Ellipsoids, material points and material segments

Simple models of potential based on material points and material segments are confronted with a homogeneous ellipsoid potential. A spheroid is approximated with a pair of material points or with one material segment. The segment model proves to be more accurate. For a triaxial ellipsoid, two models are considered: one with five material points and one with two material segments and a point. When their parameters are determined with the fourth sectorial harmonic ignored, both simple models have a similar approximation error. Numerical tests indicate that the approximate models of a triaxial ellipsoid are 5 to 10 times faster than exact formulation, whereas for a spheroid the approximate models are at most twice as fast as the exact formulae.     

图象的模型及其统计特性

本文对图象的统计特性及其适用模型作了分析。讨论了以前常用的平稳图象模型的缺陷,对实际图象的统计特性作了分析,认为它是非平稳的,不满足各态历经性,同时在空间上是高度相关的;讨论了相应的随机参数统计模型和描述性统计模型。指出作用于整幅图象的“全局”图象复原算法比每次只独立计算单个象元的“点”图象复原算法更为优越。     

Comparison of measured and simulated lateral distributions for electrons and muons with KASCADE

Lateral distributions for electrons and muons in extensive air showers measured with the array of the KASCADE experiment are compared to results of simulations based on the high-energy hadronic interaction models QGSJet and SIBYLL. It is shown, that the muon distributions are well described by both models. Deviations are found for the electromagnetic component, where both models predict a steeper lateral shape than observed in the data. For both models the observed lateral shapes of the electron component indicate a transition from a light to a more heavy composition of the cosmic ray spectrum above the knee.     

Kinetic Models of Solar and Polar Winds

The study of the solar corona has been strewn with great discoveries, surprises and controversies. The major steps since van de Hulst's (1953)and Chapman's (1957) early hydrostatic models of the extended corona, until the most recent generations of kinetic models of the coronal expansion and of the supersonic solar wind flows, are presented. These models are compared to in-situ observations. Progress in polar wind models went through a somewhat similar evolution that is outlined also. The advantages and limitations of the successive brands of solar wind and polar wind models are considered. This revised version was published online in July 2006 with corrections to the Cover Date.     

Preliminary design of pseudo satellites: Basic methods and feasibility criteria

Analytical models of weight and energy balances, aerodynamic models, and solar irradiance models to perform pseudo-satellite preliminary design are presented. Feasibility criteria are determined in accordance with the aim of preliminary design dependent on mission scenario and type of payload.     

Cosmological models with shear and rotation

Cosmological models involving shear and rotation are considered, first in the general relativistic and then in the Newtonian framework with the aim of investigating singularities in them by using numerical and analytical techniques. The dynamics of these rotating models are studied. It is shown that singularities are unavoidable in such models and that the centrifugal force arising due to rotation can never overcome the gravitational and shearing force over a length of time.     

Microinstabilities and models of the solar wind

The collision-dominated two-fluid plasma models of the solar wind are shown to become collisionless and subject to microinstabilities at a few solar radii. Assuming that once the plasma is unstable it stays close to marginal stability models of the solar wind are constructed including waves and proton heating. The resulting models have higher velocities and proton temperatures than the collision dominated two fluid models.Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16–19 September 1980, Scheveningen, The Netherlands.     

Comparative models of Uranus and Neptune

Models of Uranus and Neptune are computed based on the assumption that these planets consist of three layers: a rock core, an ice shell, and an atmosphere. Uranus models require that the ice shell have a density some 10% lower than the canonical density for an ice mixture. Two Neptune models are found, one with the canonical density in the ice shell, and one with a density 20% lower. The implications of these models are discussed.     

Interacting dark matter and holographic dark energy in an anisotropic universe

In this paper we have studied the anisotropic and homogeneous Bianchi type-I universe filled with interacting Dark matter and Holographic dark energy. Here we discussed two models, in first model the solutions of the field equations are obtained for constant value of deceleration parameter where as in the second model the solutions of the field equations are obtained for special form of deceleration parameter. It is shown that for suitable choice of interaction between dark matter and holographic dark energy there is no coincidence problem (unlike ΛCDM). Also, in all the resulting models the anisotropy of expansion dies out very quickly and attains isotropy after some finite time. The Statefinder diagnostic is applied to both the models in order to distinguish between our dark energy models with other existing dark energy models. The physical and geometrical aspects of the models are also discussed.     

Exact bianchi type-II,VIII, and IX cosmological models with matter and electromagnetic fields in Lyra's manifold

Spatially-homogeneous and anisotropic Bianchi type-II, VIII, and IX stiff-fluid cosmological models in the presence of source-free electromagnetic fields are obtained in Lyra's manifold. Some properties of the models are also discussed.     

From discs to planetesimals: Evolution of gas and dust discs

I review the processes that shape the evolution of protoplanetary discs around young, solar-mass stars. I first discuss observations of protoplanetary discs, and note in particular the constraints these observations place on models of disc evolution. The processes that affect the evolution of gas discs are then discussed, with the focus in particular on viscous accretion and photoevaporation, and recent models which combine the two. I then discuss the dynamics and growth of dust grains in discs, considering models of grain growth, the gas–grain interaction and planetesimal formation, and review recent research in this area. Lastly, I consider the so-called “transitional” discs, which are thought to be observed during disc dispersal. Recent observations and models of these systems are reviewed, and prospects for using statistical surveys to distinguish between the various proposed models are discussed.     

Equilibrium and Stability of Prominence Flux Ropes

Realistic models of solar prominence flux ropes are numerically constructed. The models are in 2.5 dimensions, including the effects of non-isothermal temperature, density and gravity, and pressure. Stability of the equilibria to pressure- and gravity-driven instabilities is numerically investigated, using the ballooning formalism of fusion plasma theory. The equilibrium models can become unstable to pressure- and gravity-driven modes at plasma parameters characteristic of prominences.     

Magnetostatic models filled with dust and disordered radiation

Magnetostatic models filled with dust and disordered radiation in which the distribution is that of perfect fluid, are obtained. Some physical and geometrical aspects of the models are investigated. In the last section of the paper, the Newtonian analogues of forces are also obtained.     

On the X-ray emission from massive star clusters and their evolving superbubbles – II. Detailed analytics and observational effects

In this work, we present a comprehensive X-ray picture of the interaction between a super star cluster and the interstellar medium. In order to do that, we compare and combine the X-ray emission from the superwind driven by the cluster with the emission from the wind-blown bubble. Detailed analytical models for the hydrodynamics and X-ray luminosity of fast polytropic superwinds are presented. The superwind X-ray luminosity models are an extension of the results obtained in Paper I. Here, the superwind polytropic character allows us to parametrize a wide variety of effects, for instance, radiative cooling. Additionally, X-ray properties that are valid for all bubble models taking thermal evaporation into account are derived. The final X-ray picture is obtained by calculating analytically the expected surface brightness and weighted temperature of each component. All of our X-ray models have an explicit dependence on metallicity and admit general emissivities as functions of the hydrodynamical variables. We consider a realistic X-ray emissivity that separates the contributions from hydrogen and metals. The paper ends with a comparison of the models with observational data.     

Circumstellar dynamics and transfer

This paper is both a review and a presentation of new models. Observation and modelization of circumstellar envelopes of early type or late type stars are now quickly evolving because of new techniques and facilities for observations, and increased power of computers. More and more complex physical phenomena involved in mass driving can now be modelized, at many different size scales. While most of models were previously based on informations derived from spectrophotometric data only or on measurements concerning objects observed with no spatial resolution, observations at much increased angular resolution can provide constraints on models of these phenomena. Theory and modelization must take this new situation into account. Two approaches are possible and effectively used. On the one hand, dynamical/physical self consistent models can be built; on the other hand, elaborate semi-empirical models including complicated distributions of matter with asymmetries (3D models) can be built and fitted for direct comparison with results of High Angular Resolution Measurements. Adding such constraints to classical constraints leads to a new insight in the physics of circumstellar matter and, through it, of stellar and interstellar evolution. Two examples have been chosen, in which new models are presented and assuming or not spherical symmetry is carefully discussed:

?Circumstellar matter around evolved stars

?Shock waves propagating in the circumstellar matter around evolved stars.

     

Brane cosmology from observational surveys and its comparison with standard FRW cosmology

Several simple dark energy models on the brane are investigated. They are compared with corresponding models in the frame of 4d Friedmann-Robertson-Walker cosmology. For constraining the parameters of the models considered, recent observational data, including SNIa apparent magnitude measurements, baryon acoustic oscillation results, Hubble parameter evolution data and matter density perturbations are used. Also, explicit formulas of the so-called state-finder parameters in teleparallel theories are found that could be useful to test these models and compare Loop Quantum Cosmology and Brane Cosmology. The conclusion is reached that a joint analysis as the one developed here allows to estimate, in a very clear way, possible deviation of our cosmology from the standard Friedmann-Robertson-Walker one.     

On the structure and composition of Uranus and Neptune

We present a series of models of Uranus and Neptune in which the relative amounts of (1) rock, (2) ices, and (3) hydrogen and helium are allowed to vary. By fitting the density and the gravitational quadruppole moment, the model composition can be determined. Because of the ambiguity in the rotation periods of these planets, several possible models are presented and discussed.     

Exact bianchi type-II,VIII and IX cosmological models in scale-covariant theory of gravitation

Exact Bianchi types-II, VIII and IX cosmological models are obtained in the scale covariant theory of gravitation. Some physical and geometrical properties of the models are studied.     

Peculiar shapes of asteroids: Implications for light curves and periods of rotation

This paper presents some simple geometrical models of asteroids with theoretical light curves similar to the observed ones. In some cases the results suggest rotation periods to be double those one can obtain adopting two- or three-axial ellipsoids as models.A possible model, not in terms of a binary system, for asteroids with light curves like those of eclipsing binary stars, is also given.It should be stressed that the models studied in this paper are probably not very similar to real asteroids, but the principal conclusions should not be changed when more sophisticated models are considered.The work is to be a starting point for future researches on laboratory models of asteroids, in order to define, in a quantitative way, how the light curves are affected by the surface roughness and/or the large scale irregularities of the shape of an asteroid.