Showing papers in "Astrophysics and Space Science in 1988"

Contributing factors to flux changes in nonradial stellar pulsations

Abstract: The linearized analytic representation of observed flux changes due to nonradial pulsations is examined under conditions applicable to a variety of pulsating star types. Histograms are presented as a function ofl, documenting the relative importance of local temperature, geometry, pressure, and limb-darkening variations, which are all contributing factors to the predicted flux changes. The most favourable situation for light and colour data to assist in mode discrimination for lowl requires, amongst other things, that the geometry-related factor be significant. This is noted to occur only if theQ value of the pulsation is not grossly different from theQ value of a radial fundamental pulsation. Published light and colour data for β Cephei, 53 Persei, δ Scuti, rapidly oscillating Ap, Cepheid, and ZZ Ceti variables are compared with model predictions on an amplitude ratio versus phase difference plane\(\left( {A_{co1} /A_{\lambda _1 } ,\phi _{co1} - \phi _{\lambda _1 } } \right)\). With the notable exception of the rapidly oscillating Ap stars, these comparisons suggest consistency of the nonradial flux change model with other known constraints on the different variable star groups.

Unsteady hydromagnetic free-convection flow with radiative heat transfer in a rotating fluid

Abstract: We study the unsteady free-convection flow near a moving infinite flat plate in a totating medium by imposing a time-dependent perturbation on a constant plate temperature. The temperatures involved are assumed to be very large so that radiative heat transfer is significant, which renders the problem very nonlinear even on the assumption of a differential approximation for the radiative flux. When the perturbation is small, the transient flow is tackled by the Laplace transform technique. Complete first-order solutions are deduced for an impulsive motion.

Theory of the radio emission of pulsars

Abstract: A consistent theory of excitation, stabilization, and propagation of electromagnetic oscillations in a relativistic one-dimensional electron-positron plasma flowing along curved magnetic field lines is presented. It is shown that in such a medium which is typical of the magnetosphere of a neutron star there exist unstable natural modes of oscillations. Nonlinear saturation of the instability leads to an effective energy conversion into transverse oscillations capable of leaving the magnetosphere of a pulsar. The polarization spectrum and the directivity pattern of generated radiation are determined. A comparison with observations has shown that the theory makes it possible to explain practically all the basic characteristics of observed pulsar radio emission.

Catalogue of unambiguous (Faraday-thin, one-component, spectrum-selected) rotation measures for galaxies and quasars

Abstract: This all-sky catalogue of unambiguous rotation measure (from a Faraday-thin, one-component, spectrum selection) for 674 galaxies or quasars has been compiled, ordered, and edited from the available literature. All the known applications of the RM distribution toward foreground objects in the Galaxy (i.e., magnetic field in 4 nearby spiral arms and in 4 nearby interstellar magnetic bubbles) have also been catalogued.

Dynamical aspects of electrostatic double layers

Abstract: Electrostatic double layers have been proposed as an acceleration mechanism in solar flares and other astrophysical objects. They have been extensively studied in the laboratory and by means of computer simulations. The theory of steady-state double layers implies several existence criteria, in particular the Bohm criteria, restricting the conditions under which double layers may form. In the present paper several already published theoretical models of different types of double layers are discussed. It is shown that the existence conditions often imply current-driven instabilities in the ambient plasma, at least for strong double layers, and it is argued that such conditions must be used with care when applied to real plasmas. Laboratory double layers, and by implication those arising in astrophysical plasmas often produce instabilities in the surrounding plasma and are generally time-dependent structures. Naturally occuring double layers should, therefore, be far more common than the restrictions deduced from idealised time-independent models would imply. In particular it is necessary to understand more fully the time-dependent behaviour of double layers. In the present paper the dynamics of weak double layers is discussed. Also a model for a moving strong double layer, where an associated potential minimum plays a significant role, is presented.

A revised reference catalogue of galactic supernova remnants

Abstract: This paper reviews observations made over recent years that have led to the identification of many new galactic supernova remnants (SNRs) and have clarified the nature of several objects that had been thought to be SNRs. A revised reference catalogue of 155 galactic SNRs is presented, complete with notes on the newly identified remnants, those objects no longer thought to be SNRs, the questionable objects in the catalogue, and possible candidate SNRs. The selection effects applicable to the identification of galactic SNRs are discussed, with particular reference to the limitations that they impose upon statistical studies.

Charged dust in the Earth's magnetosphere. I. Physical and dynamical processes.

Abstract: On considere les processus electrodynamiques associes a la charge electrostatique de grains de Al 2 O 3 deverses dans la magnetosphere durant la combustion de propergols solides dans les moteurs des fusees

Global wave patterns in the Sun's magnetic field

Abstract: When the observed pattern of solar magnetic fields is decomposed in its spherical-harmonic components and a time series analysis is performed, a resonant global wave pattern is revealed. The power spectrum indicates modes with discrete frequencies, obeying a strict parity selection rule in the case of the zonal, rotationally-symmetric modes (with spherical-harmonic order m = 0). For instance, the 22 yr resonance that dominates for the anti-symmetric modes (with odd values of the spherical-harmonic degree l) is completely absent for the symmetric modes, which instead exhibit a number of resonances having frequencies increasing with l.

Alfvén Waves in a Space Plasma and its Role in the Solar Wind Interaction with Comets

Abstract: The importance of Alfven wave generation in interacting plasmas is discussed in general and illustrated by the example of solar wind interaction with cometary plasma. The quasi-linear theory of Alfven wave generation by cometary ions at distances far from the cometary nucleus is reviewed. The incorporation of a diabatic plasma compression effects into this theory modifies the spectrum of Alfven waves and the integral intensity of magnetic field fluctuations previously published. These results are in quantitative agreement with the in situ observations near the comets Giacobini-Zinner and Halley. However, the polarization of quasi-linearly excited waves needs further detailed comparison with observations.

Interaction of interstellar pick-up ions with the solar wind

Abstract: The interaction of interstellar pick-up ions with the solar wind is studied by comparing a model for the velocity distribution function of pick-up ions with actual measurements of He+ ions in the solar wind. The model includes the effects of pitch-angle diffusion due to interplanetary Alfven waves, adiabatic deceleration in the expanding solar wind and the radial variation of the source function. It is demonstrated that the scattering mean free path is in the range ≤0.1 AU and that energy diffusion can be neglected as compared with adiabatic deceleration. The effects of adiabatic focusing, of the radial variation of the neutral density and of a variation of the solar wind velocity with distance from the Sun are investigated. With the correct choice of these parameters we can model the measured energy spectra of the pick-up ions reasonably well. It is shown that the measured differential energy density of the pick-up ions does not vary with the solar wind velocity and the direction of the interplanetary magnetic field for a given local neutral gas density and ionization rate. Therefore, the comparison of the model distributions with the measurements leads to a quantitative determination of the local interstellar gas density.

Metallic Particles in Astronomy

Abstract: We discuss some implications of iron whiskers condensing in supernova ejecta and being expelled into interstellar and intergalactic space. The supposition that the universal microwave background is a relic from an early hot state of the Universe is shown to be not as secure as is commonly supposed.

Hydromagnetic flow past a continuously moving semi-infinite plate for large suction

Abstract: The similarity solution for hydromagnetic flow of an incompressible viscous electrically conducting fluid past a continuously moving semi-infinite porous plate in the presence of a magnetic field has been obtained for the case of small magnetic Reynolds number. The perturbation method has been used to solve the similarity equations at large suction. The resulting equations have been solved by analytical method. The effect of the magnetic parameter is to increase the skin-friction coefficient while it has no significant effect on the Nusselt number.

Plasma Flow Nonstationarity in Pulsar Magnetospheres and Two-Stream Instability

Abstract: The electron-positron plasma creation near a pulsar may be a very nonstationary process. Then, the plasma that flows out from the pulsar environment is nonhomogeneous and gathers into separate clouds. The dispersion of plasma clouds during the outflow, their overlapping and the development of a two-stream instability are considered.

Electron Heating in Superhigh Mach Number Shocks

Abstract: Fluid and MHD models, as well as direct extrapolation of the Earth’s bow shock measurements in the high Mach number (HMN) range (3 ≤ M F ≤ 12) to the superhigh Mach number (SHMN) range (M F > 30–40) predict that the downstream electron pressure p e2 is only a negligible fraction of the Rankine-Hugoniot downstream pressure p 2, i.e., P e2/p 2 ≈ M F 2 )−1. However, the interpretation of X-ray supernovae emissions, due to SHMN shock heating requires p e2/p 2 ≈ 0(1). Following Alfven we have used plasma physics experimental-theoretical data combined with magnetospheric observations to probe the physics of the SHMN shocks. It is shown below that inclusion of proper plasma physics considerations in the interaction of the reflected and transmitted ions and the electrons at the ‘foot’ of the shock leads to the surprising result that electron heating can dominate in the SHNM range. A stationary model of the shock structure is derived and shown to be the result of extrapolation of the high Mach number shock physics with incorporation of collective interactions at the foot.

Hydromagnetic waves associated with possible flux transfer events

Abstract: Magnetic fluctuations in the hydromagnetic frequency band ~ 0 to ~ 005 Hz are examined at magnetospheric cusp latitudes during two times when ionospheric signatures of possible flux-transfer events were evident in the data Ultralow frequency power is found to be very broad band in the range ~ 002–005 Hz and to be more narrowly confined at a frequency ~ 00025 Hz At lower latitudes, the higher frequency (broad-band) power excites narrower-band field line resonances at the fundamental frequency of the respective field line — a standing Alfven wave The narrow-band power in the lower frequency band (period around 400 s) is approximately that expected for a field line resonance on a closed field line near the magnetopause; it also corresponds approximately to the width of the convected field-aligned current filament as observed on the ground The reconnection process at the dayside magnetopause evidently plays an important role in the generation of low-frequency (≲, 0008 Hz) hydromagnetic energy in the dayside magnetosphere, energy which can produce Alfven waves deeper in the magnetosphere

A unified theory of polynomial expansions and their applications involving Clebsch-Gordan type linearization relations and Neumann series

Abstract: Motivated by their potential for applications in several diverse fields of physical, astrophysical, and engineering sciences, this paper aims at presenting a unified study of various classes of polynomial expansions and multiplication theorems associated with the general multivariable hypergeometric function (studied recently by A. W. Niukkanen and H. M. Srivastava), which provides an interesting and useful unifiation of numerous families of special functions in one and more variables, encoutered naturally (and rather frequently) in many physical, quantum chemical, and quantum mechanical situations. Several interesting applications of these general polynomial expansions are considered, not only in the derivations of various Clebsch-Gordan type linearization relations involving products of several Jacobi or Laguerre polynomials, but also to associated Neumann expansions in series of the Bessel functionsJ v (z) andI v (z) (and of their suitable products).

The joint evolution of normal and compact magnetized stars in close binaries: Analytical description and statistical simulation

Abstract: The joint evolution of normal and compact stars in binaries of low and moderate masses (≤10M ⊙) is considered. The classification and analytical description are given for such evolution. The Monte-Carlo simulation of great number of binaries (∼ 500 000) with different initial parameters allows to evaluate the relative amount of binaries of each evolutionary stage in the Galaxy. The simulation of binary evolution in an elliptical galaxy reveals evolutionary properties of a number of type I supernova explosions in ellipticals. The gravitational wave spectrum in the simulated galaxy has also been obtained, the maximum metric strain amplitudeh being 10−19.5 at frequencesv≃10−4 Hz.

Charged dust in the Earth's magnetosphere

Abstract: We have considered the electrodynamic effects on small A12O3 spherules dumped into the Earth’s magnetosphere in large quantities during solid rocket propellant burns. The charges acquired by these grains in all regions of the terrestrial environment (plasmasphere, magnetosphere, and solar wind) are modest. Consequently electrodynamic effects are significant only at the lower end of the dust size spectrum (R g ≲ 0.1 µm). In that case, the electrodynamic forces conspire with solar radiation pressure to eliminate the grians from the magnetosphere in a comparatively short time. Although not studied here in detail, we anticipate a similar fate for fine micrometeoroids entering the Earth’s magnetosphere, with the electrodynamic effects playing an even more important role.

Acceleration of Auroral Particles by Magnetic-Field Aligned Electric Fields

Abstract: Measurements on the S3-3 and Viking satellites appear to show that at least a large fraction of magnetic field-aligned potential drops are made up of multiple double layers. Solitons and double layers in U-shaped potential structures give rise to spiky electric fields also perpendicular to the magnetic field in agreement with satellite measurements. The large scale potential structures associated with inverted V-events are built up of many similar short-lived structures on a small scale. Viking measurements indicate that electric fields parallel to the magnetic field are almost always directed upward.

Cosmic electric currents and the generalized Bennett relation

Abstract: A generalized form of the Bennett pinch is studied in both cylindrical geometry and plane-parallel geometry. In this kind of pinch electromagnetic forces, kinetic pressure gradient forces, centrifugal forces, and gravitational forces may act. For each of the two geometries considered a generalized Bennett relation is derived. By means of these relations it is possible to describe among other things the pure Bennett pinch, Jean's criterion in one and two dimensions, force-free magnetic fields, gravitationally balanced magnetic pressures, and continuous transitions between these states. The theory is applied to electric currents in the magnetosphere, in the solar atmosphere, and in the interstellar medium. It is pointed out that the currents in the solar atmosphere and in the interstellar medium may lead to pinches that are of vital importance to the phenomena of solar flares and star formation, respectively.

The ratio of total to selective absorption in the Carina OB2 association

Abstract: Several previous studies in and around the Great Carina nebula (NGC 3372) have strongly indicated an abnormal interstellar extinction law with a high ratio of total to selective absorptionR. In the present study, newUBV photometric data and accurate MK spectral-types of stars in the region of the Carina OB2 association are used to show that (1) Car OB2 is a genuine stellar association located at a distance of 3.2 kpc, and (2) the interstellar extinction law seems to be normal throughout most of the region. A representative value ofR = A v/E(B − V) = 3.0 is derived for Car OB2 from the variable extinction method.

The high amplitude delta Scuti star SZ Lyncis revisited

Abstract: Nine new times of light maximum are given for SZ Lyn and the time base of observations is herewith extended to 26 years. 154 light maxima have been used to calculate the orbital elements and the secular change in the period of the pulsating component. The results provided here support the post-Main Sequence, high mass hypothesis for the evolutionary state of this dwarf cepheid.

Unsteady MHD free-convection flows on a porous plate with time-dependent heating in a rotating medium

Abstract: The three-dimensional unsteady free-convection flows of a viscous fluid near a porous infinite vertical plate in a rotating medium in the presence of a constant transverse magnetic field are investigated under an arbitrary time-dependent heating of the plate. By using the Laplace transform technique, the Green function of the problem is determined and exact solutions are obtained for special cases of the impulsive and the accelerated heating effect for an arbitrary Prandtl number. The thermal influence on skin friction at the plate and the displacement thickness of the boundary layers are discussed.

Auroral and magnetic variations in the polar cusp and cleft — Signatures of magnetopause boundary-layer dynamics

Abstract: By combining continuous ground-based observations of polar cleft/cusp auroras and local magnetic variations with electromagnetic parameters obtained from satellites in polar orbit (low-altitude cleft/cusp) and in the magnetosheath/interplanetary space, different electrodynamic processes in the polar cleft/cusp have been investigated. One of the more controversial questions in this field is related to the observed shifts in latitude of cleft/cusp auroras and the relationship with the interplanetary magnetic field (IMF) orientation, local magnetic disturbances (DP2 and DPY modes) and magnetospheric substorms. A new approach which may contribute to clarifying these complicated relationships — simultaneous ground-based observations of the midday and evening-midnight sectors of the auroral oval — is illustrated. A related topic is the spatial relationship between the cleft/cusp auroras and the ionospheric convection currents. A characteristic feature of the polar cusp and cleft regions during negative IMF B Z is repeated occurrence of certain short-lived auroral structures which seem to move in accordance with the local convection pattern. Satellite measurements of particle precipitation, magnetic field and ion drift components permit detailed investigations of the electrodynamics of these cusp/cleft structures. Information on electric field components, Birkeland currents, Poynting flux, height-integrated Pedersen conductivity, and Joule heat dissipation rate has been derived. These observations are discussed in relation to existing models of temporal plasma injections from the magnetosheath.

Hugoniot adiabates for the waves of combustion in degenerate matter of supernovae

Abstract: Results of computations of equilibrium parameters of steady detonation, deflagration and spontaneous flame propagation both in degenerate12C+16O mixtures and helium4He are presented in a wide density range from 106 to 5×109 g cm−3. The limiting regimes of supersonic and subsonic burning are discussed. The maximum velocity of deflagration is obtained as a function of initial density of matter.

The Temperatures of the Nuclei of High-Excitation Planetary Nebulae

Abstract: An empirical relationship between the ratio of the intensities of emission lines in spectra of planetary nebulae, 4686 Heii/Hβ andN1+N2[Oiii]/4868 Heii, is established (see Equation (1), curve in Figure 1). A new statistical temperature scale based on this empirical relationship is proposed for the determination of lower limits of the temperatures of the nuclei of planetary nebulae. The well-known method 4686 Heii/Hβ gives the upper level of the temperature of the nucleus. A simultaneous application of both methods has been carried out for 97 planetary nebulae, in order to determine both the upper and lower limits of the temperature of their nuclei (last two columns in Table I). A new quantitative system for the determination of excitation classes of nebulae is proposed (Tables IV and V).

The third catalogue of nearby stars with special emphasis on wide binaries: Introductory lecture

Abstract: SummaryThirty-six percent of the 1641 visual components nearer than 22.5 parsecs listed in 1969 were members of systems. Meanwhile for an extension to 25 pc of the Sun about 2000 additional objects are found, however, only 22% of them are known today as members of systems. Angular separations are observed from a few tenths of an arcsec to more than one degree. The widest linear separations at the sphere seem to be larger than 105 astronomical units. The observed frequency distribution of the projected separations for all nearby stars is given and compared with the frequency distribution of more complete subsamples. The 32 presumable systems with the widest separations are discussed with respect to their astrometric and spectroscopic properties.

Free Convection and Mass Transfer Effects on the Magnetohydrodynamic Flows Near A Moving Plate in A Rotating Medium

Abstract: An exact solution of the free-convection flow near an infinite vertical plate moving in a rotating medium in the presence of foreign mass and a transverse magnetic field is presented under a constant heating of the plate It is apparent from this solution that the effects of the motion, the temperature, and the mass transfer are linear and, hence, can be studied independently Three applications of physical interest are discussed The non-magnetic case and non-rotating case are are also discussed

The Role of Magnetohydrodynamics in Heliospheric Space Plasma Physics Research

Abstract: Magnetohydrodynamics (MHD) is a fairly recent extension of the field of fluid mechanics. While much remains to be done, it has successfully been applied to the contemporary field of heliospheric space plasma research to evaluate the ‘macroscopic picture’ of some vital topics via the use of conducting fluid equations and numerical modeling and simulations. Some representative examples from solar and interplanetary physics are described to demonstrate that the continuum approach to global problems (while keeping in mind the assumptions and limitations therein) can be very successful in providing insight and large scale interpretations of otherwise intractable problems in space physics.

Birkeland currents in the Earth's magnetosphere

Abstract: As a result of his polar expeditions at the beginning of this century, Kristian Birkeland determined that intense ionospheric currents were associated with the aurora. Birkeland suggested that these currents originated far from the Earth and that they flowed ointo and away from the polar atmosphere along the geomagnetic field lines. The existence of such field-aligned or ‘Birkeland’ currents was disputed because it was not possible to unambiguously identify current systems that are field-aligned (as suggested by Alfven, 1939, 1940) and those which are completely contained in the ionosphere (as developed by Vestine and Chapman, 1938) with surface magnetic field observations. The presence of Birkeland currents has been absolutely confirmed with satellite-borne particle and magnetic field experiments conducted over the past two decades. These satellite observations have determined the large-scale patterns, flow directions, and intensities of Birkeland currents in the auroral and polar regions, and their relationship to the orientation and magnitude of the interplanetary magnetic field. The Birkeland currents are directly associated with visible and UV auroral forms observed with satellites. The results obtained from a variety of recently launched satellites are discussed here. These include Sweden’s first satellite, VIKING, which has provided evidence for resonant Alfven waves on the same geomagnetic field lines that guide stationary Birkeland currents. These observations demonstrate the important role that these currents play in the coupling of energy between the interplanetary medium and the lower ionosphere and atmosphere.