Showing papers in "Astrophysics and Space Science in 1992"

Solar photo rates for planetary atmospheres and atmospheric pollutants

Abstract: Unattenuated solar photo rate coefficients and excess energies for dissociation, ionization, and dissociative ionization are presented for atomic and molecular species that have been identified or are suspected to exist in the atmospheres of planets, satellites (moons), comets, or as pollutants in the Earth atmosphere. The branching ratios and cross sections with resonances have been tabulated to the greatest detail possible and the rate coefficients and excess energies have been calculated from them on a grid of small wavelength bins for the quiet and the active Sun at 1 AU heliocentric distance.

Stimulated scattering of electromagnetic waves in dusty plasmas

Abstract: The nonlinear coupling between a large amplitude electromagnetic wave and the slow background motion in a dusty plasma is considered. Stimulated scattering instabilities are investigated. The relevance of our investigation to cometary and astrophysical plasmas is pointed out.

The angular width of the coherent back-scatter opposition effect: An application to icy outer planet satellites

Abstract: It has been suggested recently that coherent back-scattering of light from powder-like regolithic surfaces can explain remarkable opposition brightening of some atmosphereless solar system bodies In this paper, a dense-medium light-scattering theory is used to calculate the half-width at half-maximum (HWHM) of the coherent back-scattering peak for a number of scattering models We demonstrate that HWHM strongly depends on the optical properties of the scattering medium and can serve as a critical test in comparing alternative models It is shown that coherent back-scaterring may be a likely explanation of the opposition effect exhibited by icy outer planet satellites

An analysis of active close binaries (CB) based on photometric measurements. III : The inverse-problem method―an interpretation of CB light curves

Abstract: In order to enable a successful application of the realised CB models (presented in the two former articles of the series) in the analysis of the observed light curves, an efficient method unifying the best properties of the gradient method and of the differential-corrections one into a single algorithm (Djurašević, 1991) is proposed. This method is realised by modifying the Marquardt (1963) algorithm. The inverse problem is solved in an iterative cycle of corrections to the model elements based on a nonlinear least-square method.The interpretation of photometric observations is based on the choice of optimal model parameters yielding the best agreement between an observed light curve and the corresponding synthetic one. Some of these parameters can be determineda priori in an independent way, while the others are found by solving the inverse problem.The programmes for analysing light curves find the optimal system parameters rapidly, reliably and correctly. The corresponding programme support enables an evident graphic presentation of the results.

An analysis of active close binaries (CB) based on photometric measurements

Abstract: For the purpose of analysing light curves of active CB with an accretion disc being at the evolutional phase of an intensive matter exchange between the components a model for light-curve synthesis has been realized where the attention is given to systems like W Ser not sufficiently examined yet with regard that in them the accretion disc is formed around an ordinary star. In the paper one uses the elements presented in the first paper of this series. The model can successfully describe the essential characteristics of the observed light curves due to existence of an accretion disc and a hot spot, as well as those originated in the temperature distribution along the disc radius. The system components are considered in the framework of the nonsynchronous Roche model and the accretion disc of a constant thickness lies in the orbital plane around the star capturing the matter of the neighbouring component.

The Mass-Ratio Distribution of Spectroscopic Binary Stars

Abstract: In order to determine the mass-ratio distribution of spectroscopic binary stars, the selection effects that govern the observations of this class of binary systems are investigated. The selection effects are modelled numerically and analytically. The results of the models are compared to the data inThe Eighth Catalogue of the Orbital Elements of Spectroscopic Binary Stars (DAO8) compiled by Battenet al. (1989). The investigations involve binary systems with Main-Sequence primary components only, in order to avoid confusion of evolutionary and selection effects.

Linear stability of the triangular equilibrium points in the photogravitational elliptic restricted problem, I

Abstract: The linear stability of the triangular equilibrium points in the photogravitational elliptic restricted problem is examined and the stability regions are determined in the space of the parameters of mass, eccentricity, and radiation pressure. It is found that radiation pressure of the larger body for solar system cases exerts only a small quantitative influence on the stability regions.

MHD thermal-diffusion effects on free-convective and mass-transfer flow over an infinite vertical moving plate

Abstract: The Soret effect on MHD free-convective and mass-transfer flow of an incompressible, viscous, and electrically-conducting fluid, past a moving vertical infinite plate is studied. The flow is assumed to be at small Reynolds numbers so that the induced magnetic field is neglected. The problem is solved with the help of the Laplace transform method for two different values of the dimensionless functionf(t) signifying two different cases, e.g., (i) when the boundary surface, the flat plate, is impulsively started, moving in its own plane (I.S.P.) and (ii) when it is uniformly accelerated (U.A.P.). The effects on the velocity field as well as on the skin-friction of the various dimensionless parameters occurring into the problem, especially the magnetic parameterM and Soret number So, are discussed with the help of graphs.

Supernovae as sources of interstellar diamonds

Abstract: Small hydrocarbon grains in the vicinity of a supernova could be annealed by the absorption of several far-ultraviolet photons to produce the tiny diamonds found in meteorites. These freshly-synthesized diamond grains would be bombarded by the heavy ions and neutrals in the supernovae outflow and would thereby acquire the distinctive noble-gas isotopic signature by which they were first isolated. Only diamonds formed relatively close to supernovae would acquire such a signature, since grains formed farther out would be subjected to a much diluted and less energetic plasma environment.

Potential energy curves and dissociation energies of NbO, SiC, CP, PH+, SiF+, and NH+

Abstract: The potential energy curves for the electronic ground states of astrophysically important NbO, SiC, CP, PH+, SiF+, and NH+ molecules are constructed by the RKRV method. The dissociation energies are determined by curve-fitting techniques using the five-parameter Hulburt-Hirschfelder function. The estimated dissociation energies are 7.86±0.16, 3.66±0.09, 5.12±0.12, 3.08±0.09, 6.46±0.14, and 3.02±0.09 eV for NbO, SiC, CP, PH+, SiF+, and NH+, respectively. The estimatedD0 values are in reasonably good agreement with literature values. If we utilizeD0 values of PH+, SiF+, and NH+, ionization potentials for PH, SiF, and NH are derived. The ionization potentials are 10.12, 7.13, and 13.66 eV, respectively, for PH, SiF, and NH. Dissociation energies for the above molecules are also estimated by use of the Birge-Sponer extrapolation and Hildenbrand and Murad methods.

A new scalar tensor theory for gravity and the flat rotation curves of spiral galaxies

Abstract: In this paper we reproduce flat rotation curves of spiral galaxies by discussing a scalar tensor theory of gravity which includes the Higgs field as scalar field. The galaxy density distribution is assumed to be homogeneous or polytropic and in the galaxy core there exists a large concentrated mass.

Brans-dicke cosmology and the cosmological constant: The spectrum of vacuum solutions

Abstract: We study the Brans-Dicke vacuum field equations in the presence of a cosmological term A. Considering a Friedmann-Robertson-Walker metric with flat spatial sections (k=0), we provide a qualitative analysis of the solutions and investigate its asymptotic properties. The general solution of the field equations for arbitrary values ofw and A is obtained.

Arbitrary amplitude double layers in a multi-species electron-positron plasma

Abstract: The existence and properties of arbitrary amplitude double layers in a four-component electron-positron plasma, consisting of two species of hot electrons, a hot and a cold positron species, are investigated as functions of plasma properties such as density and temperature ratios. Their behaviour for other plasma models is also discussed. Applications to the polar-cusp region of pulsars is considered.

Bianchi type-II, VIII, and IX in certain new theories of gravitation

Abstract: Bianchi type-II, VIII, and IX models have been investigated in scalar-tensor theories developed by Saez (1985), Saez and Ballester (1985), and Lau and Prokhovnik (1986) (under the assumption of a certain relationship between the cosmological term λ and the scalar field ψ). The dynamical behaviour of these models has been studied.

Thermal-diffusion effects on MHD free-convective and mass-transfer flow past a moving infinite vertical plate in a rotating fluid

Abstract: An analytical study of MHD free-convective and mass-transfer flow past a moving infinite vertical plate, in a rotating fluid, is presented, taking into account the thermal diffusion effects. The solution of the problem is obtained with the help of the Laplace transform technique. Analytical expressions are given for the velocity field and for skin-friction for two different cases, e.g., when the plate is impulsively started, moving on its own plane (case I) and when it is uniformly accelerated (case II). The effects on the velocity field and skin-friction, of the various parameters entering into the problem, are discussed with the help of graphs.

On the restricted three-body problem with generalized forces

Abstract: By introducing general functions which depend on distance, a general scheme which determines the equilibrium solutions for the generalized restricted three-body problem is given. Applications to problems such as primaries considered as rigid bodies, influence of the radiation pressure of the primaries, and a combination of radiation pressure and rigid body are presented.

Sound waves in an electron-positron plasma

Abstract: By means of a kinetic description, we obtain the sound wave velocity in an electron-positron plasma at relativistic temperatures.

The amplitude of the opposition effect due to weak localization of photons in discrete disordered media

Abstract: Weak localization of photons in discrete disordered media is considered as a possible physical mechanism of the opposition effect of some atmosphereless bodies. The amplitude of the opposition effect is calculated by using the rigorousvector multiple-scattering theory and thescalar approximation. It is shown that the scalar approximation can significantly overestimate the amplitude of the opposition effect. Thus, this approximation should not be used in interpreting the observational data, and some previous results obtained with this approximation may require substantial revision.

Equilibrium structure of differentially rotating polytropic models of the stars

Abstract: In this paper we propose a method for computing the equilibrium structure of differentially rotating polytropic models of the stars. A general law of differential rotation of the type ω2=b 0+b 1 s 2+b 2 s 4, which can account for a reasonably large variety of possible differential rotations in the stars has been used. The distortional effects have been incorporated in the structure equations up to second order of smallness in distortion parametersb 0,b 1, andb 2 using Kippenhahn and Thomas' averaging approach in conjunction with Kopal's results on Roche equipotentials in manner similar to the one earlier used by Mohan and Saxena for computing the equilibrium structure of polytropes having solid body rotation. Numerical results have been obtained for various types of differentially rotating polytropic models of stars of polytropic indices 1.5, 3, and 4. Certain differentially rotating models of the Sun which are possible with such a type of law of differential rotation, have also been computed.

An analysis of active close binaries (CB) based on photometric measurements. II : Active CB with accretion discs

Abstract: For the purpose of analysing light curves of active close binaries with an accretion disc being at the evolutional phase of an intensive matter exchange between the components a model for light-curve synthesis has been realized where the attention is given to systems like W Ser not sufficiently examined yet with regard that in them the accretion disc is formed around an ordinary star

The projection of fractal objects

Abstract: Motivated by several recent studies on the geometry of molecular cloud structures, we perform two experiments that consider the projection of fractal images onto a two-dimensional screen. Evidence is presented to support the proposition that the dimensionDP of the contours surrounding the projected images are given byDs−1, whereDs>2 is the fractal dimension of the object itself. A brief survey of the geometry of astrophysical cloud structures is also presented.

Some unified presentations of the Voigt functions

Abstract: The principal object of this note is to provide a natural further step toward the unified presentations of the Voigh functionsK(x, y) andL(x, y) which play a rather important role in such diverse fields of physics as astrophysical spectroscopy and the theory of neutron reactions. Explicit representations for these functions, given in terms of some relatively more familiar special functions of one and two variables, are potentially useful in finding many other needed (numerical or analytical) properties of the Voigt functions. Several erroneous recent contributions to the theory of Voigt functions, including (for example) the main result of A. Siddiqui (1990), are also corrected here.

Three-dimensional supershells in differentially-rotating galactic disks

Abstract: The evolution of remnants produced by the correlated supernovae explosions in OB-associations is investigated. The 2.5-dimensional numerical hydrodynamic scheme based on thin layer approximation is proposed. Inhomogeneity of the gas density distribution, galactic disk shear, gravity, and continuous energy input from SNe explosions are taken into account.

Laminar convection in binary mixture of hydromagnetic flow with radiative heat transfer, I

Abstract: The paper studies the hydromagnetic flow of a thermally-radiating binary mixture of an incompressible fluid. Both the Soret and Dufour effects are considered when the fluid is not chemically reacting. When the gas is optically thin the resultant system of ordinary differential equations could be solved in a closed form. Generally the problem is reduced to a set of integral equations which are tackled by perturbation and iteration. The problem is worthy of note in the re-entry problem.

A note on the very small grains (VSGs) observed at Halley's comet

Abstract: The most striking feature in the spatial distribution of the smallest dust grains observed at Halley's comet by the VEGA-1 spacecraft is the sharp glitch at a cometocentric distance of about 180,000 km, which approximately corresponds to the cometopause inside which the contaminated solar-wind plasma was rapidly cooled This glitch is theorized to have been caused by the electrostatic disruption of larger composite grains which rapidly charged up as they traversed the cometopause The clear asymmetry in the distribution between the inbound and outbound portion of the spacecraft trajectory is also consistent with the dynamical effects of grain charging

An equation of state for radiating dissipative spheres in general relativity

Abstract: The effects of viscosity and thermal conduction on the evolution of radiating fluid spheres are studied. Our proposition for studying dissipative fluids in a general relativistic approach consists of a prescription of an equation of state which specifies the degree of anisotropy induced by the viscosity. An explicit Schwarzschild-like model is worked out, using the Herrera-Jimenez-Ruggeri method extended to the case of dissipative radiating fluids.

Exact Bianchi-type VIII and IX models in the presence of the self-creation theory of cosmology

Abstract: Exact Bianchi-type VIII and IX models in the presence of Barber's second self-creation theory of cosmology are presented, when the source of the gravitational field is a perfect fluid withP=ρ. Some physical and geometrical properties of the models are discussed.

Inflation and compactification from Galaxy redshifts

Abstract: The distribution of galaxies in the pencil-beam surveys of Broadhurstet al. which proved periodical across 8–10 consecutive steps in a flat dust model withq 0=0.5 is found to reveal extended periodicity up to 16–17 phase-coherent steps, covering the total sample, in a flat, moderately inflationary model withq 0=−0.5 (vacuum/dust ratio 2/1). In the latter model the vacuum component helps to reach the critical density and lengthens the expansion time-scale. It is shown that the explanation of the found periodicity as a consequence of space compactification as suggested by G. Paal twenty years ago in connection with apparent quasar periodicities is still possible.

Ion-acoustic solitons in a warm plasma including negative ions in the vicinity of critical density

Abstract: Ion-acoustic solitons in a warm plasma with different adiabatic ion constituents (with the same temperature) and isothermal electrons in the vicinity of critical negative ion density are studied by use of the reductive perturbation theory. The basic set of equations is reduced to an evolution equation which includes quadratic and cubic nonlinearities. The effective potential of this equation agress exactly, for small wave amplitudes, with the Sagdeev potential obtained from the exact stationary solution of the original fluid equations. This implies that the evolution equation holds not only in the vicinity of the critical ion density, but also in the whole range of the negative ions under the condition of small wave amplitude.

Nonlinear ion-acoustic solitons in a warm plasma with adiabatic positive and negative ions and hot non-isothermal electrons

Abstract: The propagation of an ion-acoustic soliton in a collisionless plasma with adiabatic positive and negative ions (with equal ion temperature) and hot non-isothermal electrons is studied by use of the renormalization method introduced by Kodama and Taniuti in the reductive perturbation method. The basic set of fluid equations describing the system is reduced to a Korteweg-de Vries (K-dV)-type equation for the first-order perturbed potential and to a linear inhomogeneous differential equation to the second-order of the perturbed potential. A stationary solution of the coupled equations is obtained.