bluuebunny/crossref_metadata_2025_split huggingface dataset:
Parallel/Vector Integration Methods for Dynamical Astronomy
Authors: Fukushima Toshio 
Published: July 2016

AbstractThis paper reviews three recent works on the numerical methods to integrate ordinary differential equations (ODE), which are specially designed for parallel, vector, and/or multi-processor-unit (PU) computers. The first is the Picard-Chebyshev method (Fukushima, 1997a). It obtains a global solution of ODE in the form of Chebyshev polynomial of large (> 1000) degree by applying the Picard iteration repeatedly. The iteration converges for smooth problems and/or perturbed dynamics. The method runs around 100-1000 times faster in the vector mode than in the scalar mode of a certain computer with vector processors (Fukushima, 1997b). The second is a parallelization of a symplectic integrator (Saha et al., 1997). It regards the implicit midpoint rules covering thousands of timesteps as large-scale nonlinear equations and solves them by the fixed-point iteration. The method is applicable to Hamiltonian systems and is expected to lead an acceleration factor of around 50 in parallel computers with more than 1000 PUs. The last is a parallelization of the extrapolation method (Ito and Fukushima, 1997). It performs trial integrations in parallel. Also the trial integrations are further accelerated by balancing computational load among PUs by the technique of folding. The method is all-purpose and achieves an acceleration factor of around 3.5 by using several PUs. Finally, we give a perspective on the parallelization of some implicit integrators which require multiple corrections in solving implicit formulas like the implicit Hermitian integrators (Makino and Aarseth, 1992), (Hut et al., 1995) or the implicit symmetric multistep methods (Fukushima, 1998), (Fukushima, 1999).
Motion near The Unit Circle in The Three-Body Problem
Authors: Broucke Roger A. 
Published: July 2016

Many of the important applications of the circular planar restricted problem of three bodies involve motion in the vicinity of the unit circle, (as defined in canonical units). It is then of interest to develop simplified models which are valid in this region. These models preserve the gross characteristics of the original system but they possess simpler equations of motion.We will also show that several simplified models can be seen as a perturbation of a very well known simple linear system: the Clohessy-Wiltshire equations used by NASA in all their rendezvous operations. These are actually very close to the well-known Hill problem. We will thus consider the Restricted problem as a perturbed Hill or Clohessy-Wiltshire problem. We also introduce the Clohessy-Wiltshire Lagrangian in polar coordinates.
On The Critical Phenomena in The Dynamics of Asteroids
Authors: Shevchenko Ivan I. 
Published: July 2016

AbstractTwo statistical effects in the long-term chaotic asteroidal dynamics are considered, namely the power-law character of the dependence of recurrence times on local Lyapunov times and the power-law decay in the tails of the recurrence distributions. The dependences in both cases are shaped by effects of anomalous transport, due to the presence of the chaos border in phase space, and by statistical selection effects.
Analytical Approximation of the Dissipative Standard Map
Authors: Celletti Alessandra , Penna Gabriella Della , Froeschlé Claude 
Published: July 2016

We investigate the dynamics of a dissipative standard mapping defined by the equationswhere y ∈ R, x ∈ T and ε is a real parameter, we refer to 0 < α < 1 as the “dissipative parameter” and to ψ as the “dissipative coefficient” (ε = α = 0 provides an integrable mapping). Notice that the dynamics is contractive, since the jacobian of the above mapping equals to 1 − α. In particular, we want to compare (see Celletti et al., 1997) the solutions associated to the conservative map (i.e., α = 0) with that related to (1) (α ≠ 0). For simplicity, we consider the case when α = ε2 and construct explicit approximate solutions to the conservative and dissipative systems, using a suitable parametrization like in (Celletti and Chierchia, 1988).
Fundamental position astronomy and transportable Zenith Cameras
Authors: Pilowski K. 
Published: July 2016

AbstractIn continuation of previous works of the author (Astronomische Station of the Technical University of Hannover: Monographie 1973; Supplement I, no. 9, 1974; Suppl. II, no. 10, 1976; Suppl. Ill in preparation), a completely systematic solution of the “Fundamental Positional Astronomy” and “Transportable Zenith Cameras” is given as a new form of a purely stellar Spherical Astronomy, in theoretical foundation, instrumentation and observation in a very simple way, as with traditional means, based on geographical position determination.There are two possibilities:A purely stellar absolute fundamental system (fundamental catalogue and absolute inertial systems) and a stellar-geographical fundamental system (fundamental catalogue, absolute inertial systems, equator position in time), serving additionally for the determination of the geographical position and the earth’s rotation (absolute earth time). The first theory is based on two selected zenith fields, the second on one selected zenith field and the earth’s rotation.
Views of the future development of Fundamental Astrometry
Authors: Tucker R. H. , Teleki G. , Counselman C. C. , Hog E. , Marcus E. , Débarbat S. , Fogh Olsen H. J. , Gliese W. , Klock B. L. , Murray C. A. , Nikoloff I. , Potter H. I. , Tavastsherna K. N. , Wood H. 
Published: July 2016

SummaryThe paper presents various views of the future of fundamental astrometry, arranged under the following headings:1 General and Introductory Remarks2 The New Techniques2.1 Modernised Classical Astrometry2.2 Laser Ranging2.3 Radio Astrometry2.4 Space Astrometry2.5 Other Techniques3 The Impacts and Implications3.1 Prospects for Classical Astrometry3.2 Criteria for cessation of observations3.3 Need for adequate connection of observational series3.4 Planning of Observational Programmes4 Points Emerging4.1 Fundamental and Relative Astrometry4.2 Fundamental and Inertial Systems4.3 Data Processing4.4 The Role of IAU Commission 84.5 Proposed Observational Programmes5 Bibliography
Problems with Maximum Entropy
Authors: Ponsonby J.E.B. 
Published: July 2016

AbstractTwo rival expressions for entropy have been used in maximum entropy image formation. Attention is drawn to the fact that both have been shown to be limiting cases of a general expression for the entropy of the photon distribution swept up during an observation. All measures of entropy are based on some physical model of the radiation mechanism. It is argued that knowledge of its character should be fully included in the image formation process.
The Introduction of A Priori Knowledge in Certain Processing Algorithms
Authors: Högbom J.A. 
Published: July 2016

A common problem in radio synthesis work is that of determining the brightness at N grid points in the map domain when there are only n<N independent interferometer measurements available. The missing (N-n) equations can in principle be replaced by an equivalent amount of information in the form of a priori knowledge about the brightness distribution. One way of doing this is to add an equation of the formi.e. some function of the brightness distribution is maximized subject to the condition that the map be compatible with the n measurements. This automatically gives (N-n) new equations, leaving us in the pleasant situation of having as many equations as there are unknowns.
Instabilities in a Polytropic Atmosphere
Authors: Antia H.M. , Chitre S.M. 
Published: July 2016

AbstractThe density in the outer layers of stars varies by several orders of magnitude and it is desirable to include the full effects of compressibility in any study of instabilities arising in stellar convection zones. In an unstable compressible fluid-layer that is thermally conducting both the oscillatory and non-oscillatory motions can simultaneously arise. The conditions under which the oscillatory acoustic modes can be overstabilized in a polytropic atmosphere are examined. It is argued that the linearized perturbation theory breaks down when applied to an inviscid complete polytrope which has vanishing density and temperature at the top for both optically-thin and optically-thick approximations. However, the linearized theory is demonstrated to be self-consistent when viscosity and thermal conductivity are included in the study of complete polytropes.
Thermal and Continuum Driven Convection in B-Stars
Authors: Nelson George Driver 
Published: July 2016

AbstractTwo regions of convective instability are present in the photosphere of a typical B-star (Teff = 30,000 K Log g = 4.0). One is the usual thermal instability caused by the helium ionization. The other is driven by the continuum radiation pressure in a thermally stable layer. Mixing length and anelastic modal representations of these unstable regions show that the rapid radiative cooling of temperature fluctuations limits the velocities to an amplitude of a few meters per second, much too small to account for the observed line broadening and asymmetries.
Differential Rotation and Magnetic Activity of the Lower Main Sequence Stars
Authors: Belvedere G. , Paterno L. , Stix M. 
Published: July 2016

AbstractWe extend to the lower main sequence stars the analysis of convection interacting with rotation in a compressible spherical shell, already applied to the solar case (Belvedere and Paterno, 1977; Belvedere et al. 1979a). We assume that the coupling constant ε between convection and rotation, does not depend on the spectral type. Therefore we take ε determined from the observed differential rotation of the Sun, and compute differential rotation and magnetic cycles for stars ranging from F5 to MO, namely for those stars which are supposed to possess surface convection zones (Belvedere et al. 1979b, c, d). The results show that the strength of differential rotation decreases from a maximum at F5 down to a minimum at G5 and then increases towards later spectral types. The computations of the magnetic cycles based on the αω-dynamo theory show that dynamo instability decreases from F5 to G5, and then increases towards the later spectral types reaching a maximum at MO. The period of the magnetic cycles increases from a few years at F5 to about 100 years at MO. Also the extension of the surface magnetic activity increases substantially towards the later spectral types. The results are discussed in the framework of Wilson’s (1978) observations.
Major Observatories Versus Economy-Class Observatories in Space
Authors: Dyson Freeman J. 
Published: July 2016

AbstractBig space-instruments are essential for observing fine details of selected objects. Small space-instruments are essential for scanning large areas of sky, for discovering and cataloguing new objects, and for monitoring variable or transient sources of radiation. If astronomy is to flourish, both types of instrument must be provided. The problem is not to choose between big and small. The problem is to find the optimum mixture of big and small. Various examples from past history and from future plans illustrate the importance of maintaining a variety of different types of instrument.
The UV Imager for the Israeli Scientific Satellite
Authors: Brosch N. 
Published: July 2016

AbstractIsrael will orbit a satellite dedicated to scientific research. One of the two experiments studied for deployment on this platform is a three-channel imager in the ultraviolet proposed by Tel Aviv University and designed jointly by staff of the Wise Observatory and of El-Op (Electro-Optical Industries, Ltd.). The design provides very significant scientific returns in a small payload and for a moderate cost.
A Theoretical Review of Diffusive Shock Acceleration
Authors: Jones Frank C. 
Published: July 2016

AbstractWe discuss the fundamental ideas of particle acceleration in plasma shocks with emphasis on those features that are required to produce the “universal” power-law spectrum. We compare shock acceleration with the more familiar second-order or stochastic acceleration and see that they are not too different in many respects. We discuss the features of shock acceleration that make it appealing and some of its problems as well.Subject headings: acceleration of particles — MHD — plasmas — shock waves
Acceleration of Electrons at a Curved Shock
Authors: Vandas M. 
Published: July 2016

AbstractThe paper deals with the acceleration of electrons at a nearly perpendicular curved shock wave and results are compared with the case of a plane shock wave. We formulate the equations of motion of an electron in adiabatic approximation and solve them in a special case. The results show that the energy gain of reflected electrons at a curved shock depends on the ratio of the radius of curvature to the shock thickness; for a low ratio the energy gain is significantly lower than at a plane shock, while the energy gain of transmitted electrons is approximately the same as at a plane shock. The analytical solution provides a tool for direct calculations of fluxes of accelerated electrons near the curved shock of given parameters.Subject headings: acceleration of particles — shock waves
Gamma-Ray Precursors of Solar Flares
Authors: Rieger E. 
Published: July 2016

AbstractBursts have been observed by the gamma-ray spectrometer on SMM at medium- and high-energy gamma-rays that precede the flare maximum. The negligible contribution of nuclear lines in the spectra of these events and their impulsive appearance suggests that they are hard-electron-dominated events superposed on the flares. Spatial resolution at gamma-ray energies will be necessary to decide whether this kind of bursts is cospatial with the flares or whether they occur in the flares’ vicinity.Subject headings: Sun: flares — Sun: X-rays, gamma rays
Acceleration from Field-Aligned Potential Drops
Authors: Haerendel Gerhard 
Published: July 2016

AbstractUnstable field-aligned currents are seen as the origin of field-aligned potential drops. They convert energy stored in magnetic shear stresses into kinetic energy. A good fraction of this energy is carried by runaway electrons and ions out of the acceleration region. The paper emphasizes the analogy with mechanical fractures. Simple expressions for the energy conversion rate and the parallel potential drop are derived, the two being linked by the critical current density needed for instability. The origin of the currents (generator) lies mostly in a region remote from that of energy conversion (fracture zone). The transmission of shear stresses and energy from the generator plasma, where the primary forces are applied to the fracture zone is also considered. A closed set of relations allows quantitative evaluation of the energetic particle production efficiency. The decoupling of the plasma on either side of the fracture zone which allows fast stress relief is described in detail, as well as a stationary model for the Alfvén wave interaction between fracture zone and generator plasma. A simple concept of the nature of the anomalous resistivity generated by the unstable current leads to an expression for the magnetic diffusivity inside the fracture zone and an estimate of the latter’s extent parallel to the magnetic field, whereas its width and length transverse to B follow from the macroscopic relations. Finally and as an example, the theory is applied to the problem of fast electron (and ion) acceleration well above 1 MeV seen to occur in many solar flares. It is obvious that this process belongs to the most powerful production processes of high-energy particles in stellar magnetic fields.Subject headings: acceleration of particles — MHD — plasmas
Gamma Rays from the Geminga Pulsar: Variations with time and Phase
Authors: Grenier I. A. , Bennett K. , Buccheri R. , Gros M. , Henriksen R. N. , Hermsen W. , Kanbach G. , Sacco B. 
Published: July 2016

AbstractPulsed γ radiation from the Geminga pulsar was recorded by the COS B satellite from 50 MeV to 5 GeV between 1975 and 1982. It has been analysed to derive the source spectral properties as a function of time and phase. The two main peaks are separated by 0.50 ± 0.01 in phase. Significant pulsed emission has also been detected in both phase regions between the two main peaks. Significant spectral differences with phase have been found; the emission from the first peak is slightly softer than from the second one and the Interpeak 2 radiation is extremely soft. The first peak emission remained stable within the statistics over 7 years while the flux from the other phase intervals changed significantly with time. The phase dependence of the variability implies that all the observed source emission should be pulsed and that it consists of four discrete beams with different apertures and spectra. The beams characteristics are strikingly similar to those of four γ-ray beams generated by the Vela pulsar.Subject headings: gamma rays: observations — pulsars: individual (Geminga, Vela)
Gamma-Ray Burst Spectral Breaks and Source Beaming
Authors: Baring Matthew G. 
Published: July 2016

AbstractThe principal discovery of the ComptonGamma-Ray Observatory(GRO) concerning gamma-ray bursts is that these sources are isotropic but with a comparative deficiency of fainter sources, suggesting that they are probably cosmological in origin. If they are at such large distances from Earth then they are extremely luminous and compact. A consequence of this is that two-photon pair production attenuation of the gamma-ray continuum cannot be avoided unless the source radiation is substantially beamed. Most sources do not display gamma-ray turnovers although a few GRB detected byGROexhibit distinct spectral breaks in the MeV range. A derivation of the relationship defining of the degree of beaming in burst sources with spectral breaks due toγ-γattenuation, as a function of source spectral index and break energy, is presented. It is found that sources at distances of ~1 Gpc must typically be beamed with bulk Lorentz factors of around 103-104, indicating powerful bulk acceleration in bursts, although these Lorentz factors are reduced markedly for steep source spectra. Since the source spectra are not strongly Comptonized, such beaming will blueshift the γ-γ attenuation breaks to energies much higher than 1 MeV; an absolute lower bound to the source bulk Lorentz factor is determined from this additional constraint. This blueshifting suggests that those sources with MeV breaks may not be cosmological, or that their breaks are produced by a mechanism that dominates γ-γ attenuation at these energies.Subject headings:gamma-rays: bursts — radiation mechanisms: nonthermal
On The Location of the Acceleration and Emission Sites in Gamma-Ray Blazars
Authors: Dermer Charles D. , Schlickeiser Reinhard 
Published: July 2016

AbstractCompton scattering of external radiation by nonthermal particles in outflowing blazar jets is dominated by accretion-disk photons rather than scattered radiation to distances ~ 0.01-0.1 pc from the central engine for standard parameters, thus clarifying the limits of validity of the model by the present authors and the model of Sikora, Begelman, & Rees. On the basis of contemporaneous Ginga X-ray and EGRET gamma-ray observations, we estimate the radius of 3C 279’s gamma-ray photosphere to be smaller than estimated by Blandford. There is thus no need to require that the acceleration and emission sites of gamma-ray blazars to be located farther than ~ 102 - 103 gravitational radii from the central engine. We argue that lineless BL Lac objects, rather than quasars, are more likely to be detected in the TeV energy range.Subject headings: acceleration of particles — BL Lacertae objects: general — galaxies: jets — gamma rays: theory — radiation mechanisms: nonthermal
Oblique Magnetohydrodynamic Cosmic-Ray-Modified Shocks: Two-Fluid Numerical Simulations
Authors: Frank Adam , Jones T. W. , Ryu Dongsu 
Published: July 2016

AbstractWe present the first results of time-dependent two-fluid cosmic-ray (CR) modified MHD shock simulations. The calculations were carried out with a new numerical code for one-dimensional ideal MHD. By coupling this code with the CR energy transport equation we can simulate the time-dependent evolution of MHD shocks, including the acceleration of the CR and their feedback on the shock structures. We report tests of the combined numerical method including comparisons with analytical steady state results published earlier by Webb, as well as internal consistency checks for more general MHD CR shock structures after they appear to have converged to dynamical steady states. We also present results from an initial time-dependent simulation which extends the parameter space domain of previous analytical models. These new results support Webb’s suggestion that equilibrium oblique shocks are less effective than parallel shocks in the acceleration of CR. However, for realistic models of anisotropic CR diffusion, oblique shocks may achieve dynamical equilibrium on shorter timescales than parallel shocks.Subject headings: cosmic rays — methods: numerical — MHD — shock waves
Cataclysmic Variables: A ’SWOT’ Analysis
Authors: Warner Brian 
Published: July 2016

AbstractA brief review of the nature and subtypes of cataclysmic variables (CVs) is given. The catalogue of CVs is still very incomplete. All-sky surveys should add large numbers of CVs, which will improve knowledge of the space density of these systems. It is pointed out that the nova-like variables, which are the most difficult to discover, are the subtype having the highest space density. Their discovery is therefore the highest priority – they fix the frequency of CVs, which is important in population syntheses of binary stars.
The Rotation and Variability of T Tauri Stars: Results of Two Decades of Monitoring at Van Vleck Observatory
Authors: Herbst W. 
Published: July 2016

AbstractA brief history of photometric monitoring of T Tauri stars with a 0.6 m telescope at Wesleyan University is given. This is followed by discussion of three recent results: 1) the mass dependence of stellar rotation rates in the Orion Nebula Cluster, 2) a check on stellar radii estimates of pre-main sequence stars, and 3) the nature of a very peculiar object in NGC 2264.
Project MONICA for the Study of Time-Variable Phenomena of the Jovian Sodium Cloud and the Io Plasma Torus
Authors: Chang Chien-Pang , Ip Wing 
Published: July 2016

AbstractBecause of active volcanism, large amounts of gas and dust particles are being injected from the Galilean satellite, Io, into the Jovian system. The neutral cloud of sodium atoms and the plasma torus of sulfur ions provide very useful information on Io’s interaction with the Jovian magnetosphere. A program called MONICA (Monitoring of Neutral and Ionized Atoms Clouds) was established at NCU with a view to participate in an international campaign during the flyby of the Jovian system by the Cassini spacecraft in December, 2000. Spectrographic observations were carried out using the 2.16m spectrograph of the Beijing Astronomical Observatory in Xing-Long. A progress report is presented here.
Long-Term Coudé Radial-Velocity Studies With a 1.2-m Telescope
Authors: Scarfe C.D. 
Published: July 2016

AbstractI have used the 1.2-m telescope and coudé spectrograph of the Dominion Astrophysical Observatory for more than 30 years in a program of radial-velocity observations of binary stars. The program was begun with photographic plates as detectors, but for 20 years the primary detector has been the radial-velocity scanner, which cross-correlates stellar spectra with an artificial mask.Since some of the binaries under observation have periods of several years, the instrument’s stability is an important consideration. I have therefore been obliged to observe standard stars and asteroids to check its performance. These observations are of relevance to efforts to improve the IAU standard star system.I will describe the telescope, spectrograph and scanner, and will briefly discuss some of the results obtained for a selection of binary and multiple stars.