Showing papers in "Astrophysics and Space Science in 1985"

Absolute Astronomical Accelerometry

Abstract: The principle of our proposed accelerometers (first described at the ESA DISCO Utrecht Workshop three years ago) has now been discussed in two brief publications (IAU Symposium 112 on “Search for Extraterrestrial Life”, 1984, and IAU Colloquium 88 on “Radial Velocities”, 1984), plus a detailed one (Astrophys. and Sp. Sc., 110, 211, 1985); hence a mere abstract will suffice here. Essentially, one makes use of a variable path-difference FP interferometer, whose bandpasses are constrained by a first servo-loop to track the wavelength fluctuations of the studied lines (only one in the solar case, and all of them within a wide range in the stellar case). Through a second independent servo-loop, a tunable laser tracks the FP bandpasses. The final output is the beat frequency between this laser and a stabilized laser (of frequency NO); one shows that if the measured radial velocity changes from V1 to V2 and the corresponding beat frequencies are BN1 and BN2, then V2 − V1 = c(BN2 − BN1)/N0. The result is directly comparable with that given by Doppler radar; one has basically transferred a problem from the incoherent to the coherent optics domain. All spectral or instrumental characteristics drop out; the result is independent of wavelengths, calibrations, spectral or instrumental line profiles etc… The FP itself (together with the grating spectrometer required for error signal generation) and the detectors, are used solely as null-checking devices. Altogether, one may hope for an unusually low level of systematic errors both in the stellar oscillations frequency range, and in the ULF domain required for extrasolar planetary detection.

Nonlinear propagation of electromagnetic waves in magnetized electron-positron plasmas

Abstract: The nonlinear interaction of the external magnetic field-aligned circularly polarized electro-magnetic radiation with a cold electron-positron plasma is considered. A set of nonlinear equations is derived describing the coupling of the radiation with cold electrostatic oscillations. Modulational instabilities and wave localization are discussed.

On the initial phase of interaction between expanding stellar envelopes and surrounding medium

Abstract: The initial stages of deceleration in the circumstellar medium of a stellar envelope, thrown off by a shock wave, are investigated. The equations of spherical-symmetric adiabatic hydrodynamics are shown to have a similarity solution in the case of the density of the expanding envelope being approximated by a reasonable power law. The overall flow pattern has such a form that the stellar material is decelerated in the internal shock wave while another shock propagates through the circumstellar matter. Between the shocks there is a contact discontinuity separating the circumstellar and stellar matter. The characteristics of the similarity solution are calculated for various exponents in the density laws of an expanding envelope and circumstellar matter and for two values of the adiabatic index (γ=5/3, 4/3). Some parts of the flow exhibit Rayleigh-Taylor instability.

Nonlinear wave conversion in electron-positron plasmas

Abstract: Wave conversion mechanisms causing large-frequency shifts are considered for an electron-positron plasma in a strong magnetic field. In particular, we discuss the effects of the nonlinear Cerenkov as well as the cyclotron resonances in order to associate pulsar radio-emissions with our present model for nonlinear conversion of high-frequency radiation into the low-frequency region.

Drift theory of charged particles in electric and magnetic fields

Abstract: In this paper we review the drift theory of charged particles in electric and magnetic fields. No new physical interpretations are added to this classical topic, but through an alternative, simplified derivation of the guiding centre velocity, several complexities are eliminated and possible misconceptions of the theory are clarified. It is shown that:

Vortex lines in neutron star superfluids and decay of pulsar magnetic fields

Abstract: We adopt that in the interior of neutron stars both the proton and neutron superfluids are in the vortex state. Thus, in the superconducting core the magnetic field is expected to be organized in the form of quantized fluxoids. It is shown that fluxoids are buoyant. This gives rise to a rapid (≲5×104 yr) expulsion of the magnetic field out of the superconducting core to the subcrustal region, and a subsequent decay within the outer crust. The effect considered may be the physical reason why the characteristic decay-time of pulsar magnetic fields (∼106 yr) corresponds to the ohmic dissipation time within the neutron star crust. The intersection of two types of vortex lines with each other and its possible consequence for pulsars is briefly discussed.

Gamma / Quanta Conversion Into Positronium Atoms in a Strong Magnetic Field

Abstract: The curvature γ-quanta emitted in the pulsar magnetospheres tangentially to the curved lines of force of the magnetic field are shown to be later canalized along the magnetic field—if it is strong enough,B≳0.1Bcr=4×1012G—by gradually converting into a mutually bound electron-positron pair, i.e. a positronium atom. This happens before the photon reaches the threshold of free-pair creation. The positronium thus arising is stable against the ionizing action of the electric field near the pulsar unless it reaches a critical value about 4×107 CGSE forB≃1013G. This prevents the screening of the electric field up to the distances from the pulsar, where the magnetic field is already below the value of 0.1Bcrand the free pair creation may become essential. This effect provides, at least within the Arons model, a higher theoretical estimate for the total luminosity of pulsars whose field at the surfaceBsexceeds 0.1Bcras compared with the conventional one.

Structure and origin of the cygnus superbubble

Abstract: This paper summarizes and analyzes the results of radio optical, infrared, and X-ray observations of a large sector of the sky in the constellation Cygnus (α≈19h20m-22h, σ=30–50°;lII=65–90°, |bII|≲10°). This region is associated with an extended X-ray source referred to as the Cygnus superbubble. About a quarter of the superbubble region is occupied by the extensively investigated multicomponent thermal radio source Cyg X. The region contains eight OB-associations which, when projected on the sky, duplicate the outline of the X-ray superbubble. These associations contains 110 stars of high luminosity (about 40 Wolf-Rayet and Of stars). The observations suggest that the X-ray superbubble is not a single object. Between 50 and 75% of its X-ray emission can be ascribed to discrete sources, the rest being probably due to regions of coronal gas about 100 pc in diameter, created by stellar winds and, possibly, supernova explosions in individual associations. The objects that produce the X-ray and optical radiation of the presumed superbubble are located at distances from 0.5 to 2.5 kpc from the Sun in the Carina-Cygnus spiral arm. The eastern portion of the region presumed superbubble contains the associations Cyg OB7 and Cyg OB4 and is generally less than 1 kpc distant, while the western portion contains the associations Cyg OB1, 2, 3, 8, and 9 and is 1 to 2 kpc distant.

Exact cosmological solutions in the scalar-tensor theory with cosmological constant

Abstract: Under the assumption of a power-law between the expansion factor of the Universe, and the scalar field (πan=c=const.) tensor theory with cosmological constant are reduced to quadrature. Several exact solutions are obtained, among them inflationary universes that have barotropic equation of state.

Formation of relativistic electron-photon showers in compact X-ray sources

Abstract: The possibility of relativistic electron-photon shower formation in compact X-ray sources is discussed and the results of Monte-Carlo calculations are reported

Exact Self-Creation Cosmological Solutions

Abstract: Under the assumption of a power law between the expansion factor of the Universe and the scalar field of the second self-creation theory proposed by G. A. Barber, the cosmological equations are reduces to quadratures. Several exact solutions are obtained, among them linearly expanding and inflationary universes with a barotropic equation of state.

A note on the equation of state of a scalar field

Abstract: The energy momentum tensor of a scalar field is considered as being that of a perfect fluid with equation of statep=p(ρ). In the extreme case that the field energy is purely kinetic,p=p, whereas if it is purely potential,p=−ρ.

New photoelectric observations of four W UMa systems: OO Aql, V839 Oph, V566 Oph, and SW Lac

Abstract: The W UMa-type systems (OO Aql, V839 Oph, V566 Oph, and SW Lac) were observed photoelectrically in two wavebands (B andV) during the years 1982–1983. The light curves of these systems are analysed using Kopal's frequency-domain technique and the optimisation method. New geometrical and physical elements have been determined. The absolute elements and the period variations of these systems are discussed.

Effects of rotation and tidal distortion on the periods of small adiabatic oscillations of the polytropic models of the stars

Abstract: The averaging technique of Kippenhahn and Thomas (1970) has been used in conjunction with Kopal's method of evaluating various parameters on the Roche equipotentials to determine the effects of rotation and tidal distortions on the periods of small adiabatic radial and nonradial modes of oscillations of polytropic models of the stars.

The reflection effect in eclipsing binary systems

Abstract: The weakest point in the modern models of eclipsing binary systems (EBS) is the treatment given to the effects of mutual irradiation. In this review, which does not have a similar one in the literature, I tried to collect all the work done on the irradiation problem until the middle of 1984, in order to make possible an evaluation of the present status of problem. Special emphasis is given to the applicability of the results to the analysis of EBS. The treatment given to the effect by the early studies as well as by practically all the modern models of EBS is described, and special attention is given to works analysing the problem using stellar model atmospheres. It turns out that the effect is more complex than suspected earlier, but that significant progress has been made recently.

Free-convection flow past an impulsively started vertical plate in a porous medium by finite difference method

Abstract: A numerical solution for the effects of the free-convection currents of a viscous fluid through a porous medium bounded by a vertical moving infinite vertical plate is considered, when the flow is unsteady. The graphs of velocity profiles for different values of permeability parameter of the porous medium and the Grashof number are discussed.

Larmor radius effects on the instability of a rotating layer of a self-gravitating plasma

Abstract: The instability of a stratified layer of a self-gravitating plasma has been studied to include jointly the effects of viscosity, Coriolis forces and the finite Larmor radius (FLR). For a plasma permeated by a uniform horizontal magnetic field, the stability analysis has been carried out for a transverse mode of wave propagation. The solution has been obtained through variational methods for the case when the direction of axis of rotation is along the magnetic field. The analysis for the case when the direction of rotation is transverse to the magnetic field has also been considered and the solutions for this case have been obtained through integral approach. The dispersion relations have been derived in both the cases and solved numerically. It is found that both the viscous and FLR effects have a stabilizing influence on the growth rate of the unstable mode of disturbance. Coriolis forces are found to have stabilizing influence for small wave numbers and destabilizing for large wave numbers.

The ellipsoidal variables

Abstract: By considering the degree of orbital circularity and synchronism an estimate of the degree of tidal interaction in the ellipsoidal variable systems is attained. We find that the orbits are (with a few exceptions) circularized and synchronized forP(d)≲2. The highly inhomogeneous nature of the ellipsoidal variable class becomes clear within the framework of analysis and we discuss the evolutionary status of the more extreme systems.

A class of exact solutions of the unsteady magnetohydrodynamic free-convection flows

Abstract: The unsteady free-convection flow of an electrically-conducting fluid near a moving vertical plate of infinite extent is investigated in the presence of uniform transverse magnetic field fixed to the fluid or to the plate. Exact solution of this problem is obtained with the aid of the Laplace transform technique, when the plate is moving with a velocity which is an arbitrary functiuon of time. The solution is exemplified for three particular cases of physical interest; the non-magnetic case is also discussed.

On the origin of soft gamma-rays from cyg x-1

Abstract: A hypothesis is suggested that the observed excess atE≥300 keV in the spectrum of Cyg X-1 has a nonthermal origin and is associated with the development of a relativistic electromagnetic cascade, initiated by accelerated particles in the accretion plasma, surrounding a black hole.

Neutron capture measurements on radioactive93Zr

Abstract: Neutron capture measurements made on a sample of fission-product zirconium containing 20%93Zr(t1/2=1.5×106a) at the Oak Ridge Electron Linear Accelerator time-of-flight facility resulted in the identification of 138 resonance peaks for the93Zr isotope at neutron energies up to 21.5 keV. Average capture cross sections from 20 to 300 keV were derived by subtracting neutron capture yields of the stable zirconium isotopes90, 91, 92, 94, 96Zr and additional backgrounds. The average cross sections found were significantly less than those of JENDL-1. While generally 30% higher than those of ENDF/B —V below 60 keV, the binned data overlapped the smooth ENDF/B —V curve. The average for a Maxwellian neutron spectrum withkT=30 keV is (95±10) mb and the resonance contribution to the capture resonance integral is (15.0±0.5)b.

Self-modulation of pulsar radiation in strongly magnetized electron-positron plasmas

Abstract: A nonlinear Schrodinger equation is obtained for linearly polarized electromagnetic waves propagating across the ambient magnetic field in an electron-positron plasma. The nonlinearities arising from wave intensity induced particle mass modulation, as well as harmonic generation are incorporated. Modulational instability and localization of pulsar radiation are investigated.

Electron capture decay of cosmic rays

Abstract: Cosmic-ray nuclei are close to fully ionized during their passage through the Galaxy. Electron capture decay is rare among these nuclides because most do not have bound electrons. Under certain conditions, specifically low energies and/or high charges, electron capture becomes an essential factor in determining cosmic-ray composition. In this paper we discuss the general nature of electron capture decay in cosmic rays and describe specific measurements which can reveal the existence of electron capture decay, and energy and density-dependent processes in the interstellar medium.

The mass-radius relation in binary systems

Abstract: The mass-radius relation is determined for Main-Sequence stars from observational data of well-detached binary systems. The results are compared to previous empirical relations and we discuss their application to the study of the light curves of eclipsing binaries. A comparison with theoretical Main-Sequence models has been carried out.

A constraint on boundary data for magnetic solenoidality in MHD calculations

Abstract: Recent numerical calculations of multi-dimensional MHD solutions revealed the problem that the solutions contain fictitious magnetic monopoles. Their amounts are larger than attributable to numerical discretization. We show that the fictitious presence of magnetic monopoles is due to improper prescription of the boundary data. For a solution of an initial-boundary value problem of magnetohydrodynamics to satisfy the requirement of magnetic solenoidality, the initial data must be constrained by Gauss's law of magnetic solenoidality and the boundary data must be constrained by Faraday's law of magnetic induction. These are necessary and sufficient conditions. Otherwise, magnetic monopoles will appear erratically in the domain of computation, either being contained in the initial data or being introduced from the boundary data.

Vacuum energy in cosmic dynamics

Abstract: The analysis of the Th/U ratio in meteorites and the evolutionary ages of globular clusters favour values of the cosmic age of (19±5)×109 yr. This evidence together with a Hubble parameterH 0>70 km s−1 Mpc−1=(14×109 yr)−1 cannot be reconciled in a Friedmann model with Λ=0. It requires a cosmological constant in the order of 10−56 cm−2, equivalent to a vacuum densityρ v =10−29 g cm−3 The Friedmann-Lemaitre models (Λ>0) with a hot big-bang have been calculated. They are based on a present value of the baryonic matter density ofρ 0=0.5×10−30 g cm−3 as derived from the primordial4He and2H abundances. For a Hubble parameter ofH 0=75 km s−1 Mpc−1, our analysis favours a set of models which can be represented by a model with Euclidean metric (density parameter Ω0=1.0, deceleration parameterq 0=−0.93, aget 0=19.7×109 yr) and by a closed model with perpetual expansion (Ω0=1.072,q 0=−1.0, aget 0=21.4×109 yr). A present density parameter close to one can indeed be expected if the conjecture of an exponential inflation of the very early universe is correct. The possible behaviour of the vacuum density is demonstrated with the help of Streeruwitz' formula in the context of the closed model with an inflationary phase at very early times.

Decametric survey of discrete sources in the northern sky. IX. Source catalogue in the declination range 52° to 60°

Abstract: Le catalogue contient les positions et les densites de flux de 313 radiosources detectees a 10; 12; 6; 14,7; 16,7; 20 et 25 MHz

Hydrodynamic models for population-II cepheids

Abstract: The nonlinear self-excited pulsations of population-II stars with mass 0.6M⊙ and luminosities from 128 to 1280L⊙ are studied. The pulsation periods are found to be in the range of 1.3 to 19 days. An increase of the stellar luminosity is shown to be accompanied by an increasing nonadiabaticity and decreasing efficiency of the radiative damping region. This leads to both an increase of the growth rate while pulsations are exciting and an increase of the oscillation amplitude of the limit cycle. In the models withL≳800L⊙ the efficiency of the radiative damping region becomes so small that amplitude growth ceases due to a dissipation of the mechanical energy by shocks in the stellar atmosphere. The models with periods of from 1.3 to 3 days show the bump on their light curves. The bump is connected with a travelling pulse generated at the antinode of the second overtone at maximum compression. The time delays estimated for the pulses reflected of the stellar core are in a good agreement with the pulse resonance condition proposed by Aikawa and Whitney (1983). The model with the period of 2.1 days revealed double resonance π0 = 2π2, 2π0 = 3π1 causing alternating oscillations with slightly different periods and amplitudes. The models with period of 10 days and longer reveal the resonance π0 = 2π1. This resonance causes the flat top on the light curve at a period of about 10 days and appearance of a shallow alternating minimum at longer periods, as is observed in RV Tau variables. The theoretical period-luminosity relation proposed for population-II cepheids is in good agreement with that obtained from observations.

Stellar luminosities and radii from uvby β photometry

Abstract: A linear relation is found to exist between the Barnes-Evans visual surface brightness parameterF V and the (b-y)0 colour index ofuvbyβ photometry for spectral types later than G0. Using this relation, and a previously determined relation for spectral types B0-G0, tables of intrinsic colours and indices, absolute magnitude and stellar radius are given for the ZAMS and luminosity classes Ia-V over a wide range of spectral types. Comparison of absolute magnitudes and radii calculated fromuvbyβ photometry with values obtained using the visual magnitude, parallax and angular diameter indicates thatuvbyβ photometry can be used to determineM V to ±0 m .2 and log(R/R ⊙) to ±0.05.

Distribution of polarization and intensity of radiation across the stellar disk and numberical values of atmospheric characteristics governing this distribution

Abstract: Computations of polarization and intensity of radiation from a unit stellar surface area are presented, as well as a study of the numerical characteristics of atmospheres — single-scattering albedo Ωλ and the initial source functionλ(δλ), which define the polarization behaviour of atmospheres. The radiatively stable models of stellar atmospheres presented by Kuruczet al. (1974) and Kurucz (1979) have been used for calculations. Since the Ωλ versus optical depth τλ dependence is rather weak, it has been assumed that Ωλ(τλ=cost. With a fixed effective temperatureTeff maximum values of Ω are characteristic of stars featuring the lowest surface gravity accelerationg. Among stars with radiatively stable atmospheres, maximum values of Ω (λ=5000 A) ≈ 0.4–0.6 are exhibited by supergiants withTeff=8000–20 000 K. The plot of Ω(λ) is characterized by discontinuities at the boundaries of spectral series for hydrogen and, sometimes, for helium. Maximum Ωλ are attained in the Lyman region of λ=912–1200 A, where Ωλ can reach the value 0.7–0.9 for supergiants, this value being ≳ 0.3 for Main-Sequence stars. For stars withTeff ≳ 35 000 K, high values of Ωλ also are attained for λ<912 A. Within the infrared region, Ωλ is always small because of bremsstrahlung absorption.