609 research outputs found
The effects of a background potential in star cluster evolution: a delay in the relaxation time-scale and runaway collision processes
Runaway stellar collisions in dense star clusters are invoked to explain the
presence of very massive stars or blue stragglers in the center of those
systems. This process has also been explored for the first star clusters in the
Universe and shown to yield stars that may collapse at some points into an
intermediate mass black hole. Although the early evolution of star clusters
requires the explicit modeling of the gas out of which the stars form, these
calculations would be extremely time-consuming and often the effects of the gas
can be accurately treated by including a background potential to account for
the extra gravitational force. We apply this approximation to model the early
evolution of the first dense star clusters formed in the Universe by performing
-body simulations, our goal is to understand how the additional
gravitational force affects the growth of a very massive star through stellar
mergers in the central parts of the star cluster. Our results show that the
background potential increases the velocities of the stars, causing an overall
delay in the evolution of the clusters and in the runaway growth of a massive
star at the center. The population of binary stars is lower due to the
increased kinetic energy of the stars, initially reducing the number of stellar
collisions, and we show that relaxation processes are also affected. Despite
these effects, the external potential enhances the mass of the merger product
by a factor 2 if the collisions are maintained for long times.Comment: 16 pages. Accepted for publication in Astronomy and Astrophysic
The physical origin of super-competitive accretion during the formation of the first supermassive black holes
Numerical simulations have shown the occurence of a scenario termed
''super-competitive accretion'', a term that describes a situation where only
the central few objects grow supermassive while a larger number of stars
compete for the reservoir, with significant accretion flows of
M yr. This scenario particularly implies that the presence of
fragmentation will not necessarily impeed the formation of a central massive
object. We here explore this phenomenon using analytical estimates for growth
via collisions and accretion, considering accretion due to self-gravity as well
as Bondi-Hoyle accretion. Particularly, we explore under what conditions the
accretion onto the central massive object breaks down, and derive a criterion
that depends on the mass of the most massive object and the mass in fragments.
For compact clusters with sizes about ~pc, we further find that the mass
growth by collisions is comparable to the growth via accretion. Our results are
validated through the comparison with numerical simulations, and we overall
conclude that super-competitive accretion is a valid mechanism for the
formation of very massive objects in the early Universe.Comment: 9 pages, 5 figures. Accepted for publication in MNRA
A micromechanical analysis of inter-fiber failure in long reinforced composites based on the phase field approach of fracture combined with the cohesive zone model
A comparative evaluation of interest point detectors and local descriptors for visual SLAM
Abstract In this paper we compare the behavior of different interest points detectors and descriptors under the
conditions needed to be used as landmarks in vision-based simultaneous localization and mapping (SLAM).
We evaluate the repeatability of the detectors, as well as the invariance and distinctiveness of the descriptors,
under different perceptual conditions using sequences of images representing planar objects as well as 3D scenes.
We believe that this information will be useful when selecting an appropriat
Weather radar for urban hydrological applications: lessons learnt and research needs identified from 4 pilot catchments in North-West Europe
International audienceThis study investigates the impact of rainfall estimates of different spatial resolutions on the hydraulic outputs of the models of four of the EU RainGain project’s pilot locations (the Cranbrook catchment (UK), the Herent catchment (Belgium), the Morée-Sausset catchment (France) and the Kralingen District (The Netherlands)). Two storm events, one convective and one stratiform, measured by a polarimetric X-band radar located in Cabauw (The Netherlands) were selected for analysis. The original radar estimates, at 100 m and 1 min resolutions, were aggregated to a spatial resolution of 1000 m. These estimates were then applied to the high-resolution semi-distributed hydraulic models of the four urban catchments, all of which have similar size (between 5 and 8 km2), but different morphological, hydrological and hydraulic characteristics. When doing so, methodologies for standardising rainfall inputs and making results comparable were implemented. The response of the different catchments to rainfall inputs of varying spatial resolution is analysed in the light of model configuration, catchment and storm characteristics. Rather surprisingly, the results show that for the two events under consideration the spatial resolution (i.e. 100 m vs 1000 m) of rainfall inputs does not have a significant influence on the outputs of urban drainage models. The present study will soon be extended to more storms as well as model structures and resolutions, with the final aim of identifying critical spatial-temporal resolutions for urban catchment modelling in relation to catchment and storm event characteristics
Global instability by runaway collisions in nuclear stellar clusters: Numerical tests of a route for massive black hole formation
The centers of galaxies host nuclear stellar clusters, supermassive black
holes, or both, but the origin of this dichotomy is still a mystery. Nuclear
stellar clusters are the densest stellar system of the Universe, so they are
ideal places for runaway collisions to occur. In these dense clusters it is
possible that global instability occurs, triggered by collisions and mergers
forming a massive black hole. Here we test a new mechanism to form massive
black holes through runaway stellar collisions in nuclear stellar clusters,
performing N-body simulations using the code nbody6++gpu. Our idealized models
show that there is a critical mass where collisions become very efficient
making it possible to form massive black holes in nuclear stellar clusters. The
most massive objects reach masses of the order of . We
find that our highest black hole formation efficiency is up to of the
stellar mass at the end of the simulation. In real astrophysical systems, the
critical mass scale for this transition is expected to occur in stellar
clusters of , implying the formation of quite massive
central objects.Comment: 11 pages, 6 figure
Crack arrest through branching at curved weak interfaces: an experimental and numerical study
The phenomenon of arrest of an unstably-growing crack due to a curved weak
interface is investigated. The weak interface can produce the deviation of the
crack path, trapping the crack at the interface, leading to stable crack growth
for certain interface geometries. This idea could be used as a technical
solution for a new type of crack arrester, with a negligible impact on the
global stiffness, strength and weight of the structure. In order to exploit
this concept, an experimental campaign based on photo-elasticity and digital
image correlation is carried out, showing the capability of curved weak
interfaces to arrest cracks. The experiment is repeated for several geometrical
configurations through the modification of the interface curvature radii. The
phenomenon of crack deviation and subsequent arrest at the interface is also
investigated with the assistance of a computational model based on the finite
element method. The computational predictions provide the rationale for the
interpretation of the experimental observations, and distinguish between the
different behaviour of concave and convex interfaces. Consequently, as is shown
in the present study, the curved interface concept fosters new routes for the
attainment of structures with enhanced fracture resistance capacities, which
are of paramount importance for materials and components used in extreme
conditions.Comment: 19 pages, 13 figure
Magnetism and electrode dependant resistive switching in Ca-doped ceramic bismuth ferrite
Here we report on the preparation and structural, magnetic and electrical characterization of BiFeO 3 and Bi 0.9Ca 0.1FeO 3 ceramic multiferroic samples. We suggest that Ca-doping creates oxygen vacancies and destabilizes the BiFeO 3 spiral magnetic structure. We also study resistive switching effects in Bi 0.9Ca 0.1FeO 3 with metallic electrodes, finding that the appearance of the effect is dependant on the fabrication procedure of the metallic electrode. On the basis of these observations, we critically revise some assumptions in currently available models of resistive switching of complex oxides.Fil: Rubi, Diego. Comisión Nacional de Energía Atómica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de San Martín; ArgentinaFil: Marlasca, F. G.. Comisión Nacional de Energía Atómica; ArgentinaFil: Reinoso, M.. Comisión Nacional de Energía Atómica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Bonville, P.. No especifíca;Fil: Levy, Pablo Eduardo. Comisión Nacional de Energía Atómica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin
Impactos en el consumo de energía eléctrica por el uso de refrigeradores eficientes - caso Ecuador
This article presents an analysis of the impacts that the Renova Refrigerator program will have, on electricity consumption in Ecuador; the main objective of this program is to replace refrigerators that are more than 10 years old with efficient class “A” domestic refrigerators. The development and implementation of the program reduces energy consumption by 8\%, motivates people to use electric energy efficiently and encourages the national industry to use new technologies, which increases productivity and domestic product consumption.Additionally, in compliance with the current Ecuadorian and international regulations, the variation in electricity consumption of a domestic refrigerator is evaluated when it runs at the different temperatures determined by the thermal floors, which are very distinct in the Coast, Sierra and eastern regions of Ecuador.En este artículo se presenta un análisis de los impactos que tendrá el programa «Renova Refrigerador», en el consumo de energía eléctrica en el Ecuador; programa que tiene como principal objetivo la renovación de refrigeradores domésticos eficientes etiquetados con A, por refrigeradores ineficientes y con una vida útil que ha sobrepasado los diez años. Su desarrollo e implementación contribuye a la reducción de consumo energético en un 8 %, motiva a la población a utilizar la energía eléctrica de manera eficiente e impulsa a la industria nacional en el uso de nuevas tecnologías incrementando así la productividad y el consumo de productos nacionales.Bajo el cumplimiento de la normativa vigente ecuatoriana y sus similares internacionales; se considera, además, la variación en el consumo eléctrico que presenta un refrigerador doméstico cuando trabaja a distintas temperaturas, determinadas por los pisos térmicos, marcados plenamente en el Ecuador por las regiones Costa, Sierra y Oriente
Worldwide Incidence and Surgical Costs of Tendon Injuries: A Systematic Review and Meta-Analysis
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