bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic Instabilities in Stellar Atmospheres
Authors: Priest E.R.
Published: July 2016

The extensive theory for magnetohydrodynamic instability of a flux tube is briefly reviewed, together with its application to tokamaks and solar flares. In a star a single coronal loop whose footprints are anchored in the dense photosphere may become unstable to the kink instability when it is twisted too much. Magnetic arcades may also be subject to an eruptive instability when they are sheared too much. After the eruption the magnetic field closes back down by reconnection and continues to heat the plasma long after the impulsive phase. Global instability of a large part of the coronal magnetic field is also possible when the stored energy is too great.
Flaring Processes in Stellar Atmospheres: The Importance of Electromagnetic Coupling
Authors: Spicer D.S.
Published: July 2016

Linsky: We do not know all of the energy released in the course of a dMe star flare. What we actually measure however is often 104 to 105 times larger than what is seen in large solar flares. I have two questions. Why do you think flares in dMe stars are so much more energetic than in the solar case? Secondly, why are solar flares so much less energetic than those in dMe stars?
Interacting Magnetospheres in RS CVn Binaries – Coronal Heating and Flares
Authors: Uchida Y. , Sakurai T.
Published: July 2016

Coronae and flares of RS CVn systems are interpreted as due to gradual and sudden releases of magnetic free energy which is built up throught the interaction of magnetic fields of stars in these close binary systems.
Ejection of Particles from Comet Lexell: The Gravitational Influence of Jupiter
Authors: Kresáková M. , Carusi A. , Valsecchi G.B.
Published: July 2016

The orbital evolution of comet P/Lexell has been characterized by a sequence of three close planetary encounters within 12 years; the first of these encounters was with Jupiter, 1767, and lead to a reduction of the perihelion distance from 2.9 AU to 0.67 AU. Then the comet encountered the Earth at a distance of 0.015 AU, in 1770, and was discovered. The final, very close encounter with Jupiter, in 1779, removed the comet from observability, sending it into an orbit of perihelion distance in excess of 5 AU (Kazimirchak-Polonskaya, 1967).
The Stability of Some Asteroids
Authors: Vicente R.O.
Published: July 2016

The utilization of two different stability criteria, namely, Hill's modified stability criterium and the method of surface of section, has been employed for asteroid orbits. The idea is to compute different criteria of stability for the same asteroids in order to compare the results and see the practical interest of the computations for researches about evolutionary trends of individual asteroids, groups and families of asteroids.
On the Stability of Resonant Asteroid Orbits
Authors: Hadjidemetriou J.D. , Ichtiaroglou S.
Published: July 2016

The stability of the asteroid orbits has been studied by the method of surface of section. Families of simple symmetric periodic orbits of the asteroid and their stability have been computed and this served as a guide for the selection of the energy levels for the surface of section. In this way all possible cases for the structure of phase space have been obtained. It was found that the region in phase space around the resonant orbits at the resonances 1/3, 3/5, 5/7,.... is unstable, but small stability regions of doubly symmetric periodic orbits near the above resonances are also present. At the resonances 1/2, 2/3, 3/4, .... it was found that there exist two separate regions in phase space at about the same resonance 1/2, 2/3, 3/4,...., respectively, one being stable and the other unstable. At certain energy levels only the stable region appears. The above results are consistent with the observed distribution of the asteroids.
Collisional Origin of Asteroid Families: Effects of the Target's Gravity
Authors: Zappalà Vincenzo , Farinella Paolo , Paolicchi Paolo
Published: July 2016

The outcomes of asteroidal catastrophic collisions are strongly affected by the target asteroid's gravity, since only the fragments escaping with initial velocities higher than the target's escape velocity are not reaccumulated into “rubble pile” remnants. This idea can be compared with the observational evidence on the properties of family asteroids in several ways : (1) the shape and spin period of the “reaccumulated” family asteroids will roughly fit the relationships valid for self-gravitating fluid bodies; (2) the relative velocities of the few escaping fragments arising from a breakup event marginally overcoming self-gravity will often have an anisotropic distribution, affecting the final distribution of orbital elements; (3) the amount of mass which in a given family escaped to “infinity” will be correlated with the target's size, since only for objects larger than ~ 100 km self-gravity plays an important role. These predictions are discussed and compared with the available data.
Boundaries for the Equipotential Curves in the Elliptic Restricted Three-Body Problem
Authors: Delva Magda
Published: July 2016

In the elliptic restricted three body problem an invariant relation between the velocity square of the third body and its potential is studied for long time intervals as well as for different values of the eccentricity. This relation, corresponding to the Jacobian integral in the circular problem, contains an integral expression which can be estimated if one assumes that the potential of the third body remains finite. Then upper and lower boundaries for the equipotential curves can be derived. For large eccentricities or long time intervals the upper boundary increases, while the lower decreases, which can be interpreted as shrinking respectively growing zero velocity curves around the primaries.
The eclipse of CI Cygni in 1980 on the objective prism spectra
Authors: Mikołajewska J. , Mikołajewski M.
Published: July 2016

The observations of CI Cyg were carried out with the Toruń 60/90 cm Schmidt telescope with the objective prism giving a dispersion of 250 A/mm at H-gamma. Intensity traces of 20 selected spectra , made on the Kodak IIa-O plates, were made. The results were corrected for all instrumental effects and the interstellar extinction. The relative emission lines intensities have been obtained from the tracings after normalization to H-beta = 100.

The spectra were typical for the quiet symbiotic star. So, all changes in the spectrum were caused by geometrical effects (eclipse).
Introductory report on V1016 Cygni
Authors: Cassatella A.
Published: July 2016

The formation of a new planetary nebula should be a rare event, and the case of V1016 Cyg may represent such an example. This is the reason of the considerable interest of the astronomers in this star since the discovery of its outburst in 1965.
Long term trends in the 3.5 μ light curve of RX Puppis
Authors: Whitelock Patricia A. , Catchpole R.M.
Published: July 2016

SAAO photometry of RX Pup at 3.5 μ between 1972 and 1981 shows a very well defined period of 580 days with an amplitude of 0.35 mag, which is attributed to a Mira variable component. The infrared variability of RX Pup was demonstrated by Feast, Robertson and Catchpole (1977), who also pointed out that the infrared colours were indicative of the presence of a Mira variable with a dust excess. Further evidence for the Mira comes from the observations of H2O in the infrared spectrum by Barton, Phillips and Allen (1979).
Spectral evolution of HM Sagittae in the ultraviolet
Authors: Kindl Ch. , Nussbaumer H.
Published: July 2016

We have observed the ultraviolet spectrum of HM Sge with the International Ultraviolet Explorer during 1978-1980.
Helium and Hydrogen of the Local Interstellar Medium Observed in the Vicinity of the Sun
Authors: Bertaux Jean-Loup
Published: July 2016

The Sun is moving in respect to the nearby stars with a velocity of 20 km.s-1 in the direction of the Apex, α = 271° and δ = 30° (celestial coordinates). As the lights of a car illuminate the water droplets when driving in the fog, the Sun illuminates the Hydrogen and helium atoms of the interstellar medium which it travels through. As a result, the sun and the whole solar system are imbedded in a glow of the resonance lines of hydrogen (H Lyman α ; 121.6 nm) and helium (58.4 nm), which have been studied by several space instruments in the last 14 years.

From the intensity distribution of the glow in the solar system, one can derive the density of H and He in the L1SM and the direction of the relative motion between the sun and the LISM in the very vicinity of the sun. The velocity module Vw and the LISM temperature T are more adequately found from a measurement of the Lyman α line shape, which is an image of the velocity distribution of H atoms.

A summary of results will be presented, together with a discussion of the methods of interpretation and their difficulties. The vector is found to be 20 ± 1 km.s-1 in the direction α = 254 ± 3°, δ = - 17 ± 3°, quite different from the Apex direction. This means that the LISM is moving also in respect to the local frame of reference giving rise to the socalled Interstellar Wind. This wind blows in the galactic plane at 16 km.s-1, in the direction , significantly different from the direction found by interstellar absorption lines on stars within ≃ 100 pc, pointing to a local significance of this flow. The temperature of the LISM is T = 8,000 ± 1,000 K, the density n (H) ≃ 0.04 to 0.06 cm-3, and the helium density n (He) ≃ 0.015 to 0.020. The high helium/hydrogen ratio, in respect to the cosmological ratio, would imply that a substantial part of the hydrogen is ionized. Temperature, density and degree of ionization of the LISM are suggesting that the sun is now in an intermediate phase of the interstellar medium, at the Interface between a hot and tenuous gas, and a dense and cold cloud of gas.
Absorption Line Studies and the Distribution of Neutral Gas in the Local Interstellar Medium
Authors: Bruhweiler Frederick C.
Published: July 2016

Previous published absorption line studies performed at ultraviolet and visual wavelengths are combined with new ultraviolet data in order to map out the distribution of HI within 150 pc of the Sun. Newly presented data for distances less than 50 pc further support the local cloud model as presented by Bruhweiler (1982). The Sun is embedded, near the edge of a diffuse cloud with total column density of 2 x 1019cm-2. Most observed directions within 50 pc away from the cloud body reveal trace amounts of gas (N)HI) ~ 1018 cm-2) presumably arising in the outer skin of the local cloud. At greater distances (50 ≲ d ≲ 150 pc) most directions show significant absorption with N(HI)>1019 cm-2. Two directions, one toward the northern galactic pole (NGP), the other toward β CMa exhibit unusually low HI column densities out to distances of 150-200 pc. However, substantial amounts of gas, N(HI) > 1019 cm-2, are seen toward the NGP at greater distances. The implications of these results on astronomy at wavelengths shortward of 912Å are discussed.
Optical Emission Line Studies and the Warm Ionized Component of the Local Interstellar Medium
Authors: Reynolds R.J.
Published: July 2016

Observations of diffuse, galactic Hα, [NII]λ6583, and [SII]λ6716 emission lines provide evidence for a warm (~ 104 K), primarily ionized component of the interstellar medium distributed throughout the galactic disk. This component of the interstellar gas has an electron density ≈ 0.1-0.2 cm-3 and occupies about 10-30% of the interstellar volume. Interstellar Hα emission near the galactic poles, the dispersion measure of a nearby pulsar, and observations of interstellar gas flowing into the solar system indicate that this ionized component is an important constituent of the interstellar medium in the solar neighborhood. The intensity of the Hα background at high galactic latitudes implies that this component is maintained by an average hydrogen ionization rate in the vicinity of the Sun of (2-4) x 106 s-1 per cm2 of galactic disk. The emission measure is 1.3-2.3 cm-6 pc toward the galactic poles. The sources of this ionization have not yet been identified but may include escaping Lyman continuum radiation from planetary nebulae, hot white dwarfs, and early type stars. Investigations of the regions surrounding ζ Oph (09V), the nearest (d ≃ 140 pc) 0 star, and a Vir (Bl IV), one of the nearest (d ≃ 87 pc) early B stars, have revealed areas of enhanced Hα emission extending 6°-12° from each star. However, it appears that these stars do not contribute significantly to the more diffuse ionization within the local interstellar medium.
Observations of Absorption Lines from Highly Ionized Atoms
Authors: Jenkins Edward B.
Published: July 2016

In the ultraviolet spectra of hot stars, we can see absorption lines from highly ionized species in the interstellar medium. Observations of these features have been very influential in revising our perception of the medium's various physical states and how gases in space are heated and ionized. The pervasiveness of 0 VI absorption lines, coupled with complementary observations of a diffuse background in soft x-rays and EUV radiation, shows that there is an extensive network of low density gas (n ≈ fewx10-3cm-3) existing at “coronal” temperatures, 5.3 ≲ log T ≲ 6.3. Thus, while we once thought that x-rays and cosmic rays were the dominant sources of excitation and heating of the interstellar gas, we now realize that shocks created by supernova explosions or mass loss from early-type stars can propagate freely through space and eventually transfer a large amount of energy to the medium. To create the coronal temperatures, the shocks must have velocities in excess of 150 km s-1; shocks at somewhat lower velocity (v ≲ 100 km s-1) can be directly observed in the lines of Si III. Observations of other lines in the ultraviolet, such as Si IV and C IV, may highlight the widespread presence of energetic uv radiation from very hot, dwarf stars. More advanced techniques in visible and x-ray astronomical spectroscopy may open up for inspection selected lines from atoms in much higher stages of ionization.
A Wide Latitiude CO Survey of Molecular Clouds in the Northern Milky Way
Authors: Dame T.M. , Thaddeus P.
Published: July 2016

It is now well established that molecular clouds are an important part of the interstellar medium, containing much or most of the dense, cold component of the gas, and producing the massive stars and supernovae responsible for the diffuse, hot component. It would therefore appear essential in formulating a complete picture of the local interstellar medium to have some knowledge of the distribution and properties of nearby molecular clouds. To this end we have used the Goddard-Columbia 1.2-meter telescope to carry out a wide latitude, low angular resolution survey of CO along most of the first galactic quadrant and a small part of the second. The survey is uniform and fully sampled in galactic longitude from 12° to 100°, and in latitude from -5° to +6°, with extensions to as high as +11° to include specific dark clouds; the total area surveyed, 1128 deg2, is much larger than any region previously studied in CO or any other interstellar molecule. In order to fully sample such a large area in a reasonable amount of time, angular resolution has been sacrificed to coverage and speed; an angular resolution of 1° was obtained synthetically by simply scanning a square 8 x 8 raster of points separated by 1/8°, the size of the primary beam, and summing the resulting 64 spectra at the end of an observation.
Modelling the Local Interstellar Medium As A Supernova Remnant In A Multiphase Gas
Authors: Cowie Lennox L.
Published: July 2016

Trying to understand the local interstellar gas in detail may be a hopeless task for a theorist. In the interstellar medium as a whole, we can at least address global properties and perhaps come to some reasonable “time averaged” conclusions such as those of Cox and his collaborators (e.g. Cox and Smith 1974, Cox 1979) or McKee and Ostriker (1977). Even this is quite uncertain of course, both because the ISM gas has structure on scales from at least 1 Pc (and probably much smaller) all the way up to the size of the galaxy, and because none of us are quite sure which physical processes (such as thermal evaporation or heating of cooler gas by magnetohydrodynamic processes) are really important. However, in the local ISM things are significantly worse in that we no longer have even the ergodic hypothesis available to us – rather we have to try and deal with individual events and structures. On the other hand, we do have more detailed observations and hence a laboratory to try to decide on the importance of the various physical processes.
Large-Scale Bubble Structure of the Interstellar Medium (ISM) and Properties of the Local Spiral Arm (LSA)
Authors: Bochkarev Nikolay G.
Published: July 2016

Bubbles are very common structure units in the Galaxy and galaxies. Collection of radio, optical, infrared and x-ray observations of the Cyg superbubble (CSB) region of the sky show that the CSB is not a single bubble object. Between 50 to 75 percent of its x-ray emission can be ascribed to discrete sources. The other 25 to 50% x-ray emission, probably originates from bubbles around 8 OB associations of the region. All bubbles located within the spiral structure of Galaxy, M31 and M33 have diameter ≲ 300pc.

The large distance of stellar association from the galactic plane (GP) combined with picture of the gas distribution within the LSA shows that a Reyleigh-Taylor instability in the LSA can develop and give use to the formation of compact steller clusters, such as the Cyg OB2 association. Development stages of the Reyleigh-Taylor instability, some peculiarities of the dust distribution and departures of the local structure from the galactic grand design suggest the absence of a spiral Shockwave in the LSA.
Kinematics of Nearby Gas and Stars
Authors: Goulet Thomas , Shuter William L.H.
Published: July 2016

The kinematic properties of gas and stars in the Solar neighborhood are described in terms of the line-of-sight component of a three-dimensional first order Taylor series expansion of the local velocity field. The types of object analysed are (1) 21 cm absorbing clouds (2) intercloud medium (3) main sequence B stars closer than 200 pc (4) B stars of luminosity class ranging from I to IV (5) main sequence A stars (6) K-giant stars. The least squares fitting procedure used to derive the 10 coefficients describing each of the six velocity fields was essentially the same, so that a valid comparison could be made. Marked departures from circular motion are found in most cases, but the only systematic trend is a correlation between ∂u/∂x (u being the velocity component along the x-axis directed towards the Galactic center) and stellar spectral type, where the gas behaves like a medium ‘younger’ than the early type stars. Our analysis of the gas indicated that the standard plane-parallel model provided a good description for the intercloud medium, but was inadequate for the absorbing clouds. A velocity ellipsoid description of the residuals is presented for each type of object. The influence of the Gould belt on local kinematics is discussed.
The Discovery of a Spectroscopic Binary in M 3
Authors: Latham David W. , Hazen-Liller Martha L. , Pryor Carleton P.
Published: July 2016

Binary stars appear to be a pervasive feature of the sidereal universe. In every stellar population or habitat that has been investigated, except globular clusters, from 10 to 40 percent of the objects observed prove to have variable radial velocities that suggest they are spectroscopie binaries. To extend the search for spectroscopie binaries in the globular cluster M3, we have obtained more than 300 new radial velocities for the 111 giants previously observed by Gunn and Griffin (1979). For one of the stars, von Zeipel 164, four observations spanning ten years show a velocity variation with an amplitude of at least 18 km s−1 and a period of perhaps a few years. We believe this is a strong candidate for the first spectroscopie binary to be found in a globular cluster.
Radial Velocity Variations in CN-Peculiar and Normal K Giant Stars
Authors: Aikman G.C.L. , McClure R.D.
Published: July 2016

A sample of 54 CN-enriched G and W giant stars monitored for radial velocity variations for the last four years seems to have a binary frequency comparable to a similar sample of normal giant stars.
A New Radial Velocity Survey at the NGP
Authors: Adamson A.J. , Hilditch R.W. , Hill G. , Fisher W.A.
Published: July 2016

A classical astronomical problem has been to derive the total density of matter local to the Sun from observations of the velocity and density distributions of well-defined “tracer groups” of stars near the Galactic poles. Since the original work by Oort (1932: ρ0=0.15 M⊙/pc.3), the dynamical value of ρ0, has consistently been in excess of the amount of matter actually observed ; a substantial part of the mass would seem to exist in a form which renders it undetectable.

Our earlier survey of the dynamics of NGP A and F stars yielded 0.14 M⊙/pc.3 for ρ0 , (Hill et al. 1979) but employed an analytical potential derived by Camm (1950,52) requiring a (physically implausible) increase in the velocity dispersion with z-distance, although no such departure from the isothermal state was apparent in the velocity data for the F stars to z=200 pc. Such problems could well arise from the numerous simplifying assumptions involved in solving the combined Poisson and Boltzmann equations analytically. Recent studies (eg. Bahcall 1984) have used more sophisticated multicomponent mass models and solved numerically for the potential, taking as input the observed velocity dispersions and scale parameters for the various components.
The Binary Population of the Hyades
Authors: Mathieu Robert D. , Stefanik Robert P. , Latham David W.
Published: July 2016

The nature of the binary population in clusters is of interest for numerous reasons. Through their study we gain insight into the star-formation process, dynamical interactions in the clusters, mass-luminosity-age relations, chromospheric activity and more. We have made a detailed analysis of the binary population of the Hyades, using radial-velocity, visual and proper-motion data in the literature and new multiple high-precision (σ = 0.85 km/sec) radial-velocity observations of over 100 Hyads with 6 < V < 14 (see Latham and Stefanik 1985 and this colloquium). Complete details are given in Mathieu 1983 and Mathieu, Stefanik and Latham 1985. In this report we focus our analysis on the 95 Hyads found by van Bueren (1952) with 6 < V < 9 (0.9 to 1.7 M⊙), thus providing a stellar sample analogous to that of the classic study of the field binary population by Abt and Levy (1976).
The Temperatures of White Dwarfs in Accreting Binaries
Authors: Szkody Paula , Sion Edward M.
Published: July 2016

Through the use of accreting binary systems, it is possible to study the effects of the deposition of matter and energy on the surface of a white dwarf. The observed atmospheric properties of composition and temperature obtained from direct observation of the spectral lines and the continuum flux can be used to compare with those of single white dwarfs to understand the consequences of mass accretion on binary evolution.

Cataclysmic variables provide one of the best targets for this type of study because a) the primaries are all white dwarfs b) the level and the timescale of the accretion cover a large range from the high rate, relatively steady novalike accretors to the dwarf novae systems which are modulated on short timescales in a quasi-periodic manner. Unfortunately, due to the mass transfer process, an accretion disk builds up to the point where its radiation overwhelms the white dwarf light in most cases. Thus, to study the effects on the stellar primary, systems must be found which have low mass transfer rates (generally the short orbital period systems (Patterson 1984)) and/or high inclinations (since most of the disk flux emerges perpendicular to the plane of the disk). The best identification of the white dwarf emerges from IUE spectra which show a broad Lyman α absorption profile (in contrast to the normal emission lines from a disk at quiescence). The shape of this profile provides a sensitive indicator of the temperature and gravity. In some cases, broad absorption lines are also evident in the optical Balmer lines, although the broad emission lines from the disk usually make these difficult to detect. The steeply falling flux distribution of a white dwarf throughout the optical region, combined with a flat disk distribution usually means that the white dwarf contributes a minor amount to the optical flux. However, in the ultraviolet, the rising energy distribution of the white dwarf easily dominates the falling energy distribution of a low accretion rate disk (Mateo and Szkody 1984). White dwarfs are generally acknowledged to be prominent in the dwarf novae U Gem (Panek and Holm 1984), VW Hyi (Mateo and Szkody 1984) and Z Cha (Marsh, Horne and Shipman 1987) and suggested in EK TrA and WZ Sge (Verbunt 1987). In addition, the white dwarf has been seen in some novalike systems which sporadically turn off their mass transfer, (resulting in the disappearance of most of the disk and the resulting appearance of the white dwarf). This has been the case in TT Ari (Shafter et al. 1985) and some limits have been determined for MV Lyr (Szkody and Downes 1982) and V794 Aql (Szkody, Downes and Mateo 1988). Several magnetic white dwarfs have also been seen when the mass transfer ceases in the AM Her systems (summarized in Szkody, Downes and Mateo 1988).
FAR-Ultralviolet Spectrophotometry of two do White Dwarfs from Voyager
Authors: Poulin E. , Wesemael F. , Holberg J.B. , Fontaine G.
Published: July 2016

While the observed number of hot, helium-rich degenerates is noticeably larger than that of their hydrogen-rich counterparts, the calibration of their effective temperatures has been comparatively much less trustworthy. The spectroscopic classification scheme introduced three years ago by Wesemael, Green, and Liebert (1985, hereafter WGL), and the crude temperature domains associated with each class remain, to this date, the only comprehensive effort at defining a temperature scale for DO stars. The current uncertainty in this is perhaps best epitomized by two objects, HD149499B and PG1034+001. The former belongs to a binary system which also contains a KO V primary, 2” away. The temperature determined for the degenerate secondary ranges from 85,000±15,000 K (Wray, Parsons, and Henize 1979) to 55,000±5000 K (Sion, Guinan, and Wesemael 1982, hereafter SGW). PG1034+001, on the other hand, is the prototype of the so-called hot DO spectroscopic class; WGL assign an uncertain temperature of 80,000±20,000 K to this object.
The Phase Diagram of High Density Binary Mixtures and the Luminosity Function of Single White Dwarfs
Authors: Isern J. , Garcia-Berro E. , Hernanz M. , Mochkovitch R.
Published: July 2016

For the last two decades, plasma physics developments have led to a better understanding of physical conditions in white dwarfs interiors. Following the pioneering work of Mestel (1952), the problem of white dwarf cooling has been a subject of continuous interest until the present time. In the early sixties, Kirzhnits (1960), Abrikosov (1960), and Salpeter (1961) recognised the importance of Coulomb interactions in the dense plasma which forms the white dwarf interior. A first-order transition from liquid to solid phase was predicted and the resultant release of latent heat was shown to somewhat affect the cooling rate (Mestel and Ruderman, 1967). Subsequently, improved theoretical luminosity functions (number of white dwarfs per pc9 and per magnitude interval as a function of luminosity) taking into account not only Coulomb interactions but also neutrino losses, and using detailed atmosphere models (Van Horn, 1968; Koester, 1972; Lamb and Van Horn, 1975; Shaviv and Kovets, 1976; Sweeney, 1976). Recently, Iben and Tutukov (1984) have discussed the evolution of a 0.6 M⊙ carbon-oxygen white dwarf from its nuclear burning stages to complete crystallization. Their luminosity function agrees reasonably well with observations in the range −4 ≤ log(L/L⊙) ≤ 4 but it predicts an excess of white dwarfs at low luminosities. Indeed, the luminosity function derived from observations grows monotonically until log(L/L⊙) ≃ −4.5 (Mv ≤ 16) and then makes an abrupt shortfall (Liebert, Dahn and Monet, 1988). The agreement between theory and observations is so good in the aforementioned range luminosity that we can wonder as to whether it is possible not only to test the theory of white dwarf cooling but also to obtain information on the galactic structure and evolution. One example of that is the use of the cutoff in the white distribution to determine the age of the galactic disk (Schmidt, 1959). Using this method, Winget et al. (1987) have found that the galactic disk age could be of the order of 9 Gyr old, in agreement with some predictions from nucleocosmochronology (Fowler et al. 1987).
Magnetic Fields in White Dwarfs
Authors: Schmidt Gary D.
Published: July 2016

Just one year ago at the 2nd Conference on Faint Blue Stars, two review papers were presented summarizing the observational characteristics and attempts at spectral modeling of isolated magnetic white dwarfs (see Schmidt 1987 and Wickramasinghe 1987, respectively). Because much of that information is still relevant, this paper will concentrate on some of the progress which has occurred since the time of that meeting, and on highlighting current problems in the area of research. Using specific examples for illustration, the discussion concentrates on the single stars. Magnetic binary systems are considered from the point of view of the white dwarfs themselves.
Evidence of Complex Field Structure in the Magnetic White Dwarf in EXO 033319–2554.2
Authors: Ferrario Lilia , Wickramasinghe D. T.
Published: July 2016

We present detailed theoretical models for polarisation and spectroscopic data from EXO 033319–2554.2 that suggest the presence of two cyclotron emission regions of field strengths B ∼ 5.6 × 107 G and B ∼ 2.8 × 107 G, and which are separated by about 90° on the surface of the white dwarf. The large difference in magnetic field strength and the proximity of the two regions imply a more complex field structure than that of a centred dipole field distribution if it is assumed that the regions are connected by closed field lines.
Strrtified Model Atmospheres for Hot Da White Dvhrfs
Authors: Vennes S. , Fontaine G. , Wesemael F.
Published: July 2016

Observations of hot DA white dwarfs in the EUV/soft X-ray range have revealed that, in a majority of cases, the detected flux is less than that expected from pure hydrogen atmospheres. This implies an extra opacity source which must be due to the presence of small traces of heavier elements. These elements are generally not spectroscopically detected in hot DA white dwarfs, but the large sensitivity of the EUV/soft X-ray broad-band flux to the presence of extra absorbers can be used with profit to Infer their abundances. For simplicity, it has been assumed that only helium provides the required opacity source in the majority of the analyses carried out so far. In this context, Vennes et al. (1988a) have recently reviewed in details the mechanisms that could be responsible for the presence of small traces of helium in the atmospheres of hot DA white dwarfs. They favor a model in which these stars are interpreted as stratified objects with an outer layer of hydrogen which is sufficiently thick that radiation in the visible escapes only from H-rich regions, and yet sufficiently thin that the EUV/soft X-ray radiation escapes from deeper layers, polluted by the tail of the helium distribution which extends upwards. This model accounts naturally for the positive correlation observed between the inferred helium abundance and the effective temperature in hot DA stars studied at short wavelengths. If the model is correct, hot DA white dwarfs as a class must have very thin outer hydrogen layers with estimated masses in the range–13 > log q(H) = log (M(H)/M) > –15.
High Frequency Oscillations in SS Cygni
Authors: Arévalo M.J. , Solheim J.E. , Lázaro C.
Published: July 2016

Because of its relative brightness (mv about 11.8 in quiescence and mv about 8.6 in outburst), SS Cygni is the Dwarf Novae most extensively studied since 1896 (Mattei et al., 1985; Bath and van Paradijs, 1983).

Concerning its long-term behaviour, SS Cygni has short, long and anomalous out-bursts. Correlations between outburst characteristics and periodicities have been studied by van Paradijs (1983), Campbell (1934), and Sterne and Campbell (1934).

SS Cygni, as any cataclysmic variable, also presents rapid variations. Since the discovery of this variability (Warner and Robinson, 1972) many long runs of high-speed photometry have been performed for different Dwarf Novae, both during out-bursts and in quiescence stages. Regarding to SS Cygni we can summarize these variations as:
Search for Pulsating Stars in Multiple Stellar Systems
Authors: Antonello E. , Fracassini M. , Pasinetti L.E. , Pastori L.
Published: July 2016

The importance of the search for pulsating stars of the δ Scuti type in multiple stellar systems is emphasized, in particular for the following objects: the evolutionary status of these systems, the interconnection between multiplicity and pulsation (Frolov et al., 1980), a verification of the theory of the turbulent mixing (Vauclair, 1976). A list of possible pulsating companions in visual and spectroscopic systems with foreseeable period and amplitude is given. Some preliminary results of a spectrophotometric survey begun at the Observatories of Milano - Merate and Bologna - Loiano are also given.
Supernovae in Binary Systems : Production of Run-Away Stars and Pulsars
Authors: De Cuyper Jean-Pierre
Published: July 2016

The effects of an instantaneous asymmetric supernova explosion in an eccentric binary system are analyzed, taking into account the mass loss out of the system, the influence of the impact of the supernova shell on the companion star and the extra “kick” velocity which a collapsed star might receive in an asymmetric supernova explosion. For a random orientat-ion in space of this asymmetric kick velocity, the survival probability and the runaway velocities are derived and their properties discussed for an explosion occurring at a given position in the initial keplerian orbit and the mean and extreme values of these quantities over one orbit are derived. As an example, the outcome of a possible supernova explosion in the ten best known WR+OB binaries is studied and a comparison is made with the observed run-away OB stars, radio pulsars and binary X-ray pulsars.
Les occultations du 15 Août 1980 et du 22 Avril 1982 par les anneaux d’Uranus
Authors: Sicardy Bruno , Bouchet Patrice , Perrier Christian , Brahic André , Lecacheux Jean , Laques Pierre
Published: July 2016

Les occultations stellaires du 15 Août 1980 et du 22 Avril 1982 par les anneaux d’Uranus ont pu être observées avec les télescopes de 3.6 m et de 1 m de l’ESO, les télescopes de 2 m et de 1 m du Pic du Midi et le télescope de 0.8 m de l’Observatoire de Haute-Provence. Le télescope de 1 m du Pic du Midi était équipé d’un photomètre centré sur la bande d’absorption du méthane à 0.89 µm, et les autres observations ont été réalisées dans la bande d’absorption à 2.2 µm. Ces observations permettent d’étudier des corrélations sur des distances de 800 m à 10.000 km.
Physical properties of Saturn's rings
Authors: Cuzzi Jeffrey N.
Published: July 2016

A review is given of important features of the rings, touching only lightly on aspects covered by other speakers (Spokes, E ring). This extended abstract will only convey the high points of the talk.

Most of the material in Saturn's rings is concentrated in the B ring, with a lesser amount in the A ring and only small amounts in the C ring and Cassini Division. There is a very different character to these classical ring regions; the C and Cassini particles are darker and more neutral in color; (Smith et al. 1981, 1982). The A and B regions contain nearly all of the “small” particles, from microns to millimeters. Overall, however, the particles are fairly well characterized by Voyager radio occultation results as roughly following an r-3powerlaw between about 1 cm and a few meters (Tyler et al. 1982, Marouf et al. 1982). A fairly sharp cutoff in the size distribution is seen at radii varying with location from about 1 to about 5 meters. The material of the ring particles is probably mostly water ice (see e.g., Pollack 1975) but the redness of the rings requires the presence of minor constitutents. Combinations of ground-based radar and radio emission observations (Pollack 1975, Cuzzi and Pollack 1978; Pettengill, this issue) strongly indicate that the non-icy component comprises a small fraction of the total bulk material. In fact, mass densities derived from density waves (e.g. Holberg et al. 1982) and CRAND measurements (Cooper et al. 1982) combined with Voyager particle size measurements indicate a particle density more like that of snow or frost than that of pure ice.
The Kinematics of Spokes
Authors: Porco C.C. , Danielson G.E.
Published: July 2016

The results of a study of the long term behavior of spokes are presented. Previous work on a Voyager 1 imaging data set spanning 12 Saturn rotations led to the discovery of a variability in the appearance of spokes on the morning half of the rings, with a period of 621 ± 22 minutes (Porco and Danielson,A. J., May, 1982; hereafter PD). It was found that maximum spoke activity is associated with the same sector of the magnetic field which gives rise to the emission of SKR (Saturn Kilometric Radiation). In addition to suggesting that the spokes have their origin in electromagnetic processes related to Saturn's field, these results have provided another indirect piece of evidence for a possible inhomogeneity in the near field of Saturn.
Voyager UVS observations of Saturn's rings and the ring atmosphere
Authors: Holbelg J.B.
Published: July 2016

During the Voyager 1 and 2 Saturn encounters the Voyager ultraviolet spectrometers (UVS) made numerous observations of Saturn's rings in the extreme and far ultraviolet. HI Lyman a (1216 Å) observations of the rings from a number of different aspects are used to define the extent and density of the neutral hydrogen “atmosphere” associated with the rings. Voyager 2 observations of the 520 to 1700 Å spectrum of the rings (~20 Å resolution) are used to derive the albedo of particles in the B ring at these wavelengths. These albedo measurements are compared with the laboratory reflectance spectrum of water ice longward of 1200 Å. The significance of the lack of ring reflectance in the Voyager 1 UVS data is also discussed. Finally, UVS spectra of the rings obtained in Saturn's shadow are used to establish upper limits on the presence of any emission from the neutral and ionic states of oxygen possibly associated with the Saturn electrostatic discharges (SED) discovered by the Voyager Planetary Radio Astronomy experiment.
Particle sizes in planetary rings from forward-scattering and spectral-extinction observations
Authors: Marouf E.A. , Tyler G.L. , Eshleman V.R.
Published: July 2016

The forward scattering geometry is favored for observing the size distributions of particles in planetary rings. One reason is the dominance of the collective diffraction lobe of the particles in the near-forward scattering. For tenuous rings, like the Jovian ring, Saturn's ring C, ring F, and Cassini division, observation of the shape of this lobe as a function of scattering angles decouples the size information from the exact particle shape, surface structure, and material. For more opaque rings, such as Saturn's ring A and possibly some of the Uranian rings, the shape of the forward lobe can be contaminated by multiple scattering which must be deconvolved before size information can be recovered. Knowledge of the opacity of the rings is required to carry out the deconvolution.with the degree of success largely dependent on the apparent ring opening. Here, the forward geometry provides a convenient means for determining the “true” opacity by measurements of the extinction of the direct ray between the spacecraft source and receiver. At the present time, only the radio occultation technique has the capability of separating this direct signal component from the forward-scattered signal. This separation is based on the distinct spectral signatures of the direct and scattered components, and permits simultaneous and independent estimates of the opacity and of the strength and shape of the forward-scattering phase function. Because opacity is an area-weighted measure of the particle distribution , while the diffraction lobe is an area-squared weighted measure of the distribution these two observables complement each other by binding the size distribution over different size ranges.
The W-shaped occultation signatures of planetary ring systems: Inference of size-sorted particles in charged, tilted ringlets
Authors: Eshleman V.R. , Tyler G.L. , Marouf E.A.
Published: July 2016

A chain of inferences concerning plausible characteristics of certain planetary ring structures is initiated by the observed W-shaped radio-occultation signatures of imbedded ringlets in the C-ring of Saturn. These distinctive shapes, and similar stellar occultation signatures of the rings of Uranus, imply concentrations of material near the sharp radial edges of the ringlets. This bunching could be a natural consequence of particles moving in elliptical orbits in such a way that the same particles that define the outer edge of the ringlet at a given longitude, also define the inner edge 180° from that longitude. Collisions could be avoided, and hence ringlet lifetime could be very long, if the orbits had small inclinations and the same fixed argument of periapse, ω, together with other orbital parameters that would cause the particles to move “in formation” in helical paths within a torus. Perturbations also need to be considered. The oblateness of Saturn would cause ω to increase with time and to pass through particular values for which the torus cross-section would collapse to a line, so that collisions would again occur. However, since this change is due in equal parts to an extremely small perturbation acceleration (proportional to orbit inclination) toward the equatorial plane and a much larger radial acceleration, it would be possible to stop ω with a very small force per unit mass which doubly cancels the weaker of the two oblateness effects. Mutual electrostatic repulsion of like-charged particles could do this. It appears that the cancellation could be self-adjusting to provide ringlet stability over a limited range of ω, despite large changes in the electric effects. Moreover, if the charge on a particle is proportional to its surface area, the electrostatic acceleration would be inversely proportional to particle size in a given field, causing the noncolliding particles to be sorted by size with the smaller ones tending to the exterior surface of the torus. The occultation measurements at two radio wavelengths suggest such sorting.
The adiabatic invariant: a tool for the study of the dynamics of the solar system
Authors: Henrard J.
Published: July 2016

A summary of the main results of the theory of the Adiabatic Invariant is presented. They are applied to a simple one degree of freedom Hamiltonian system which is a rood model for many resonance problems in Celestial Mechanics. As an illustration of the previous concepts, the capture into resonance of the pair of satellites Enceladus-Dione is analysed.
Macrostructure and microphysics of Saturn's E-ring
Authors: Pang Kevin D. , Voge Charles C. , Rhoads Jack W.
Published: July 2016

All observed optical and infrared properties of Saturn's E-ring can be explained in terms of Mie scattering by a narrow size distribution of ice spheres of 2 - 2.5 micron diameter. The spherical shape of the ring particles and their narrow size distribution imply a molten (possibly volcanic) origin on Enceladus. The E-ring consists of many layers, possibly stratified by electrostatic levitation.
X-ray Observations of SNR E0102.2-72.2 in the SMC
Authors: Hughes John P.
Published: July 2016

The supernova remnant (SNR) E0102.2-72.2 is the brightest in the Small Magellanic Cloud (SMC) at X-ray wavelengths. This object, which is remarkable because of its high velocity (∼4000 km s−1) oxygen-rich optical emission, appears to be similarly remarkable at X-ray wavelengths. The high resolution imager (HRI) data can be quite well described by a thick ring with a radius of ∼19" (6 pc at a distance of 63 kpc). The imaging proportional counter (IPC) X-ray spectral data can be best fit by a single emission line of energy ∼0.9 keV. It seems likely that this is the emission from a plasma of almost pure neon.
Observations of “Dents” in the Radio Shells of SNRs Expanding into Dense Clouds
Authors: Velusamy T.
Published: July 2016

High resolution (∼36“×90”) radio maps of SNRs W28 and W44 observed with the OSRT at 327 MHz are presented. The distortions or dents in their shell structure are associated with the parts of the shells expanding into dense molecular clouds in their vicinity and provide direct observational evidence for interaction between them.
Aperture Synthesis Observations of the Supernova Remnant G73.9+0.9
Authors: Chastenay P. , Pineault S.
Published: July 2016

Radio continuum observations, obtained with the DRAO Synthesis Telescope, are presented of the new supernova remnant (SNR) G73.9+0.9. Our map at 1420 MHz shows indications of spatially resolved knots of emission in the brightest part of the remnant. The 408 MHz map, although of lower resolution, shows the same general morphology. The spectral index α (Sv∝v−α) between 1420 and 408 MHz is about 0.5, a value typical for shell type SNRs. The morphology however is more suggestive of a filled centre SNR.
Small-Scale Structure in Young Supernova Remnants
Authors: Reynolds Stephen P.
Published: July 2016

Recent VLA observations of the shell supernova remnant SN 1006 AD (Reynolds and Gilmore 1986) and the Crablike remnant 3C 58 (SN 1181 AD?; Reynolds and Aller 1987) show features at high resolution that contain information on details of particle acceleration and transport in the remnants. Thin arcs at the edge of SN 1006 require time-variable particle acceleration and/or magnetic field amplification. Filaments in 3C 58 probably result from interaction of pulsar-generated relativistic fluid with filaments of thermal gas formed early in the remnant’s life by cooling or dynamical instabilities. Their sharp edges imply efficient scattering by Alfvén waves; as much as 1% of the large-scale magnetic energy density may be in magnetic turbulence on length scales of 1011 cm.
Stability of Radiative Shocks with Time-Dependent Cooling
Authors: Gaetz T.J.
Published: July 2016

The stability of a radiative shock subject to nonequilibrium cooling is Investigated. It is found that high velocity shocks (> 140 km/s) are subject to oscillational and condensational instabilities.
Collective Effects of Supernova Explosions in a Starburst Nucleus
Authors: Tomisaka Kohji , Ikeuchi Satoru
Published: July 2016

The collective effects of supernova (SN) explosions in the nucleus of a starburst galaxy are studied. It is shown that a large wind bubble with the size of a few hundred parsecs in the nucleus of a starburst galaxy will expand for SN explosion rates of 0.1 per year or greater. The bubble gradually elongates due to density stratification. Finally, the shell breaks near the top and the hot matter flows through the cylindrical shell up to 1-2 kpc above the disk plane. We will discuss the X-ray emission in the halos of galaxies such as M82 and NGC 253 and the distribution of molecular gas in such galaxies in relation to the starburst phenomenon.
The Distribution and Birthrate of Galactic SNRs
Authors: Wu Xinji , Leahy D.A.
Published: July 2016

In this paper we find that the spatial distribution of SNRs shows a peak in the range R = 4–6 kpc; the total number of SNRs in the Galaxy is 749±104; and the birthrate of SNRs is one in every 18±3 years.