Luis A. Núñez

Sound speeds, cracking and the stability of self-gravitating anisotropic compact objects

Using the concept of cracking we explore the influence that density fluctuations and local anisotropy have on the stability of local and non-local anisotropic matter configurations in general relativity. This concept, conceived to describe the behavior of a fluid distribution just after its departure from equilibrium, provides an alternative approach to consider the stability of self-gravitating compact objects. We show that potentially unstable regions within a configuration can be identified as a function of the difference of propagations of sound along tangential and radial directions. In fact, it is found that these regions could occur when, at a particular point within the distribution, the tangential speed of sound is greater than the radial one.

A variational principle and the classical and quantum mechanics of the damped harmonic oscillator

A variational principle is presented, by means of which the equation of motion of the damped harmonic oscillator is found. Starting from this variational principle a systematic reformulation of the classical mechanics leads us to a Hamilton–Jacobi equation with an additional term, which is proportional to the action. The quantization of this Hamilton–Jacobi equation, following a method originally due to Schrodinger, gives the Langevin–Schrodinger equation.

Symmetries and conserved quantities in geodesic motion

Recently obtained results linking several constants of motion to one (non‐Noetherian) symmetry to the problem of geodesic motion in Riemannian space‐times are applied. The construction of conserved quantities in geodesic motion as well as the deduction of geometrical statements about Riemannian space‐times are achieved.

VAMDC--The Virtual Atomic and Molecular Data Centre--A New Way to Disseminate Atomic and Molecular Data--VAMDC Level 1 Release

The Virtual Atomic and Molecular Data Centre (VAMDC, http://www.vamdc.eu/) is a European‐Union‐funded collaboration between groups involved in the generation, evaluation, and use of atomic and molecular data. VAMDC aims to build a reliable, open, flexible and interoperable e‐science interface to existing atomic and molecular data. The project will cover establishing the core consortium, the development and deployment of the infrastructure and the development of interfaces to the existing atomic and molecular databases. This paper describes the organisation of the project and the achievements during its first year.

Ricci and contracted Ricci collineations of the Robertson–Walker space‐time

Ricci collineations and contracted Ricci collineations of the Robertson–Walker metric, associated with a vector field of the form ξ=(ξ0(t,r),ξ1(t,r),0,0) are presented.

NON-LOCAL EQUATION OF STATE IN GENERAL RELATIVISTIC RADIATING SPHERES

We show that under particular circumstances a general relativistic spherically symmetric bounded distribution of matter could satisfy a non-local equation of state. This equation describes, at a given point, the components of the corresponding energy-momentum tensor not only as a function at that point, but as a functional throughout the enclosed configuration. We have found that these types of dynamic bounded matter configurations, with constant compactness or gravitational potentials at the surface, admit a conformal Killing vector field and fulfil the energy conditions for anisotropic imperfect fluids. We present several analytical and numerical models satisfying these equations of state which collapse as reasonable radiating anisotropic spheres in general relativity.

Portal of damage: a web-based finite element program for the analysis of framed structures subjected to overloads

This paper describes a web-based finite element program called portal of damage. The purpose of the program is the numerical simulation of reinforced concrete framed structures, typically buildings, under earthquakes or others exceptional overloads. The program has so far only one finite element based on lumped damage mechanics. This is a theory that combines fracture mechanics, damage mechanics and the concept of plastic hinge. In the case of reinforced concrete frames, the main mechanism of deterioration is cracking of concrete. Cracking in a frame element is lumped at the plastic hinges. Cracking evolution in the plastic hinge is assumed to follow a generalized form of the Griffith criterion. The behavior of a plastic hinge with damage is described via the effective stress hypothesis, as used in continuum damage mechanics. The portal that can be accessed using any commercial browser (explorer, netscape, etc.) allows to: (a) Create an account in a server.(b) Make use of a semi-graphic pre-processor to create an input file with a digitalized version of the structure.(c) Run a dynamic finite element program and monitor the state of the process.(d) Download or upload input and output files in text format.(e) Make use of a graphic post-processor.

Ricci Collineations for Non-degenerate, Diagonal and Spherically Symmetric Ricci Tensors

The expression of the vector field generator of a Ricci collineation for diagonal, spherically symmetric and non-degenerate Ricci tensors is obtained. The resulting expressions show that the time and radial first derivatives of the components of the Ricci tensor can be used to classify the collineation, leading to 64 families. Some examples illustrate how to obtain the collineation vector.

Ricci Collineations for Type B Warped Space-times

We present the general structure of proper Ricci Collineations (RC) for type B warped space-times. Within this framework, we give a detailed description of the most general proper RC for spherically symmetric metrics. As examples, static spherically symmetric and Friedmann-Robertson-Walker space-times are considered.

Cracking of Self-Gravitating Compact Objects with Local and Non Local Equations of State

We discuss the effect that small fluctuations of density and local anisotropy (principal unequal stresses) may have on the occurrence of cracking in spherical compact objects having a non local and local politropic equations of state. A non local equation of state provides, at a given point, the radial pressure not only as function of the density at that point, but its functional throughout the enclosed distribution. It is shown that departures from the equilibrium may lead to the appearance of cracking and it seams to be related not only to fluctuations on the local anisotropy but also to the density. We have found that these fluctuations should have the same sign and the effect of the perturbations in density qualitatively different to the variations in anisotropy.