Showing papers in "Astrophysics and Space Science in 1991"
TL;DR: In this paper, the mass-luminosity and mass-radius relations (MLR and MRR) for the Main-Sequence (MS) stars have been used together with new theoretical estimates in the study of mass luminosity.
Abstract: The new binary star data have been used together with new theoretical estimates in the study of mass-luminosity and mass-radius relations (MLR and MRR) for the Main-Sequence (MS) stars. The slopes of the relations change at certain critical values, e.g., at ∼0.5M⊙ for MLR and at ∼1.7M⊙ for MRR. The first point is the indication of transition from fully-convective internal structure to the developing radiative core, and the second point, the transition from proton-proton cycle to the carbon-nitrogen cycle as the dominant source of nuclear energy in these stars. It seems that these critical points are important in constraining on the stellar models. Improved set of more parameters for stars particularly around the critical points where the slopes of the relations change, may help bringing the theory and observations closer.
206 citations
TL;DR: In this article, high linear polarization (up to 5-8%) discovered in the deep minima of isolated AE-Herbig stars is discussed in the framework of the model of "zodiacal light" produced by scattering matter in a circumstellar dust envelope (probably in protoplanetary discs).
Abstract: High linear polarization (up to 5–8%) discovered by our group in the deep minima of isolated AE-Herbig stars is discussed in the framework of the model of ‘zodiacal light’ produced by scattering matter in a circumstellar dust envelope (probably in protoplanetary discs). The numerical simulations of polarizational and colourimetrical properties of the scattered light based on the Mie theory permit to obtain from the observations some important parameters of circumstellar dust: the size distribution of grains, their rough chemical composition (silicate/graphite), the flatness of the dust envelope.
182 citations
TL;DR: In this paper, a Monte-Carlo simulation of the neutral interstellar atoms penetration into the solar system is considered, and the ionization and charge exchange processes with due regard for the plasma interface structure as well as the gravitational force and the radiation pressure are taken into account.
Abstract: The Monte-Carlo simulation of the neutral interstellar atoms penetration into the solar system is considered. The ionization and charge exchange processes with due regard for the plasma interface structure as well as the gravitational force and the radiation pressure are taken into account. The Monte-Carlo scheme with splitting of atom trajectories is developed, and its high efficiency is shown against the imitation Monte-Carlo scheme. The main algorithms of the splitting scheme and the possible applications of the suggested method are described. In particular calculation of the mass, momentum and energy sources appearing in the plasma gasdynamical equations is considered. The gasdynamical approach for these terms is shown to be inconsistent with the Monte-Carlo results.
87 citations
TL;DR: Bianchi type I, III, V, VI0, and Kantowski-Sachs type models have been investigated in a scalar tensor theory developed by Saez and Ballester as discussed by the authors.
Abstract: Bianchi type I, III, V, VI0, and Kantowski-Sachs type models have been investigated in a scalar tensor theory developed by Saez and Ballester (1985) and Saez (1985). The dynamical behaviour of the models has also been analyzed.
85 citations
TL;DR: In this paper, an analytical study was performed to examine the mass transfer characteristics on the free-convection flow of an incompressible viscous fluid past an exponentially accelerated infinite vertical plate under the action of constant heat flux.
Abstract: An analytical study is performed to examine the mass-transfer characteristics on the free-convection flow of an incompressible viscous fluid past an exponentially accelerated infinite vertical plate under the action of constant heat flux. The effects of various parameters occurring into the problem, is extensively discussed.
66 citations
TL;DR: Very high and ultra-high-energy gamma-ray fluxes expected from giant molecular clouds, and the prospects of detection of these fluxes by means of air Cherenkov telescopes and EAS arrays of new generation are discussed in this paper.
Abstract: Very high and ultra-high-energy gamma-ray fluxes expected from giant molecular clouds, and the prospects of detection of these fluxes by means of air Cherenkov telescopes and EAS arrays of new generation are discussed.
60 citations
TL;DR: In this article, a spatially homogeneous and anisotropic Bianchi type-II and III cosmological models are obtained in Barber's second self-creation theory of gravitation both in vacuum and in the presence of perfect fluid with pressure equal to energy density.
Abstract: Spatially-homogenous and anisotropic Bianchi type-II and III cosmological models are obtained in Barber's second self-creation theory of gravitation both in vacuum and in the presence of perfect fluid with pressure equal to energy density. Some properties of the models are discussed.
56 citations
TL;DR: In this paper, the effect of a uniform transverse magnetic field on the free-convection and mass-transform flow of an electrically-conducting fluid past an infinite vertical plate for uniformly accelerated motion of the plate through a porous medium is discussed.
Abstract: The effect of a uniform transverse magnetic field on the free-convection and mass-transform flow of an electrically-conducting fluid past an infinite vertical plate for uniformly accelerated motion of the plate through a porous medium is discussed. The magnetic lines of force are assumed to be fixed relative to the plate. Expression for the velocity field and skin-friction are obtained by the Laplace transform technique. The influence of the various parameters, entering into the problem, on the velocity field and skin-friction is extensively discussed.
53 citations
TL;DR: In this article, general equations for the conductivity and resistivity tensors in a multicomponent plasma of strongly degenerate fermions with a magnetic field were derived in terms of rates of momentum transfer in particle collisions.
Abstract: General equations for the conductivity and resistivity tensors in a multicomponent plasma of strongly degenerate fermions with a magnetic fieldB are derived in terms of rates of momentum transfer in particle collisions. For two-, three-, and four-component plasmas, the equations are obtained in closed form. Scaling relations are presented which exhibit the universal temperature and magnetic field dependence of the conductivity and resistivity. When the magnetic field is high and an electrically neutral particle component is present, the resistivity transverse toB, ℛ⊥, suffers an enhancement ℛ⊥∼B2. When the neutral component is absent, ℛ⊥ is field-independent asB→∞. The expressions for the momentum transfer rates are simplified for further application to the neutron star interiors in the next paper (this issue). The derivation of the expressions for the particle diffusion tensor is outlined in the Appendix.
43 citations
TL;DR: In this paper, the inflationary solutions for the Bianchi-IX space-time in presence of a massless scalar field with a flat potential were studied for pure massive strings following the Takabayashi equation of state ρ=(1+w)λ.
Abstract: In this paper the inflationary solutions are studied for the Bianchi-IX space-time in presence of a massless scalar field with a flat potential. Also a class of cosmological solutions of massive strings are obtained following the techniques used by Letelier and Stachel. Some solutions are calculated for pure massive strings following the Takabayashi equation of state ρ=(1+w)λ.
40 citations
TL;DR: In this paper, an analytical study was performed to examine the heat source characteristics on the free-convection and mass transfer flow past an impulsively started infinite nonconducting vertical plate of a viscous, incompressible electrically-conducting fluid under the action of a uniform magnetic field through porous medium.
Abstract: An analytical study is performed to examine the heat source characteristics on the free-convection and mass transfer flow past an impulsively started infinite non-conducting vertical plate of a viscous, incompressible electrically-conducting fluid under the action of a uniform magnetic field through porous medium. The effects of various parameters on the velocity field are extensively discussed.
TL;DR: In this paper, the mergers of unequal-mass components of a binary globular cluster were studied. But the authors only considered the case of mass ratio 1∶0.5 by means of an N-body code with 6144 particles in total.
Abstract: Merger process of binary globular cluster is discussed for a pair of unequal-mass components. We calculated the case of mass ratio 1∶0.5 by means of anN-body code with 6144 particles in total. We have found the followings. The mass exchange between the components takes place through the Roche-lobe overflow. In the early stages, however, the dynamical evolution is mainly governed by escape of particles from the system. As the particles escape carrying angular momentum with them, the separation between the component cluster shrinks. The time-scale of this shrinkage depends upon the size of the clusters. When a critical separation is reached, the orbital angular momentum is transferred unstably to the spins of the component clusters. This is the process of the synchronization instability which was found in a previous study on binary cluster of equal masses. As a result the component clusters merge into a single cluster. The structures of the mergers are quite similar among different cases except for the central cores which retain their initial central concentrations. In particular, the ellipticity and the rotation curve are quite close each other among models of different initial radii and of different mass ratios.
TL;DR: In this paper, the role of field emission in mitigating the process of electrostatic disruption of charged cosmic dust grains is discussed, and it is shown that this process prevents the disruption of even the smallest grains independent of the plasma environment if the tensile strength of such grains is greather than about 3.6×107 dynes cm−2.
Abstract: The role of field emission in mitigating the process of electrostatic disruption of charged cosmic dust grains is discussed. It is shown that this process prevents the disruption of even the smallest grains independent of the plasma environment if the tensile strengthF
t
of such grains is greather than about 3.6×107 dynes cm−2. On the other hand, if such grains are very fragile (i.e.,F
t
≲106 dynes cm−2, as in ‘cometary’ grains) even field emission cannot always prevent their electrostatic disruption. In this case it is required that the magnitude of the potential of the grains is ≲0.15 V, which places stringent upper limits on the ambient plasma temperatureT. For pure hydrogen and pure oxygen plasmas,T≲700 K andT≲460 K, respectively.
TL;DR: In this paper, a study of free convection in hydromagnetic flows through a porous medium of a heat generating fluid past an infinite vertical porous plate is made, where a strong magnetic field is imposed in a direction which is perpendicular to the free stream and makes an alge α to the vertical direction.
Abstract: A study is made of the free convection in hydromagnetic flows through a porous medium of a heat generating fluid past an infinite vertical porous plate. A strong magnetic field is imposed in a direction which is perpendicular to the free stream and makes an alge α to the vertical direction. The governing equations for the hydromagnetic fluid flow and the heat transfer are solved analytically. The influence of Hall currents, the permeabilityK and the inclination α upon the velocity field are shown in figures.
TL;DR: In this article, the authors presented further results of the calculation of anomalous extinction laws following Steenman and The (1989, Astrophys. Space Sci., 159), which were calculated by enlarging the lower size cutoff of the particle size distribution.
Abstract: In this paper we present further results of the calculation of anomalous extinction laws following Steenman and The (1989, Astrophys. Space Sci., 159). The extinction laws in this article are calculated by enlarging the lower size cutoff of the particle size distribution, as opposed to the laws in the previous paper, which were obtained by making the upper size cutoff larger.
TL;DR: In this paper, general equations for the conductivity and resistivity tensors in a multicomponent plasma of strongly degenerate fermions with a magnetic field B are derived in terms of rates of momentum transfer in particle collisions.
Abstract: General equations for the conductivity and resistivity tensors in a multicomponent plasma of strongly degenerate fermions with a magnetic field B are derived in terms of rates of momentum transfer in particle collisions. For two-, three-, and four-component plasmas, the equations are obtained in closed form. Scaling relations are presented which exhibit the universal temperature and magnetic field dependence of the conductivity and resistivity.
TL;DR: In this paper, a statistical study of 228 flares on the three UV Ceti-type stars, i.e., YZ CMi, AD Leo, and EV Lac, is presented.
Abstract: A statistical study of 228 flares on the three UV Ceti-type stars, i.e., YZ CMi, AD Leo, and EV Lac, is presented. Observations were gathered by Ichimura and Shimizu over a total monitoring time of 907 hours distributed over 18 years (1971 to 1988). Period analysis of flare activity was performed, and no periodicity was detected on the three stars for either the flare number rate or the energy rate in time-scales ranging from a year up to 14 years. Average colour of flares at peak was (U-B)=−0.98±0.17 and (B-V)=0.05±0.13. Cumulative number distributions of flare event time-integrated energies were solved by a least-squares method on a log-log plot for a power-law function to get both the constant of and the gradient β, which were found to be similar among the three stars. The gradient showed that rare large flare events radiate most of the energy released by all the flare events in the monitoring time. The flare number rate and energy rate are similar if the power-law distributions are extended up to a specific maximum energy. In reality, the Kolmogorov-Smirnov test showed that the observed cumulative number distributions of flare event energy were not necessarily a power-law function. The monte-Carlo simulation, however, indicates that the monitoring time and/or the patrol observation time interval may not be long enough to get the average flare number rate and energy rate, especially at the upper energy limits which are statistically unreliable.
TL;DR: In this article, the effective temperatures of 211 early-type stars found in a previous investigation were combined with the effective temperature of other 313 early type stars found from literature to derive a new stellar temperature scale and the standard deviation of the MK spectral classification.
Abstract: Empirical effective temperatures of 211 early-type stars found in a previous investigation (Kontizas and Theodossiou, 1980; Theodossiou, 1985) are combined with the effective temperatures of other 313 early-type stars found from literature. From these effective temperatures of a total number of 524 early-type stars of spectral types from O8 to F6 we derive a new stellar temperature scale and the standard deviation of the MK spectral classification.
TL;DR: In this article, a theoretical model of shock wave propagation in a self-gravitating radiative magneto-hydrodynamic medium has been studied, and the effects of the magnetic field, radiation, and gravitation have been discussed separately.
Abstract: A theoretical model of shock wave propagation in a self-gravitating radiative magneto-hydrodynamic medium has been studied. The effects of the magnetic field, radiation, and gravitation have been discussed separately. The results discussed depend upon the numerical variations of flow variables behind the shock.
TL;DR: In this paper, a prescription originally conceived for perfect fluids is extended to the case of anisotropic pressure, and the method is used to obtain exact analytical solutions of the Einstein equations for spherically-symmetric self-gravitating distribution of the anisotropes.
Abstract: A prescription originally conceived for perfect fluids is extended to the case of anisotropic pressure. The method is used to obtain exact analytical solutions of the Einstein equations for spherically-symmetric self-gravitating distribution of anisotropic matter. The solutions are matched to the Schwarzschild exterior metric.
TL;DR: In this paper, it was shown that a bounded interior static sperical-symmetric charged dust, if exists, can only be of electromagnetic origin, which is a continuation of our earlier work on gravitational sources of purely electromagnetic origin.
Abstract: The present paper is a continuation of our earlier work on gravitational sources of purely electromagnetic origin, known in the literature as electromagnetic mass models. Here we have shown that a bounded (regular) interior static sperical-symmetric charged dust, if exists, can only be of electromagnetic origin.
TL;DR: In this paper, a method is proposed for computing the eigenfrequencies of small adiabatic barotropic modes of oscillations of rotationally and tidally-distorted stars.
Abstract: In this paper a method is proposed for computing the eigenfrequencies of small adiabatic barotropic modes of oscillations of rotationally and tidally-distorted stars. The method utilizes Kippenhahn and Thomas approach and concepts of Roche equipotentials to incorporate up to second-order the effects of rotation and tidal distortion terms on the eigenfrequencies. The proposed method has also been used to compute the eigenfrequencies of certain barotropic modes of oscillation of some rotationally and tidally distorted models of 10M⊙, and 2.5M⊙ Main-Sequence stars.
TL;DR: In this paper, it has been shown that when the metric potentials are functions of only one of the coordinates, the interior charged dust becomes purely of electromagnetic origin, in the sense that the physical quantities like the energy density, the effective gravitational mass, etc., are dependent only on the charge density and vanish when this charge density vanishes.
Abstract: The Einstein-Maxwell field equations for charged dust corresponding to static axially-symmetric metric of Levi-Civita have been studied. It has been shown that when the metric potentialsgij are functions of only one of the coordinates, viz.,r, the interior charged dust becomes purely of electromagnetic origin, in the sense that the physical quantities like the energy density, the effective gravitational mass, etc., are dependent only on the charge density and vanish when this charge density vanishes. Such models are known as electromagnetic mass models in the classical electrodynamics. An interior charged dust solution corresponding to this case has been obtained which, in a sense, represents an infinite dust distribution of electromagnetic origin. In the second case, viz., when the metric potentials are functions of the coordinatesr andz both, it has been shown that some of the situations correspond to electromagnetic mass models. An example to illustrate this case has been obtained. This represents the source of the ‘Reissner-Nordstrom-Curzon’ field (an analogue of the Reissner-Nordstrom solution obtained by Curzon) which according to Curzon describes the exterior field of an electron.
TL;DR: In this paper, the relativistic polytropic equation of state of spherical-symmetric massive structures has been studied in detail and various static parameters have been calculated under all possible variations of σ 0 = P0/E0∶0 pertaining to the central value.
Abstract: Spherical-symmetric massive structures with a relativistic polytropic equation of state have been studied in detail. Thevarious static parameters have been calculated under all possible variations of σ0 (=P0/E0∶0 pertaining to the central value) and the polytropic indexn. In particular, the values of the static parameters have been identified under the extreme relativistic conditions and for the most bound structures. The structures have been used to model neutron stars and the various neutron parameters have been calculated. Radial pulsation and rotation of these configurations have also been discussed.
TL;DR: The authors showed that considering the torsion in early universe, we are led to an inflationary expansion with only a massless scalar field, so avoiding all physical questions that we are facing when working with massive scalar fields.
Abstract: We show that considering the torsion in early universe, we are led to an inflationary expansion with only a massless scalar field, so avoiding all physical questions that we are facing when working with massive scalar field.
TL;DR: In this article, the effects of energy losses on free-free emission and Comptonization of a typical WR+O binary system were investigated in the model of two colliding stellar winds.
Abstract: Two-dimensional calculations in the model of two colliding stellar winds are provided. The effects of energy losses on free-free emission and Comptonization are investigated. The expected X-ray emission of a typical WR+O binary system is calculated.
TL;DR: In this paper, the maximum acceleration at the Planck epoch is shown to be related to the maximum magnetic field and curvature as well as temperature in that era, and spin-torsion effects at that epoch also lead to same value.
Abstract: The maximum acceleration at the Planck epoch is shown to be related to the maximum magnetic field and curvature as well as temperature in that era. Spin-torsion effects at that epoch also lead to same value.
TL;DR: In this paper, the authors argue that a major star formation in giant galaxies is preceded by an evolutionary phase at which a strong galactic wind driven by the initial burst of star formation enriches the gaseous protogalaxy with metals and heats it up, so that the latter turns over from contraction to expansion.
Abstract: We argue that a combined evidence from galactic and extragalactic studies suggests that a major star formation in giant galaxies is preceded by an evolutionary phase at which a strong galactic wind driven by the initial burst of star formation enriches the gaseous protogalaxy with metals and heats it up, so that the latter turns over from contraction to expansion. The result is the ejection of enriched material from the outer part of the protogalaxy into the intergalactic space, while the inner part, after a delay of about one to a few Gyr, finally contracts and cools down to form the galactic major stellar component (the ‘hot’ model of galaxy formation). The paper presents a specific mechanism to produce a hot protogalaxy according to which an early galactic wind is imparting energy and momentum into a collapsing protogalaxy whose mass is contained mainly in clouds and only a small portion is in the intercloud gas that provides pressure confinement for the clouds. The model is then capable of accounting for the nearly equal mass and iron abundance in cluster giant galaxies and the intracluster gas provided the observationally plausible input parameters for giant galaxies and early galactic winds are adopted. It also predicts the formation of long-lived X-ray coronae with characteristics similar to those observed around giant ellipticals. The model specifies a characteristic length-scale that can be very naturally interpreted as a size for a stellar system to come; a very encouraging result is that it perfectly fits in with a typical size of giant ellipticals.
TL;DR: In this article, large-scale morphological waves with wavelengths of order 30° and amplitude 1/8λ exist in several long Hi filaments in a 540 square degrees of sky aroundl=230°,b=+40°.
Abstract: Large-scale morphological waves with wavelengths of order 30° and amplitude=1/8λ exist in several long Hi filaments in a 540 square degrees of sky aroundl=230°,b=+40°. The extent of the longest filament is greater than 72°, the limit set by the boundaries of the area surveyed. TheHi gas appears to be controlled by magnetic fields and the motion within flux tubes, determined by analysis of velocity along the filament axes, shows the presence of wave patterns with amplitude 5 to 6 km s−1 on an angular scale similar to that seen in the spatial structure projected on the sky.