bluuebunny/crossref_metadata_2025 huggingface dataset:
High Resolution Infrared Spectroscopy and the Nature of Mira Variability
Authors: Hall Donald N. B. , Hinkle Kenneth H. , Ridgway Stephen T.
Published: July 2016

High resolution visible spectra of Mira variables are extremely complex and despite major observing programs both by Merrill and his colleagues and more recently Wallerstein, the relation between the emission and absorption line velocities measured at optical wavelengths and either the nature of the optical variability or the observed microwave features remains elusive.
Multicolour Photoelectric Photometry of Magellanic Cloud Cepheids
Authors: Feast M.W. , Martin W.L. , Warren P.R.
Published: July 2016

Photoelectric 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 < logP < 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

The 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.
RSCVn Stars in the Southern Hemisphere
Authors: Hearnshaw J.B.
Published: July 2016

RSCVn stars are fully detached binary stars which show intrinsic small amplitude (up to 0.3 amplitude peak-to-peak) light variations, as well as, in most of the known cases, eclipses. The spectra are F to G, IV to V for the hotter component and usually KOIV for the cooler. They are also characterised by abnormally strong H and K emission from the cooler star, or, occasionally, from both components. The orbital and light curve periods are in the range 1 day to 2 weeks. An interesting feature is the migration of the light variations to earlier orbital phase, as the light variation period is shorter than the orbital period by a few parts in 10+4to a few parts in 10+3.
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

Radio 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

The 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).
Systematic effects in observed parallaxes
Authors: Hanson R. B.
Published: July 2016

Several outstanding problems affecting the existing parallaxes should be resolved to form a coherent system for the new General Catalogue proposed by van Altena, as well as to improve luminosity calibrations and other parallax applications. Lutz has reviewed several of these problems, such as: (A) systematic differences between observatories, (B) external error estimates, (C) the absolute zero point, and (D) systematic observational effects (in right ascension, declination, apparent magnitude, etc.). Here we explore the use of cluster and spectroscopic parallaxes, and the distributions of observed parallaxes, to bring new evidence to bear on these classic problems. Several preliminary results have been obtained.
Summary remarks to discussion on parallaxes
Authors: van Altena W.
Published: July 2016

A number of general conclusions can be drawn from this morning’s discussions. Unfortunately most of them have been well known for many years. That fact alone should cause us to seriously consider Dr. Lutz’s suggestion that standard parallax regions be adopted for the continuous calibration of accidental and systematic errors of parallax observations now being made. In this way we will be able to put our modern observations on a standard system with some degree of confidence. On the other hand the problem is now to make the best possible use of the previously published data for which no such reliable calibration exists.
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.
Present status of radio astrometry with baselines ≤ 35 km
Authors: Johnston K. J.
Published: July 2016

A summary of results for radio astrometry with baselines ≤ 35 km and priorities for future work are given.
Resolution des Plaques de la Carte du ciel par L’AGK/3 dans les Zones situees entre + 31° et - 2°
Authors: Valbousquet A.
Published: July 2016

The independent re-reduction of the plates of the Astrographic Catalogue between declination 31° and -2° which was started by Lacroute and Valbousquet (1972) is now complete. Misprints, mistakes and such other errors and blunders that vere found were removed. The latest Strasbourg version of the AGK2-3 was used as reference material. Field corrections were determined and removed.
Some aspects of developing new meridian instruments
Authors: Kharin A. S.
Published: July 2016

At present and evidently in the immediate future, the traditional fundamental coordinate system created with the help of the earth-based meridian observations cannot be substituted by an astronomical reference coordinate system realized by means of radiointerferometers or some other new techniques. Compact radiosources are too few to be the basis of a fundamental sytem and the proposed satellite projects are too expensive and too complex to be realised with sufficiently high accuracy in the next decade. At the same time, newly automated meridian instruments will give a good opportunity to solve the most urgent problems of fundamental astrometry, namely, improving the accuracy of observations, substantially increasing the number of observations and observing more fainter objects.
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.
The I.N.A.G. Schmidt Telescope
Authors: Heudier J. L.
Published: July 2016

The French National Institute for Astronomy and Geophysics (I.N.A.G.) has designed and built a Schmidt Telescope which is located at Calern Observatory, the observing station of the Center for Studies and Research in Astronomy and Geophysics (C.E.R.G.A.).
Optical interferometers in astrometry
Authors: Shao M.
Published: July 2016

Long 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

The 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”.
Plan of sky coverage by using wide-angle astrographs
Authors: Kolchinsky I. G. , Gavrilov I. V. , Onegina A. B.
Published: July 2016

In this paper we present information on the Soviet project to photograph the sky (epoch 1980-1985) using wide-angle astrographs (Kolchinsky I.G., Onegina A.B., 1977). The feasibility of such modern surveys has been discussed earlier, especially by R.H.Stoy (1975). Detailed information on a similar program for the northern hemisphere to be made with the new zone astrograph of the Hamburg Observatory has been given by Chr.de Vegt (1974).
A project of a new photographic catalogue of positions and proper motions of stars of the Southern Sky
Authors: Polozhentsev D. D. , Potter H. I.
Published: July 2016

The reference photographic catalogue of the southern sky (RPC) will be a basis for investigations in the fields of astrometry, stellar astronomy, geodesy, space studies etc. in the southern hemisphere. It will be a catalogue of high precision, containing both bright and faint stars, reliably connected to the fundamental catalogue and giving an inertial coordinate system for a wide range of southern stars. The RPC will be an extension of the Goloseevo Plan (1) for the southern hemisphere. However, our project has its own peculiarities, mainly because there is no AGK3 type catalogue of the southern sky for the present epoch.
Status of “AGK4” Project
Authors: de Vegt Chr.
Published: July 2016

The present accuracy limit for the majority of fainter stars on the northern hemisphere is set by the AGK2/3-catalogue, recently completely finished, but it should be noted that its epoch is much earlier (1960). Furtheron the AGK3-catalogue is a direct repetition of the AGK2, the plates have been taken with the same astrograph in a broad blue spectral bandpass and measured visually with the same equipment, therefore virtually an instrumental standard of 1930 is realized again. Figure 1 shows the mean errors of the AGK2/3 catalogue positions as a function of magnitude. The best accuracy for the AGK2/3 data is obtained for the stars of about ninth magnitude: 017 (AGK2) and 020 (AGK3) but decreases for the faint stars with mpg11 to 019 (AGK2) and Pg 027 (AGK3). Here the AGK3 data are even less accurate. With increasing distance to the catalogue epochs, the accuracy of positions decreases due to the proper motion errors. In the upper part of figure 2 the dependence of the AGK2/3 catalogue accuracy on time is shown for the faint stars separately and an averaged value.
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 < 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 < 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 < 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

The 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.
Fundamental Aspects of Aperture Synthesis with Limited or No Phase Information
Authors: Baldwin J.E. , Warner P.J.
Published: July 2016

Much ingenuity has been devoted to the determination of mutual coherence functions at radio wavelengths. Measurements of the amplitude and phase of the interference fringes with uncertainties of only 1 percent are now attainable. Unfortunately the atmosphere intervenes between the measured quantities and the mutual coherence function itself and the phase uncertainties introduced by the ionospheric and tropospheric irregularities are often the factor limiting the quality of radioastronomical maps.
Reduction of Phase Noise in Interferometry with State-space Analysis
Authors: Okatan Ali , Basart J. P.
Published: July 2016

In radio mapping, one of the problems encountered is the random bias in the visibility estimate. The bias can be divided into two parts: (a) the positive bias due to the common sky background seen by all elements of the interferometer, and (b) the negative bias due to phase noise present in the system. The first kind of bias can be easily removed by subtracting the correlation between the signals at two interferometer sites when the source is not in the antenna beams from that measured with the source in the antenna beams. This bias will therefore not be considered here. In contrast, the second kind of bias is more difficult to remove. When the signal-to-noise ratio of the interferometer system is high, incoherent averaging techniques can be utilized in the fringe frequency or in the time domain (Clark et al., 1969; Moran, 1973).
Digital Processing Methods for Aperture Synthesis Observations
Authors: Clark B. G.
Published: July 2016

The rapid growth, over the last two decades, of the amount of digital arithmetic capability available for a given cost is obvious to all, and is apparently not yet at an end. Therefore, any discussion of the best equipment and even of the best algorithms with which to attack a given problem will soon become obsolete as decreasing costs bring things previously considered inconceivable within economic feasibility. The general trend is that, as the cost of hardware decreases, the discussions of algorithms and procedures becomes simpler, as fewer approximations have to be made, and the mathematically simple correct formulae may be directly implemented.
The Maximum Entropy Method
Authors: Gull S.F. , Daniell G.J.
Published: July 2016

We have heard so far at this meeting from the practical-minded men of radio astronomy. Faced with the problem: ‘my map has sidelobes and noise on it’, they promptly reach for the nearest available weapons – ‘cleaners, polishers and kneaders’. All these use varying amounts of subjective human judgement, as for example in ‘window cleaning’.
The Information of Images and Its Dependence on Noise and Spatial Resolution
Authors: Kalberla Peter M.W.
Published: July 2016

Radioastronomical observations are often incomplete in the sense that either the angular resolution desired or the signal-to-noise ratio required are not adequate. Attempts to fill in the missing information by either smoothing or deconvolving the observed data date back to Bracewell (1956, 1958). More recently the need for more sophisticated restoration procedures has increased since observations with radioastronomical interferometers frequently suffer from incomplete coverage of the u – v plane. Procedures like CLEAN (Högbom 1974, Schwarz 1978) and MEM (Wernecke 1977, Gull and Daniell 1978) have been developed to interpolate the unknown data in the u – v plane.
The Method CLEAN – Use, Misuse and Variations
Authors: Schwarz U.J.
Published: July 2016

The 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

The 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

The 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.
Observations of Extended Radio Sources Using a Multiple-beam Technique
Authors: Emerson D.T. , Klein U.
Published: July 2016

The limiting sensitivity for conventional single-dish radio continuum observations of extended objects at wavelengths of a few cm is set mainly by weather; for much of the time a varying component of atmospheric attenuation, and hence thermal emission, causes deviations in the response of the receiver far in excess of any deviations due to receiver noise or receiver instabilities.
Generation of Oscillatory Motions in the Stellar Atmosphere
Authors: Osaki Yoji
Published: July 2016

Thermal 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

The 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

Analyses 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.
Turbulence in Main Sequence Stars
Authors: Gehren Thomas
Published: July 2016

It has for long been realized that the interpretation of stellar spectral lines normally requires broadening velocities well in excess of purely thermal motions. The origin and structure of such velocity fields, which have usually been summarized as turbulence, still appear to be a subject of controversy.
Observational Aspects of Macroturbulence in Early Type Stars
Authors: Ebbets Dennis
Published: July 2016

The basic premise of this paper is that the atmospheres of supergiants of spectral type 0, B, and A are not homogeneous static and stable. Observations show effects in the spectrum which seem to indicate velocity fields of many different scales, which probably are variable with depth and are certainly variable with time. Some effects can be understood in terms of global, steady, symmetric velocities, namely rotation and expansion. Those effects which still cannot be accounted for are generally attributed to turbulent velocities.
Numerical Simulation of Granular Convection: Effects on Photospheric Spectral Line Profiles
Authors: Nordlund Åke
Published: July 2016

The 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

Ladies 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

Asymmetries 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.
Collapse of White Dwarfs in Close Binary Systems
Authors: Canal R. , Isern J.
Published: July 2016

The presence of neutron stars in close binary systems, shown by the pulsating X-ray sources, poses the problem of their origin. In the case of the low-mass (M1 + M2 ≤ 5 M⊙) X-ray binaries, the neutron star might have originated from a massive white dwarf, driven over the Chandrasekhar limit by mass transfer (Schatzman 1974). A similar scenario had been put forward by Whelan and Iben (1973) for type I supernovae. To solve the problem of the very low eccentricities observed for the orbits, and to facilitate keeping the system bound after neutron star formation, Canal and Schatzman (1976) suggested a non explosive collapse of the white dwarf to a neutron star. The occurence of this kind of collapse depended on the possibility of avoiding thermonuclear ignition by means of neutronization. Since there is a density interval where the electron captures on carbon go faster than the pycnonuclear reactions, just above the critical density for the beginning of the collapse, there seemed also to be a chance of escape from thermonuclear runaway. A closer examination of this picture leads, however, to significant changes.
Some Properties of Hot, High-Gravity, Pure Helium Atmospheres
Authors: Wesemael F. , Van Horn H.M.
Published: July 2016

Model atmosphere analyses of white dwarf spectra have contributed significantly to our understanding of the properties of degenerate stars.: In particular, the pioneering investigations of Bues (1970), Strittmatter and Wickramasinghe (1971) and Shipman (1972) have provided the first reliable determinations of the effective temperature and surface gravity of these objects (see Shipman 1979 and Weidemann 1978 for recent results). We now know with certainty that the hydrogen-rich white dwarf sequence extends at least over the range Te ∽ 6000 – 60.000K. In contrast, the hottest identified helium-rich white dwarfs seem to reach Te ~ 25.000K only, a puzzling result since the progenitors of DB white dwarfs should presumably also be helium-rich.
The Quadratic Zeeman Effect for CI Lines-Application to the Spectrum of EG 129
Authors: Bues I. , Varvatis G.
Published: July 2016

Garstang and Kemic (1974) derived formulae for the splitting of hydrogen and helium lines in the presence of large magnetic fields. We applied their method in calculating the eigenvectors for some transitions of neutral carbon, where a correction factor for the coupling and mixing of the energy levels has to be applied. For CI – lines in the visible region of the spectrum, well separated components can be expected for a field strength up to 2 • 107G. Fig. 1 shows the splitting of three lines for B = 107 G.
Analysis of the Soft X-Ray Pulsations of Dwarf Novae
Authors: Córdova F. A.
Published: July 2016

Both SS Cyg and U Gem show enhanced soft X-ray emission (0.1–0.5 keV) during optical outbursts. In extended HEAO-1 pointings, it was discovered that a large fraction of this emission is pulsed (see Fig. 1). This confirms the general expectation that high energy pulsations are the ultimate source of the optical pulsations observed during outbursts of dwarf novae. For SS Cyg the average soft X-ray pulsation amplitude is 30%; this is to be contrasted with the upper limit of 10% to a hard X-ray (2–25 keV) pulsed component for the same observation (Swank 1979). Thus the origin of the oscillation is in soft X-rays, and the direct implication of such a large pulsed amplitude is that the oscillation itself must be an intrinsic part of the X-ray production mechanism.
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.
The Parallaxes of U Gem Variables
Authors: Kamper Karl W.
Published: July 2016

An Allegheny parallax series of SS Cyg, consisting of 52 exposures obtained on 15 nights, was recently measured on the PDS microphotometer at the David Dunlap Observatory, and a value of (m.e.) derived for the absolute parallax. This is close to the mean of the two previous discordant measures for this star given in the table below. The weighted mean of the three determinations implies that the absolute magnitude, at quiescent phase, of the star is between 7.0 and 9.0 formally at a 90% confidence level. Recent parallax determinations made at Lick by Vasilevskls et al. (1975) for three other stars, listed below along with the Mt. Wilson value for U Gem, imply even fainter absolute magnitudes.
Observations of Southern Dwarf Novae
Authors: Vogt N.
Published: July 2016

Preliminary results of spectroscopic and photometric data for five dwarf novae are presented :

1) V 436 Cen. The orbital period of 0.0669 days was determined from radial velocity variations. The RV half amplitude of the primary, K1. = 159 km/s, implies very small masses of M1 ≲ 0.20 M⊙ and M2 = 0.18 M⊙ for the binary components.

2) Z Cha. Broad emission (Hβ, Hγ, Hδ) and superimposed narrow absorption lines of Hβ-HII, HeI 4471, Cal 4427 and Call K characterize the spectrum during quiescence. Apparently, the cool, optically thin outer disc is seen on the background of a hot continuum, originating from the white dwarf or the inner disc. The RV half amplitude K1 = 87 km/s results in masses of M1 = 1.10 M⊙ and M⊙ = 0.21 M⊙.

3) EX Hya. The RV half amplitude K1 = 68 km/s reveals masses of M1 = 1.4 M⊙ and M2 = 0.19 M⊙ The equivalent widths of the emission lines of H, HeI 4471 and HeII 4686 vary with the phase of the recently detected 67 minute cycle (maximal EW coincides nearly with maximal continuum intensity).

4) 0Y Car is an eclipsing binary with an orbital period of 0.0631 days. The eclipses show strong variations in shape and amplitude in the course of an outburst, similar as those of Z Cha. The observations seem to confirm that the location of the eruption is the central part of the disc which increases in size and luminosity.

5) EK TrA shows periodic superhumps (P = 0.0645 days) during supermaximum, and therefore belongs to the SU UMa sub-group of dwarf novae which are also characterized by a quasi-periodic occurence of super-maxima. The SU UMa sub-group comprises 70% of the ultra-short period cataclysmic binaries, and at least 18% of all dwarf novae.
A Diagnostic for Models of U Geminorum
Authors: Paczyński B. , Schwarzenberg-Czerny A. , Alcock Charles
Published: July 2016

Models of U Geminorum fall into two basic types. Both involve a Roche-lobe filling main sequence dwarf orbiting a white dwarf, around which there is an accretion disk. In one type of model there is continuous mass accretion from the inner Lagrange point through the disk to the white dwarf at all times, and the outburst is produced by varying the mass transfer across the inner Lagrange point (Bath et al., 1974). In the second type of model, there is not any accretion from the outer edge of the disk to the white dwarf between outbursts, and matter streaming from the inner Lagrange point accumulates in a torus (Osaki 1974, Paczyński 1978). The outburst is a result of an instability in the disk that suddenly increases the viscosity. A simple diagnostic test of the former type of model is to model the luminosity produced from the accretion disk at minimum light. Following Smak (1976) we adopt a white dwarf mass of 0.9 M⊙, inner and outer radii of the disk of 6.1 × 108 cm and 3.6 × 1010 cm, inclination 67°, and velocity of the stream hitting the disk of 400 km sec-1. Following Wade (1979) we adopt a distance modulus of 4m.4 and a maximum brightness in the visual band of 14m.76. Averaging this radiation over angles and using the velocity information gives a lower limit to the mass transfer rate of 10–10 M⊙ yr-1. Assuming that the disk is optically thick, and taking into account limb darkening in an approximate fashion, yields an expected brightness in the visual band Vdisk ≤ 12m.6. This is two magnitudes too bright. A crude analysis of optically thin disks indicates that stable configurations only exist for disk temperatures close to 1.5×10 °K. This temperature is too low for the accretion rate inferred here, if the disk is optically thin. For this reason the disk must be optically thick. Since the visual magnitude of the disk is too high if it is accreting, we conclude that continuous accretion does not occur at minimum light and that matter accumulates in a torus.
Z Cha – Another Case of Gravitational Radiation?
Authors: Ritter H.
Published: July 2016

By combining spectroscopic data obtained by Vogt (1979) and photometric data obtained by Bailey (1979) and Fabian et al. (1979) it is possible to determine the physical parameters of the southern dwarf nova Z Cha (Ritter, 1979). The resulting values are: mass of the white dwarf primary M1=(0.035±0.06)M⊙, radius of the white dwarf R1=(1.8 10-2)R⊙, mass of the secondary M2=(0.16±0.02)M⊙, radius of the secondary R2=(0.19±0.01)M⊙, inclination i = 748±0.8°. Accordingly the primary conforms to the theoretical M-R relation of white dwarfs and the secondary to the theoretical low mass main sequence. The mass exchange rate derived from the luminosity of the accretion disk and from the hot spot respectively turns out to be Ṁ ≈ 4.10-11 M⊙/year, about 102 times more than is expected from the nuclear evolution of the secondary. On the other hand, angular momentum losses due to gravitational radiation predicts a mass loss rate (Faulkner, 1971) which is almost identical to the one derived from observations. Therefore after the binary pulsar (Taylor et al., 1979), Z Cha seems to be another case presenting strong evidence for the action of gravitational waves.
Dwarf Nova EX Hydrae
Authors: Vogt N. , Krzeminski W. , Sterken C.
Published: July 2016

Extensive photoelectric observations of EX Hya were obtained between 1962 and 1976 at different telescopes and observatories. Evaluation of the data reveals a decrease of the orbital period at the rate Ṗ = −5.2 × 10−12. This period change is likely caused by mass transfer with angular momentum exchange between orbital motion and disc rotation on a time scale which is considerably shorter than the corresponding Kelvin – Helmholtz time scale. A rough estimate of the mass transfer rate gives Ṁ ⋍ 2 × 10−9 M⊙/yr. Gravitational radiation losses are negligible in the evolution of EX Hya. Furthermore we report the detection of a periodic modulation of the light-curve with a period of 0.046546447 days and mean amplitudes between 0.2 and 0.4 mag. (“67-minute cycle”). This oscillation remained coherent over the 14 years covered by our observations. The 67-minute variability resembles superhumps seen at supermaxima in SU UMa-type stars with the difference that it shows a 2:3 commensurability to the orbital period. We mention either (i) asynchronous rotation or pulsations of the red component, or (ii) asynchronous rotation of the magnetic primary, or (iii) resonances in the outer regions of the accretion disc as possible sources for this periodicity.
Soft X-Ray Observations of Cataclysmic Variable Stars
Authors: Córdova France
Published: July 2016

HEAO-1 observations of a large number of cataclysmic variable stars demonstrated that most of these objects are not bright X-ray sources, even during their eruptive phases.

HEAO-1 scanned about 20 dwarf novae while they were undergoing optical outbursts, yet soft X-ray emission (0.18-0.43 keV) was detected only from SS Cyg and U Gem. The 3σ upper limits to the flux from the nondetected objects, several of which were observed by HEAO-1 during more than one outburst, range from 1 to 10% of U Gem’s mean flux of ∽ 1 × 10-10 erg cm-2 s-1. SS Cyg itself was detected during outburst at an intensity 20% of this value.
Rapid Oscillations in DQ Her and UX Uma
Authors: Petterson Jacobus A.
Published: July 2016

Although the novalike variable UX UMa strongly resembles the classical nova DQ Her in color, emission spectrum, and optical lightcurve, the properties of the rapid oscillations in both systems are quite different. The oscillations differ in period, amplitude, and phase stability, but most remarkably they differ in the characteristics of the “eclipse related phase shift.” The phase shift in DQ Her is explainable by partial obscuration of the disk during eclipse, together with the idea that the oscillating light does not reach us directly from the white dwarf, but is reflected by the disk. It comes from a rotating UV beam originating near the white dwarf surface, which is reflected better by the backside of the disk than by the front side. We show that the phase shift in UX UMa is explainable by the same model, viewed at a different inclination angle i, if it is assumed that at this value of i reflection from the frontside of the disk is better than from the backside. There may be different ways to accomplish this preference. The results suggest that no retrograde rotation of the white dwarf (or retrogradely rotating nonradial pulsation) is needed to explain UX UMa’s eclipse related phase shift. These phase shifts provide a new (and quite accurate) way to determine a system’s inclination angle. Specific predictions are made for the behaviour of amplitude and phase of the oscillations in other eclipsing systems.
Ultraviolet Studies of Novae
Authors: Friedjung M. , Rocca B.
Published: July 2016

High dispersion spectra of WZ Sagittae and Nova Cygni 1978, taken by IUE, have been studied by us.

Of the two spectra of WZ Sagittae in our possession, the long wave region one shows little besides the interstellar Mg II doublet. The more interesting short wavelength region has relatively wide (130-250 km/s) resonance absorption lines sometimes at the center of wider emission. No clear sign of ejection is seen. The study of the profiles of the CIV, NV, OI, and SiIV resonance lines and of Hell 1640 Å indicates:

a) The absorption lines are probably not interstellar (widths, depths).

b) A standard accretion disk model can be used. The high ionization lines are probably formed in a region ionized by the boundary layer. The line width indicates that they are formed near the edge of the disk (top of raised edge?), illuminated by radiation from a radius a factor of 10 smaller.
Magnetic Guiding of Mass Flow in Eruptions of Cataclysmic Variables
Authors: Meyer F.
Published: July 2016

It is suggested that the superhump phenomenon in eruptions of cataclysmic variables like VW Hyi indicates a continuous white light flaring in magnetic flux tubes connecting a magnetic spot on the convective dM secondary and the disk whose Keplerian rotation provides the driving energy by twisting up the magnetic field. Due to the gravitational (and centrifugal) potential difference between the surface of the secondary and the “deeper” disk material a continuous evaporation-condensation mass flow from the secondary to the disk must occur. This can account for the 100-fold increase in mass flow in the disk despite the fact that the ordinary Roche lobe overflow mass flow is not affected, as indicated by the unvaried brightness of the regular accretion spot at the disk. This model interprets the supermaxima eruptions as a consequence of a magnetic cycle on the secondary that periodically immerses the accretion disk in strong magnetic fields. Due to the strong dependence of dipolar field strength on distance from the secondary surface this might explain why the superhump phenomenon is mainly found in the closer dwarf novae. Required field strengths and observed spot sizes are in good agreement with expectations and observations from smaller dM stars like BY Draconis-type stars and flare stars.
Defining Characteristics of Fast Novae
Authors: Truran James W.
Published: July 2016

Scrutiny of the light curves of the common novae yields important clues concerning both the nature of the nova outburst and the characteristics of the underlying white dwarfs. Ultraviolet and infrared observations have served to make available essentially complete bolometric light curves for several recent novae. These data confirm our earlier prediction that, following maximum, both fast and slow novae experience an epoch (of varying duration) of substantially hydrostatic evolution defined by thermonuclear burning of the residual hydrogen fuel at constant bolometric luminosity. Theoretical studies reveal that the luminosity during this phase of a nova’s evolution is well represented by the Paczynski core mass-luminosity relation for such shell burning configurations involving degenerate stellar cores. This luminosity represents, as well, an increasingly significant fraction of the Eddington luminosity with increasing white dwarf mass.
The Hubble Space Telescope
Authors: Boggess Albert
Published: July 2016

The Hubble Space Telescope was launched from the Kennedy Space Center on April 24, 1990. Its initial check-out indicates that all sub-systems of the satellite are working very well, with two key exceptions: The line-of-sight pointing is subject to occasional jitter apparently induced by thermal stresses in the solar arrays; this is expected to be overcome. The telescope mirrors are found to contain approximately 0.5 wave rms of spherical aberration which cannot be overcome by any controls on board the satellite. This defect will limit the scientific performance of the telescope in the short run. However, the aberration can be fully corrected in the optical designs of future replacement instruments, and the delivery schedules of these instruments are being accelerated.