bluuebunny/crossref_metadata_2025 huggingface dataset:
Pulsation Hydrodynamics of Luminous Blue Variables and Pulsation-Driven Winds
Authors: Guzik Joyce A. , Cox Arthur N. , Despain Kate M. , Soukup Michael S.
Published: July 2016

Many physical factors, including radial and nonradial pulsation, rotation, radiation pressure, convection, magnetic fields, or dynamical instabilities may play important roles in the hydrodynamics of Luminous Blue Variables. We review the current status of hydrodynamic modeling of LBV envelopes, and describe results of our models using the one-dimensional nonlinear hydrodynamics code of Ostlie and Cox. We find that the models pulsate in several simultaneous radial modes, driven by the helium and Fe ionization к effect. The pulsations have quasi-periods between 5 and 80 days, with radial velocity amplitudes of 50-200 km/sec, and may be identified with the LBV microvariations. In some cases, depending on luminosity-to-mass ratio and helium abundance, deep layers in the model can periodically exceed the Eddington luminosity limit. The key to exceeding LE is the inclusion of the time dependence of convection: Near the regions of opacity peaks produced by Fe and helium ionization, convection is turning on and off during each pulsation cycle. If convection cannot turn on rapidly enough to transport the required luminosity through the region, the Eddington limit is exceeded. If this region of the star is sufficiently adiabatic, an “outburst” may occur. In the hydrodynamic models, an outburst is indicated by the photospheric radial velocity suddenly becoming very large, and the photospheric radius increasing monotonically over several pulsation cycles. Such pulsation-triggered outbursts may be responsible for the driving of variable, nonspherical winds. If large and infrequent enough, these outbursts may be identified with the classic LBV eruptions accompanied by episodic mass loss.
Instabilities in LBVs and WR Stars
Authors: Ødegaard Knut Jørgen Røed
Published: July 2016

Using a dynamical stellar code, detailed evolutionary models have been computed that follow the evolution and nucleosynthesis of massive stars from the pre-MS (accretion phase) up to the onset of O-burning. Various instabilities occur during the evolution. As an example, the evolution of a star with maximum mass 120 Mʘ is discussed.
Chaos in Relativity and Cosmology
Authors: Contopoulos G. , Voglis N. , Efthymiopoulos C.
Published: July 2016

Chaos appears in various problems of Relativity and Cosmology. Here we discuss (a) the Mixmaster Universe model, and (b) the motions around two fixed black holes, (a) The Mixmaster equations have a general solution (i.e. a solution depending on 6 arbitrary constants) of Painlevé type, but there is a second general solution which is not Painlevé. Thus the system does not pass the Painlevé test, and cannot be integrable. The Mixmaster model is not ergodic and does not have any periodic orbits. This is due to the fact that the sum of the three variables of the system (α + β + γ) has only one maximum for τ = τm and decreases continuously for larger and for smaller τ. The various Kasner periods increase exponentially for large τ. Thus the Lyapunov Characteristic Number (LCN) is zero. The “finite time LCN” is positive for finite τ and tends to zero when τ → ∞. Chaos is introduced mainly near the maximum of (α + β + γ). No appreciable chaos is introduced at the successive Kasner periods, or eras. We conclude that in the Belinskii-Khalatnikov time, τ, the Mixmaster model has the basic characteristics of a chaotic scattering problem, (b) In the case of two fixed black holes M1 and M2 the orbits of photons are separated into three types: orbits falling into M1 (type I), or M2 (type II), or escaping to infinity (type III). Chaos appears because between any two orbits of different types there are orbits of the third type. This is a typical chaotic scattering problem. The various types of orbits are separated by orbits asymptotic to 3 simple unstable orbits. In the case of particles of nonzero rest mass we have intervals where some periodic orbits are stable. Near such orbits we have order. The transition from order to chaos is made through an infinite sequence of period doubling bifurcations. The bifurcation ratio is the same as in classical conservative systems.
Origin and Evolution of Near Earth Asteroids
Authors: Morbidelli A.
Published: July 2016

The present paper reviews our current understanding of the origin and evolution of NEAs, at the light of the results of recent quantitative numerical simulations that have revolutioned the previously accepted scenario.
A Symplectic Mapping Model as a Tool to Understand The Dynamics of 2/1 Resonant Asteroid Motion
Authors: Hadjidemetriou John D.
Published: July 2016

We present a 3-D symplectic mapping model that is valid at the 2:1 mean motion resonance in the asteroid motion, in the Sun-Jupiter-asteroid model. This model is used to study the dynamics inside this resonance and several features of the system have been made clear. The introduction of the third dimension, through the inclination of the asteroid orbit, plays an important role in the evolution of the asteroid and the appearance of chaotic motion. Also, the existence of the secondary resonances is clearly shown and their role in the appearance of chaotic motion and the slow diffusion of the elements of the orbit is demonstrated.
Escape from a Crisis in Fokker-Planck Models
Authors: Portegies Zwart Simon F. , Takahashi Koji
Published: July 2016

Recent N-body simulations have shown that there is a serious discrepancy between the results of N-body simulations and the results of Fokker-Planck simulations for the evolution of globular and rich open clusters under the influence of the galactic tidal field. In some cases, the lifetime obtained from Fokker-Planck calculations is more than an order of magnitude smaller than those from N-body simulations. In this paper we show that the principal cause for this discrepancy is an oversimplified treatment of the tidal field used in previous Fokker-Planck simulations. We performed new Fokker-Planck calculations using a more appropriate implementation for the boundary condition of the tidal field. The implementation is only possible with anisotropic Fokker-Planck models, while all previous Fokker-Planck calculations rely on the assumption of isotropy. Our new Fokker-Planck results agree well with N-body results. Comparison of the two types of simulations gives a better understanding of the evolution of such clusters.
Conditional Entropy: A Tool to Explore the Phase Space
Authors: Cincotta P. , Simó C.
Published: July 2016

In this paper we show that the Conditional Entropy of nearby orbits may be a useful tool to explore the phase space associated to a given Hamiltonian. The arc length parameter along the orbits, instead of the time, is used as a random variable to compute the entropy. In the first part of this work we summarise the main analytical results to support this tool while, in the second part, we present numerical evidence that this technique is able to localise (stable) periodic and quasiperiodic orbits, ‘aperiodic’ orbits (chaotic motion) and unstable periodic orbits (the ‘source’ of chaotic motion). Besides, we show that this technique provides a measure of chaos which is similar to that given by the largest Lyapunov Characteristic Number. It is important to remark that this method is very simple to compute and does not require long time integrations, just realistic physical times.
Interaction of Resonances and Yarkovsky Non-Gravitational Effects in The Asteroid Belt
Authors: Vokrouhlický David , Farinella Paolo
Published: July 2016

The interaction between resonances and dissipative effects has long been investigated in relationship with the dynamics of dust particles or the evolution of planetesimals in the primordial solar nebula. More recently, it has been shown that another important application of this kind of dynamics is the delivery of asteroid fragments (precursors of meteorites and NEAs) to Earth-crossing orbits. The relevant dissipative mechanism, the so-called Yarkovsky effect (Öpik 1951; Peterson 1976; Burns et al. 1979) has been known for a long time, but its “seasonal” and “diurnal” variants, as well as its interaction with the collisional process taking place in the asteroid belt, have been studied quantitatively only in the last few years (Rubincam 1995, 1998; Hartmann et al. 1997, 1998; Farinella et al. 1998; Vokrouhlický and Farinella 1998a,b; Vokrouhlický 1998a,b; Farinella and Vokrouhlický 1998a,b).

In the Farinella et al. (1998) paper we have provided a unified discussion of the Yarkovsky effect in both its variants. After computing the rate of the corresponding semimajor axis drift as a function of size and spin rate, and comparing the relevant timescales with those for collisional disruption and spin axis reorientation, we have rediscussed some issues in meteorite science which are put in a new light by the relevance of the Yarkovsky effect. In particular, this mechanism provides a good explanation for the fact that meteorite cosmic ray exposure ages (in particular for irons) are much longer than the dynamical lifetimes of objects delivered to the Earth-crossing region through resonances. Thanks to the Yarkovsky effect, small asteroid fragments in the belt undergo a slow drift in semimajor axis (with a random-walk component related to their rotational state) and therefore have enough mobility to reach the resonances after comparatively long times spent in nonresonant main-belt orbits. Metal-rich fragments have slower Yarkovsky drift rates than stones, but their much longer collisional lifetimes can explain why iron meteorites appear to sample a larger number of asteroid parent bodies compared to ordinary chondrites.
Detailled Study of The Dynamics of Fragments of Comet C/1996 B2 Hyakutake
Authors: Desvoivres E. , Klinger J. , Levasseur-Regourd A.C.
Published: July 2016

The fragmentation of cometary nuclei is a frequent phenomenon, but the dynamics of the fragments is not yet well understood. During the close approach of comet C/1996 B2 Hyakutake to the Earth (0.1 AU) on late March 1996, images were taken with the 1 meter telescope of Pic du Midi observatory. Bright condensations were observed near the nucleus on images taken between March, 22,1996 and March, 31, 1996. It was suggested that these features were mini-comæ surrounding fragments receding from the nucleus (Lecacheux et al., 1996). A model was developped for the motion of cometary fragments in the orbital plane of the comet, and the simulations were compared with the observations (Desvoivres et al, 1998).

In the model, we consider that the nucleus of the comet and a fragment are under the influence of the gravity of the Sun, of their mutual gravity, and of non-gravitational forces (NGF) due the loss of mass induced by solar heating. From an estimation of those NGF, we compute numerically the trajectories of the fragment and of the nucleus with respect to their common center of mass (CoM). Then, the motion of the center of mass is studied in an heliocentric reference frame using the theory of perturbed keplerian motion.
Effect of Chaotic Orbits on Dynamical Friction
Authors: Cora Sofía A. , Marcela Vergne M. , Muzzio Juan C.
Published: July 2016

When a body moves through a medium of smaller particles, it suffers a deceleration due to dynamical friction (Chandrasekhar 1943). Dynamical friction is inversely proportional to the relaxation time, which can be defined as the time needed for the orbits to experiment an energy exchange of the order of their initial energies, as a result of the perturbations produced by stellar encounters. Chaotic orbits, present in non-integrable systems, have exponential sensitivity to perturbations, a feature that makes them to relax in a time much shorter than regular ones, which suggests that dynamical friction would increase in the presence of chaotic orbits (Pfenniger 1986). We present preliminary results of numerical experiments used to check this idea, investigating the orbital decay, caused by dynamical friction, of a rigid satellite which moves within a larger stellar system (a galaxy) whose potential is non-integrable. Triaxial models with similar density distributions but different percentages of chaotic orbits are considered. This last quantity depends on the central concentration of the models. If the potential corresponds to triaxial mass models with smooth cores, the regular orbits have shapes that can be identified with one of the four families of regular orbits in Stäckel potentials (box and three types of tubes). Chaotic orbits behave very much like regular orbits for hundreds of oscillations at least. In this case, the galaxy is represented by the triaxial generalization of the γ-models with γ = 0 (Merritt & Fridman 1996). However, the situation is very different in triaxial models with divergent central densities (cusps) or black holes, a feature that is in agreement with the observations. While the tube orbits are not strongly affected by central divergencies, the boxlike orbits are often rendered chaotic (Gerhard & Binney 1985). The rimescale in which the chaos manifests itself in the orbital motion is short compared to a Hubble time. In this models, the compact object is taken as a Plummer sphere.
The Case for Surface Helium Enhancement in Cepheids
Authors: Cox Arthur N.
Published: July 2016

Cepheid masses obtained from pulsation theory are almost always less than those derived from observed luminosities and the no-mass-loss stellar evolution theory mass-luminosity relation. This may indicate mass loss in the red giant region. However, evolution theory indicates that more than ten percent mass loss will prevent the blue loops that move most of the stars into the Cepheid pulsation instability strip. The mass anomaly problem is about thirty percent for the long period (> 15 days) Cepheids with observed Baade-Wesselink method radii and for those with bumps in their light and velocity curves (5.5 - 13 day periods). Chemically homogeneous models produce the correct bump phases only with lower than evolution theory masses. The mass anomaly is a factor of three or four for the double mode Cepheids and AC Andromedae, the only triple mode Cepheid. The long standing problem is to reconcile the differing masses obtained from evolution and pulsation theories.
UBV Photometry of RR Lyrae Variables in the Globular Cluster M4
Authors: Cacciari Carla
Published: July 2016

M4, one of the closest globular clusters to the Sun, presents some peculiar features that make it very interesting to study. The first C-M diagram, published by Greenstein (1939), showed a well populated blue-horizontal branch, a characteristic which usually, but not always, indicates low metal abundance (Hartwick 1968). Kinman (1959) found however that it contains strong lined red giants and classified it as type A, indicating normal metal abundance. More recent C-M diagrams (Moshkalev 1975, Alcaino 1975, Lee 1977, Lloyd Evans 1977) substantially confirmed both these characteristics.

One of the major problems in the study of this cluster is the high and possibly non-uni form reddening, due to its location behind the Scorpius-Ophiuchus dark clouds.
External and systematic parallax errors
Authors: Schmidt-Kaler T.
Published: July 2016

The external and internal mean errors (Table 1), and the corrections to the parallaxes of the four most important parallax series A, M, Y, C (Table 2 and 5) are given, as determined by comparison among the various parallax series or by comparison with spectroscopic/photometric parallaxes. The zero-point correction to the GCTP system (Allegheny parallaxes) is found to be ±0”.0023±.”0003.
Parallaxes and the significance of their errors
Authors: Upgren A. R.
Published: July 2016

This is a brief report whose principal purpose is to summarize the present status of trigonometric parallaxes and to present the results of a new analysis of their errors. The review articles by Gliese, Strand, Upgren and Vasilevskis fully describe the methods used by investigators of parallax errors and their results; here we shall update the tabulation of the parallaxes and describe some recent results and conclusions. Although the pages on the left side of the most recent edition of the Yale Parallax Catalogue and its Supplement have been available in machine-readable form for years, the pages on the right side listing the individual parallax determinations by each observatory became similarly available only last year at the Van Vleck Observatory. Thus it has now become a simple matter to tabulate and analyze the parallaxes of each observatory separately without lengthy tabulation procedures being made by hand. One study has already been made from these data in which the frequency distribution of all trigonometric parallaxes in the Yale Catalogue was fit to a Gaussian distribution using a nonlinear least—squares procedure. This distribution is shown in Figure 1. The major conclusion was that the best estimate of the mean error of all of the parallaxes is about +0.”016, in agreement with a much simpler approach made by Hertzsprung. The large parallaxes did not affect the ability of the program to fit a Gaussian distribution to the smaller parallaxes.
Astrometrical and geodetical applications of VLBI “ARC METHOD”
Authors: Dravskikh A. F. , Finkelstein A. M. , Kreinovich V. Ja.
Published: July 2016

The geodetic and astrometric potentialities of radiointerferometric “arc-method” are considered. This method is a logical generalization of differential astrometry method, achieved through the expulsion of all non-invariant with respect to 3 - dimensional rotations and non - constant in time parameters. The brief theory of method is given. An algorithme of processing of radiointerferometric information for the construction of a catalogue on arcs, cited as an example for two - element interferometer, is brought. The question of optimal interferometer’s base orientations and observational regimes has been studied analytically. The estimations of accuracy are given for the determination geodesic and geodynamical parameters through the standard arcs. The question of application of arc - method for verification of relativistic gravitation theories is discussed.
The image trailer
Authors: Gatewood G. D. , Goebel R. W. , Stein J. W.
Published: July 2016

To further our understanding of the limitations of ground based astrometry and to test the application of electronic centering techniques to the determination of star positions, we have assembled a device which can follow the position of a star as it is trailed across the focal plane of a telescope. A uniform above atmosphere motion is achieved by pointing the instrument slightly ahead of a star image and turning off the telescope’s drive. As the star moves into the acquisition circle of the Image Trailer (IT) the device commences a steady motion at a rate predetermined by the focal plane scale of the 76-cm Thaw photographic refractor, the star’s distance from the optical axis, and the star’s declination. Utilizing a time resolution of better than one millisecond, the IT can follow variations from the predicted rate with a precision of better than 40 milliarcseconds(mas) per second and can track stars fainter than the 16th photographic magnitude.
New methods for analysing proper motion fields
Authors: Meurers J.
Published: July 2016

Proper motions are some of most important results of astrometry. Therefore it seems to be suitable, to exhaust the scientific contents here with all means and so far as possible. Of course, this is a general principle for each scientific doing. But concerning the proper motions is a singular situation: There are necessary epoch-differences, and the problem is the error as especially of a single proper motion as of star-collectives.
Critique and systematic classification of the concepts and phenomena of kinematic astronomy
Authors: Eichhorn H. K.
Published: July 2016

The coordinate systems and transformations used in kinematic astronomy are rigorously and generally defined (so that they apply for instance to a nonrigid, nonuniformly rotating Earth), and the coordinates and transformation parameters, as well as the phenomena which they describe, are critically discussed.
Some results of observations with the Scott Seven - Inch Transit Circle at Leoncito
Authors: Smith C.
Published: July 2016

Recently observed mean positions based on a fundamentally derived instrumental system were obtained for all FK4, FK4-Supp, and SRS stars in the declination zones +30 to -75SP, +30 to -75SP, and +5 to -75SP, respectively, at a mean epoch of about 1971.2.
Modern programs of meridian and photographic observations
Authors: Duma D. P. , Polozhentsev D. D. , Potter H. I. , Tavastsherna K. N.
Published: July 2016

There are 44 active meridian instruments in the world ( 5 great transit instruments, 6 vertical circles, 33 meridian circles), including 8 in the Southern Hemisphere and 16 in the USSR. 10 instruments are new ones - less than 12 years old. We have also more then 20 widefield astrographs which are suitable for catalogue work.

There are another 100 astrometric instruments (Danjon Astrolabe, ZT, Time Service TI, PZT and so on) which are used for catalogue purposes.
Exploring Methods of Effective Data Display in an Interactive Astronomical Data-processing Environment
Authors: Allen R.J.
Published: July 2016

Many of the image restoration algorithms discussed during these past days work best on fields which are virtually empty except for a few discrete sources. But the sky also has faint regions of extended emission; these regions invariably turn out to be of great astrophysical interest and must be represented as accurately as possible in the maps. The methods chosen to present the data in pictorial form can have an important effect on the speed and efficacity with which the astronomer can extract the useful information. I would be afraid of boring all of you with this discussion of display methods if I did not know that, besides being well versed in subjects of probability, statistics, and applied mathematics (and in some cases philosophies and polemics), you are also astronomers with a strong motivation to sift out of your data the useful information on the physics of cosmic radio sources, and to do it in the most efficient way possible.
Imaging with a Coherence Interferometer in Optical Astronomy
Authors: Roddier C. , Roddier F.
Published: July 2016

In optical astronomy, images are highly degraded by atmospheric turbulence. The diameter of the seeing disk may vary from 0.2 arcseconds to 10 arcseconds or more according to weather conditions. A typical diameter is of the order of 2 arcseconds which is the angular resolution of a 6cm lens at the diffraction limit.

Large telescopes have been built for their light collecting capabilities, not for their resolving power which is, in general, of the order of 0.2 arcseconds, that is far from the diffraction limit.
Diagnostic Use of FE II H & K Wing Emission Lines
Authors: Cram Lawrence E. , Rutten Robert J. , Lites Bruce W.
Published: July 2016

The Fe II λ3969.4 line is one of the weak lines in the wings of Ca II H and K that appear in emission near the solar limb, and in the flux spectra of cool giants. In spatially resolved solar spectrograms the line shows very pronounced small-scale spatial intensity variation, which is strongly correlated to the line structure of the local H-wing background, and not at all to the chromospheric structure seen in the H & K cores. A 15-level atomic model computation for iron shows that this behaviour is due to pumping by photons in the wings of the strong Fe II resonance lines near 2600 Å, in the deep photosphere. The λ3969.4 line is therefore deeply controlled, with large sensitivity to photospheric inhomogeneities, while its background is formed much higher. This makes the line a useful diagnostic of stellar photospheric line structure, in contrast to the adjacent H core for which emission indicates chromospheric line structure.
Differential Line-Shifts
Authors: Buscombe William
Published: July 2016

In the beginning of this review, I paraphrase three statements from Paul W. Merrill (1960). (1) In the spectra of long-period variables, absorption lines of neutral metals show a shortward displacement increasing algebraically with excitation potential, probably due to atoms moving outward in the star’s upper atmosphere with an expansion velocity of the order of 20 km s-1. (2) About 6 weeks after maximum light emission components appear within the broad absorption at H and K of Ca II, but displaced about -100 km s-1. A similar, but less extreme, behaviour is known for classical cepheids. (3) In quasi-constant red giants, the resonance lines of Ca II, Ca I, and SR II show circumstellar components displaced shortward relative to the normal stellar photospheric lines by a velocity which is correlated with the spectral type, typically -8 km s-1 for M6 II to -25 km s-1 for class M0 III.
I U E Observations of Circumstellar Lines and Mass Loss from B-Star
Authors: Tarafdar S.P. , Swamy K.S. Krishna , Vardya M.S.
Published: July 2016

The circumstellar lines due to A lΠ - X1Σ transition of CO has been identified for the first time in the spectra of an early type star. The spectrum, taken from the International Ultraviolet Explorer between 1150 - 1900 Å, of 9 Cep, a B2Ib star with E(B-V) = 0.47, shows not only the interstellar absorption lines of CO but also absorption features shifted towards short wavelength relative to the interstellar lines. The amount of shifts towards short wavelength from the 1 - 0, 2 - 0, 3 - 0, 4 - 0, & 6 - 0 interstellar absorption bands of CO has been found to be fairly close and corresponds to a velocity of 450 km sec-1. All attempts to identify these set of lines in a consistent way with atomic or ionic lines have yielded essentially negative results. This has led us to tentatively identify these lines due to circumstellar ∞ around 9 Cep. An estimate of rate of mass loss from the circumstellar ∞-column density leads to a value of Ṁ ≥ 10-11 M⊙ yr-1.
Effects of Flux Tubes on Conventional Chromospheric Diagnostics
Authors: Ayres Thomas R.
Published: July 2016

Magnetic flux tubes are usually envisioned as small discrete structures sparsely distributed throughout an otherwise uniform “intertube” medium. An important distinction between the flux tubes and the surrounding atmosphere is the presence of a strong chromospheric temperature inversion at high pressures in the flux tubes, while the intertube component has only a mild, low pressure chromosphere, if any at all. This implies that the flux tubes will be enormously brighter in conventional chromospheric diagnostics than the intertube component. However, the unresolved magnetic elements cover perhaps only 10% of the “quiet” Sun at chromospheric heights. Consequently the intense K and k emission cores are severely diluted. The net result is a weak emission reversal that is not characteristic of either the flux tube or intertube chromosphere. Even in the thermal microwave continuum longward of 100 μm, the flux tubes can contribute significantly to low spatial resolution spectra. Consequently, the spatially averaged microwave emission is also not characteristic of either of the distinct components.

If magnetic flux tubes are indeed the dominant class of atmospheric inhomogeneity in the Sun and other cool stars, then single-component interpretations of spatially unresolved data can be completely misleading, especially for inferring important auxiliary quantities, for example chemical abundances, line broadening parameters and chromospheric energy budgets. In the latter case, chromospheric radiative cooling rates derived from empirical mean models could be overestimated by up to an order of magnitude.
Masses, Radii, Temperatures, Luminosities and Surface Compositions of White Dwarfs
Authors: Greenstein Jesse L.
Published: July 2016

My title is so inclusive that it is not possible to find significant subject matter not in direct conflict with the specialists’ talks scheduled throughout this Colloquium. I intend, however, to emphasize areas where it is probable that much remains to be done. From the Kiel group come printouts for a large family of models, first for hydrogen and recently for helium-dominated atmospheres, which D. Koester has kindly made available. Hydrogen atmospheres for a wide variety of temperatures by Shipman and McGraw and Shipman exist also in computer printout form. Models of mixed He/H composition are less available, but are being prepared. Some hydrogen-deficient stars are the richest in metals, so that models for a variety of metal/helium ratios will need to be computed, and synthetic spectra. Clearly, further models with differing C/He ratios are needed. The successful observations in the ultraviolet with the IUE also demand computations of Lyman-α profiles using modern broadening theory, and a few now exist (Koester 1979). Present observational capabilities require quite detailed predicted fluxes and He II, He I line profiles.
Spectra of White-Dwarf Stars at Wavelengths < 3000 A: A Theoretical Perspective
Authors: Shipman Harry L.
Published: July 2016

Astronomers studying objects outside the solar system first used the ultraviolet, extreme ultraviolet, and x-ray regions of the electromagnetic spectrum in the 1970s. The exploration of these wavelength regions has produced considerable improvements in our understanding of these objects. The achievements of x-ray astronomy are perhaps the best known. With the advance of satellite technology, other wavelength regions begin to play a role, and x-ray astronomy moves into the luminosity domain where quiescent as well as violent astrophysical processes can produce detectable amounts of radiation. This paper reviews the current state of our interpretation of white-dwarf stars at wavelengths less than 3000 A.
New Model Atmospheres for DB White Dwarfs and Comparison with Observations
Authors: Koester D.
Published: July 2016

About eight years have elapsed since the publication of an extensive grid of model atmosphere calculations for DB white dwarfs by Wickramasinghe (1972), and some selected data on colours and line strengths by Bues (1970), Shipman (1971, 1972), Strittmatter and Wickramasinghe (1971), and Wickramasinghe and Strittmatter (1970). Since then only rather few theoretical results applicable to individual objects have been published by e.g. Wickramasinghe and Whelan (1977), Shipman, Greenstein, and Boksenberg (1977). On the other hand, a large amount of observational material on DB stars has accumulated, mainly through the work of Greenstein (1976), Wickramasinghe and Bessell (1977), Hintzen and Strittmatter (1974), Wickramasinghe et al. (1975), Wegner (1973, 1979), Schulz (1979), Bessell and Wickramasinghe (1978), Oke (1974), and Liebert (1977).
Spectroscopy of WZ Sagittae
Authors: Brunt C.C. , Whelan J.A.J.
Published: July 2016

WZ Sge was observed spectroscopically during quiescence in August 1978 and July 1979, before and after the December 1978 outburst. The same instrumental set up was used, namely the Image Photon Counting System on the Anglo-Australian Telescope. Time resolved spectra were obtained in phase over several orbital periods, in wavelength ranges covering Hβ and Hγ and also separately Hα. These data are being used to study the “before and after” changes in the emission line profiles, in the radial velocities of the S-wave and of the emission lines, and in the broad absorption line profiles. Additionally these features are being studied as a function of orbital phase in order to improve the determination of system parameters. When completed, the results will be published elsewhere.
Simultaneous Ultraviolet and Optical Spectrophotometry of Dwarf Novae
Authors: Bath G.T. , Pringle J.E. , Whelan J.A.J.
Published: July 2016

We present spectrophotometric observations of three southern dwarf novae – BV Cen, EX Hya and VW Hyi - in the wavelength range 1250 – 7500Å. Because of the rapid variability of these objects, spectra were obtained simultaneously using the IUE satellite and the Anglo-Australian Telescope. BV Cen and EX Hya were observed in quiescence and VW Hyi on the decline from outburst.
The Luminosity Function and Evolution of Cool White Dwarfs
Authors: Liebert James
Published: July 2016

In the review, I attempt to focus on what we have recently learned from the observations about three general topics of great theoretical interest:

1. evidence for evolution of white dwarf surface abundances,

2. the special problems of determining atmospheric parameters and abundances at the cool end of the sequence, and

3. evidence for a peaking of the white dwarf luminosity function at Mbol = + 15 ± 1, with possible implications for cooling theory and the history of galactic star formation.
Diffusion in White Dwarfs
Authors: Muchmore David
Published: July 2016

Two decades ago it was realized (Schatzman, 1958) that metals should be rapidly removed from the atmospheres of white dwarfs by diffusion through the steep pressure gradients present. Yet metals are observed. Their presence thus prompts us to consider the diffusion process in greater detail and to seek mechanisms which might deposit metals in WD atmospheres.
Quasi-Periodic and Periodic Oscillations in the Dwarf Nova AH Her
Authors: Duthie J.G. , McMillan R.S.
Published: July 2016

With a high speed photometric system we have made extensive observation of the dwarf nova AH Her. We have detected rapid oscillations on two consecutive nights during the recovery portion of the light curve of a typical outburst during the spring of 1979. The periods and the sense of the change of period are consistent with previous observations of rapid oscillations of AH Her. On another occasion we detected quasi-periodic oscillations during the rising portion of an outburst. These are similar in appearance to quasi-periodic oscillations found in other cataclysmic variables in that they appear with high statistical significance in the autocorrelation function but are not apparent in power spectra (Robinson and Nather 1979). However they are different from those reported by Robinson and Nather in the sense that they have the same order of period as the periodic rapid oscillations typical of AH Her. These results will be presented together with a discussion of their Implications.
Two-Fluid Calculations with Conduction of X-Ray Emission from Accreting Nonmagnetic Degenerate Dwarfs
Authors: Imamura J. N. , Durisen R. H. , Lamb D. Q. , Weast G. J.
Published: July 2016

Kylafis and Lamb (1979, hereafter KL) have recently completed a comprehensive study of X-ray emission from accreting nonmagnetic degenerate dwarfs. Their calculations assumed a common temperature for the electrons and the ions, and neglected electron conduction. However, when Compton cooling of the X-ray emission region becomes important at high masses, electron conduction begins to play a role (Kylafis 1978).
The Formation of White Dwarfs in Close Binary Systems
Authors: Webbink Ronald F.
Published: July 2016

Three major modes of interaction in binary systems are identified, corresponding to (I) physical contact, (II) tidal mass loss from a star with a radiative envelope, and (III) tidal mass loss from a star with a convectlve envelope. The qualitative evolution of binary systems in each of these cases is outlined, leading to the formation of single white dwarfs, long-period remnants of quasi-conservative mass transfer, or short-period cataclysmic binaries. The origin of short-period double white dwarfs in a second phase of mass transfer is outlined. Implications for the frequency of different systems of these types, and of planetary nebulae with close binary nuclei, are discussed.
An Analysis of the Light Curves of Z Cha
Authors: Smak J.
Published: July 2016

Photometric results by Warner (.1974) and Bailey (1979) are re-analysed to derive geometrical and relative photometric parameters of the system at minimum light. Relative mass of the secondary component is found between μ = 0.1 and 0.2, while its photometric contribution at the bottom of eclipse is between 20 and 40 percent.
Nova Explosion of Mass Accreting White Dwarfs
Authors: Nomoto Ken’ichi , Nariai Kyoji , Sugimoto Daiichiro
Published: July 2016

Evolution of a mass accreting white dwarf has been computed from the onset of accretion through nova explosion. We have considered a white dwarf of 1.3M⊙ with the accretion rate of 1×10-10M⊙ yr-1. Because the thermal structure during the accretion phase has been fully taken into computation, the mass of the accreted hydrogen-rich envelope and the corresponding temperature distribution in the envelope have been determined. When the hydrogen-rich envelope of mass ΔMH = 1.63 × 10-4M⊙ has been formed, a hydrogen-shell flashes commences. The flashing shell lies midway between the bottom of the envelope and the stellar surface; the mass lying above this shell is 5.7 × 10-5M⊙
Nucleosynthesis During Nova Explosions and Grain Formations in Ejected Envelopes
Authors: Fujimoto Masayuki Y.
Published: July 2016

Recent observations have revealed the existence of infrared brightening in some nova explosions, and its absence in others. These infrared excesses are ascribed to thermal emission from grains which are considered to consist of graphite. Such nova explosions are widely accepted to be triggered by hydrogen shell-flashes on the surface of white dwarfs which accrete matter in close binary systems. As for the hydrogen shell-flash, recently, a general theory applicable even to the case of finite amplitude has been developed. According to this theory, the progress of a shell-flash is determined only by the mass of the white dwarf MWD and the mass of the accreted hydrogen-rich envelope ΔMH.
The Automatic Spacecraft Granat
Authors: Sunyaev R.
Published: July 2016

The participants in the project are:

– USSR, by developing the Granat spacecraft (Scientific and Production Association S.A. Lavochkin, G.N. Babakin Research Center) and the instruments in the scientific equipment packages

– ART-P, ART-S, KONUS-V, ‘Podsolnukh’ (Sunflower), KS-18M (IKI of the Academy of Sciences of the USSR, Leningrad Institute of Physics and Technology of the Academy of Sciences of the USSR, Nuclear Physics Science and Research Institute of the Moscow State University);

– France (CNES), by developing and delivering the gamma ray telescope SIGMA, the high energy spectrometer Phoebus, a memory device with 128 Mbit capacity, and a system for determining the current orientation of the telescope (star sensor);

– Denmark (Danish Institute for Space Research), by developing the wide-angle X-Ray telescope VOTCh;

– Bulgaria (TsLKI Bulgarian Academy of Sciences), by developing and delivering certain control instruments for the ‘Podsolnukh’ apparatus.
The Hipparcos Mission: Will it be a Scientific Success?
Authors: Perryman M.A.C.
Published: July 2016

The Hipparcos astrometry satellite was launched on 8 August 1989, and after spacecraft and payload commissioning, commenced the routine data acquisition phase on 26 November 1989. Having failed to reach its planned geostationary orbit, major revisions in the mission operations were made, and the post-launch expectations of the mission were strongly degraded with respect to the original goals - principally due to the greatly reduced observational efficiency (caused by the lack of ground station coverage) and the anticipated degraded mission lifetime (as a result of the high-energy particle degradation of the solar arrays in the geostationary transfer orbit).

The final astrometric accuracies attainable by the Hipparcos mission will be influenced by the spacecraft and payload performances on the one hand, and by fraction of useful data and mission lifetime on the other. It will be shown that the elemental observational measurements correspond very closely to the predictions, and the data recovery fraction now stands at around 60–70 per cent, so that the ultimate scientific value of the Hipparcos results will be tied directly to the satellite lifetime. A measurement duration of at least 18 months is mandatory if the astrometric parameters (parallaxes and proper motions) are to be decoupled through the data reductions procedures. A somewhat longer lifetime (2.5–3 years) is necessary in order to reduce the errors on the astrometric parameters to the astrophysically-significant accuracies of around 2 milli-arcsec.

It will be shown that the present indications of the satellite performances, and the significant progress already made in the data reductions, indicate that the difficulties of the ‘revised’ Hipparcos mission have been largely overcome, and that these target accuracies could still be achievable.
The International Ultraviolet Explorer (IUE)
Authors: Kondo Yoji
Published: July 2016

The International Ultraviolet Explorer (IUE) was launched into a geosynchronous orbit on 26 January 1978. It is equipped with a 45-cm mirror and spectrographs operating in the far-ultraviolet (1150–2000 Å) and the mid-ultraviolet (1900–3200 Å) wavelength regions. In a low-dispersion mode, the spectral resolution is some 6–7 Å. In a high-dispersion echelle mode, the resolution is about 0.1 Åat the shortest wavelength and about 0.3 Åat the longest. It is a collaborative program among NASA, ESA and the British SERC. The IUE is operated in real time 16 hours a day from NASA Goddard Space Flight Center near Washing ton, D.C. and 8 hours daily from ESA’s Villafranca groundstation near Madrid, Spain. By the end of 1989, 1870 papers, using IUE observations, have been published in refereed journals. During the same period, over 1700 different astronomers from all over the world used the IUE for their research.
The Infrared Space Observatory
Authors: Cesarsky C.J. , Kessler M.F.
Published: July 2016

The Infrared Space Observatory (ISO), a fully approved and funded project of the European Space Agency (ESA), is an astronomical satellite, which will operate at wavelengths from 3–200 μm. ISO will provide astronomers with a unique facility of unprecedented sensitivity for a detailed exploration of the universe ranging from objects in the solar system right out to distant extragalactic sources. The satellite essentially consists of a large cryostat containing at launch about 2300 litres of superfluid helium to maintain the Ritchey-Chrétien telescope, the scientific instruments and the optical baffles at temperatures between 2K and 8K. The telescope has a 60-cm diameter primary mirror and is diffraction-limited at a wavelength of 5μm. A pointing accuracy of a few arc seconds is provided by a three-axis-stabilisation system consisting of reaction wheels, gyros and optical sensors. ISO’s instrument complement consists of four instruments, namely: a photo-polarimeter (3–200μm), a camera (3–17μm), a short wavelength spectrometer (3–45μm) and a long wavelength spectrometer (45–180μm). These instruments are being built by international consortia of scientific institutes and will be delivered to ESA for in-orbit operations. ISO will be launched in 1993 by an Ariane 4 into an elliptical orbit (apogee 70000km and perigee 1000km) and will be operational for at least 18 months. In keeping with ISO’s role as an observatory, two-thirds of its observing time will be made available to the european and american astronomical community.
Santa Maria: An Orbiting Multispectral Observatory
Authors: Morales C. , Sabau L. , Gimenez A. , Broadfoot A.L. , Sandel B.R. , Stalio R. , Talavera A. , Bucconi A.
Published: July 2016

SANTA MARIA is a program of international scope, being a collaboration between scientific groups in Italy, Spain and the United States. The experiment consists of a cooperative space project with the objective of designing, developing, launching, data processing and scientific exploitation of an astronomical satellite under the name of SANTA MARIA.

The three research organizations will contribute to the project with tasks shared in the most practical way to guarantee the success of SANTA MARIA.
Cosmic Rays and the Dynamic Balance in the Large Magellanic Cloud
Authors: Fichtel Carl E. , Ozel Mehmet E. , Stone Robert G.
Published: July 2016

Present and future measurement of the Large Magellanic Cloud (LMC) particularly in the radio and high energy gamma ray range offer the possibility of understanding the density and distribution of the cosmic rays in a galaxy other than our own and the role that they play in galactic dynamic balance. After a study of the consistency of the measurements and interpretation of the synchrotron radiation from our own galaxy, the cosmic ray distribution for the LMC is calculated under the assumption that the cosmic ray nucleon to electron ratio is the same and the relation to the magnetic fields are the same, although the implications of alternatives are discussed. It is seen that the cosmic ray density level appears to be similar to that in our own galaxy, but varying in position in a manner generally consistent with the concept of correlation with the matter on a broad scale.
Evidence for Particle Acceleration During Magnetospheric Substorms
Authors: Lopez Ramon E. , Baker Daniel N.
Published: July 2016

Magnetospheric substorms represent the episodic dissipation of energy stored in the geomagnetic tail that was previously extracted from the solar wind. This energy release produces activity throughout the entire magnetosphere-ionosphere system, and it results in a wide variety of phenomena such as auroral intensifications and the generation of new current systems. All of these phenomena involve the acceleration of particles, sometimes up to several MeV. In this paper we present a brief overview of substorm phenomenology. We then review some of the evidence for particle acceleration in Earth’s magnetosphere during substorms. Such in situ observations in this most accessible of all cosmic plasma domains may hold important clues to understanding acceleration processes in more distant astrophysical systems.

Subject headings: acceleration of particles — Earth — solar wind
Electron-Beam Evolution in Partially Ionized Hydrogenic Plasma with Return Currents
Authors: Syniavskii D. V. , Zharkova V. V.
Published: July 2016

This paper presents the kinetic equation solution for beam electrons injected during solar flares from the corona to the chromosphere, consisting of hydrogenic plasma with partial ionization. The electrons are considered to lose their energy both in collisional processes with the charged and neutral species of ambient plasma and in ohmic heating by return currents induced in the plasma by beam electrons. The evolution of the energy and angular distributions of energetic electrons is calculated as functions of the column density. The bulk of the electron-beam energy stored in low-energy electrons is shown to be lost in the ambient plasma heating, less via Coulomb collisions at the lower corona and more via ohmic dissipation at the upper chromosphere. More energetic electrons with energies above 120 keV can reach the chromospheric levels with a weak ionized plasma, where a decrease of the Coulomb collisions and the induced electric field of a return current produce beams as well directed as on the top boundary. The X-ray bremsstrahlung polarization is shown to be positive in the range 5%-10%. It is very sensitive to the emergent photon energies below 40 keV and to angles of view for all of the X-ray radiation range.

Subject headings: acceleration of particles — plasmas — Sun: flares — Sun: X-rays, gamma rays