs show a low cell adherence that decreases with the thermal annealing, as observed from the reduction in the average cell spreading area and more rounded cell morphology. The adhesion of S. aureus (Gram-positive) and E. coil Gram-negative) bacteria strains was quantified by optical microscopy,. counting the number of colony-forming units and measuring the light scattering of bacteria suspension after detachment from the PEM surface. A 20% decrease in bacteria adhesion was selectively observed in the S. aureus strain after annealing. The changes in mammalian cell and bacteria adhesion correlate with the changes in topography of the chitosan/hyaluronic PEMs from a rough fibrillar 3D structure to a smoother and planar surface after thermal annealing. (C) 2017 Published by Elsevier B.V
Large-scale study of the NGC 1399 globular cluster system in Fornax
We present a Washington C and Kron-Cousins R photometric study of the
globular cluster system of NGC 1399, the central galaxy of the Fornax cluster.
A large areal coverage of 1 square degree around NGC 1399 is achieved with
three adjoining fields of the MOSAIC II Imager at the CTIO 4-m telescope.
Working on such a large field, we can perform the first indicative
determination of the total size of the NGC 1399 globular cluster system. The
estimated angular extent, measured from the NGC 1399 centre and up to a
limiting radius where the areal density of blue globular clusters falls to 30
per cent of the background level, is 45 +/- 5 arcmin, which corresponds to 220
- 275 kpc at the Fornax distance. The bimodal colour distribution of this
globular cluster system, as well as the different radial distribution of blue
and red clusters, up to these large distances from the parent galaxy, are
confirmed. The azimuthal globular cluster distribution exhibits asymmetries
that might be understood in terms of tidal stripping of globulars from NGC
1387, a nearby galaxy. The good agreement between the areal density profile of
blue clusters and a projected dark-matter NFW density profile is emphasized.Comment: 9 pages, 9 figures. Accepted for publication in A&
Multifractal Scaling, Geometrical Diversity, and Hierarchical Structure in the Cool Interstellar Medium
Multifractal scaling (MFS) refers to structures that can be described as a
collection of interwoven fractal subsets which exhibit power-law spatial
scaling behavior with a range of scaling exponents (concentration, or
singularity, strengths) and dimensions. The existence of MFS implies an
underlying multiplicative (or hierarchical, or cascade) process. Panoramic
column density images of several nearby star- forming cloud complexes,
constructed from IRAS data and justified in an appendix, are shown to exhibit
such multifractal scaling, which we interpret as indirect but quantitative
evidence for nested hierarchical structure. The relation between the dimensions
of the subsets and their concentration strengths (the "multifractal spectrum'')
appears to satisfactorily order the observed regions in terms of the mixture of
geometries present: strong point-like concentrations, line- like filaments or
fronts, and space-filling diffuse structures. This multifractal spectrum is a
global property of the regions studied, and does not rely on any operational
definition of "clouds.'' The range of forms of the multifractal spectrum among
the regions studied implies that the column density structures do not form a
universality class, in contrast to indications for velocity and passive scalar
fields in incompressible turbulence, providing another indication that the
physics of highly compressible interstellar gas dynamics differs fundamentally
from incompressible turbulence. (Abstract truncated)Comment: 27 pages, (LaTeX), 13 figures, 1 table, submitted to Astrophysical
Journa
GBStools: A Statistical Method for Estimating Allelic Dropout in Reduced Representation Sequencing Data
Reduced representation sequencing methods such as genotyping-by-sequencing (GBS) enable low-cost measurement of genetic variation without the need for a reference genome assembly. These methods are widely used in genetic mapping and population genetics studies, especially with non-model organisms. Variant calling error rates, however, are higher in GBS than in standard sequencing, in particular due to restriction site polymorphisms, and few computational tools exist that specifically model and correct these errors. We developed a statistical method to remove errors caused by restriction site polymorphisms, implemented in the software package GBStools. We evaluated it in several simulated data sets, varying in number of samples, mean coverage and population mutation rate, and in two empirical human data sets (N = 8 and N = 63 samples). In our simulations, GBStools improved genotype accuracy more than commonly used filters such as Hardy-Weinberg equilibrium p-values. GBStools is most effective at removing genotype errors in data sets over 100 samples when coverage is 40X or higher, and the improvement is most pronounced in species with high genomic diversity. We also demonstrate the utility of GBS and GBStools for human population genetic inference in Argentine populations and reveal widely varying individual ancestry proportions and an excess of singletons, consistent with recent population growth.Facultad de Ciencias Naturales y MuseoInstituto Multidisciplinario de BiologĂa Celula
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