Showing papers in "Astrophysics and Space Science in 1995"

The AM canum venaticorum stars

Abstract: The photometric and spectroscopic properties of the six known AM CVn stars are reviewed. This gives support to the idea that they are helium-transferring double degenerate binaries with orbital periods in the range 1000-3000 s. The photometric variations arise predominantly from the same ‘superhump’ phenomenon that is seen in superoutbursts of the SU UMa subclass of H-rich cataclysmic variable stars. This enables tentative masses and mass ratios to be obtained for some of the AM CVn systems. An evolutionary scheme is proposed, linking the various behaviours of the AM CVn systems. AM CVn and EC15330-1403 are the equivalent of nova-like variables, with high mass transfer stable accretion discs. CR Boo, V803 Cen and CP Eri are VY Scl analogues, with unstable mass transfer caused by irradiative effects. GP Com is the equivalent of an SU UMa star at quiescent, and would be expected to show superoutbursts on intervals of decades. The AM CVn stars should be the strongest gravitational radiation emission line sources in the 0.7–2.0 mHz range.

A Revision of the Ephemeris-Curve Equations for Eclipsing Binaries with Apsidal Motion

Abstract: An improved expression for the prediction of eclipse times in eccentric eclipsing binaries showing apsidal motion is presented. Truncated series expansions have been extended to take into account up to the sixth power of the eccentricity, and all the calculations have been thoroughly checked by means of the Mathematica computer tool. This supersedes the previous equations by Gimenez & Garcia-Pelayo (1983). A new description of the process to derive the given equations is made in order to facilitate the verification of the numerous terms involved as well as to correct some misprints which were found in our first paper. Nevertheless, despite some doubts due to the difficulty to check all the terms involved, Equation 20 by Gimenez & Garcia-Pelayo (1983) has been confirmed to be free of misprints and, as claimed, was found to be accurate to the fifth power of the eccentricity. This equation has been adopted by many authors in their analysis of apsidal motion elements.

Systematics of superoutbursts in dwarf novae

Abstract: Photometric properties of known and suspected members of the SU UMa subclass of dwarf novae are tabulated and discussed The precessing disc model of superoutbursts gives a satisfactory quantitative explanation of the periods of superhumps and their changes during superoutburst The systems such as WZ Sge and HV Vir that have very long intervals between superoutbursts are deduced to be beyond the orbital period minimum, and have degenerate secondaries The systems such as V1159 Ori that have extremely short recurrence times have high rates of mass transfer and are the equivalent of the Z Cam subclass that occurs at longer orbital periods A simplified analytical approach to the theory of accretion discs is able to explain the correlations between normal and superoutburst recurrence times It also explains the slope of the plateau region of superoutbursts, and why the slope is shallower in the very short recurrence time systems

Plateau De Bure Observations of IRC+10216: High Sensitivity Maps of SiC2, SIS, and CS

Abstract: We have obtained high angular resolution ~ 3”), and high sensitivity maps of IRC+10216. SiC2 is found both in a spherical shell and in the very central region, indicating it is formed both in the inner envelope close to the star, and in the outer shell. The molecules SiS and CS are mostly found in the inner parts of the envelope, but are still detectable in the outer region (r ~ 15”) where the products of photochemistry are found. The maps show that IRC+10216 has a very clumpy envelope, with strong departures from spherical symmetry; an axis oriented NS-SW (P.A. 20°) can be seen in all maps. The radial brightness distribution of CS has secondary maxima, at the radius where the SiC2 shell has its peak emission. A preliminary map shows CN in the same shell, but also in a still larger outer shell. Time variations in the mass loss rate, could be invoked to explain the multiple shell structure of this envelope.

EGRET observations of > 30 MeV emission from the brightest bursts detected by BATSE

Abstract: EGRET on the Compton Gamma Ray Observatory has detected 5 gammaray bursts above 30 MeV. The sub-MeV emission, as detected by BATSE, for these 5 bursts has the largest fluence,F, and peak intensity,I, of any of the BATSE detected bursts within the EGRET field of view. The BATSE data reported in the 2B catalog and the EGRET exposure,E, are combined to select additional bursts with largeF ×E andI ×E. The EGRET data for these bright BATSE bursts are searched for prompt, as well as delayed, emission above 30 MeV. The average properties of the >30 MeV emission are obtained by adding the EGRET data from the 5 bursts. On average the fluence is greater than 15% of the fluence detected by BATSE below an MeV, and the average spectrum is flatter than the spectrum from 1-30 MeV.

BACODINE, the real-time BATSE gamma-ray burst coordinates distribution network

Abstract: The celestial coordinates of gamma-ray burst sources observed with BATSE on GRO are automatically determined and distributed in real time to members of the global scientific community. These data are now being used by more than 20 operations to enable searches for associated transients in a variety of other wavelength or energy regimes to identify the burst source objects. The minimum total delay time from the onset of a burst to the receipt of its coordinates by distant experimenters can be under 4 sec, less than the duration of a typical GRB, and the maximum total delay is 7 sec, or longer, depending on the distribution method. Some improvements to the BACODINE system and a summary of the follow-up observations made by some of the sites are given.

Grain destruction in interstellar shock waves

Abstract: Interstellar shock waves can erode and destroy grains present in the shocked gas, primarily as the result of sputtering and grain-grain collisions. Uncertainties in current estimates of sputtering yields are reviewed. Results are presented for the simple case of sputtering of fast grains being stopped in cold gas. An upper limit is derived for sputtering of refractory grains in C-type MHD shocks: shock speeds v s ≳ 50kms−1 are required for return of more than 30% of the silicate to the gas phase. Sputtering can also be important for removing molecular ice mantles from grains in two-fluid MHD shock waves in molecular gas. Recent estimates of refractory grain lifetimes against destruction in shock waves are summarized, and the implications of these short lifetimes are discussed.

Steady-State Drift-Dominated Modulation Models for Galactic Cosmic Rays

Abstract: A number of steady-state drift-dominated modulation models has been developed by the Potchefstroom modulation group. In this review a selection of these models is discussed and briefly compared. A short overview of the relevant drift theory incorporated into the models is also given.

Dark matter in the Milky Way

Abstract: If the Newtonian inverse-square law is an appropriate description of gravity, then disk galaxies, and the Milky Way in particular, contain large amounts of dark matter: in general, the observed shapes of rotation curves do not correspond to the gravitational potential of the observed stars and gas. The nature of this dark matter is still a topic of debate, the outcome of which will have important consequences for our ideas of how galaxies and larger structures in the universe formed.

Low-Mass Protostars and Protostellar Stages

Abstract: The use of sensitive receivers on large ground-based radiotelescopes such as the JCMT, the IRAM 30 m MRT, and the VLA has recently yielded significant progress in our observational understanding of low-mass protostars. Submillimeter continuum observations suggest that the youngest stellar objects detected in the near-/mid-IR range —the so-called Class I sources or “infrared protostars” — have only residual amounts of circumstellar material and are thus relatively evolved. At the same time, a smaller number of colder and more obscured YSOs — designated “Class 0” — characterized by virtually no emission below 10 µm but strong submillimeter emission have been identified. These Class 0 or “submillimeter protostars” have not yet assembled the bulk of their final stellar mass, and correspond to the youngest protostar stage known to date (probable age ≲ 104 yr). Direct evidence for gravitational infall has been found in some of these sources confirming their protostellar nature. However, most (if not all) Class 0 protostars already drive highly collimated CO outflows.

A heuristic remark on the periodic variation in the number of solar neutrinos detected on Earth

Abstract: Four operating neutrino observatories confirm the long standing discrepancy between detected and predicted solar neutrino flux. Among these four experiments the Homestake experiment is taking data for almost 25 years. The reliability of the radiochemical method for detecting solar neutrinos has been tested recently by the GALLEX experiment. All efforts to solve the solar neutrino problem by improving solar, nuclear, and neutrino physics have failed so far. This may also mean that the average solar neutrino flux extracted from the four experiments may not be the proper quantity to explain the production of neutrinos in the deep interior of the Sun. Occasionally it has been emphasized that the solar neutrino flux may vary over time. In this paper we do address relations among specific neutrino fluxes produced in the proton-proton chain that are imposed by the coupled systems of nonlinear partial differential equations of solar structure and kinetic equations by focusing our attention on a statistical interpretation of selected kinetic equations of PPII/PPIII branch reactions of the protonproton chain. A fresh look at the statistical implications for the outcome of kinetic equations for nuclear reactions may shed light on recent claims that the7Be-neutrino flux of the Sun is suppressed in comparison to the pp- and8B neutrino fluxes and may hint at that the solar neutrino flux is indeed varying over time as shown by the Homestake experiment.

Fine-scale structure in relativistic jets and rapid variability in blazars

Abstract: Several versions of the shock-in-jet model of the blazars' variability have been proposed in recent years. This family of models resorts to the interaction between a relativistic shock and a feature in the parsec-scale jet associated to an AGN in order to explain the origin of the rapid variability recently observed in many extragalactic radio sources. The nature of the involved features varies from one particular model to another. Here I discuss a possible origin for this variety of subparsec-sized features in the frame of the two-fluid model of extragalactic jets. Macroscopic Kelvin-Helmholtz instabilities are shown to be capable of producing the required plurality of morphologies in the innermost part of the beams, in such a way that the different models have particular ranges of applicability according to the differencies in the parameters of the flows.

Solar wind-magnetosphere interaction as simulated by a 3-D em particle code

Abstract: We have studied the solar wind-magnetosphere interaction using a 3-D electromagnetic particle code. The results for an unmagnetized solar wind plasma streaming past a dipole magnetic field show the formation of a magnetopause and a magnetotail, the penetration of energetic particles into cusps and radiation belt and dawn-dusk asymmetries. The effects of interplanetary magnetic field (IMF) have been investigated in a similar way as done by MHD simulations. The simulation results with a southward IMF show the shrunk magnetosphere with great particle entry into the cusps and nightside magnetosphere. This is a signature of a magnetic reconnection at the dayside magnetopause. After a quasi-stable state is established with an unmagnetized solar wind we switched on a solar wind with an northward IMF. In this case the significant changes take place in the magnetotail. The waving motion was seen in the magnetotail and its length was shortened. This phenomena are consistent with the reconnections which occur at the high latitude magnetopause. In our simulations kinetic effects will determine the self-consistent anomalous resistivity in the magnetopause that causes reconnections.

Does the Gravitational ``Constant'' Increase?

Abstract: The possibility that the gravitational coupling “constant”G is an increasing function of the cosmic timet is discussed

Twin Universes Cosmology

Abstract: Starting from the field equationS =χ(T -A(T)), presented in a former paper, we present a test result, based on numerical simulations, giving a new model applied to the very large structure of the Universe. A theory of inverse gravitational lensing is developed, in which the observed effects could be due mainly to the action of surrounding ‘antipodal matter’. This is an alternative to the explanation based on dark matter existence. We then develop a cosmological model. Because of the hypothesis of homogeneity, the metric must be a solution of the equationS = 0, although the total mass of the Universe is non-zero. In order to avoid the trivial solutionR = constant ×t, we consider a model with ‘variable constants’. Then we derive the laws linking the different constants of physics:G, c, h, m; in order to keep the basic equations of physics invariant, so that the variation of these constants is not measurable in the laboratory, the only effect of this process being the red shift, due to the secular variation of these constants. All the energies are conserved, but not the masses. We find that all of the characteristic lengths (Schwarzschild, Jeans, Compton, Planck) vary like the characteristic lengthR, from where all the characteristic times vary like the cosmic timet. As the energy of the photonhν is conserved over its flight, the decrease of its frequencyν is due to the growth of the Planck constanth ≈ t. In such conditions the field equations have a single solution, corresponding to a negative curvature and to an evolution law:R ≈ t2/3.

The redshift revisited

Abstract: We analyse the history of modern cosmology based on the redshift phenomenon and on the cosmic background radiation (CBR). We show the models of different authors for the interpretation of the redshift and how the tired light models predicted the correct value of 2.7 K temperature previous to Gamow and collaborators.

On the General Properties of the Secondaries of Cataclysmic Variable Stars

Abstract: The mass-radius, mass-orbital period, spectral type-orbital period and absolute magnitude-orbital period relationships for the secondary star components of cataclysmic variable (CV) binary stars are discussed. By comparing these with those expected for systems containing Roche lobe-filling main sequence stars, using empirical data for low-mass main sequence stars, it is shown thatas a group the CV secondaries do not differ significantly from main sequence stars.

Theoretical Interpretation of Cosmic Magnetic Fields

Abstract: The paper discusses the possibilty of interpreting the magnetic fields of astronomical bodies in the framework of a unified field theory. Using one of the solutions of the generalized field theory, a direct relation between the polar magnetic field, the angular velocity and the gravitational potential of the body considered, is obtained. The geometric model used for applications has spherical symmetry and is of the type (FIGI). The predictions of the theoretical formula, obtained from the model, are compared with available observational data, and with the empirical formula of Blackett. The theoretical formula gives a possible interpretation of a seed magnetic field which will develop and produce the large-scale magnetic field observed for celestial objects. The formula shows that the field is generated as a result of rotation of a massive object.

New path equations in absolute parallelism geometry

Abstract: The Bazanski approach, for deriving the geodesic equations in Riemannian geometry, is generalized in the absolute parallelism geometry. As a consequence of this generalization three path equations are obtained. A striking feature in the derived equations is the appearence of a torsion term with a numerical coefficients that jumps by a step of one half from equation to another. This is tempting to speculate that the paths in absolute parallelism geometry might admit a quantum feature.

Spectral Evolution of GRB Pulses

Abstract: We perform time-resolved spectral analysis on 26 multi-pulse BATSE GRBs. By fixing two parameters of a 3-parameter fit to the time-integrated values, we achieve a robust spectral hardness measure and physical fluxes. 28 pulse decay phases in 15 GRBs show significant correlation between peak power energy and instantaneous energy flux (characterized byFE∝E1.7 p ). We compare peak energy fluxes, peak hardnesses, and pulse durations and separation for pulse pairs in our sample. There may be some indication of a “recovery timescale” for the bursters' energy source or environment.

Shock High-Energy Emission Mechanisms Applied to SGRs and GRBs

Abstract: We briefly discuss possible applications of the mechanism of relativistic shock emission to soft-gamma-ray repeaters and gamma-ray burst sources.

Internal shocks in a relativistic wind as a source for gamma-ray bursts?

Abstract: We investigate the recent proposal made by Rees and Meszaros (1994) that GRBs result from internal shocks in the relativistic wind emerging from two coalesced neutron stars. Using a simple model of that wind, where a large number of layers with different Lorentz factors interact through a series of mildly relativistic shocks we compute the efficiency of the process and the time profile of the resulting burst. We show that a great variety of profiles can be obtained using plausible initial distributions of Lorentz factors in the wind. However, the rather low overall efficiency (< 10%) of the process and limits imposed on the burst duration can be used to put severe constraints on the nature of the energy source.

Gamma-ray pulsars: polar cap or outer gap emission?

Abstract: The power law spectra of the six knownγ-ray pulsars are seen to cut off at energies < 1 TeV so that no steady pulsed TeV component is observed from any knownγ-ray pulsar. In some cases we show that the cutoff is steeper than exponential, which is consistent with magnetic pair production above the polar cap region. The small upper limits on the ratio of TeV energy flux to optical - GeV energy flux is again consistent with the polar cap model, but appears to be inconsistent with inverse Compton controlled outer gaps.

A model cosmology based on gravity-electromagnetism unification

Abstract: In this article the GEM (Brandenburg, 1992; Brandenburg, 1988) theory is applied to the problem of the cosmos in which most of the matter is hydrogen, spacetime is flat, and a Cosmic Background Radiation CBR field exists. Using the two postulates of the GEM theory: 1. That gravity fields are equivalent to an array of E × B drifts or a spacially varying Poynting field, such that spacetime is determined by EM fields so that the stress tensor of ultrastrong fields is self-canceling; 2. That EM and gravity fields and protons and electrons are unified at the Planck scale of lengths and energies and split apart to form distinct fields and separate particles at the “Mesoscale” of normal particle rest energies and classical radii. A new derivation is made of the formula for G found previously: G = e 2/(m p m e )α exp(−2R 1/2) = 6.668 × 10−8 dynes cm2 g−2where m p and m e are the proton and electron masses respectively, R = m p /m e and α is the fine structure constant, shows that quantum processes may occur which make the vacuum unstable to appearance of hydrogen thus allowing matter creation and a steady state universe to occur. The value for the Hubble Time calculated from this model is T o = (3/((2α)(αR1/2)4))1/3(r e /c)(e 2/Gm p m e ) = 19 Gyr where r e = e 2/(m e c 2) and follows the form first hypothesized by Dirac(1937). The CBR is traced to this process of matter creation and its temperature is calculated as being T CBR = ((3/4)Gm e 2 c/(σ2σo))1/4 = 2.66K where σ is the Thomson cross section of the electron and σo is the Stefan-Boltzman constant.

Cosmological models in certain scalar-tensor theories

Abstract: Exact solutions of Einstein field equations are obtained in the scalar-tensor theories developed by Saez and Ballester (1985) and Lau and Prokhovnik (1986) when the line-element has the form $$ds^2 = \exp \left( {2h} \right)dt^2 - \exp \left( {2A} \right)\left( {dx^2 + dy^2 } \right) - \exp \left( {2B} \right)dz^2 $$ whereh, A andB are functions oft only. The solutions are spatially homogeneous, locally rotationally symmetric and admit a Bianchi I group of motions on hypersurfacest = constant. The dynamical behaviours of these models have also been discussed.

Magnetic field versus gas density, in different physical conditions

Abstract: A search is made here for possible variations in the behavior of magnetic field valuesB at various gas density valuesn, when comparing low density gas versus high density gas, and when comparing compressed gas versus quiescent gas. (a) For thequiescent microturbulent interstellar gas (e.g., clouds, interclumps — see TableI), the statistical relationB ~n k yieldsk = 0.46 ± 0.07 forhigh gas densityn > 100 cm−3, andk = 0.17 ± 0.03 forlow gas densityn < 100 cm−3 (see Figure 1). (b) For thecompressed macroturbulent interstellar gas (e.g., masers, expanding shells — see Table II), the statistical relationB ~n K yieldsK = 0.61 ± 0.09 forhigh gas densityn > 100 cm−3, andK = 0.37 ± 0.2 forlow gas densityn < 100 cm−3 (see Figure 2). (c) The separation betweenlow density gas andhigh density gas is statistically significant. The 2 different physical behaviors (below and above the break at 100 cm−3) are confirmed statistically (about 2 to 4 sigma away for the quiescent gas alone; about 3 to 6 sigma away for the combined quiescent plus compressed gas). (d) The separation betweencompressed gas andquiescent gas is not statistically significant now (see Figure 3). Atn > 100 cm−3, a comparison of quiescent gas versus compressed gas shows no statistically significant differences in behavior (they are only about 1 sigma away). Atn < 100 cm−3, a comparison of quiescent versus compressed gas also shows no statistically significant differences in behavior (less than 1 sigma away). (e) A relation between the densityn and the galactic-wide Star Formation Rate (SFR) can be made for galactic magnetic fields, i.e.: (SFR) ~n n . For galactic-wide parameters using quiescent, low densityn < 100 cm−3, and the known relationshipsB ~n k/j withk = 0.17,B ~ (SFR) j withj = 0.13, then one gets here a lawSFR ~n k/j with an exponentk/j = 1.3. This is in rough accord with known data for the Milky Way and for NGC6946.

The production of circumstellar26Al by massive stars

Abstract: We investigate the production of26Al during hydrogen burning and its ejection by massive single and binary stars. Effects of convection and rotation are studied. We discuss the importance of RSGs, LBVs and WR stars to the total Galactic26Al production, and the detection probability of the26Al decay in individual objects as P Cygni,γ Velorum andη Carinae.

Astronomical time series analysis in South Africa

Abstract: This review is essentially a bibliography. In the first part, work done locally on period-finding methods is discussed. Some of the topics referred to are: periodogram significance testing, identification of non-sinusoidal periodicities, and computational shortcuts useful for calculating periodograms of large data sets. A number of aspects of period-finding inγ ray astronomy are briefly mentioned: these include various test statistics for the presence of a periodicity, the influence of oversampling on significance levels, estimation of the pulse shape, and the specification of a flux limit in the case of a non-detection. The second part of the review deals with the analysis of stochastic astronomical time series. Topics dealt with are ARMA modelling, Kalman filtering, problems associated with O-C analyses, and continuous time ARMA modelling. Two aspects of bivariate astronomical time series are touched on, namely time domain transfer function modelling and the estimation of the lag between two irregularly observed series.