bluuebunny/crossref_metadata_2025 huggingface dataset:
Voyager EUV and FUV Observations
Authors: Holberg J.B.
Published: July 2016

The Voyager 1 and 2 ultraviolet spectrometers are sensitive over the wavelength range 500 to 1700 A. In the EUV, at wavelengths shortward of the Lyman limit (912 A), Voyager observations have detected emission from three out of a sample of 11 nearby hot DA white dwarfs. These observations imply very low HI column densities in the directions of the three stars detected. In the FUV, at wavelengths between 912 and 1200 A, Voyager observations of O and B stars can be used to study interstellar reddening at the shortest wavelengths and to provide useful estimates of interstellar H2 column densities.
Detection of Narrow C IV and Si IV Absorption Features in Spectra of Stars within 200 pc of The Sun
Authors: Molaro P. , Beckman J.E. , Franco M. , Morossi C. , Ramella M.
Published: July 2016

Detection of narrow ( δλ< 0.5 A ) absorption features in C IV at λ λ 1548 and 1550 have been made in the spectra of 4 late B dwarfs within 200 pc of the sun; the Si IV doublet at λ λ 1393 and 1403 shows up in two of them. We argue that it is difficult to account for the strengths, widths, shapes , and C IV/Si IV ratios in terms consistent with a circumstellar origin except possibly for an asymmetric C IV component in one star (HD 135037). The most probable source is “semi-torrid” gas (cf. Bruhweiler et al. 1980) in the 50,000 K range forming the interfaces between cooler H I clouds and the ambient medium at coronal temperatures. Our technique, using late B rapid rotators, is useful for LISM probing of this kind.
Ultrasoft X-Ray Background Observations of the Local Interstellar Medium
Authors: Sanders W.T. , Snowden S.L. , Bloch J.J. , Juda M. , Jahoda K.M. , McCammon D.
Published: July 2016

Preliminary results from a May 8, 1984 sounding rocket survey of the soft X-ray background are presented. The X-ray detectors are sensitive to X-rays in three soft X-ray bandpasses: 80-110 eV, 90-188 eV, and 284-532 eV (at 20% of peak response). The lowest energy X-rays in this range have a mean free path of order 1019 cm-2 and provide information about the local interstellar medium. The count rate in the 80-110 eV energy band (the Be band) tracks the 90-188 eV band (the B band) very well, indicating that the same ~1 million degree gas that is responsible for the B band emission may be responsible for the bulk of the Be band X-rays as well. We estimate for the flux in the Be band ~1 photon cm-2 s-1 sr-1 eV-1 , about a factor of four lower than that found by Stern and Bowyer (1979) and Paresce and Stern (1981) over a similar energy band.
Molecular Clouds Within 100 pc
Authors: Blitz Leo , Magnani Loris , Mundy Lee
Published: July 2016

Observations at the 2.6 mm line of CO reveal the presence of a large number of molecular clouds at high galactic latitude. If the velocity dispersion of the clouds is a measure of their scale height, the mean distance of the ensemble we have detected is 100 pc. The clouds are unusual in that either they are not gravitationally bound or they are very deficient in CO relative to molecular hydrogen. These clouds represent a heretofore unrecognized component of the local interstellar medium. If they are pervasive in the Milky Way, they probably represent the small molecular cloud component of the interstellar medium.
Stromgren Trails of Hot White Dwarfs
Authors: Raymond John C.
Published: July 2016

High resolution IUE spectra of Feige 24 reveal sharp C IV absorption features at both the stellar velocity and the velocity of the interstellar lines. Dupree and Raymond have shown that the ionization zone created by Feige 24 itself accounts for the interstellar components of C IV and Si IV, and that the density of hot white dwarfs in the galaxy implies that similar ionized zones around hot white dwarfs make a significant contribution to the C IV column densities observed toward distant O and B stars. Typical white dwarf velocities of around 30 km/s and C IV ionization zone diameters less than 1 pc imply that the crossing time is comparable to the ionization time, so that non-equilibrium ionization models are required. Models of the resulting elongated trails are presented.
Measuring EUV Line Emission from the Hot Interstellar Medium
Authors: Labov Simon , Martin Christopher , Bowyer Stuart
Published: July 2016

In the local interstellar medium, the presence of hot gas (log T ~ 5 to 6) has been inferred from measurements of oxygen VI absorption in the ultraviolet, and from diffuse emission in the extreme ultraviolet (EUV) and soft X-ray regions. Here we describe a spectrometer that is very sensitive to gas in this temperature range. The spectrometer uses an array of plane-ruled gratings at grazing incidence in the extreme off plane mount. A set of Kirkpatrick-Baez mirrors focuses the conically diffracted light on to one of two microchannel plate detectors. The field of view of the instrument is 0.2° by 12°. The predicted sensitivity ranges from 50 to 200 ph/(cm2 sec str) with a resolving power (λ/Δλ) of 15 to 50 over the 50 to 700A wavelength band. The instrument is currently under construction for a sounding rocket flight.
The McDonald Observatory High Precision Radial Velocity Spectrometer
Authors: Cochran William D. , Young Brenda W.
Published: July 2016

We are developing a prototype instrument for McDonald Observatory designed to measure stellar radial velocity variations to a precision of a few meters per second. The instrument will be used to study low amplitude stellar oscillations, to search for binary stellar systems with large mass ratios, and possibly to search for extra-solar planetary systems. A fixed gap Fabry-Perot etalon, used in reflection, imposes a set of fixed reference absorption lines on the stellar spectrum before it enters the McDonald Observatory 2.7m coudé spectrograph. The spectrum, covering 1500 Å at 0.13 Å resolution, is recorded on a set of eight Reticon arrays, placed end-to-end. Doppler shifts of the stellar spectral lines with respect to the fixed Fabry-Perot orders are measured by cross-correlation techniques. Calibration methods have been developed to measure any long-term drifts within the system.
Absolute Astronomical Accelerometry
Authors: Connes Pierre
Published: July 2016

A new technique is proposed, leading to two closely related instruments, the solar and stellar accelerometers. The principal fields of application are solar/stellar seismology and the search for extrasolar planets. The technique is fully specific: it cannot give the velocities themselves. The results are absolute : the measured accelerations are directly related to primary length and frequency standards. Essentially one makes use of a variable path Fabry-Pérot interferometer whose channelled spectrum is constrained by a servo loop to track the Doppler shifted astronomical spectrum. Then a tunable laser tracks the interferometer. The ultimately measured quantity is the beat frequency between the tunable laser and a stabilized one.

A fundamental limit to the precision of radial velocity measurements, set by photon counts and the spectral profile itself, is established, and it is shown that the proposed technique closely approaches that limit. Hence it is considerably more sensitive than any other used so far, and holds the promise of making both planetary searches and the study of oscillations practical for a large number of stars.
Current Programs of Local Galactic Structure and Evolution
Authors: Andersen J. , Nordström B.
Published: July 2016

We present a progress report on some current radial-velocity observing programs aiming to provide complete data for selected samples of stars covering the whole sky. The velocities are based on ESO coudé spectra as well as CORAVEL observations obtained in both hemispheres. As a first step, the Bright Star Catalogue has been completed in radial velocities ( ~1500 stars or ~l/3 of the southern BS stars). Currently, we are approaching completion of some 4000 dwarf F stars from Olsen’s (1983) uvbyß photometric survey. The data will be used to study the velocity dispersion of these stars as a function of age and metal abundance from a kinematically unbiased sample. They will also provide a basis for an improved determination of Kz. Extension of the program to the G dwarfs is planned for the near future.
Image Tube Radial Velocities of Southern Stars
Authors: Lu Phillip K.
Published: July 2016

Radial velocities have been determined for 243 southern barium and high proper motion stars based on 346 image-tube (IT) spectra. These spectra were obtained using the Yale 1-m telescope at CTIO with a dispersion of 43A/mm near H-gamma. All spectra were widened to 0.3 mm for MK spectral classification. Analysis of the measurements yields the following results:

1.

Of the 91 spectra scanned and reduced using the Yale PDS and cross-correlation (CC) procedures, the internal standard errors for stars of similar spectral type to the template or standard star are ±12 km/sec.

2.

Spectra of 130 southern high proper motion stars measured with a PSK-2 and a Grant comparator, the linear least squares solution yielded a mean internal error of ±7.5 km/sec.

3.

For the 125 IT spectra of southern barium stars scanned using a PDS at KPKO, radial velocities obtained using the linear portion of the s-curve and a Gaussian line fit procedure, the accuracy is about ±5.5 km/sec.

4.

Using standard stars and some published radial velocities, the comparison has shown that no systematic error exists between this study and those published data.
Stellar Kinematics in Star-Forming Regions
Authors: Mathieu Robert D. , Latham David W. , Hartmann Lee
Published: July 2016

High-precision radial-velocity studies of three young, star-forming regions - λ Orionis, NGC 2264 and Taurus-Auriga - are under way; study of the Orion cluster (with L. Marschall, in conjunction with the Yale proper-motion study) is beginning this winter. Single-order (~50 A, central wavelength 5200A) echelle spectra have been obtained for late-type pre-main sequence stars. Due to the spectral peculiarities and stellar rotation associated with pre-main sequence stars, our radial-velocity precisions are often somewhat poorer than those obtained for late-type dwarfs or giants. Measurement errors of 0.7-1.5 km/sec are typical, although some stars do not permit any radial velocity measurement at all. We summarize preliminary findings for each region:
Mean Stellar Radial, Velocities Catalogue
Authors: Barbier M.
Published: July 2016

The accumulation of new data requires that the existing catalogues (Wilson 1951 - Evans * 1970, still not completed) be supplemented by a new mean radial velocities catalogue; a need which is particularly pressing for astronomers, especially statisticians.

Today, such work cannot be done by one person only, as has been done previously. Therefore¿ in 1976, during the IAU General Assembly at Grenoble, Commission 30 made up a working-group (consisting of J. Andersen, M. Barbier, A. H. Batten, M. Duflot, D. S. Evans, R. Griffin), under my responsibility, with a mandate to examine the possibility of making a new mean radial velocities catalogue using computers, so that the catalogue could regularly be brought up to date. In 1982, at Patras, our working-group submitted the results of its work and proposed the following set of rules to plan their catalogue. Commission 30 asked us to test them. This work has been done at the Marseille Observatory by M. Duflot and myself in collaboration with P. Figon, who worked on the programming for the catalogue.
Radial Velocity and Four-Color Measures of Halo A-Type Stars
Authors: Philip A.G. Davis
Published: July 2016

The literature has been searched for A-type, halo stare for which both radial velocity measures and four-color photometric measures have been made. Data were found in Drilling and Pesch (1973) Philip (1969, 70), Philip and Drilling (1985), Rodgers (1971) and Stetson (1981). The greatest number of stars are from the finding list of Philip and Sanduleak (1968) of A-type stars at the SGP. Rodgers obtained spectra and determined radial velocities for most of the stars in the SGP list. Philip and Drilling have measured all the stars in Table II in the four-color system. The next largest group of stars is from the 4 HLF 4 region (finding list of Philip and Drilling 1970) for which photometry and and radial velocity measures were made by Drilling and Pesch. The rest of the stars were found in other high galactic latitude areas.
Radial Velocities of F and G Dwarfs in the Orion Cluster Region
Authors: Mermilliod J.-C. , Mayor M.
Published: July 2016

The interest in the distribution of rotational velocities of red dwarfs in open clusters has been recently enhanced by the discovery of W dwarfs in the Pleiades showing light variability of short period (van Leeuwen and Alphenaar, 1982) and correspondingly large Vsini values (Stauffer et al., 1984b). Below a given mass limit, slow and rapid rotators coexist at the same B-V index. Therefore, the distribution of Vsini presents two components: one very peaked at low values and a second one, rather broad. The reality of this unique feature has been beautifully confirmed by the observations of faint stars in the α Persei cluster (Stauffer et al., 1984a).
Pre-White Dwarf Evolution: Up to Planetary Nebulae
Authors: Mazzitelli Italo
Published: July 2016

The main evolutionary phases having some interest for the formation of the remnant white dwarf are discussed, starting from the core helium burning phase, in the attempt of evaluating a theoretical relation between initial main sequence mass and final white dwarf mass. Several difficulties, mainly due (but not only) to uncertainties in the theory of mass loss, have been met, so that only a fiducial bona fide correlation can be drawn. The mass function of population I white dwarfs has probably a secondary maximum at M = 0.9 – 1 Me.
The Effect of Coulomb Interactions on the Helium Flash
Authors: Harpaz A. , Kovetz A.
Published: July 2016

Detailed evolutionary calculations show that Coulomb interactions between the charged particles of a stellar plasma reduce the core mass at which a low mass red giant undergoes the helium flash (contrary to a recent claim). This has implications for the determination of the rate of mass loss from red giants.
Cluster analysis of the hot subdwarfs in the PG survey
Authors: Thejll Peter , Charache Darryl , Shipman Harry L.
Published: July 2016

The Palomar Green survey (Green et al., 1986) of faint blue, high galactic-latitude objects, turned up several interesting new classes of objects, such as gravitational lenses (Weyman et al., 1980), the ultra hot star H1504+65 (Nousek et al., 1986), and the PG 1159-035 variables (McGraw et al., 1979). The PG survey forms a mainstay in the investigations of late stages of stellar evolution. Work (Flemming et al., 1986) has already cast light on the important question of the space density of DA’s and continuing work is seeking values for similar population parameters for the subdwarfs: Evolutionary links between the subdwarf stage and white dwarfs will thereby be illuminated.
Helium Abundance in the Photospheres of Hot Da White Dwarfs
Authors: Holberg J. B. , Kidder K. , Liebert J. , Wesemael F.
Published: July 2016

We have used optical and UV spectroscopy to determine He abundances and upper limits to He abundances in the photospheres of a selected sample of very hot hydrogen-rich white dwarfs. He abundances in the range log(He/H) -3 to -1.5 are observed in several of these DAs and upper limits of -3 determined for the remainder. In apparent contradiction to the relatively large He abundances inferred from soft X-ray observations for the hot DA G191 B2B, we find no evidence of He in the optical and UV.
Origin of the DA and Non-DA White Dwarf Stars
Authors: Shipman Harry L.
Published: July 2016

Understanding the origin of DA and non-DA stars — the bifurcation of the white dwarf cooling sequence — is one of oldest and perniciously difficult problems in the white dwarf research field. Processes which could play a role have been discussed for years. For example, the idea of diffusive element separation was suggested by Schatzman (1958). The suggestion of convective mixing goes back at least as far as Strittmatter and Wickramasinghe (1971) and Shipman (1972), and this process was actively discussed by a number of authors in the 1970s (see the excellent resume by D’Antona and Mazzitelli 1979).
Model Atmospheres for Very Cool Hydrogen-Rich White Dwarfs
Authors: Allard France C. , Wehrse Rainer
Published: July 2016

In recent years cool white dwarfs have been studied for various aspects ( see e.g.Winget et al.,1987 Winget and van Horn, 1987, Koester, 1987, Llebert, 1980) and much effort has been Invested in attempts to interpret the energy distributions of these stars ( Greenstein, 1984, Zeldler-K.T. et al, 1986, Llebert et al., 1987, and others). However, it seems that in spite of these efforts the spectra in particular of the very cool objects with effective temperatures below about 6000 K are not yet fully under-stood, since they are extremely diverse and each objects needs special consideration. In addition, the analyses are extremely difficult because the principal constiuents of the atmospheres ( H, He ) and elements, which may donate the majority of electrons, are essentially invisible. Since usually only one ionlsatlon stage of an element is present, this implies that the gas pressure Pg is high ( compared e.g. to the solar photosphere ), the accurate value of Pg, however, cannot be determined reliably.
Finite Element Analysis of Diffusion Processes in White Dwarfs
Authors: Pelletier C. , Fontaine G. , Wesemael F.
Published: July 2016

The spectral evolution of white dwarfs is governed by diffusion processes which enter into competition with mechanisms such as mass loss, convective mixing, and accretion from the interstellar medium in various phases of the evolution. Until recently, our theoretical understanding of the chemical evolution of these stars has been limited by the very severe numerical difficulties which plague a time-dependent description of the problem. Indeed, diffusion problems in white dwarf interiors and envelopes are particularly demanding from a computational standpoint: they involve relative chemical abundances spanning many orders of magnitude, time integration length of a few billion years, and many physical processes operating with greatly different time constants. We have already introduced in the field a robust numerical technique based on an implicit finite difference scheme designed for nonlinear two-point boundary value problems (Pelletier 1986). This method has been used to investigate a number of problems related to the spectral evolution of white dwarfs (Pelletier 1986; Pelletier et al. 1986; Dupuis et al. 1987). As requirements for further progress in the field become more exacting and in the interest of improving the efficiency, we have sought to develop even more powerful numerical techniques. We briefly introduce here an efficient computational approach to diffusion problems in white dwarfs based on a Galerkin finite element method to solve the convective-diffusion equation in an evolving white dwarf model. As an illustrative example, we discuss some sample results of a detailed investigation of the problem of chemical sedimentation (H, He, and C) in the envelopes of hot white dwarfs and the formation of DA stars.
Are Pulsations a Useful Probe of the Structure of the Outer Layers of White Dwarfs ?
Authors: Brassard P. , Fontaine G. , Vesemael F. , Kawaler S.D.
Published: July 2016

The most fundamental aspect of white dwarf seismology is the determination of the gravity-mode (g-mode) period structures of models of isolated pulsating white dwarfs. These stars show multiperiodic luminosity variations which result from the superposition of excited pulsation modes. Among the many oscillation modes available in the very rich nonradial g-mode spectra of white dwarfs, the observed modes are selectively chosen by a filtering mechanism. Although the period evolution is strongly tied to the core temperature evolution in a white dwarf, the period structure remains largely specified by the mechanical properties of the star. The most basic structural feature of a white dwarf is its highly degenerate interior, which leads to nearly isothermal and nearly isentropic stratifications in the core region containing more than 99% of the mass of the star. In particular, because the density gradient is almost adiabatic throughout the interior of a white dwarf, the Brünt-Väisälä frequency (see below) is very small there and low-order g-modes cannot propagate. As a result, g-modes are essentially envelope modes in white dwarfs, with large amplitudes occuring only in the non-degenerate outer layers. One can thus expect that g-modes in white dwarfs are extremely sensitive to envelope properties such as compositional stratification and partial ionization mechanisms. Compositional stratification is, in fact, the second structural feature of a white dwarf model which has strong effects on the period structure. Indeed, trapped modes result when a resonance or near-resonance occurs between the local g-mode radial wavelength and the thickness of one of the composition layers. This results in a period structure which strongly bears the signature of compositional stratification in the outer layers. Thus, it has been widely accepted that white dwarf pulsations probe primarily the outer layers of these stars. This point of view has been borne out by detailed pulsation calculations carried out by several independant groups (see Winget 1987 and references therein).
Thermonuclear Explosion in Binaries: White Dwarf Helium Star
Authors: Ergma E. , Fedorova A.V. , Khohlov A.M.
Published: July 2016

Evolutionary sequences of a binary system consisting of a helium star in the stage of core helium burning and a white dwarf are presented. Different ways of He burning in the envelope of a dwarf are discussed. Incomplete helium burning is discussed with relation to occurence of possible observational features.
Phase Correlated Spectra of Magnetic White Dwarfs
Authors: Bues I. , Pragal M.
Published: July 2016

New photometric and polarimetric observations of the magnetic white dwarfs G 99–37 and G 99–47 are in accordance with a period of 4.117h and 0.97h respectively. Spectra taken at various phases could be correlated and analyzed by model atmosphere technique.
Time-Dependent Studies of the Settling of Heavy Elements in the Envelopes of Cool White Dwarfs
Authors: Dupuis J. , Pelletier C. , Fontaine G. , Wesemael F.
Published: July 2016

Gravitational settling is widely accepted as being a fundamental physical process acting upon superficial layers of white dwarfs and resulting in an important alteration of their atmospheric composition. Several investigators have been interested by the problem of gravitational settling in white dwarfs (Fontaine and Michaud 1979; Vauclair, Vauclair, and Greenstein 1979; Alcock and Illarianov 1980; Muchmore 1984; Paquette et al. 1986). As pointed out in Paquette et al. 1986, they all reached the same qualitative conclusion: the gravitational settling time scales of metals in cool white dwarfs are small compared to their evolutionary time scales. These stars should therefore have their photospheres depleted of metals if there is no extrinsic source such as accretion for example. This is consistent with the observational fact that most of the cool white dwarfs spectra just show hydrogen and helium lines while the absence of metallic lines indicates a strong depletion of metals. Although the qualitative agreement between theory and observations is satisfactory, only time-dependent calculations can lead to a thorough understanding of the heavy element abundance patterns in cool white dwarfs. In particular, the predicted abundance of an element within the framework of the accretion-diffusion model does depend explicitly on the results of such calculations. We have already presented some preliminary results of numerical simulation of accretion episodes of heavy elements into white dwarfs (Dupuis et al. 1987). As part of an ongoing detailed investigation of these processes, we focus here exclusively on the mechanism of gravitational settling in white dwarfs in order to clear some confusion which has appeared in the literature.
The Nearby Binary, Stein 2051 (G175–34AB)
Authors: Strand K. Aa. , Kallarkal V. V.
Published: July 2016

Photographic observations of the large proper motion binary, Stein 2051, extended over the period 1966–87, do not support that it is a triple system as previously reported (Strand, 1977). The orbital motion is nearly linear over this interval? however, when results of plates from the Vatican Astrographic Zone from 1908–11 are included, a mass ratio is obtained, leading to a mass of 0.50©for the white dwarf component, given the calculated mass of 0.24© for the red dwarf component.
Toward a Determination of the Helium Abundance in Cool Da White Dwarfs
Authors: Bergeron P. , Wesemael F. , Fontaine G.
Published: July 2016

Connective mixing between the thin superficial hydrogen layer and the more massive and deeper helium layer is generally believed to be responsible for the increased number of non-DA white dwarfs relative to the number of DA below 10000K (see Sion 1984 and references therein). However, because of the spectroscopic invisibility of the helium lines at effective temperatures below 13000K, the true atmospheric composition of these cool stars remains somewhat uncertain. On theoretical grounds, studies of the evolution of white dwarfs on the cooling sequence have shown that if the hydrogen layer is thicker than ~10”6Me, convective mixing does not occur (Tassoul, Fontaine, and Winget 1988). Furthermore, the exact amount of helium pollution is very sensitive to the thickness of the hydrogen layer. It seems therefore imperative to evaluate to what extent DA stars below 13000K truly are hydrogen-rich. In line with our previous efforts geared toward an understanding of the atmospheric properties of the cool DA white dwarfs, we present new insights into the spectroscopic modelling of these cool stars, and also demonstrate, for a particular object, how the helium abundance might be determined.
Time Resolved Spectroscopy of AM CVn
Authors: Lázaro C. , Solheim J.-E. , Arévalo M.J.
Published: July 2016

The preliminary results of time resolved spectroscopy of the He White Dwarf system AM CVn are presented. We have searched for spectral variations at different photometric periods. These are found at different time scales although this does not particularly favour any of the searched periods.
Binary, Pulsating, and Irregular Variables Among Planetary Nuclei
Authors: Bond Howard E. , Ciardullo Robin
Published: July 2016

For the past two years, the authors have been carrying out a program of CCD photometry of planetary-nebula nuclei (PNNs), using the 0.9-m telescopes at Kitt Peak National and Cerro Tololo Inter-American Observatories. The aim of this program is to investigate the variability of PNNs on timescales of minutes to days, in order to search for close binaries and pulsators, as well as unexpected new classes of variable stars.

The program represents an extension of the photoelectric photometry of PNNs carried out by Bond and Grauer (1987). Use of the two-dimensional CCD detector allows us to model and subtract the nebulosity surrounding a PNN, in addition to providing exactly simultaneous observations of the PNN and several nearby comparison stars. The latter allow us to compensate for variable atmospheric transparency, permitting accurate differential photometry to be obtained even when the observing conditions are not photometric.
Evolution of Stars in the Post-Main Sequence Stage
Authors: Kippenhahn R.
Published: July 2016

I would like to start with a short historical introduction. In 1938 thermonuclear reaction rates for hydrogen burning became available. This made it possible to fit a convective core into the point source model of stellar structure integrated by Cowling three years earlier. The free parameter in the fitting process could be fixed with the thermonuclear reaction rates, the first realistic stellar model for a massive main sequence star was constructed! After the war electronic computers became available, and one was able to do more complicated models like those on the lower main sequence, like realistic models for the sun with its helium enriched interior, and one tried already to follow in time the exhaustion of hydrogen in the central regions of stars numerically. There was not too much progress for stars at the upper end of the main sequence. As soon as the stellar model tried to leave the main sequence and to march towards the region of the red supergiants the methods known at that time failed to produce models. For less massive stars the exhaustion of hydrogen could be followed up more easily and, in 1955, the great paper by Hoyle and Schwarzschild came out, which showed how these stars from the main sequence move into the red giant branch and move up parallel to what we now call the Hayashi line (which was not yet known at that time). But when helium started to burn the methods also failed.
Disk Instabilities in Hubble-Sandage Variables?
Authors: Meyer F. , Meyer-Hofmeister E.
Published: July 2016

The brightest individual objects in extragalactic nebulae are the Hubble-Sandage variables. They were first investigated by Hubble and Sandage (1953). Observations by Tamman and Sandage (1968) and Rosino and Bianchini (1973) followed. The main characteristics are high luminosity (L/Lʘ ≈ 105), blue color indices, F type spectra and irregular variability.

Bath (1979) has suggested that the Hubble-Sandage variables contain an accreting main-sequence star with a Roche Lobe filling companion in a wide binary system. Based on this model we derive theoretical color indices for disks and determine the mass in the disk for different mass accretion rates. Further we discuss an instability of the disk which could explain the change in the color index observed for Var A in M33.
A Note about Multiple Systems of Dense Molecular Clouds
Authors: Vanýsek Peter
Published: July 2016

The space distribution of some small dense clouds with point-like IR sources, resembles the clustering of young OB stars. It can be assumed that such objects contain heavy obscured high-luminosity stars on the ZAMS. From the comparison of infrared and radio data it follows that in typical cases, only one B star is the source of the radiation of the cloud. The total mass of the cloud is of the order of one solar mass. If the individual fragments of the cloud are gravitationally unstable, then in the later stage of the evolution only low-mass stars are formed. One can therefore expect that young OB stars are most frequently accompanied by low-mass pre-Main Sequence stars.
A Hypothesis on the Richness of a Galaxy in Binary Stellar Systems
Authors: Zinnecker Hans
Published: July 2016

This contribution is concerned with the origin of binary stellar systems (cf. Abt 1977, Huang 1977). The hierarchical fragmentation scheme of rotating interstellar clouds, proposed by Bodenheimer (1978), is combined with the role of magnetic fields in the early stages of star formation (Mouschovias 1978, Dorfi 1981). The possible influence of the local strength of the mean interstellar magnetic field on the local fraction of binary systems in a galaxy is stressed.
Photometric Investigation of Visual Binaries with a Component above the Main Sequence
Authors: Franz O.G.
Published: July 2016

Visual binaries containing a component above the main sequence in combination with a main-sequence star provide an excellent opportunity for determining absolute magnitudes of evolved stars. Some aspects of this technique are discussed, and preliminary results are presented.
Observational Selection in Spectroscopic Binary Eccentricities
Authors: Staniucha M.S.
Published: July 2016

The discovery-and-identification probability for different shaped orbits of spectroscopic binary stars is estimated. The eccentricity distribution observed in the sample of ~1000 binaries with known orbits and appearing as strongly peaked toward e=0 is corrected for observational selection effects. The resulting e-distribution seems to be flat for e in the range ~0.05-0.6 with some excess of circular (or almost circular) orbits and a deficiency of orbits with e ≲ 0.6.
Ultraviolet Light Curves Of U Geminorum and VW Hydri
Authors: Wu Chi-Chao , Panek R.J. , Holm A.V. , Schiffer F.H.
Published: July 2016

Ultraviolet light curves have been obtained for the quiescent dwarf novae U Gem and VW Hyi. The amplitude of the hump associated with the accretion hot spot is much smaller in the UV than in the visible. This implies that the bright spot temperature is roughly 12000 K if it is optically thick. A hotter spot would have to be optically thin in the near UV. The flux distribution of U Gem in quiescence cannot be fitted by model spectra of steady state, viscous accretion disks. The absolute luminosity, the flux distribution, and the far UV spectrum suggest that the primary star is visible in the far UV. The optical-UV flux distribution of VW Hyi could be matched roughly by our model accretion disks, but the fitting is poorly constrained due to the uncertainty in its distance.
Numerical Calculations for Accretion Disks in Close Binary Systems
Authors: Hensler G.
Published: July 2016

A numerical method for 3D magnetohydrodynamical investigations of accretion disks in close binary systems is presented, which allows for good spatial resolution of structures (hot spot, accretion column). The gas is treated as individual gas cells (pseudo-particles) whose motion is calculated within a grid consisting of one spherical inner part for 3D MHD and two plane outer parts. Viscous interactions of the gas cells are taken into account by a special treatment connected with the grid geometry.

We present one result of 2D hydrodynamical calculations for a binary applying the following parameters which are representative for Cataclysmic Variables: M1= 1 Mʘ, r1= 10-2Rʘ, M2= 0.5 Mʘ, p = 0.2 d, M = 10-9Mʘy-1.

Column density and radiative flux distributions over the disk are shown and briefly discussed by comparison with the theoretical understanding of these Dwarf Novae drawn from observations.
S Cnc as a Case Study of Classical Algol Evolution
Authors: Awadalla N.S. , Budding E.
Published: July 2016

Basic information on empirical determinables for the system S Cnc are reviewed. Whilst the photometric information can be clearly analysed along well known lines, the spectrographic information is far from clear and requires further study, though a semi-detached configuration, at least, can be confidently deduced.

The potentially important question of some physical relationship of the variable to the open cluster Praesepe is considered, but the result is in the negative.

The evolutionary status of the binary is examined in a general way by making use of some summary formulae, some guidelines for which were taken from previous more general work of Refsdal and Weigert. S Cnc is in good overall agreement with the low mass Case B theoretical mode of binary evolution, and even the absence of a detectable rate of period variation is shown to be not in serious conflict with this picture. Interesting close comparisons with the well known example AS Eri are possible.
The Very Unusual Ultraviolet Spectra of R Arae
Authors: Kondo Yoji , McCluskey George E.
Published: July 2016

The high resolution ultraviolet spectra of the 4.4-day period binary R Arae, observed in 1980 with the International Ultraviolet Explorer (IUE), show that its continuum flux level varied outside of eclipse by more than a factor of two in ten days, and by over 50 percent within a same orbital cycle. The flux level varied non-monotonically at different wavelengths. The resonance lines of Mg II and Si IV exhibited shortward–shifted absorption components near phase 0.4, indicating the presence of a gas stream toward the observer at a velocity of some –450 to –500 km s–1. Our observations of R Arae with the Einstein satellite show it to be an X–ray source.
The Short-Period Non-Contact Binary Systems UU Lyn and GR Tau
Authors: Yamasaki A. , Okazaki A. , Kitamura M.
Published: July 2016

Two binary systems UU Lyn (P = 0d.468) and GR Tau (P = 0d.430) are discussed on the basis of BV photoelectric observations and spectroscopic observations which were made between 1980 and 1981 at the Tokyo Astronomical Observatory. The component stars in both systems are found to be still inside their Roche lobes but very close to them.
Binary Stars and SS433
Authors: Ruffini Remo
Published: July 2016

Some of tile most unique experimental features of the source SS433 are outlined as well as some implications of the theoretical models of this source.
Observations of Secular Changes in the Kinematic Model of SS433
Authors: Collins G.W. , Newsom G.H.
Published: July 2016

In this paper we present evidence that several of the defining parameters of the Kinematic Model for SS433 are not constant but rather exhibit long term systematic changes. Recent data confirm the existence of the previously reported decrease in the precessional period. The value for this period change, when combined with the observed change in the period of the synodic spectral variations, implies that the orbital period is not significantly changing on a time scale less than 1000 years.

In addition we find mounting evidence for a statistically significant (40) secular change in the cone angle θ at a rate of about -1.5 x 10-3 deg/day. However, t the surprisingly short time scales implied by the observed values of P and θ when combined with estimates of the system age suggest the possible existence of detectable higher time derivatives. This view is supported by the most recent data which suggest a value for P ~ 10-5 (days)-1. It is possible to understand these secular changes in terms of the motions to be expected from an object exhibiting classical precession in response to an external torque.
The Formation of Massive White Dwarfs in Cataclysmic Binaries
Authors: Law Wai-Yuen , Ritter Hans
Published: July 2016

In contrast to the mass spectrum of single white dwarfs which has a single narrow peak at ~0.6 M⊙, the observed mass spectrum of white dwarfs of cataclysmic binaries (CB's) shows a rather uniform distribution of the masses in the range ~0.3 M⊙, to ~1.3 M⊙. The formation of CB's with white dwarfs of less than about 0.8 M⊙ can be understood as the result of a binary evolution according to low mass Case B or Case C with a subsequent spiraling-in in a common envelope. On the other hand the formation of massive white dwarfs of M ≲ 1 M⊙, can be explained as the result of a massive Case B mass transfer yielding a helium star which subsequently undergoes a second Case B mass transfer (so called Case BB evolution). The ultimate product of such an evolution is either a CO-white dwarf with a mass up to the Chandrasekhar limit or a neutron star. The formation of CB's via Case BB evolution requires the binary to undergo at least one, most probably two separate phases of spiraling-in in a common envelope.
The Subdwarf Eclipsing Binary LB3459 (AA Dor)
Authors: Hilditch R.W. , Hill Graham , Kilkenny D.
Published: July 2016

A review was presented of the currently available observational data on this evolved system, namely, uvby photometry (Kilkenny et al. 1978; paper II) radial velocities (Kilkenny et al. 1981; paper III) and a non-LTE atmosphere analysis of the primary component (Kudritski et al. 1981). Kudritski et al. conclude that T(pr) = 40000 ± 2500°K and log g = 5.3 ± 0.2 and from the spectroscopic data and light curve analysis (papers II, III) derive masses and radii for the primary and secondary components of M(pr) ≃ 0.25 M⊙, M(sec) ≃ 0.04 M⊙, R(pr) ≃ 0.16R⊙, R(sec) ≃ 0.09R⊙ respectively. Thus the primary component is a normal sd0 star whilst the secondary component is a most enigmatic object, perhaps a normal composition degenerate dwarf but of too low a mass to be on the main sequence, perhaps an evolved degenerate object but then the radius is too large. The published evolutionary models for this system (Paczynski 1980; Conti et al 1981) do not adequately describe its current status.
A model containing “blobs” for the A ring of Saturn
Authors: Thompson Willian , Blerkom David van , Irvine William
Published: July 2016

Observations of the brightness distribution of Saturn’s ring A from four separate years, including specifically the azimuthal variations, are calibrated and interpreted through the use of a scattering model. In this model the A ring is composed of elongated density enhancements (“blobs”) embedded in a ring of uniform density. The blobs are approximated by biaxial ellipsoids. Intensity observations at one tilt angle can be used in the context of the model to predict the intensity at other tilt angles. The analysis shows that there must be matter between blobs characterized by an optical thickness τo≃0.45 and single scattering albedo ωo≃0.70. A rough estimate, based on what appears to be shadowing of one bfob by another, gives the ring area occupied by blobs as 5%.
Indications of twist in the epsilon ring of Uranus
Authors: Vasundhara R. , Bhattacharyya J.C.
Published: July 2016

A comparison of light curves during occultation of stars by ε ring of Uranus indicate that this ring assembly possesses stable fine structures. The light curves observed during several events show almost similar fine structure across its width. Nicholson et al. (1982) quantitatively compared the separation of the five optical depth maxima, and concluded that no systematic variation of the normalised spacing, either with ring segment width or with data of observation was apparent. We have investigated the possibility of this having structure of a twisted bundle of narrow lanes of condensed material similar to that of Saturn’s F ring (Ref 4) and report the findings.
Saturn electrostatic discharges
Authors: Warwick J.W. , Romig J.H. , Evans D.R.
Published: July 2016

Electrostatic discharges were recorded by the Planetary Radio Astronomy (PRA) instrument aboard both Voyager 1 and Voyager 2 at Saturn closest approach. Two distinct kinds of data are involved: spectral scans covering the range from 20 kHz to about 40 MHz and discrete frequency, high time resolution, scans of duration about 48 seconds near 10 MHz. Both sets of data exhibit lightning-like bursts of ragged noise lasting typically for a few tens of milliseconds. The spectral data are the most abundant, covering several days near the Saturn encounters, and show that these individual events occur in episodes of a few hours duration that repeat in a period of about 10h. The two spacecraft actualLy gave quite different stetistics of burst occurrence, and in other respects, for example, burst poLarization, were also different.
Solved and unsolved problems in planetary ring dynamics
Authors: Borderies N. , Goldreich P. , Tremaine S.
Published: July 2016

We discuss a number of new ideas related to planetary ring dynamics and apply them to Saturn's rings. The topics are ordered in terms of increasing spatial scale. For simplicity, the ring particles are taken to be identical, smooth, spheres composed of dense and chemically pure ice.
Dynamics of eccentric and inclined ringlets
Authors: Borderies N. , Goldreich P. , Tremaine S.
Published: July 2016

The Saturn and Uranian ring system contain a number of narrow eccentric ringlets. Several of the Uranian ringlets are also inclined to the planet's equator. We show that the self-gravity is probably responsible for maintaining apse and node alignment across these ringlets. This hypothesis leads to the prediction that within each ringlet both the eccentricity and the inclination increase with semimajor axis.
Instabilities and the fine structure of Saturn's rings
Authors: Bobrov M.S. , Fridman A.M. , Polyachenko V.L.
Published: July 2016

In the opinion of the most of authors, reasons, leading to formation of observational complex structure of Saturn's rings, can be conditionally classified into three types. Type I, distinguished for a long time, is resonances. Types II and III are young; they were born in 1981. Type II is the kinematic instability due to differentiality of angulare velocity and to inverse dependence of kinematic viscosity on the particle density. Type III is collective processes due to selfgravitation of the medium.
Diffusion of dust around a central body
Authors: Barge P. , Millet J. , Pellat R.
Published: July 2016

The dynamics of particles moving in an external field of force and submitted to a stochastic perturbative force is relevant to a number of astrophysical problems. The Fokker-Planck equation describing the evolution of the distribution function of particles in phase space can be reduced to a diffusion equation through a formalism coming from the quasi-linear theory in plasma physics, (Laval and Pellat, 1974). The diffusion coefficients, expressed in terms of power spectral densities of the fluctuating quantity, are shown to have a resonant behaviour. This aspect of the diffusion and the simplifications due to the formalism could be interesting in the study of planetary rings.
Spoke electrodynamics
Authors: Acterberg A. , Blandford R. , Goldreich P.
Published: July 2016

Electrons are ejected by solar ultraviolet photons from the illuminated parts of Saturn's rings. Consequently, these portions of the rings possess significant electrical conductivity. Wc evaluate the height integral surface conductivity tensor and show that the main contribution is provided by charged dust grains. The conductivity tensor is applied to estimate the pattern of current flow along the surface of the rings and between the rings and the planet's ionosphere. Mechanisms for elevating grains along nearly radial spokes are explored.
Saturn's E, G and F rings: modulated by the plasma sheet
Authors: Grün E. , Morfill G.E. , Johnson T.V. , Schwehm G.H.
Published: July 2016

Saturn's broad E ring, the narrow G ring and the structured and apparently time variable F ring(s), contain many micron and sub-micron sized particles, which make up the “visible” component. These rings (or ring systems) are in direct contact with magnetospheric plasma. Fluctuations in the plasma density and/or mean energy, due to magnetospheric and solar wind processes, may induce stochastic charge variations on the dust particles, which in turn lead to an orbit perturbation and spatial diffusion. It is suggested that the extent of the E ring and the braided, kinky structure of certain portions of the F rings as well as possible time variations are a result of plasma induced electromagnetic perturbations and drag forces. The G ring, in this scenario, requires some form of shepherding and should be akin to the F ring in structure. Sputtering of micron-sized dust particles in the E ring by magnetospheric ions yields lifetimes of 102to 104years. This effect as well as the plasma induced transport processes require an active source for the E ring, probably Enceladus.