Showing papers in "Astrophysics and Space Science in 1998"
TL;DR: In this article, a general planar configuration consisting of n-1 primaries arranged in equal arcs on an ideal ring and a central body of different mass located at the centre of mass of the system is proposed.
Abstract: Our intention in this article is to present a new model for the investigation of the motion of a particle of negligible mass in a multibody surrounding The proposed general planar configuration consists of ν = n-1 primaries arranged in equal arcs on an ideal ring and a central body of different mass located at the centre of mass of the system We formulate the general equations of motion and we study the stationary solutions and the zero-velocity contours for various values of ν
83 citations
TL;DR: In this article, spectra obtained with the SWS on board of ISO of dust shells around O-rich objects were used to reveal the presence of many new emission features between 10 and 45 µm.
Abstract: This paper reviews spectra obtained with the SWS on board of ISO of dust shells around O-rich objects. These spectra reveal the presence of many new emission features between 10 and 45 µm. These bands are generally much narrower than the well-known 10 and 20 µm silicates features. The strength of these features relative to the underlying broad continuum varies from source to source (≃ 5–50%). The 10 µm region shows evidence for the presence of Al2O3 grains. At longer wavelength, the spectra are dominated by features due to crystalline olivine and pyroxene. The exact peak position of these features shows that the emitting grains consist of the Mg-rich end-members of these minerals with an Fe-content of <10 %. The underlying continuum is attributed to amorphous silicate grains.
79 citations
[...]
TL;DR: The CASTLES (CfA-Arizona-Space-Telescope-LEns-Survey) project as mentioned in this paper collected a sample of NICMOS images of gravitational lenses and a brief list of the preliminary findings.
Abstract: We describe the goals of the CASTLES (CfA-Arizona-Space-Telescope-LEns-Survey) project including a sample of NICMOS images of gravitational lenses and a brief list of the preliminary findings.
68 citations
TL;DR: In this paper, the authors investigated the effects of quintessence fluctuations on the background cosmological evolution, the cosmic microwave background (CMB) and mass power spectrum, finding a broad range of cosmologically viable models.
Abstract: We examine the possibility that a substantial fraction of the total energy density in a spatially flat Universe is composed of a time-dependent and spatially inhomogeneous component whose equation-of-state differs from that of baryons, neutrinos, dark matter, or radiation. In this lecture, we report on our investigations of the case in which the additional energy component, dubbed "quintessence", is due to a dynamical scalar field evolving in a potential. We have computed the effects on the background cosmological evolution, the cosmic microwave background (CMB) and mass power spectrum, finding a broad range of cosmologically viable models. We stress three important features of the quintessence or Q-component: the time evolution of the equation-of-state; the length-scale dependence of the speed of propagation of the fluctuations in the Q-component; and, the contribution of quintessence fluctuations to the CMB anisotropy spectrum.
62 citations
TL;DR: In this article, a rigorous theoretical investigation has been made of obliquely propagating dust-acoustic solitary structures in a cold magnetized two-ion-temperature dusty plasma consisting of a negatively charged, extremely massive, cold dust fluid and ions of two different temperatures.
Abstract: A rigorous theoretical investigation has been made of obliquely propagating dust-acoustic solitary structures in a cold magnetized two-ion-temperature dusty plasma consisting of a negatively charged, extremely massive, cold dust fluid and ions of two different temperatures. The reductive perturbation method has been employed to derive the Korteweg-de Vries (K-dV) equation which admits a solitary wave solution for small but finite amplitude limit. It has been shown that the presence of second component of ions modifies the nature of dust-acoustic solitary structures and may allow rarefactive dust-acoustic solitary waves (solitary waves with density dip) to exist in such a dusty plasma system. The effects of obliqueness and external magnetic field on the properties of these dust-acoustic solitary structures are also briefly discussed.
54 citations
TL;DR: In this paper, the problem of magnetohydrodynamic free-convection flow, with radiative heat transfer in porour media subject to time-dependent suction of an incompressible and optically transparent medium has been solved making a fairly realistic assumption.
Abstract: The problem of magnetohydrodynamic free-convection flow, with radiative heat transfer in porour media subject to time-dependent suction of an incompressible and optically transparent medium has been solved making fairly realistic assumption. For a small-time-dependent perturbation of the fluid velocity and temperatures, the nonlinear problem is tackled by asymptotic approximation, giving solutions for steady-flow on which a first-order transient component is superimposed. The effect of heat radiation and free convection on the flow of the fluid is demonstrated analytically and quantitatively. The flow field is seen to be affected mainly by radiation and convection parameters, in addition to the porosity and magnetic factors.
44 citations
Journal Article•
41 citations
TL;DR: In this paper, the potential energy curves for the electronic ground states of astrophysically important YO, CrO, BN, ScO, SiO and AlO molecules are constructed using the five-parameter Hulburt-Hirschfelder function.
Abstract: The potential energy curves for the electronic ground states of astrophysically important YO, CrO, BN, ScO, SiO and AlO molecules are constructed techniques using the five-parameter Hulburt-Hirschfelder function. The estimated dissociation energies are 7.235±0.15, 4.337 ± 0.09, 3.917 ± 0.09, 6.899 ± 0.14, 8.181 ± 0.17 and 5.202 ± 0.11 eV for YO, CrO, BN, ScO, SiO and AlO, respectively. The estimated D0 values are in reasonably good agreement with literature values. The r-Centroids and Franck-Condon factors for the bands of B2 Σ+ − X2 Σ+ of YO, B5 Π −X5 Π> of CrO, A3 Π − X3 Π of BN, B2 Σ+ −X2 Σ+ of ScO, E1 Σ+ − X1 Σ+ of SiO and D2 Σ+ − X2 Σ+ and B2 Σ+ −X2 Σ+ of AlO molecules have been determined. The Franck-Condon factors are evaluated by the approximate analytical method of Jarmain and Fraser. The absence of the bands in these systems is explained.
38 citations
TL;DR: In this article, the authors review the dynamics of radiatively driven mass loss from rapidly rotating hot-stars and show that gravity darkening effects can actually lead to a reduced mass loss, and thus lower density, in the wind from the equatorial region.
Abstract: We review the dynamics of radiatively driven mass loss from rapidly rotating hot-stars We first summarize the angular momentum conservation process that leads to formation of a Wind Compressed Disk(WCD) when material from a rapidly rotating star is driven gradually outward in the radial direction We next describe how stellar oblateness and asymmetries in the Sobolev line-resonance generally leads to nonradialcomponents of the driving force is a line-driven wind, including an azimuthal spin-down force acting against the sense of the wind rotation, and a latitudinal force away from the equator We summarize results from radiation-hydrodynamical simulations showing that these nonradial forces can lead to an effective suppressionof the equatorward flow needed to form a WCD, as well as a modest (∼ 25%) spin-downof the wind rotation Furthermore, contrary to previous expectations that the wind mass flux should be enhanced by the reduced effective gravity near the equator, we show here that gravity darkening effects can actually lead to a reducedmass loss, and thus lower density, in the wind from the equatorial region Finally, we examine the equatorial bistability model, and show that a sufficiently strong jump in wind driving parameters can, in principle, overcome the effect of reduced radiative driving flux, thus still allowing moderate enhancements in density in an equatorial, bistability zone wind
34 citations
TL;DR: In this paper, exact solutions of Einstein field equations in the scalar tensor theory developed by Saez and Ballestev are presented in vaccum and in the presence of stiff fluid.
Abstract: For the general plane symmetric metric, some exact solutions of Einstein field equations in the scalar tensor theory developed by Saez and Ballestev are presented in vaccum and in the presence of stiff fluid. The physical and kinematical features of the models are also discussed.
31 citations
TL;DR: In this paper, it was shown that dust-neutral and ion-neutral drags can produce instabilities of electrostatic waves in multicomponent self-gravitating dusty plasmas.
Abstract: It is shown that dust-neutral and ion-neutral drags can produce instabilities of electrostatic waves in multicomponent self-gravitating dusty plasmas. Explicit expressions for the growth rates of the instabilities are presented. The relevance of our investigation to fragmentation of molecular dust clouds is discussed.
TL;DR: In this paper, the effect of wave-particle in- teractions on the outflow characteristics of polar wind plasma was investigated, and the effects of WPI were introduced via a Monte Carlo technique, and an iterative approach was used in order to converge to self-consistent results.
Abstract: The energization of charged particles, due to interaction with the ambient electromagnetic turbulence, has a significant influence on the plasma transport in space. The effect of wave-particle in- teractions on the outflow characteristics of polar wind plasma was investigated. The theoretical model included gravitational acceleration .g/, polarization electrostatic field .Ep/, and divergence of the geomagnetic field. Within the simulation region (1.7 to 10 earth radii, Re) the ions were assume to be collisionless and the electrons to obey a Boltzmann relation. Profiles of altitude-dependent diffusion coefficients TD?.O C / and D?.H C /U were computed from the wave spectral density (S) observed by the Plasma Wave Instrument (PWI) on board DE-1. The effects of WPI were introduced via a Monte Carlo technique, and an iterative approach was used in order to converge to self-consistent results. The main conclusions of this study were the following. As a result of perpendicular heating, the temperature anisotropy .Tk=T?/ was reduced and even reversed .Tk 1, the heating rate becomes self-limited and the ion distribution displays toroidal features. This result is consistent with the observation of O C toroidal distribution in the high altitude ionosphere. Finally, the large variability in the wave spectral density S was studied. This variability was found to change our results only in a quantitative manner, while our conclusions remained qualitatively unchanged.
[...]
TL;DR: In this article, the authors provide a short pedagogical introduction to the consequences of a nonzero cosmological constant and provide a description of the future of the universe if the cosmology constant is nonzero and then provide a summary.
Abstract: The cosmological constant is an energy associated with the vacuum, that is, with 'empty space'. The possibility of a nonzero cosmological constant Λ has been entertained several times in the past for theoretical and observational reasons (early work includes e.g. Einstein 1917; Petrosian, Salpeter and Szekeres, 1967; Gunn and Tinsley, 1975; a popularized description of the history is found in Goldsmith, 1997). Recent supernovae results (Perlmutter et al., 1998; Ries et al., 1998) have made a strong case for a nonzero and possibly quite large cosmological constant. Their results have encouraged increased interest in the properties of a universe with nonzero cosmological constant. Several other observations of various cosmological phenomena are also planned or underway which will further constrain the range of allowed values for the cosmological constant. Given the expected quality and quantity of upcoming data, there is reason to believe that we will know soon whether or not we need to learn to 'live with lambda'. The purpose of this review is to provide a short pedagogical introduction to the consequences of a nonzero cosmological constant. Basic terms are defined in Section 1, where the equations for the time evolution of the scale factor of the universe (defined below) are given. Section 2 indicates the current theoretically expected values of the cosmological constant, introducing the theoretical 'cosmological constant problem'. Some suggestions to explain a cosmological constant consistent with current measurements are listed. In Section 3, the time evolution of the scale factor of the universe (from Section 1) is used to show how the age of the universe, the path length travelled by light, and other properties depending on the spacetime geometry vary when the cosmological constant is present. Section 4 outlines some effects of a nonzero cosmological constant on structure formation. Section 5 summarizes how some recent and upcoming measurements may constrain Λ. Several observations (including those of the supernovae) are described which have provided constraints or show promise for the future. The quality of those observations are improving rapidly. However, the current theoretical explanations for a nonzero cosmological constant consistent with the data, some of which are listed in Section 2, are not compelling. Section 6 briefly describes some suggested theoretical alternatives to a nonzero cosmological constant. Section 7 contains a description of the future of the universe if the cosmological constant is nonzero and then provides a summary. Earlier reviews, in particular the one by Carroll, Press and Turner (1992, hereafter denoted by CPT) are highly recommended for some of the in depth results and more references, as well as the books by Kolb and Turner (1990), Peebles (1993) and Padmanabhan (1993) for the basic cosmology. The referencing is indicative rather than comprehensive. For more extensive referencing consult the more in depth reviews, textbooks and articles cited.
TL;DR: In this article, the polarization behavior of radio waves propagating through an ultrarelativistic highly magnetized electron-positron plasma in a pulsar magnetosphere was investigated.
Abstract: We consider the polarization behaviour of radio waves propagating through an ultrarelativistic highly magnetized electron-positron plasma in a pulsar magnetosphere. The rotation of magnetosphere gives rise to the wave mode coupling in the polarization-limiting region. The process is shown to cause considerable circular polarization in the linearly polarized normal waves. Thus, the circular polarization observed for a number of pulsars, despite the linear polarization of the emitted normal waves, can be attributed to the limiting-polarization effect.
TL;DR: In this article, the characteristics of OSIRIS (Optical System for Imaging and low-Resolution Integrated Spectroscopy), an instrument for imaging and low resolution spectroscopy proposed for the GTC 10m telescope, are described.
Abstract: The characteristics of OSIRIS (Optical System for Imaging and low-Resolution Integrated Spectroscopy), an instrument for imaging and low resolution spectroscopy proposed for the GTC 10m telescope, are described. OSIRIS is a multi-purpose instrument that incorporates in a "classical design" several innovations, already tested in 4m-class telescopes, that will provide a powerful first light instrument for the GTC.
TL;DR: In this paper, the authors investigated the regular and chaotic motion in a model potential found using the recent developments of the Inverse Problem of Dynamics (IPoD) and showed that chaotic motion on the surface of section decreases as the angular velocity increases and finally all orbits become regular.
Abstract: We investigate the regular and chaotic motion in a model potential found using the recent developments of the Inverse Problem of Dynamics. The potential describes the motion in the central parts of a barred galaxy. In the absence of rotation chaotic motion is observed when the perturbation strength is near the escape perturbation for a fixed value of the energy. In the rotating cases one observes that the area of chaotic motion on the surface of section decreases as the angular velocity Ω increases and finally all orbits become regular. The character of motion is also checked by computing the Liapunov characteristic exponents in all cases.
TL;DR: In this article, the radiated power of a single isolated black hole was shown to be P ∈ e-2Δγ P, where e − 2 Δγ is the transmission probability.
Abstract: Small, quiescent black holes can be considered as candidates for the missing dark matter of the universe, and as the core energy source of ball lightning. By means of gravitational tunneling, unidirectional radiation is emitted from black holes in a process much attenuated from that of Hawking radiation, P, which has proven elusive to detect. Gravitational tunneling emission is similar to electric field emission of electrons from a metal in that a second body is involved which lowers the barrier and gives the barrier a finite rather than infinite width. Hawking deals with a single isolated black hole. The radiated power here is P ∝ e-2Δγ P, where e-2Δγ is the transmission probability.
TL;DR: In this article, the list of CII elements that could be used for LTE calculation of carbon abundance in hot stars is presented, and a short discussion of this result is given, along with a list of the elements that can be used in hot star counting.
Abstract: NLTE calculations of carbon abundance in γPeg atmosphere revealed a slight deficiency of this element ([C/H] =−0.25). A short discussion of this result is given. The list of CII that could be used for LTE calculation of carbon abundance in hot stars is presented.
TL;DR: In this article, the authors derive a well-posed conservative reformulation of the Euler equations from the analysis of the associated full convective-diffusive system and then propose an exact Roe-type linearization for the equivalent system of conservation laws on the basis of an original Lemma for averagings.
Abstract: This paper is devoted to the numerical approximation of the discontinuous solutions of the Euler equations for weakly ionized mixtures of reacting gases. The main difficulty stems from the non conservative formulation of these equations due to a widely used physical assumption. We show how to derive a well-posed conservative reformulation of the equations from the analysis of the associated full convective-diffusive system. We then propose an exact Roe-type linearization for the equivalent system of conservation laws on the basis of an original Lemma for averagings. Our results can be seen as an extension of the classical Roe average, for nonlinearities that cannot be recast under quadratic form.
TL;DR: In this article, the formation of dust grains in steady state dust driven winds around oxygen-rich AGB stars has been investigated to clarify the carrier of the observed 13µm feature.
Abstract: The formation of dust grains in steady state dust driven winds around oxygen-rich AGB stars has been investigated to clarify the carrier of the observed 13µm feature. In the calculations not only homogeneous Al2O3 and silicate grains but also heterogeneous grains consisting of an Al2O3 core and a silicate mantle are included simultaneously. The radiation transfer calculations based on the results of condensation calculations demonstrate that the core-mantle grains consisting of an α-Al2O3 core and a silicate mantle formed in the vicinity of the sonic point can produce a distinctive emission feature similar to the observed 13µm feature when the mass loss rate is less than 2 · 10-5 M ⊙/yr.
TL;DR: In this article, a survey of different kinds of instabilities in the context of radiative colliding flows which greatly contribute to structure formation is presented, in particular the formation of knots and filaments.
Abstract: We present a survey of different kinds of instabilities in the context of radiative colliding flows which greatly contribute to structure formation. In particular, this includes analytical results for different kinds of thin shell instabilities (DI, NDI, NTSI). New numerical results for the non-linear evolution of such instabilities in two dimensions, and their coupling with the thermal cooling instability are presented. The astrophysical implications are briefly outlined, in particular the formation of knots and filaments.
TL;DR: In this paper, the behavior of magnetic field in anisotropic Bianchi type I cosmological model for perfect fluid distribution in General Relativity is investigated, where the distribution consists of an electrically neutral perfect fluid with an infinite electrical conductivity.
Abstract: The behaviour of magnetic field in anisotropic Bianchi type I cosmological model for perfect fluid distribution in General Relativity, is investigated. The distribution consists of an electrically neutral perfect fluid with an infinite electrical conductivity. It is assumed that the component σ11 of shear tensor σji is proportional to the expansion (θ) which leads to A = (BC)n. The other physical and geometrical aspects of the model are also discussed, Bali (1986) obtained the cosmological model for n = 1 in presence of magnetic field. We have investigated the model for general values of n and discussed the particular case and general behaviour of the model.
TL;DR: In this article, the properties of the interaction region of colliding winds in hot star binaries and evolved low mass binaries are discussed in the frame of 2D and 3D hydrodynamical numerical simulations.
Abstract: The properties of the interaction region of colliding winds in hot star binaries and evolved low mass binaries are discussed in the frame of 2D and 3D hydrodynamical numerical simulations. The emission of such systems and their nebulae, in particular in X-rays, is reviewed and new results regarding orbital line profile variations are presented. The connection between colliding winds, thin shell instabilities and structure formation is outlined.
TL;DR: In this article, the authors used HIPPARCOS proper motions and the method of Statistical Parallax to estimate the absolute magnitude of RR Lyrae stars, and used the HIPParcOS parallax of RR Lyr itself to determine its absolute magnitude.
Abstract: We have used HIPPARCOS proper motions and the method of Statistical Parallax to estimate the absolute magnitude of RR Lyrae stars. In addition, we have used the HIPPARCOS parallax of RR Lyr itself to determine its absolute magnitude. These two results are in excellent agreement with each other and give a zero-point for the RR Lyrae Mv,[Fe/H] relation of 0.77 ± 0.15 at [Fe/H]= -1.53. This zero-point is in good agreement with that obtained recently by several groups using Baade-Wesselink methods which, averaged over the results from the different groups, gives Mv = 0.73 ± 0.14 at [Fe/H] = -1.53. Taking the HIPPARCOS based zero-point and assuming a value of 0.18 ± 0.03 for the slope we find the distance modulus of the LMC is 18.26 ± 0.15. This value is compared with recent estimates based on other methods. Potential problems that may affect the results are outlined.
TL;DR: In this paper, the spatial structure during the formation and decay of a turbulent wave is illustrated, and a model of the dynamics of the wave is proposed to illustrate the spatial structures during the process.
Abstract: Supersonic and super-Alfvenic turbulent motions generate an intricate pattern of shock waves through which the turbulence decays. We here illustrate the spatial structure during shock formation and decay.
TL;DR: In this article, a long-term photometric monitoring of the young single main-sequence star HD134319 is presented, which shows short-term variability of the optical-band continuum flux with a period of 4.448 days attributed to dark spots or spots groups unevenly distributed in longitude on the star's photosphere.
Abstract: We present the results of a long-term photometric monitoring of the young single main-sequence star HD134319. It shows short-term variability of the optical-band continuum flux with a period of 4.448 days. The variability is attributed to dark spots or spots groups unevenly distributed in longitude on the star's photosphere, whose visibility is modulated by the star's rotation. Maps of the photospheric spot pattern have been obtained with light curve inversion techniques based on the Maximum Entropy and the Tikhonov regularization criteria. The overall spot pattern shows evidence for two long-lasting active longitudes located about 180° apart, with a total area of at least 16% of the stellar surface (assuming an inclination of the stellar rotation axis of 90° on the line of sight). The longitude distribution of the spot pattern and its total area do not show any clear evidence for a long-term variation along the five years of observations. A comparison with recent mean field dynamo models is also addressed, suggesting a possible interpretation of such a behaviour. Singularity, spectral type, youth and a high level of photospheric and chromospheric activity make HD134319 a suitable proxy for studying the magnetic activity of the young Sun not far after its arrival on the zero age main sequence.
TL;DR: In this paper, the authors present an ongoing project of multiwavelength optical observations aimed at studying the chromosphere of active binary systems, using the information provided for several optical spectroscopic features.
Abstract: We summarize here our ongoing project of multiwavelength optical observations aimed at studying the chromosphere of active binary systems, using the information provided for several optical spectroscopic features, that are formed at different heights in the chromosphere.
TL;DR: Inversion techniques have become one of the most powerful tools to obtain, from spectropolarimetric data, the stratification of physical quantities (temperature, pressure, velocity fields, magnetic field etc.) describing different structures in the solar atmosphere as mentioned in this paper.
Abstract: During the last years, inversion techniques have become one of the most powerful tools to obtain, from spectropolarimetric data, the stratification of physical quantities (temperature, pressure, velocity fields, magnetic field, etc.) describing different structures in the solar atmosphere.
TL;DR: In this paper, the theory of equilibrium of nonrotating, spherically symmetric neutron stars and white dwarfs is revisited and the condition for equilibrium is that the total energy is a minimum, for given baryon number, amongst configurations fulfilling Einstein's equations.
Abstract: The theory of equilibrium of nonrotating, spherically symmetric neutron stars and white dwarfs is revisited We propose that the condition for equilibrium is that the total energy is a minimum, for given baryon number, amongst configurations fulfilling Einstein's equations The isotropy of the stress tensor is not assumed 'a priori' Using this condition it is shown that, in post-Newtonian gravity, there are equilibrium configurations of white dwarfs, with local anisotropy, having mass above the Chandrasekhar limit