10,138 research outputs found
Dark-Halo Cusp: Asymptotic Convergence
We propose a model for how the buildup of dark halos by merging satellites
produces a characteristic inner cusp, of a density profile \rho \prop r^-a with
a -> a_as > 1, as seen in cosmological N-body simulations of hierarchical
clustering scenarios. Dekel, Devor & Hetzroni (2003) argue that a flat core of
a<1 exerts tidal compression which prevents local deposit of satellite
material; the satellite sinks intact into the halo center thus causing a rapid
steepening to a>1. Using merger N-body simulations, we learn that this cusp is
stable under a sequence of mergers, and derive a practical tidal mass-transfer
recipe in regions where the local slope of the halo profile is a>1. According
to this recipe, the ratio of mean densities of halo and initial satellite
within the tidal radius equals a given function psi(a), which is significantly
smaller than unity (compared to being 1 according to crude resonance criteria)
and is a decreasing function of a. This decrease makes the tidal mass transfer
relatively more efficient at larger a, which means steepening when a is small
and flattening when a is large, thus causing converges to a stable solution.
Given this mass-transfer recipe, linear perturbation analysis, supported by toy
simulations, shows that a sequence of cosmological mergers with homologous
satellites slowly leads to a fixed-point cusp with an asymptotic slope a_as>1.
The slope depends only weakly on the fluctuation power spectrum, in agreement
with cosmological simulations. During a long interim period the profile has an
NFW-like shape, with a cusp of 1<a<a_as. Thus, a cusp is enforced if enough
compact satellite remnants make it intact into the inner halo. In order to
maintain a flat core, satellites must be disrupted outside the core, possibly
as a result of a modest puffing up due to baryonic feedback.Comment: 37 pages, Latex, aastex.cls, revised, ApJ, 588, in pres
Endoplasmic reticulum aminopeptidase 1 (ERAP1) polymorphism relevant to inflammatory disease shapes the peptidome of the birdshot chorioretinopathy-associated HLA-A∗29:02 Antigen
Birdshot chorioretinopathy is a rare ocular inflammation whose genetic association with HLA-A∗29:02 is the highest between a disease and a major histocompatibility complex (MHC) molecule. It belongs to a group of MHCI- Associated inflammatory disorders, also including ankylosing spondylitis, psoriasis, and Behç et's disease, for which endoplasmic reticulum aminopeptidases (ERAP) 1 and/or 2 have been identified as genetic risk factors. Since both enzymes are involved in the processing of MHC-I ligands, it seems reasonable that common peptide- mediated mechanisms may underlie the pathogenesis of these diseases. In this study, comparative immunopeptidomics was used to characterize >5000 A∗29:02 ligands and quantify the effects of ERAP1 polymorphism and expression on the A∗29:02 peptidome in human cells. The peptides predominant in an active ERAP1 context showed a higher frequency of nonamers and bulkier amino acid side chains at multiple positions, compared with the peptides predominant in a less active ERAP1 background. Thus, ERAP1 polymorphism has a large influence, shaping the A∗29:02 peptidome through length-dependent and length-independent effects. These changes resulted in increased affinity and hydrophobicity of A∗29:02 ligands in an active ERAP1 context. The results reveal the nature of the functional interaction between A∗29:02 and ERAP1 and suggest that this enzyme may affect the susceptibility to birdshot chorioretinopathy by altering the A∗29:02 peptidome. The complexity of these alterations is such that not only peptide presentation but also other potentially pathogenic features could be affectedThis work was supported by grants SAF2011/25681 (Plan Nacional de I+D+i) to JALC, Binational Science Foundation Grant 2009393 to AA, and an institutional grant of the Fundacion Ramon Areces to the CBMS
Conocimientos actuales y perspectivas de investigación de elementos traza en la nutrición infantil: parte III. Biodisponibilidad y necesidades de investigación futura
La alimentación de los lactantes mediante las fórmulas lácteas no
suplementadas, durante periodos de tiempo prolongados, presenta el riesgo del
desarrollo de deficiencias nutricionales de diversos oligoelementos; en contraste, los
lactantes alimentados con leche materna rara vez presentan alguna deficiencia mineral
Puesto que la concentración de estos oligoelementos en la leche humana con frecuencia
es más baja o igual que aquella de las fórmulas lácteas, incluso sin suplementar, está
implicada claramente una más alta biodisponibilidad de estos elementos desde la leche
humana.
Es conveniente la comparación entre la leche humana y la de vaca, ya que esta
última va a ser, en definitiva, materia prima principal de muchas fórmulas infantiles.
Ello permite utilizar sus valores de concentración con fines comparativos, en cuanto a la
cantidad de elementos traza suministrada, y el establecimiento de su distinta
distribución entre los componentes mayoritarios. Este trabajo presenta una amplia
revisión de los principales estudios de biodisponibilidad de elementos traza en leche
humana, de vaca y fórmulas infantiles.
Dado que la absorción y posterior utilización de los elementos traza por el
neonato, no solo depende del contenido total en la fórmula infantil, sino de la forma
química en la cual se encuentra y los compuestos en los que se integran, parece
oportuno caracterizar, no solo los componentes principales de la fórmula infantil, sino
los micronutrientes para encontrar las formas químicas más adecuadas y de mejor
biodisponibiliad, mediante estudios de especiación, para evitar las posibles
interacciones debido a las altas concentraciones inorgánicas añadidas, con la finalidad
de asemejar aún más si cabe, al estándar de la alimentación infantil, la leche materna
Conocimientos actuales y perspectivas de investigación de elementos traza en la nutrición infantil: parte I. Situación actual de la alimentación infantil
La nutrición infantil ha experimentado una creciente evolución en función
de los estudios de investigación desarrollados y los diferentes criterios adoptados
para establecer unas recomendaciones que satisfagan plenamente las necesidades
fisiológicas y demandas nutricionales del neonato.
La normativa de composición de las fórmulas ha sido dictadas por la
Sociedad Europea de Gastroenterología y Nutrición Pediátrica (ESPGAN) y la
Academia Americana de Pediatría (AAP). Recientemente el Panel de Expertos del
Life Sciences Resarch Office (LSRO) de la American Society for Nutritional Sciences
(ASNS) ha sido establecido las últimas disposiciones en relación a los niveles
recomendados de diferentes nutrientes en las formulas infantiles de manera que su
cumplimiento asegure y proporcione, por si mismas, la única fuente de alimento de
los lactantes a término durante su primer año de vida.
No hay duda de que la lactancia natural es la más adecuada para el recién
nacido y que es preferible a cualquier otro tipo de alimentación infantil. No obstante,
el empleo de las fórmulas infantiles se hace imprescindible en aquellos casos en que
no sea posible la alimentación del lactante con leche materna. El modelo de lactancia
natural en España es similar al de los países del área industrializada del mundo,
aunque su duración es sensiblemente inferior a la descrita para otros países del área
europea, se abandona masivamente a lo largo del primer semestre de vida.
No sólo se debe considerar la leche materna bajo aspectos analíticos
cuantitativos en cuanto a su contenido en nutrientes esenciales, sino también es
necesario que las fórmulas infantiles proporcionen una fuente de micronutrientes
biodisponibles y, en este sentido, todavía hoy los conocimientos sobre ingesta,
requerimientos, biodisponibilidad e intervalos de tolerancia de gran parte de los
oligoelementos, con respecto a su influencia en el desarrollo infantil, son todavía
limitados e insuficientes
Conocimientos actuales y perspectivas de investigación de elementos traza en la nutrición infantil: parte II. Niveles de concentración, ingesta dietética y requerimientos de la alimentación infantil.
La leche materna sirve de referencia en cuanto al contenido de
micronutrientes, ya que, por razones de ética, dichos datos no pueden ser obtenidos
de lactantes humanos en buen estado de salud. La concentración de los distintos
oligoelementos en la leche materna va disminuyendo a medida que transcurre el
tiempo de lactancia, por lo que llegado el cuarto mes, si el niño es exclusivamente
alimentado con leche materna y pertenece a un grupo particularmente predispuesto
(bajos depósitos orgánicos, incremento de las necesidades, aumento de pérdidas,
reducción de la absorción intestinal), pueden desarrollarse cuadros de deficiencias
nutricionales dependientes del oligoelemento deficitario.
El presente trabajo recoge los contenidos de los principales elementos traza
esnciales (hierro, zinc, cobre, manganeso y selenio) y aquellos potencialmente
tóxicos (aluminio, plomo y cadmio), hallados en las fórmulas infantiles en una
amplia revisión bibliográfica.
Los requerimientos nutricionales de elementos traza son específicos de cada
neonato. Las recomendaciones dietéticas deben establecerse de manera que observen
pautas bastante generosas, considerando la amplia variación interindividual, para que
se cubra las necesidades de la mayor parte de la población neonatal. Atendiendo a
esta consideración, el Comité conjunto de la OMS/FAO/IAEA incorpora dos nuevos
conceptos, no considerados anteriormente, sobre requerimientos de elementos traza:
requerimientos basales y normalizados.
Con este estudio se ha dado un gran paso en el establecimiento de
recomendaciones dietéticas para lactantes, aunque no se debe perder de vista que esta
estimación de ingesta dietética no es suficiente en si misma; siendo necesaria una
mayor investigación en los balances metabólicos de elementos traza en el organismo
neonatal y se requiere de un mayor conocimiento de las formas químicas
biodisponibles adecuadas a la nutrición pediátrica
Universal stability of coherently diffusive 1D systems with respect to decoherence
Static disorder in a 3D crystal degrades the ideal ballistic dynamics until
it produces a localized regime. This Metal-Insulator Transition is often
preceded by coherent diffusion. By studying three different paradigmatic 1D
models, the Harper-Hofstadter-Aubry-Andr\'e and the Fibonacci tight-binding
chains, and the power-banded random matrix model, we show that whenever
coherent diffusion is present, transport is exceptionally stable against
decoherent noise. This is completely at odds with what happens for ballistic
and localized dynamics, where the diffusion coefficient strongly depends on the
environmental decoherence. A universal dependence of the diffusion coefficient
on the decoherence strength is analytically derived: the diffusion coefficient
remains almost decoherence-independent until the coherence time becomes
comparable with the mean elastic scattering time. Thus, systems with a quantum
diffusive regime could be used to design stable quantum wires and may explain
the functionality of many biological systems, which often operate at the border
between the ballistic and localized regimes.Comment: Main: 8 pages, 3 figures. Supplementary: 17 pages, 10 figure
The First Galaxies: Chemical Enrichment, Mixing, and Star Formation
Using three-dimensional cosmological simulations, we study the assembly
process of one of the first galaxies, with a total mass of 10^8 M_sun,
collapsing at z = 10. Our main goal is to trace the transport of the heavy
chemical elements produced and dispersed by a pair-instability supernova
exploding in one of the minihalo progenitors. To this extent, we incorporate an
efficient algorithm into our smoothed particle hydrodynamics code which
approximately models turbulent mixing as a diffusion process. We study this
mixing with and without the radiative feedback from Population III stars that
subsequently form in neighboring minihalos. Our simulations allow us to
constrain the initial conditions for second-generation star formation, within
the first galaxy itself, and inside of minihalos that virialize after the
supernova explosion. We find that most minihalos remain unscathed by ionizing
radiation or the supernova remnant, while some are substantially photoheated
and enriched to supercritical levels, likely resulting in the formation of
low-mass Population III or even Population II stars. At the center of the newly
formed galaxy, 10^5 M_sun of cold, dense gas uniformly enriched to 10^-3 Z_sun
are in a state of collapse, suggesting that a cluster of Population II stars
will form. The first galaxies, as may be detected by the James Webb Space
Telescope, would therefore already contain stellar populations familiar from
lower redshifts.Comment: 13 pages, 9 figures, published in Ap
Aluminium content of spanish infant formula
Aluminium toxicity has been relatively well documented in infants with impaired renal
function and premature neonates.
The aims of this study were to analyse the concentration of aluminium in the majority of
infant formulae sold commercially in Spain, to determine the influence of aluminium
content in the tap water in reconstituted powder formulae and to estimate the theoretical
toxic aluminium intake in comparison with the PTWI, and lastly, to discuss the possible
interactions of certain essential trace elements added to formulation with aluminium
according to type or main protein based infant formula.
A total of 82 different infant formulae from 9 different manufacturers were studied.
Sample digestion was simulated in a closed acid-decomposition microwave system.
Aluminium concentration was determined by atomic absorption spectrophotometry with
graphite furnace.
In general, the infant formulae studied provide an aluminium level higher than that found
in human milk, especially in the case of soya, preterm or hydrolysed casein-based
formulae.
Standard formulae provide lower aluminium intakes amounting to about 4 % PTWI.
Specialised and preterm formulae result in moderate intake (11 – 12 % and 8 – 10 %
PTWI, respectively). Soya formulae contribute the highest intake (15 % PTWI).
Aluminium exposure from drinking water used for powder formula reconstitution is not
considered a clear potential risk.
In accordance with the present state of knowledge about aluminium toxicity, it seems
prudent to call for continued efforts to standardise routine quality control and reduce
aluminium levels in infant formula as well as to keep the aluminium concentration under
300 g l-1 for all infant formulae, most specifically those formulae for premature and low
birth neonates
Critical Behavior of Three-Dimensional Disordered Potts Models with Many States
We study the 3D Disordered Potts Model with p=5 and p=6. Our numerical
simulations (that severely slow down for increasing p) detect a very clear spin
glass phase transition. We evaluate the critical exponents and the critical
value of the temperature, and we use known results at lower values to
discuss how they evolve for increasing p. We do not find any sign of the
presence of a transition to a ferromagnetic regime.Comment: 9 pages and 9 Postscript figures. Final version published in J. Stat.
Mec
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