15,457 research outputs found
Taking apart the dynamical clock. Fat-tailed dynamical kicks shape the blue-straggler star bimodality
In globular clusters, blue straggler stars are heavier than the average star,
so dynamical friction strongly affects them. The radial distribution of BSS,
normalized to a reference population, appears bimodal in a fraction of Galactic
GCs, with a density peak in the core, a prominent zone of avoidance at
intermediate radii, and again higher density in the outskirts. The zone of
avoidance appears to be located at larger radii the more relaxed the host
cluster, acting as a sort of dynamical clock. We use a new method to compute
the evolution of the BSS radial distribution under dynamical friction and
diffusion. We evolve our BSS in the mean cluster potential under dynamical
friction plus a random fluctuating force, solving the Langevin equation with
the Mannella quasi symplectic scheme. This amounts to a new simulation method
which is much faster and simpler than direct N-body codes but retains their
main feature: diffusion powered by strong, if infrequent, kicks. We compute the
radial distribution of initially unsegregated BSS normalized to a reference
population as a function of time. We trace the evolution of its minimum,
corresponding to the zone of avoidance. We compare the evolution under kicks
extracted from a Gaussian distribution to that obtained using a Holtsmark
distribution. The latter is a fat tailed distribution which correctly models
the effects of close gravitational encounters. We find that the zone of
avoidance moves outwards over time, as expected based on observations, only
when using the Holtsmark distribution. Thus the correct representation of near
encounters is crucial to reproduce the dynamics of the system. We confirm and
extend earlier results that showed how the dynamical clock indicator depends
both on dynamical friction and effective diffusion powered by dynamical
encounters.Comment: 8 pages, 6 figures. Version accepted in Astronomy & Astrophysic
Fractional differential equations solved by using Mellin transform
In this paper, the solution of the multi-order differential equations, by
using Mellin Transform, is proposed. It is shown that the problem related to
the shift of the real part of the argument of the transformed function, arising
when the Mellin integral operates on the fractional derivatives, may be
overcame. Then, the solution may be found for any fractional differential
equation involving multi-order fractional derivatives (or integrals). The
solution is found in the Mellin domain, by solving a linear set of algebraic
equations, whose inverse transform gives the solution of the fractional
differential equation at hands.Comment: 19 pages, 2 figure
Fractional Spectral Moments for Digital Simulation of Multivariate Wind Velocity Fields
In this paper, a method for the digital simulation of wind velocity fields by
Fractional Spectral Moment function is proposed. It is shown that by
constructing a digital filter whose coefficients are the fractional spectral
moments, it is possible to simulate samples of the target process as
superposition of Riesz fractional derivatives of a Gaussian white noise
processes. The key of this simulation technique is the generalized Taylor
expansion proposed by the authors. The method is extended to multivariate
processes and practical issues on the implementation of the method are
reported.Comment: 12 pages, 2 figure
Observed frequency-independent torque in flagellar bacterial motors optimizes space exploration
A surprising feature of many bacterial motors is the apparently conserved form of their torque-frequency relation. Experiments indicate that the torque provided by the bacterial rotary motor is approximately constant over a large range of angular speeds. This is observed in both monotrichous and peritrichous bacteria, independently of whether they are propelled by a proton flux or by a Na+ ion flux. If the relation between angular speed ω and swimming speed is linear, a ω-independent torque implies that the power spent in active motion is proportional to the instantaneous bacterial speed. Using realistic values of the relevant parameters, we show that a constant torque maximizes the volume of the region explored by a bacterium in a resource-depleted medium. Given that nutrients in the ocean are often concentrated in separate, ephemeral patches, we propose that the observed constancy of the torque may be a trait evolved to maximize bacterial survival in the ocean.Fil: Di Salvo, Mario Esteban. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; ArgentinaFil: Condat, Carlos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentin
Has the Chilean Neo-Liberal Experiment Run Out of Fuel? A View on Specialisation, Technological Gaps and Catching-Up
Due to an extraordinary growth performance during the last two decades the Chilean neo-liberal model of development, based on the exploitation of the country’s static comparative advantages, has turned into a benchmark for most developing countries. The aim of this paper is to discuss the long term sustainability of the Chilean neo-liberal model of development. We present new empirical results obtained by using CAN2000 as well as input-output analysis that describe the Chilean model of development during the period 1986-1998. On the basis of these stylised facts, a simple ricardian-evolutionary model is developed in order to offer an interpretative framework to discuss the conditions under which Chile could maintain the current catching-up process in the long run. The main conclusion is that, with the recent ceasing of the push effects of the neo-liberal policies, it is unlikely that an increase of the export volume, given its structural characteristics, will be sufficient to this end. Indeed, an increase of the country’s sectoral industrial interdependence and an improvement of its international specialisation pattern towards goods with higher technological content and higher income elasticity of world demand are necessary conditions for maintaining the current catching-up process in the long run.Chile, Technological gaps, International specialisation, Catching-up, Neo-Liberal Structural Reforms
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