Thermal flickers: A semianalytical approach

In order to enhance physical insight into the nature of thermal oscillations arising from a thin helium burning shell, the behavior in its phase plane of a simple two zone model which, however, contains all the relevant physics is analyzed. This simple model very naturally reproduces thermal flickers and is relatively insensitive to all but two parameters

Dynamics of many-particle fragmentation in a Cellular Automaton model

A 3D Cellular Automaton model developed by the authors to deal with the dynamics of N-body interactions has been adapted to investigate the head-on collision of two identical bound clusters of particles, and the ensuing process of fragmentation. The range of impact energies is chosen low enough, to secure that a compound bound cluster can be formed. The model is devised to simulate the laboratory set-up of fragmentation experiments as monitored by 4pi detectors. The particles interact via a Lennard-Jones potential. At low impact energies the numerical experiments following the dynamics of the individual particles indicate a phase of energy sharing among all the particles of the compound cluster. Fragments of all sizes are then found to evaporate from the latter cluster. The cluster sizes, measured in our set-up by simulated 4pi detectors, conform to a power law of exponent around 2.6.Comment: 27 pages, 10 figures, submitted to Phys. Rev.

Wave chaos in rapidly rotating stars

Effects of rapid stellar rotation on acoustic oscillation modes are poorly understood. We study the dynamics of acoustic rays in rotating polytropic stars and show using quantum chaos concepts that the eigenfrequency spectrum is a superposition of regular frequency patterns and an irregular frequency subset respectively associated with near-integrable and chaotic phase space regions. This opens new perspectives for rapidly rotating star seismology and also provides a new and potentially observable manifestation of wave chaos in a large scale natural system.Comment: 5 pages, 3 figures; accepted for publication in Phys. Rev.

FACTS AND IDEAS IN MODERN COSMOLOGY

A review of the principles of observational testing of cosmological theories is given with a special emphasis on the distinction between observational facts and theoretical hypotheses. A classification of modern cosmological theories and possible observational tests for these theories is presented. The main rival cosmological models are analyzed from the point of view of observational testing of their initial hypothesis. A comparison of modern observational data with theoretical predictions is presented. In particular we discuss in detail the validity of the two basic assumptions of modern cosmology that are the Cosmological Principle and the Expanding Space Paradigm. It is found that classical paradigms need to be reanalyzed and that it is necessary to develop crucial cosmological tests to discriminate alternative theories.Comment: 84 pages, latex, figures are available to F.S.L ([email protected]). Accepted for publication in Vistas In astronomy, Vol.38, Part.4, 199

CA Simulations of 2D Stellar Atmosphere Pulsations

We develop a new version of a Cellular Automaton (CA) for the simulation of the dynamics of a stellar atmosphere sitting on top of an inert core, and specified by the following physical input parameters: mass, radius and luminosity of core, and mass of atmosphere. The CA incorporates various parametrised simulation schemes of the instability mechanism (essentially ionisation). The initial state in all of our numerical experiments is a radially symmetric atmosphere of exponential density run and uniform temperature (input parameters: density scale– height and temperature of atmosphere). The initial atmosphere is not in hydrostatic and thermal equilibrium. After a transient stage, the system relaxes, for certain ranges of the parameters of the instability mechanism, towards a state of nontrivial dynamical behaviour: Local heat–driven circulations are set up which may range from nearly stationary and spatially symmetric cellular patterns to temporally and spatially irregularly fluctuating velocity fields. The traditional radial symmetry of the density pattern is broken, so that the star acquires a globally non–spherical shape. The residual non–stationary component, when integrated over the star to produce the counterpart of an observational velocity curve of a variable star, shows an irregular cyclic behaviour which does not have the signature of low–dimensional deterministic chaos

A Case of Nonlinearity

Robert Buchler's carrier as seen by his long term friend and colleague Jean Perdang (Note of the Eds.)

Thermodynamics from three-dimensional many-body fragmentation simulations on a cellular automaton model

The thermal, equilibrium of many-body systems subject to finite: range interactions is investigated numerically, by means of a multipurpose 3D cellular automaton dynamic model developed by the authors. The numerical experiments, carried out at fixed number of bodies, volume and,energy, demonstrate the formation of an equilibrium among 3D aggregates of bodies. The distribution of the aggregates against size, obeys a power law of (negative) exponent tauapproximate to2.2 (against 1.3 in 2D). Our experiments, indicating that the exponent is insensitive to the precise parameter values and the precise parametrization of the interactions, are consistent with the idea of the existence of a universality class corresponding; to the thermal equilibrium. The numerical value for the exponent tau is in agreement with the theoretical thermal equilibrium analyses based on various other approaches, numerical and semianalytical, indicating that the cellular automaton approach provides an adequate methodology to investigate thermal equilibria. In this paper, as an illustration of this method, we refer to the problem of formation of clusters of nucleons in heavy ion collisions of nuclei leading on to-fragmentation. The theoretical tau value, however, corresponding to the thermal equilibrium among the aggregation clusters, is 15 percent lower than the empirical value (approximate to2.6), as measured in laboratory nuclear fragmentation, experiments induced by collision. There is then only a very approximate correspondence between the experimental and the thermal equilibrium value. On the basis of the results of this paper and of a previous paper of this series, we conjecture that the approximate agreement is due to a partial establishment of a thermodynamic equilibrium during the collision of the nuclei: The thermal. equilibrium gives, the main contribution to the observed tau value; the deviation from this possibly universal value is largely the consequence of the lack of full thermal equilibrium in actual laboratory experiments. This conjecture is extended to interpret,the observed ubiquity of power laws of exponents exceeding 2.2, which refer to the distribution of various types of matter in 3D space

Cellular Automaton experiments on local galactic structure. I. Model assumptions

The purpose of the present paper, combined with the companion paper (Lejeune & Perdang 1995, hereinafter Paper II), is to demonstrate that a Cellular Automaton (CA) framework incorporating detailed physical evolutionary mechanisms of the galactic components provides a straightforward approach for simulating local structural features in galaxies (such as those of flocculent spiral galaxies). Conversely, and more important, the observed local irregularities may give information on the relevant timescales of the evolutionary processes operating in these galaxies. In this paper we start out with a critical review of the more standard methods in use in galactic modelling. We insist on the fact that these models do not lend themselves to a straightforward inclusion of both the galactic dynamics and the physical evolution of the galactic components. We show that the Cellular Automaton approach can combine both effects, on condition that the dynamics is approximated by a stationary, in general space–dependent velocity field of the galactic matter. The main part of the paper addresses an extension of the Stochastic Propagating Star Formation scheme originally devised by Mueller & Arnett (1976). The model consists in a multi–state $2D$ CA specifically designed to deal with the evolutionary behaviour of an off-centre region of a galaxy, of an area of a few ${\rm kpc}^2$. The model incorporates a detailed sequence of in part parametrised stellar evolutionary processes. In the version discussed here it includes as dynamical effects the motions of galactic matter due to a stationary circulation and, to some extent, due to the proper motions of the stars. The model we present is a first nontrivial instance of a CA defined over a lattice lacking geometric symmetries (crystal symmetries of standard CA, or rotational symmetry of the Mueller–Arnett CA). The precise geometry of the CA network of cells is imposed in our model by the space–dependent stationary galactic velocity field. Numerical results are discussed in the companion paper