bluuebunny/crossref_metadata_2025_split huggingface dataset:
Rotational Motion of Celestial Bodies in the Relativistic Framework
Authors: Klioner Sergei A. , Soffel Michael 
Published: July 2016

There are several important reasons to consider relativistic effects in rotational motion of celestial bodies. General Relativity is now recommended by the International Astronomical Union and International Union of Geodesy and Geophysics as a theoretical framework for modeling of high-precision observational data. On the other hand, various geodynamical observations provide data which are widely used for testing General Relativity itself.In Newtonian mechanics it is well known how to describe rotational motion of an extended body. In General Relativity this is a rather subtle issue. The concept of a precessing extended rigid body in general relativity encounters fundamental difficulties and cannot be introduced even in the first post-Newtonian approximation. From a practical point of view, however, the rotational motion of the Earth even at the Newtonian level is defined operationally through the time-dependence of geocentric quasi-inertial coordinates of observing sites. An analogous operational definition can be applied in general relativity. To this end, we need a set of physically adequate reference systems.
A Symplectic Integration Scheme that allows Close Encounters between Massive Bodies
Authors: Chambers J.E. 
Published: July 2016

Mixed-variable symplectic integrators provide a fast, moderately accurate way to study the long-term evolution of a wide variety of N-body systems (Wisdom & Holman 1991). They are especially suited to planetary and satellite systems, in which a central body contains most of the mass. However, in their original form, they become inaccurate whenever two bodies approach one another closely. Here, I will show how to overcome this difficulty using a hybrid integrator that combines symplectic and conventional algorithms.A symplectic integrator works by splitting the Hamiltonian, H, for an N-body system, into two or more parts H = H0 + H1 + …, where є i = Hi/H0 ≪ 1 for i = 1, 2 …. An integration step consists of several substeps, each of which advances the system due to the effect of one part of the Hamiltonian only. The error incurred over the whole step is ∼ є τn1, where τ is the timestep, n is the order of the integrator, and є is the largest of єi.
Orbital Structure of the Two Fixed Centres Problem
Authors: Cordero A. , Martínez Alfaro J. , Vindel P. 
Published: July 2016

The set of orbits of the Two Fixed Centres problem has been known for a long time (Chartier, 1902, 1907; Pars, 1965), since it is an integrable Hamiltonian system.We consider a plane that contains the fixed masses. Denote by φ the angle denned by this plane and the one that contains also the third body. The momentum pφ is a first integral of the system and when pφ is different from zero, the manifold generated by the generalized coordinates and momenta are two copies of the three-dimensional sphere S3. If pφ = 0, that is to say when the planet crosses the line joining both suns, the motion is restricted to a planar one. All the equilibrium points appears in this case and therefore the phase spaces are more complex. We restrict our attention to this case which has two degrees of freedom.It is again a Bott-integrable Hamiltonian system. The set of periodic orbits of this systems can be studied from a subset of them, the Non-Singular Morse-Smale type orbits (see Casasayas, 1992). It is proved in Campos (1997) that a small perturbation of a Bott-integrable Hamiltonian system transforms it into a Non-Singular Morse-Smale system. The NMS periodic orbits belong to both the NMS system and the Hamiltonian one. Moreover, The NMS p.o. can be continued to nearly Hamiltonian systems. For instance, in our case to the Restricted Three Body Problem and in the study of the motion of a material point moving inside the gravitational field generated by two stars. This approximation is also useful when the motion of an artificial satellite around a spheroidal body is considered.
Outbursts of Southern Dwarf Novae
Authors: Bateson Frank M. 
Published: July 2016

Abstract.The mean cycles and types of maxima for a number of dwarf novae are tabulated. It is shown that supermaxima are not confined to dwarf novae of short mean cycles. The decline rates for some stars are examined to ascertain if the orbital period, accurate to within ten percent, could be derived from visual observations. It is shown that, at least for some stars, the rate of decline differs widely at different levels of brightness, and, also, between dissimilar maxima of the same star.
Nova Scorpii 1964
Authors: Przybylski A. 
Published: July 2016

SummaryNova Scorpii 1964 was discovered several months after its maximum on a photograph of the centre of the Milky Way taken in May 1964. Its light curve shows that it was a slow nova. At the time of the discovery the nova was in the transition phase between the “4640” stage and the early nebular stage.
The Spectra of Red Variables: SC Stars
Authors: Keenan Philip C. 
Published: July 2016

The value of recording the spectra of red stars depends upon one’s point of view. On the one side, to those concerned with spectral classification in general, it is of the greatest aid in calibrating the criteria of classification if those criteria are observed in a variable star in which the physical variables change by a known amount. Thus the temperature differences, amounting to three or four hundred degrees, between the maximum and minimum phases of Mira variables, have helped to establish the temperature criteria that are in general use for the coolest stars.On the other hand, I am sure that at this meeting there is more interest in the inverse problem: how to use the behavior of the spectral features to learn more about what is going on in the variable star - or, at least, at their surfaces.
Multicolour Photoelectric Photometry of Magellanic Cloud Cepheids
Authors: Feast M.W. , Martin W.L. , Warren P.R. 
Published: July 2016

AbstractPhotoelectric B, V, I observations of LMC cepheids made at SAAO are combined with previous photoelectric B, V values and analysed. It is shown that a P-L-C relation gives a much better fit to the data that a P-L relation at least in the period range 0.9 &lt; logP &lt; 1.7. Individual corrections for interstellar reddening, are derived from the B, V, I results. The colour term in the P-L-C relation is shown to be intrinsic rather than due to differential interstellar reddening. We find,
Frequency Analysis of the Early Type Delta Scuti Star HD153747
Authors: Desikachary Krishna , McInally Cameron J. 
Published: July 2016

SummaryThe early type delta Scuti star HD153747 was photo-electrically observed in the years 1973, 1975 and 1976 at the Mt. John University Observatory, Lake Tekapo. A frequency analysis of these data has shown that the frequency spectrum of this star is variable with time.
Radio Variability of X-Ray Sources
Authors: Duldig M.L. , Thomas R.M. , Greenhill J.G. , Haynes R.F. , Simons L.W. , Murdin P.G. 
Published: July 2016

AbstractRadio observations at a wavelength of 2 cm have been made in the directions of a large number of X-ray sources. Here we discuss the variability of the sources detected. Circinus X-l was clearly observed in each of the three periods 1977 June, 1977 August and 1978 April. Many of the radio sources seem to be variable, although possible source confusion makes confirmation of some of these results difficult.
Recent Photoelectric and Spectroscopic Observations of YY Orionis Stars
Authors: Mundt R. 
Published: July 2016

AbstractThe YY Orionis Stars are a subclass of the T Tauri stars, which have a particularly strong UV-excess and show spectroscopic evidence for infailing envelopes, like the occurrence of inverse P Cygni profiles. They have been interpreted as low mass protostars in the final stages of their hydrodynamic evolution (Walker 1972, Appenzeller and Wolf 1977, Wolf et al. 1977).
Review of radio astrometry II. Optical measurements
Authors: de Vegt Chr. 
Published: July 2016

The establishment of an astronomical reference frame which is based on positions of extragalactic objects is an old problem of fundamental astronomy. Radio astrometry has now achieved a level of accuracy, which rivals the best optical work, thus openingexcitingprospects to reach this aim within a relatively short time scale.To take full advantage of these encouraging developments for optical fundamental astrometry, precise positions of the faint optical counterparts of compact extragalactic radio sources and galactic radio stars have to be determined in the fundamental system of the FK4.
Concept of a new PZT
Authors: Kühne Chr. 
Published: July 2016

The PZT according to MARKOWITZ, undoubtedly one of the most accurate measuring instruments, has become an integral part of astronomy in the last years. Yet there are two reasons why I still tried to improve the MARKOWITZ basic idea.For the MARKOWITZ process the use of a lens objective is imperative. Because of the diameter limitations of the lens objectives the magnitude is reduced. Although a 65 cm PZT has been put into operation in Washington recently, the thermal difficulties encountered made it necessary to control the temperature actively. In addition, the 65 cm diameter of a lens equipped with 4 elements can be considered as the very limit, but not for a mirror system.In addition there are still more disadvantages using lens objectives. The useable field is restricted by astigmatism, reducing the range of declination. The transmission, especially in the blue wavelengths, decreases very rapidly with the thickness of lenses. For large multilens objectives the transmission is less than for a mirror system.
Optical interferometers in astrometry
Authors: Shao M. 
Published: July 2016

AbstractLong baseline stellar interferometers, for astrometry are potentially capable of extremely high accuracies, 10-4to 10-5arc sec for a single measure ment. There are three categories of errors that apply to an astrometric interferometer, instrumental, atmospheric and photon noise. Of these, only photon noise has a flat power spectrum. All of the other sources of error are systematic in the sense that the accuracy does not increase as the square root of the measurement time. The elimination of atmospheric error is the major difference between interferometers and other photoelectric astrometric instruments. Two types of interferometers, image plane and pupil plane, are described. The advantages and limitations of these two approaches are discussed.
Contribution of astro - geodetic observations to the improvement of fundamental catalogue
Authors: Gerstbach G. 
Published: July 2016

AbstractThe example of astronomical location determination by Zeiss astrolabe shows that the accuracy of many astro-geodetic observations is sufficient for improvement of star positions. An estimation yields a number of 800 visual latitude/longitude determinations with more than 20.000 star transits, annually observed at triangulation points of western Europe alone. It is proposed to forward the residuals of the observed stars to a central agency (e.g. Astronomisches Rechen-Institut Heidelberg) together with parallactic angle and standard deviation. Collecting these data over a period of 5 years the positions of most fundamental stars could be improved with an accuracy better than ± 0.05”.
Scientific impact of interferometric and space astrometry
Authors: Kovalevsky J. 
Published: July 2016

Four types of astrometry are distinguished in this paper, depending upon the field ϕ where the measurements are made. For each of them, the expectations for astronomic results due to the advent of interferometry and space techniques are discussed. In the case of very narrow field astrometry (ϕ &lt; 5”)speckle and Michelson interferometry in visual and infrared light will permit an increase by two or three orders of magnitude on the precision of double star and diameter measurements. In narrow field astrometry ( ϕ &lt; 0°,5) space telescope, mask detection systems and Michelson interferometry, the main results will be accurate’ relative parallaxes of selected objects and a search for invisible companions of stars.For large field astrometry (ø &lt; 5°), photographic techniques will be still used. Finally, while Earth based semi-global astrometry will increase the number of observed objects, the breakthrough will come from space astrometry which will permit to get 0"001 precision for parallaxes and annual proper notions.The scientific expectations for stellar and galactic astronomy, astrophysics, stellar and galactic evolution, etc… are sketched, in particular for more than 100 000 stars parallaxes with an overall precision of one to two milliseconds of arc. This should provide:- a remarkable basis for the determination of the masses about 300 double stars,- the calibration of luminosities for most main sequence stars, type III giants and population II horizontal branch,- a new cosmic distance scale, with a direct trigonometric calibration of new categories of stars that are used as distance indicators,- Using the extended distance scale and the proper motion that will be obtained for all the stars in the program, it will be possible, for the first time, to analyse the variations of the kinematic properties within a sphere of about 1000 parsecs, providing data for the analysis of the galactic gravitational potential and the kinematics of various types of stars.- In connection with VLBI and narrow field astrometry using interferometers, the catalogue will provide an accessible frame of reference for all studies where an absolute system of reference is needed.
The Short Spacings Problem in Correlation Interferometers and a Procedure for Solving It
Authors: Bajaja E. 
Published: July 2016

AbstractThe effects of the missing short spacings in correlation interferometers measurements on the synthesized antenna pattern and on the maps produced, are analysed. A method for correcting those effects, based on the obtention of the missing components from single dish observations, is described. The procedure has been developed for the WSRT observations of extended objects and it has been applied on several cases. Some results are shown for M31 and NGC4258.
The Method CLEAN – Use, Misuse and Variations
Authors: Schwarz U.J. 
Published: July 2016

SummaryThe mathematical background of the method CLEAN, as given by Schwarz (1978) is summarized. A method to derive source parameters (flux, position, extent) as a direct application of the theory is described. The ambiguity of the CLEAN method is discussed from a theoretical and practical point of view. Empirical results on the influence of the loop-gain are given. A new approach for the processing of maps with limited phase information is presented and a novel application of CLEAN for beam-switching observations (as an example for the case of a non-symmetric beam) is given.
Fitting a Gaussian Model to Aperture Synthesis Data by Akaike’s Information Criterion (AIC)
Authors: Ishiguro Masato , Ishiguro Makio 
Published: July 2016

AbstractThe problem of determining the optimum number of components in fitting a Gaussian model to aperture synthesis data is considered. As a measure of the badness of the fitted model, we propose the use of Akaike’s Information Criterion (AIC).
Application of the Simplex Algorithm to Image Restoration
Authors: Baker P.L. 
Published: July 2016

AbstractThe data measured by an interferometer define the original image through linear equations of constraint containing the Fourier coefficients. These constraints and the requirement that all image points show a non-negative flux define a problem frequently encountered in operations research and numerically tractable through the Simplex Algorithm. When the data are undersampled, a continuum of original images is compatible with the measured points. The advantage of the operations research method in this situation is that the continuum of restorations is generated rather than a single example. As a practical matter, it appears most useful to select from this continuum the maximum sharpness restoration – which like CLEAN uses reasonable a priori information to chose one of the possibilities – and to select for comparison the maximum entropy solution – which embodies diametrically opposed a priori information. The first may be considered the best restoration for an astronomical source, while the second indicates the uncertainty due to undersampling. When our astrophysical deductions depend strongly on the restored structure of a source, a comparison of alternative solutions is necessary to acquire confidence in the deductions. The maximum sharpness solutions also have the advantage that small phase errors in the measured complex points can be compensated. In principle, large phase errors can be compensated, but uniqueness may be lost, and existing numerical procedures for this case are inefficient.
Generation of Oscillatory Motions in the Stellar Atmosphere
Authors: Osaki Yoji 
Published: July 2016

AbstractThermal overstability of non-radial eigenmodes of stars is discussed as one of possible causes for generating non-thermal motions in the stellar atmosphere. The nature of oscillatory motions in stars is first considered both in the local and the global stand points. Then, the excitation of eigen-oscillations is discussed and results of numerical studies so far made are reviewed for the vibrational stability of various stellar models against non-radial oscillations. It is found that many of non-radial p-modes of high tesseral harmonics are likely excited in various stars of the HR diagram and that they possibly manifest themselves as non-thermal velocity fields in the stellar atmosphere.
Differential Line Shifts in Late Type Stars
Authors: Stawikowski A. 
Published: July 2016

AbstractThe differential line shifts method for studying convective type motions has been described in the review of Glebocki and Stawikowski at this colloquium. Here the results of differential line shifts for twenty late type stars are presented. Line shifts were determined either from published wavelength measurements (α CMi, β Peg, α Boo, α Ser, α UMa, α Per, α Sco, γ Cyg α Car and HD 19445) or from my own wavelength measurements on 8.1 Å/mm dispersion spectra (56 Ori, η Peg, η Psc, 61 Cyg A, ε Eri, δ Eri, o Tau, ε Cyg, α Aqr and ι Ari). Because of low accuracy of the wavelength determinations, mean shifts (VR) were calculated for narrow ranges of lower excitation potentials (LEP). These shifts were plotted against excitation potential and VR-LEP relations were obtained for all analysed stars. Because of the scatter of points in these diagrams linear approximation was assumed and the slopes (denoted A and expressed in Km/s/eV) of the VR-LEP relations were calculated. The stellar parameters and the derived slopes for the whole sample of investigated stars were analysed, and correlations with microturbulence, luminosity and gravity were found. These correlations demonstrate that the slope of VR-LEP relation describes some component of velocity field in late type stars.The detailed description of the procedure and results will be published in Acta Astronomica.
Temporal and Spatial Fluctuations in Widths of solar Euv Lines
Authors: Athay R.G. , White O.R. 
Published: July 2016

AbstractAnalyses of some 300 hours of time sequences of solar EUV line profiles obtained with 0S0-8 show large fluctuations in line widths. At a given location on the sun, line widths fluctuate temporally on time scales ranging from less than a minute to over an hour. At any given time, line widths fluctuate spatially on a variety of scales ranging from active region size to arc second size. Temporal and spatial fluctuations are of approximately the same amplitude. Thus, the sun can be characterized by an aggregate of small cells in each of which line widths are fluctuating in time and which have random phases with respect to each other.Spatial fluctuations in line width are correlated with large scale spatial fluctuations in brightness for some lines but not for others. Temporal fluctuations in width are sometimes correlated with either Doppler shifts or intensity fluctuations, but more often such correlations are absent.For a given line, the line width varies through an extreme range of about a factor of two. Nonthermal components of line width vary from approximately the local sound speed to a small fraction of the sound speed.
Numerical Simulation of Granular Convection: Effects on Photospheric Spectral Line Profiles
Authors: Nordlund Åke 
Published: July 2016

AbstractThe results of numerical simulations of the solar granulation are used to investigate the effects on photospheric apectral lines of the correlated velocity and temperature fluctuations of the convective granular motions. It is verified that the granular velocity field is the main cause for the observed broadening and strengthening of photospheric spectral lines relative to values expected from pure thermal and pressure broadening. These effects are normally referred to as being due to “macro-turbulence” and “micro-turbulence”, respectively. It is also shown that the correlated temperature and velocity fluctuations produce a “convective blue shift” in agreement with the observed blue shift of photospheric spectral lines. Reasons are given for the characteristic shapes of spectral line bisectors, and the dependence of these shapes on line strength, excitation potential, and center to limb distance are discussed.
Mechanical Energy Transport
Authors: Stein Robert F. , Leibacher John W. 
Published: July 2016

AbstractLadies and Gentlemen, we now reveal to you the secrets of how to create chaos out of order. The existence of a chromosphere or corona requires the existence of motions. A chromosphere or corona requires some non-radiative heat input. There has to be some kind of motion, either oscillatory or quasi-static, to transport the energy up to the chromosphere or corona. This ordered motion may be observed as chaos: microturbulence, macroturbulence, line asymmetries or shifts. Of course, it is necessary to actually compute the effects of motions on line profiles in order to see what will really happen.
Changes of Photospheric Line Asymmetries with Effective Temperature
Authors: Gray David F. 
Published: July 2016

AbstractAsymmetries in photospheric lines were described briefly in my earlier contribution to these proceedings and in more detail in the January 15, 1980 issue of the Ap. J. The cores of the stronger lines in α Boo (K2III) show a blue shift, quite opposite to the effect seen in the solar lines. Suspecting this difference to be due to the shift in ionization with effective temperature, two other stars were measured, one slightly hotter than α Boo (β Gem KOIII) and one slightly cooler (β UMi K4IV). The preliminary results seem to confirm the effective temperature dependence since no asymmetries were found for β Gem and stronger blue-shifted core asymmetries were found for β UMi.
The Equilibrium of Degenerate Configurations
Authors: Seidov Z.F. 
Published: July 2016

Great progress has been achieved in recent years in investigations of the dense degenerate matter and in calculations of white dwarf structure. Nevertheless the classical theory by Chandrasekhar (1967) describes the structure of white dwarf stars rather well, with accuracy of few precents. This theory leads to the following second-order differential equation:where p is a constant parameter, 0 ≤ y < 1. In Eq. (1) the values of the parameter n = 0,1 and 2 correspond to the plane-symmetric, cylindrical and spherical self-gravitating configurations of degenerate matter.