bluuebunny/crossref_metadata_2025 huggingface dataset:
Convection in RR Lyrae Stars
Authors: Stellingwerf R. F. , Bono G.
Published: June 2019

Convection undoubtedly plays a strong role in defining the RR Lyrae instability strip, but its effects on amplitude, mode of pulsation, and light curve have always been problematical. One reason is simply that convective models are difficult to compute, and their accuracy is difficult to ascertain. We present results of a new convective survey of RR Lyrae stars that constitute the best models available at this time, with better boundary conditions, physics, zoning, and length of computational run than previous results.
Modelling Cepheids and RR Lyrae Stars
Authors: Kovács G.
Published: June 2019

We review the recent nonlinear hydrodynamical results and the numerical problems to be solved by the next generation of codes. The behavior of the Fourier parameters and double-mode RR Lyrae pulsation will be discussed in detail. We emphasize the importance of the implementation of the various adaptive-mesh schemes for stellar pulsations. A further goal is the substitution of the artificial viscosity with modern shock capturing methods and a better treatment of radiative transfer. Convection however, must ultimately be included in order to supply the necessary physical mechanism for dissipation.
Theoretical Implications of Triple-Mode RR Lyrae Pulsations
Authors: Kovács G. , Buchler J. R.
Published: June 2019

We argue that triple-mode RR Lyrae pulsation with low amplitudes might be quite common. It is shown that until very accurate abundance data become available, triple-mode RR Lyrae stars are the only hope to estimate reliable stellar parameters from the periods alone.
Hydrogen Emission Lines from Extended Pulsating Atmospheres
Authors: de Laverny P. , Magnan C.
Published: June 2019

In order to study the basic radiative mechanisms in the extended envelopes of evolved stars (e.g. Long Period Variables), we determine the spectrum emerging from a very optically thick Non-LTE hydrogen layer surrounding a core of high temperature (see figure 1). Such a model was first proposed by Sobolev (1960) and Menzel (1946). It is consistent with the fact that LPVs are likely evolving toward planetary nebulae. The main parameters of the model are the temperature of the illuminating star (Tr), the dilution factor (ω) and the Lyman continuum optical depth of the envelope (τ1c). We examine their influence upon the emerging intensities in the lines and in the continua.

The model could explain some observations related to evolved stars and especially the presence of emission features in an otherwise cold environment. Furthermore atmospheric pulsation phenomena produce large variations in the emerging spectra: reddening, emission/absorption transition of the hydrogen lines, profile deformations…
Propagation of Radial Pulsation Modes in the Outer Atmosphere of Arcturus: First Results
Authors: Cuntz M.
Published: June 2019

I present first results of a study about propagating radial pulsation modes in the outer atmosphere of Arcturus (K1.5 III). Mechanical energy input is explicitly taken into account by treating shock wave dissipation. I investigate the influence of different wave frequencies on the mass loss behavior of the star. I show that significant time-averaged mass loss can only be produced when periods larger than 5 x 105 s (∼ 1 week) are employed. The initial atmosphere I use extends from 1.2 R* up to 11.8 RR*. All wave models are adiabatic. I found that the mass loss rates and the final flow speeds obtained are extremely sensitive to the wave periods. In a certain regime the effect of increasing the period by a factor of 4 is to increase the corresponding mass loss rate by four orders of magnitude. I note that the mass loss rate and final flow speed of the wind for a 5.6 x 105 s period wave are somewhat close to the observed values. A more complete discussion regarding mass loss generation in Arcturus has been presented by Cuntz (1990). Recent observations of low-amplitude radial velocity variations in the photosphere of Arcturus provide evidence that the theoretically predicted mass loss frequencies might exist. Belmonte et al. (1990) presented evidence for a ∼8.3 d period with an amplitude of ∼50 m s-1, which they attributed to the fundamental radial pulsation mode. Further studies are in progress (Larson et al. 1992). I note that if the 8.3 d period is real than it would be sufficiently large to support continuous mass loss. Judge & Stencel (1991) argued that the mechanical energy in the observed disturbances could be ∼15 times greater than the energy required to drive the wind. For other stars than Arcturus the required minimum mass loss periods can be estimated as PML ∼ R*.
Evolutionary Models of RR Lyrae Stars
Authors: Lee Young-Wook
Published: June 2019

As tracers of old stellar populations and as primary Population II standard candles, RR Lyrae stars have played an important role in the development of modern astronomy. Our knowledge of stellar evolution has identified these variable stars in a core helium burning phase of low-mass star evolution, the hoiizontal-branch (HB) phase. Consequently, not only to understand fully the nature of RR Lyrae stars, but also to apply them correctly as population probes and distance indicators, we must understand the underlying evolutionary effect of HB stars.

In this paper, I briefly review the most important properties of RR Lyrae stars predicted from the HB evolutionary models, and present many pieces of supporting evidence for these models. For the implications of these models on the chronology of the Galactic formation and on the cosmological distance scale, the reader is referred to several recent publications by Lee (1992a,b,c,d).
The Cepheid Instability “Wedge“
Authors: Morgan Siobahn M.
Published: June 2019

For the most part it had been assumed that the red and the blue edges of the Cepheid Instability region were parallel. However, previous work by Pel and Lub (1978) and recent work by Fernie (1990) seems to reveal a rather interesting structure to the shape of the Cepheid Instability region. Figure 1 shows the shape defined using the data from Fernie (1990) and the observational data of Gieren (1989). It is apparent that the edges defined by the distribution of these points are not parallel.

I have calculated a series of pulsation models that included varying values of the mixing length to try and produce a distribution of Cepheids as seen in Figure 1. Calculations were done using the methods outlined by Castor (1971) with a Linear Non Adiabatic pulsation code to determine the characteristics for a given model.
Masses of c-type RR Lyrae Variables in Globular Clusters
Authors: Cacciari C. , Bruzzi A.
Published: June 2019

The mass of RR Lyrae variables has been a controversial problem for about a decade: while the stellar evolution theory predicts masses ranging between 0.65 and 0.75 M⊙ for Oosterhoff type I and II clusters respectively, the stellar pulsation theory predicts smaller masses (0.55 and 0.65 respectively) using the double-mode pulsators. Simon (1990, M.N.R.A.S. 246, 70), comparing hydrodynamical models with observed stars by means of Fourier parameters, has found relations between the stellar mass and its luminosity, pulsation period, Helium content and Fourier parameter ϕ31. Combining his equations we obtain:

(1)

from which one can estimate the stellar mass by using the observable quantities P and ϕ31 and a luminosity scale, e.g. the one derived by Cacciari, Clementini and Fernley (1992, Astrophys. J. in press).
Search for Variables in the Galactic Globular Clusters NGC 6544 and NGC 6642
Authors: Hazen Martha L.
Published: June 2019

A search for variable stars in the globular cluster NGC 6544 has revealed only one possible short period variable within the tidal radius of the cluster. A search in NGC 6642 yielded 16 new RR Lyrae stars within the tidal radius and 5 new field RRs. The previously discovered (Hoffleit 1972) V1 is a slow variable, and V2 is an RR Lyrae star. Photometry of the variables within the tidal radius gives a mean B for the horizontal branch of < B > = 17.0 mag. With E(B – V) = 0.37 mag and (B – V) = 0.35 mag for RR Lyraes, a value for V(HB) = 16.3 mag is derived. This is about one mag fainter than previous estimates (Webbink 1985), and places NGC 6642 at a distance of approximately 7.9 kpc.
Cepheids in Magellanic Cloud Clusters
Authors: Welch D.L. , Mateo M. , Olszewski E.W.
Published: June 2019

The Magellanic Clouds remain an ideal place to study the properties of Cepheid variables. In this paper, we review historical and current work on Cepheids in LMC and SMC clusters, present new results for NGC 1866 and NGC 2164, and describe a new technique for automated selection of Cepheid variables using two-color photometry. We also emphasize the numerous advantages of high-precision radial velocities in the study of Magellanic Cloud variables.
CCD Photometry of Faint Cepheids
Authors: Henden Arne A.
Published: June 2019

The parameters and output products of a cepheid survey are presented.
First overtone pulsators among Cepheids
Authors: Mantegazza L. , Poretti E.
Published: June 2019

The Fourier decomposition has been successfully applied to several classes of pulsating variables. Antonello and Poretti (1986) and Antonello et al. (1990a) applied it to the Cepheids with P < 8 d. The latter authors redefined the s – Cepheids a Population I Cepheids that do not follow the Hertzsprung progression, but have progression of their own. The same authors proposed a new denomination (Antonell et al., 1990b): C-a stars to indicate the Classical Cepheids and C-b stars to indicate the redefined s–Cepheids.

The new photometric data obtained at La Silla and Merate Observatories (Mantegazza and Poretti, 1992) increase the evidence of a separation of Cepheids into two well defined subclasses on the basis of the Fourier parameters of their light curves.

In the ϕ21 – P plane, the s– and Classical Cepheids are characterized by two sequences well separated for P<5.5 d.
Harmonics and Coupling-terms in the Pulsation of the Double-mode Cepheid TU Cas
Authors: Szabados L.
Published: June 2019

Preliminary results of the period analysis of the double-mode Cepheid TU Cassiopeiae are given. Up to now 29 frequencies have been identified: the frequency of the fundamental mode pulsation and the first overtone, and their linear combinations. The amplitudes of the frequency constituents show temporal variations.
Spectroscopic Study of Cepheids in the Globular Cluster Omega Cen
Authors: Gonzalez Guillermo , Wallerstein George
Published: June 2019

Until now there have been few spectroscopic studies of cepheids in globular clusters. In this preliminary report we present sample spectra of the stars Vl and V29 in ω Cen. Eventually, we hope to use the abundance patterns, masses, and period changes of cepheids to better understand post-horizontal branch evolution in globular clusters.
Ultraviolet Studies of Cepheids
Authors: Böhm-Vitense Erika
Published: June 2019

We discuss whether with new evolutionary tracks we still have a problem fitting the Cepheids and their evolved companions on the appropriate evolutionary tracks. We find that with the Bertelli et al. tracks with convective overshoot by one pressure scale hight the problem is essentially removed though somewhat more mixing would give still better fits.

Using results of recent nonlinear hydrodynamic calculations by Morgan we find that we also have no problem matching the observed pulsation periods of the Cepheids with those expected from their new evolutionary masses, provided that Cepheids with periods less than 9 days are overtone pulsators.

We investigate possible mass loss of Cepheids from UV studies of the companion spectrum of S Mus and from the ultraviolet spectra of the long period Cepheid Carinae. For S Mus with a period of 9.6 days we derive an upper limit for the mass loss of M < 10-9 M⊙, if a standard velocity law is assumed for the wind. For Carinae with a period of 35.5 days we find a probable mass loss of M ∼ 10-5±2 M⊙.
The Stability of Cepheid Lightcurves
Authors: Fernie J.D.
Published: June 2019

Published lightcurves of 25 to 40 day classical Cepheids and of BL Herculis stars have been examined closely for evidence of alternating deep and shallow minima, as predicted by Moskalik and Buchler (1991) and Buchler and Moskalik (1992). No such effect has been detected. Two stars, SZ Mon and EN Tra, which are sometimes taken to be classical Cepheids, do show this RV Tauri-like effect, but neither star is, in fact, likely to be a classical Cepheid.
Strömgren Photoelectric Photometry of the Dwarf Cepheid Stars DY Peg and BP Peg
Authors: Peña J.H. , Peniche R. , Garrido R.
Published: June 2019

A study of short period high amplitude Dwarf Cepheid stars has been undertaken in order to discriminate between the two possible models, namely Bessel's (1969) proposal of low mass Pop. II or old Pop. I. The telecsope utilized was the 1.5 m at the SPM Observatory. The Danish spectrophotometer that allows the simultaneous acquisition of data in the uvby filters and almost simultaneously in the Hβ narrow and wide niters was attached. With this advantage of simultaneous observations, no phasing adjustment was needed, eliminating the risks of losing information due to amplitude varition explained either by multiple periodicity or by the Blazhko effect. We have restricted our discussion to the descending branch and light minimum phase interval between 0.175 to 0.725. For this interval the unreddened indices (b – y)0 and (cl)0 were calculated as in Nissen (1988), which will serve to determine the effective gravity and temperature variation of both stars, through the model atmsphere calibrations by Breger (1974), based on the ATLAS and Kurucz model atmospheres. Mean log g values of 4.0 and 4.33, and mean temperatures of 7500 and 7700 K were determined for DY Her and BP Peg, respectively. It can be concluded that both pulsating stars DY Her and BP Peg are Pop.I, normal Dwarf Cepheid stars.
The Color Dependence of Interstellar Polarization
Authors: Behr Alfred
Published: June 2019

Measurements of the Interstellar Polarization of Starlight at different wavelengths have been reported by several observers in references 1 to 8. With a few exceptions the wavelength range of the measurements is still relatively small, and up to now only a few dozens of stars have been measured with high precision. There is still a remarkable discrepancy between the results of different observers, as can be seen in figure 1. The scattering of the individual values is significantly larger than the probable error given by the respective authors. One of the most serious error sources seems to be the instrumental polarization which itself is highly color dependent. (See ref. 9.)

Nevertheless, even if a much greater individual error is assumed, a remarkable conclusion can be drawn: the wavelength dependence of polarization is not the same for all stars.
Dispersion of Interstellar Polarization
Authors: Gehrels Thomas
Published: June 2019

The Wavelength Dependence of Polarization as observed in 32 stars, for which the Henry Draper numbers are given, is shown in figure 1. Details of some of these observations are presented in reference 1.

The equipment is now being used with the new 154-cm Catalina reflector of the Lunar and Planetary Laboratory at the University of Arizona. The instrumental polarizations are nearly zero. The data processing and observing techniques have been further improved; the precision is mainly determined by statistics such that the internal probable error in the percentage polarization is ±0.03 percent (±0.0006 magnitude) for a half-hour observation per filter on objects brighter than about 7 magnitudes. The wavelength λ ranges from 0.33 to 0.95 μ covered by seven filters of bandwidth of about 0.05 μ. The wavelength range is being extended to 1.2, 1.6, and 2.2 μ and, with high-altitude ballooning, to 0.28 and 0.22 μ.
Accreted Molecules
Authors: Gehrels Thomas
Published: June 2019

In Some of the Plots of the Interstellar Polarization (for example, fig. 1 of first paper by Gehrels in the present compilation) a striking feature is noted in a discontinuity that frequently occurs near λ-1 = 1.3. The following explanation of the shape of the curve of interstellar polarization as a function of wavelength is proposed in terms of physical optics.

Far in the infrared, light passes the particle with relatively little interaction. At shorter wavelengths (red light), scattering by the molecules at the particle skin increases and consequently the amount of polarization rises. The rise with increasing values of λ-1 is steep (although not necessarily with λ-4 for polarization).

Prediction of the wavelength at which the rise becomes evident would necessitate a knowledge of the optical depth or scattering cross sections. These factors have not yet been evaluated. Qualitatively, the larger the particles, the larger the wavelength at which the polarization rise becomes manifest because of greater molecular optical depth near the skin.
Effects of Absorption Spectra of Ices on the Ultraviolet Extinction by Interstellar Grains
Authors: Field G. B. , Partridge R. B. , Sobel H.
Published: June 2019

In Spite of Many Years of Research on Interstellar Grains, their composition is still unknown. Evidently it can be found only through the interaction of the grains with radiation, and one is led to the spectroscopic study of the solids composing the grains. If discrete lines, bands, or other spectral features can be discovered, they may be of great importance in the chemical analysis of the grains. So far, data of this type include the diffuse interstellar lines, the optical lines of CH, CH +, and CN (almost certainly once part of the grains), the radio lines of OH, and the distinct “kink” or curvature in the optical extinction curve (see paper by Nandy in present compilation), which may reflect spectral variations in refractive index and hence indicate a specific material.

The advent of ultraviolet and infrared techniques suggests that consideration be given to new features to be expected in those regions.
Ultraviolet Photochemistry of Some Frozen Gases
Authors: Thompson Warren E.
Published: June 2019

Here Seems to be Agreement that laboratory studies are necessary in the pursuit of a better understanding of the interstellar medium. Herein is described some experiments performed on the effects of far ultraviolet radiation on molecules containing the astronomically abundant atoms H, O, C, and N. In particular, the attempts that have been made to prepare, isolate, and identify various free radicals and other reactive species, some of which may play a role in the processes occurring in the interstellar medium, are discussed.

In one of the studies, a frozen film of pure carbon dioxide at 77° K was irradiated with the light from a low pressure xenon discharge lamp equipped with a LiF window. This lamp is activated by radio-frequency radiation, and the emitted light is rich in the 1470-Å resonance line of xenon.
The Astronomical Potential of Spatial Interferometry in the Infrared
Authors: Low Frank J
Published: June 2019

The angular diameters ad shapes of bright infrared stars are now being measured from 2 micron to 20 microns using several newly developed techniques of high angular resolution interferometry. These objects possess circumstellar shells emitting in the temperature range 300-600K, thus the brightest of their number have sizes ranging from 0 1 to 10 arcseconds, and are readily resolved by instruments a few meters across.

The size, shape, and structure of several typesof stars will be discussed in detail as the Colloquium proceeds. It will be seen that this information can be used to help characterize the optical properties of the particulate material contained in the circumstellar envelopes. Conclusions concerning the formation and evolution of these cold outer regions of stars may also be possible It appears that the conditions in some of these circumstellar envelopes may be suitable for the formation of planets, a process which may be studied directly as these new techniques are further developed.

Although sensitivities of present instruments are still somewhat marginal, it is now possible to resolve the brightest of the extragalactic nuclei at 10 microns Future instrumental developments should bring the size and structure of many extragalactic nuclei within reach at several wavelengths, thus adding important information on the nature of these extraordinary sources of infrared emission
Amplitude Interferometry at C.E.R.G.A.
Authors: Koechlin Laurent
Published: June 2019

The 12 to 20 metre baseline interferometer at C.E.R.G.A. built by A. Labeyrie has now been operational for two years. The purpose of this instrument was initially to be a prototype showing the feasibility of long baseline coherent arrays and for testing the techniques needed for them. Yet, pending completion of large interferometers, this small 2 x 26 cm instrument has proven able to yield astrophysical data: a dozen stars up to third magnitude have been observed with a 3 x 10-3 arc second resolution on diameters and a 5 x 10-4 arc second accuracy for binary separation measurements in the case of α Aur.

What has been observed both from the technological and the astrophysical point of view will be discussed.
Interferometric Measurements of Flattend Circumstellar Envelopes
Authors: McCarthy D. W.
Published: June 2019

An incoherent spatial interferometer has been used to measure the shapes of circumsteliar dust envelopes at infrared wavelengths. The objects IRC +10216, VY CMa, and NML Cyg exhibit a large variation of fringe visibility with position angle Their circumstellar envelopes are probably flattened disks. For IRC +10216, visibility measurements from 5.0 to 20µm are discussed in detail to illustrate the physical information available from interferometric work. In VY CMa and NML Cyg, disk orientation is strongly correlated with polarimetric position angles and with the distribution of OH maser sources. Interferometric measurements are shown to be relevant to studies of star/planet formation and to studies of mass loss in long-period variables
Status and Needs of Astrometry
Authors: Hughes James A.
Published: June 2019

A review will be presented of the status of the fundamental reference system. Mention will be made of the problems limiting the accuracy of transit circles and photographic astrometry and attempts at possible improvments in the near future. The progress in the preparation of the FK5 will be briefly mentioned, as will be the various methods used to approximate an inertial reference frame. Needs for higher accuracy in Astronomy fall in the area of better modeling of the Earth's motion (rotation, nutation, precession, the ecliptic, etc.) as well as a parallax and proper motion determination for galactic structure studies, stellar dynamics and stellar evolution. It will be assumed that interferometric binary star studies will be discussed elsewhere.
On the Quenching of Single Excitation Channels in the Inner Shell Spectra of Certain Elements
Authors: Connerade J. P.
Published: June 2019

The collaboration between the University of Bonn and Imperial College on the use of synchrotron radiation for the observation of atomic absorption spectra has yielded much new data since IAU Colloquium No.27 (see also E. Radtke's paper in the present meeting). Among recently observed spectra are those of Tl I, In I, Ga I, Pb I, Sn I and Ge I, while earlier data on Zn I and Cd I have been extended to include double excitations. The first evidence of strong mixing between the d-np channel and double excitations at the same energy has come for Tl I where some of the expected d-np series are absent and, instead, other additional series appear.
Wavelength Calibration of Balloon-Borne Ultraviolet Stellar Spectrograph
Authors: Hoekstra R.
Published: June 2019

The procedure will be described that is developed for assigning wavelengths to the UV stellar spectra obtained in 1976 by means of a balloon-borne instrument consisting of telescope with starpointing system, echelle spectrograph and SEC vidicon. The instrument covers the spectral region of 2000 Å to 3400 Å with a wavelength resolution of 0.1Å. The wavelength assignment aims at an accuracy of 0.01 Å. Results will be presented and difficulties encountered, related with the use of echelle spectrograph and SEC vidicon detector, will be analysed.
Photoabsorption to Even-Parity Autoionizing Levels of Na I from the Laser-Populated 3P2p States
Authors: Sugar J. , McIlrath T.J. , Lucatorto T.B. , Weiss A.W.
Published: June 2019

A column of sodium vapor with a significant fraction of the atoms in the 3p2p state (excited state density of the order of 1015 atoms/cm3 ) was prepared by axially irradiating a sodium heat-pipe oven with a pulsed dye laser tuned to the 3s-3p resonant frequency. This technique has recently been shown to produce nearly complete ionization of the illuminated column in ∼ 600 nsec. By passing continuum VUV light down the pipe 150 nsec after initiating the laser pulse, an absorption spectrum of neutral excited state sodium is obtained in the region of autoionizing levels containing the 2p53s3p and 2p53d3p configurations. The continuum is produced by a BRV-type triggered spark that gives a light pulse of ∼ 100 nsec duration in the wavelength range of observation of ∼ 200-400 A. About 25 absorption lines are attributed to these configurations based on predicted wavelengths obtained from an intermediate coupling calculation of the autoionizing levels, including configuration interaction. This appears to be the first observation of these even parity levels, which are inaccessible to photabsorption from the ground state by electric dipole transitions.
Remarks on Orbits and Dynamical Parallaxes
Authors: Dommanget J.
Published: June 2019

In connection with statements made at the preceding session, some results presented at the Nice Conference on systematic and random errors affecting the elements a and P as well as a3/P2 may be recalled. If many orbits have been calculated for one binary, it is instructive to plot log P, log a, and log a3/P2 versus log Δt/P where Δt is the time interval covered by observations. As a general rule, when Δt ≃ P the systematic trends vanish, and the scatter of the plotted elements reaches a low value, not considerably changing thereafter. It may well be of interest, then, to have several orbits based on the same observations but computed by different authors and methods.
Les Etoiles Doubles Faibles
Authors: Couteau P.
Published: June 2019

La notion d'étoiles faibles concerne deux catégories d'objets: une étoile peut être de faible éclat parce qu'elle est loin, ou bien parce qu'elle est de faible luminosité. D'ailleurs ces deux facteurs se combinent entr'eux, selon les distances au soleil, de sorte que beaucoup d'étoiles géantes sont des étoiles faibles.

Il est évident qu'une étoile double faible n'est intéressante que dans la mesure où son mouvement est appréciable, ce qui élimine les géantes. Notre rôle est done de dissocier parmi les étoiles faibles, celles qui le sont intrinsèquement.

Avant d'entamer le vif du sujet, il est bon de rappeler que si nous étudions les étoiles doubles depuis trois siècles, ce n'est que depuis trente ans que nous avons réellement des connaissances sur les étoiles à faible luminosité. Mais faut-il encore définir ce que nous entendons par là.
Observational Procedures for Visual Double-Star Work
Authors: Franz Otto G.
Published: June 2019

When IAU Symposium No. 17, the most recent international conference on visual double stars to take place in the United States, was held in Berkeley in 1961, there was little discussion on observational techniques and no introductory remarks were presented on the subject. The reasons for this are obvious. The classical techniques of double-star observation, those that provided virtually all the data available on visual double stars, were too well known to require introduction or discussions; other methods, with the possible exception of early image tube and electronic camera experiments, did not exist.

Today, more than ten years later, most double-star work continues to be carried out by the same classical techniques. However, several methods have come into use or have recently been developed that are capable of contributing importantly to visual double-star research.
The N IV λ5820 Multiplet in WN Stars
Authors: Nussbaumer H.
Published: June 2019

Evaluating transition probabilities of N IV 2p3d 3 P ° to 2s4s 3S and 2p3p 3P and an emission feature at λ7410 it is shown that a disputed emission at λ5810 in WN stars may not be attributed to N IV.
Line Formation in Moving Atmospheres
Authors: Kalkofen W.
Published: June 2019

We discuss an integral equation method that permits the calculation of the line source functions and of the emergent profiles in finite and semiinfinite atmospheres with macroscopic motion normal to the surface. Solutions are presented for a semiinfinite atmosphere with a temperature rise in the outward direction and with a flow that decays with increasing depth. The computed profiles have the form of P Cygni lines.
Coherent Line Formation with Depth-Dependent Parameters
Authors: Carson T. R.
Published: June 2019

A discrete ordinate method is developed for solving the equation of coherent line formation, for arbitrary given variations with depth in an atmosphere of the temperature (or Planck function, assuming local thermodynamic equilibrium) and the line absorption and scattering coefficients. The direct solution thus obtained can then be used as the starting point of an iterative procedure. Results obtained for an exactly soluble case indicate the utility of the method.
Circumstellar CA II K Lines in G, K and M Giants and Supergiants
Authors: Vaughan A. H. , Skumanich A.
Published: June 2019

Tentative evidence, based on photoelectric observations of the Ca+ emission core, is presented for the existence of circumstellar envelopes in several G and K type “giants.” A significant emission asymmetry in α Tau may imply a chromospheric rather than a circumstellar source.
A Physical Mechanism for the Generation of Extended Stellar Atmospheres
Authors: Hillendahl R. W.
Published: June 2019

A physical mechanism that can result in the generation of extended expanding atmospheres is discussed. The process involves the unloading of stellar material following the arrival of a shock wave at the edge of the star. The basic principles are developed from a discussion of a simplified case that has been studied in the laboratory; they are then applied to the atmosphere of a star. A radiation-hydrodynamics computation of a model cepheid is then used to obtain quantitative atmospheric profiles. The computed continuum and spectral lines during the unloading process are then examined. A discussion of the possibility that the unloading process occurs in stars other than cepheids suggests the existence of a shock visibility factor associated with ionization or dissociation in the region behind the shock front and leads to a possible alternate interpretation of the variable star instability strips in the H-R diagram.
The Global X-Ray Burst Behaviour of KS 1731-260
Authors: Cornelisse R.
Published: June 2019

An overview is given of the type I X-ray burst properties of the transient KS 1731-260. These characteristics appear to be proto-typical for all type I X-ray bursters, and stable helium burning occuring at high mass accretion rates could explain them.
Disentangling X-Ray Reprocessing Sites in EXO 0748-676
Authors: Hynes R. I. , Haswell C. A. , Robinson E. L. , Horne K. , O'Brien K. , Charles P. A. , King A. R. , Rolfe D. J.
Published: June 2019

We present first results from a coordinated multiwavelength study of the neutron star low-mass X-ray binary EXO 0748 676. Fast UV, X-ray, and optical data were obtained including both spectral and timing information. We discuss how this study allows us to probe the temperature distribution within the binary and hence the geometry and efficiency of X-ray irradiation.
The Optical Counterpart of SAX J1808.4-3658 in Quiescence: Evidence of an Active Radio Pulsar?
Authors: Burderi L. , Di Salvo T. , D'Antona F. , Robba N. R. , Testa V.
Published: June 2019

The optical counterpart of the binary millisecond X-ray pulsar SAX J 1808.4-3658 during quiescence was detected at V = 21.5 mag, inconsistent with intrinsic emission from the fain companion star. We propose that the optical emission from this system during quiescence is due to the irradiation of the companion star and a remnant accretion disk by the rotational energy released by the fast spinning neutron star, switched on, as magneto-dipole rotator (radio pulsar). In this scenario the companion behaves as a bolometer, reprocessing in optical part of the power emitted by the pulsar. The reprocessed fraction depends only on known binary parameters. Thus the blackbody temperature of the companion can be predicted and compared with the observations. Our computations indicate that the observed optical magnitudes are fully consistent with this hypothesis. In this case the observed optical luminosity may be the first evidence that a radio pulsar is active in this system in quiescence.
The η Carinae Like Fell-Rich Spectrum of FBS0022-021
Authors: Zharikov S. , Tovmassian G. , Costero R.
Published: June 2019

We report results of Echelle and B&Ch spectroscopic observations of the emission-line object FBS 0022-021 that were obtained at SPM Observatory, Mexico. We analyze the emission-line spectra along with available optical and IR data. The Fell-rich spectrum of the object closely resembles that of the peculiar binary system η car. The discussion of the physical conditions leading to such spectrum and the nature of this peculiar object are presented.
Compact Binaries in Globular Clusters
Authors: Ivanova Natalia , Rasio Frederic A.
Published: June 2019

In dense stellar systems the frequent dynamical interactions between stars play a crucial role in the formation and evolution of compact binaries. We study these processes using a novel approach combining a state-of-the- art binary population synthesis code with a simple treatment of dynamical interactions in dense star cluster cores. Here we focus on the dynamical and evolutionary processes leading to the formation of compact binaries containing white dwarfs in dense globular clusters. We demonstrate that dynamics can increase by factors ~ 2 – 100 the production rates of interesting binaries such as cataclysmic variables, “nonflickerers” (He white dwarfs with a heavier dark companion), merging white dwarf binaries with total masses above the Chandrasekhar limit, and white dwarf binaries emitting gravitational waves in the LISA band.
Close Binary White Dwarfs and Supernovae la
Authors: Napiwotzki R. , Karl C. , Nelemans G. , Yungelson L. , Christlieb N. , Drechsel H. , Heber U. , Homeier D. , Leibundgut B. , Koester D. , Marsh T. R. , Moehler S. , Pauli E.-M. , Reimers D. , Renzini A.
Published: June 2019

We report on the current status of radial velocity surveys for white dwarf binaries (double degenerates DDs) including SPY (ESO Supernovae la progenitor survey) recently carried out at the VLT. A large sample of DD will allow us to put strong constraints on the phases of close binary evolution of the progenitor systems and to perform an observational test of the DD scenario for Supernovae of type Ia.
Long-Term Evolution of (Short) Gamma-Ray Burst Central Engines
Authors: Lee William H.
Published: June 2019

Cosmological gamma ray bursts (GRBs) possibly originate from accretion disks around stellar mass black holes. These could be formed after the merger of a double neutron star or black hole-neutron star binary. The dynamical evolution of the disk is important if one wishes to relate characteristic timescales with the observed duration and variability. We show here the results of such a set of calculations, relevant for short GRBs.
Warped Accretion Disks and Longterm X-Ray Periodicities
Authors: Clarkson W. I. , Charles P. A. , Laycock S. , Coe M. J. , Wilson-Hodge C. , Tout M. D.
Published: June 2019

A significant number of X-ray binaries are now known to exhibit long-term periodicities on timescales of ~10 - 100 days. Several physical mechanisms have been proposed that give rise to such periodicities, one of which is radiation-driven warping and precession of the accretion disk. Recent theoretical work predicts the stability to disk warping as a, function of the mass ratio, binary radius, viscosity and accretion efficiency. We investigate the stability of the superorbital periodicities in the neutron star X-ray binaries Cyg X-2, LMC X-4, SMC X-l and Her X-l, and thereby confront stability predictions with observation. We find that the period and nature of the superorbital variations in these sources is consistent with the predictions of warping theory.
XMM Observations of Polars from the SDSS
Authors: Szkody Paula , Homer Lee , Chen Bing , Henden Arne , Schmidt Gary
Published: June 2019

We condueted XMM observations of 5 faint Polars discovered by the Sloan Digital Sky Survey. The X-ray fluxes and spectra show a variety of shock regimes. The lowest accretor has X-ray properties consistent with an origin on the secondary star. The light curves of all systems show large variability, with one showing a deep eclipse, and with some evidence for orbital-related variations in 2 others.
Replaying the 2001 Outburst of WZ SGE
Authors: Steeghs D.
Published: June 2019

I present the key results of extensive optical spectroscopy obtained during the first three weeks of the 2001 super-outburst of WZ Sge. A sequence of Doppler maps of the accretion flow in various lines was constructed, and used to create real-time movies of WZ Sge as it evolved through the outburst. Main results include the detection of two-armed spiral asymmetries in the disk, emission from the irradiated donor star and an extended stream of emission around day 19 of the outburst that bears the hallmarks of a mass-transfer burst event.
On the Classification of Magnetic Cataclysmic Variables
Authors: Mason Paul A.
Published: June 2019

The classification scheme for Magnetic Cataclysmic Variables (MCVs) is addressed. When only ten or twenty MCVs had been discovered their classification was simple and based on direct observational properties. Now that the number of MCVs exceeds one hundred, the method of classification needs updating. One important consideration is the possibility that binaries might physically change their classification. For example will intermediate polars become polars as they evolve to shorter orbital periods? Or does a polar become an intermediate polar when synchronism is broken due to a nova?
V445 Puppis, A First Identified Helium Nova
Authors: Kato Mariko , Hachisu Izumi
Published: June 2019

The 2001 outburst, of V445 Puppis is an unique object, that shows absence of hydrogen, enrichment of helium and carbon, slow decay of the light curve with a small amplitude. We have calculated theoretical light curves of helium novae to confirm that V445 Pup is the first example of helium nova. From the light curve fitting we conclude that the white dwarf is very massive ≥ 1.33M⊙.
Discovery of the 2001 Outburst of WZ Sagittae
Authors: Ohshima T.
Published: June 2019

I found WZ Sagittae at mvis = 9.7 in the rising phase of the historical outburst on 2001 July 23. This article vividly reports the very exciting situation at the discovery.

WZ Sge is one of the most famous cataclysmic variables (CVs), and forms the WZ Sge sub-class with several other enigmatic SU UMa-type dwarf novae. The unusual characteristics of the WZ Sge stars are rare superoutbursts, large amplitudes, and no (or only few) normal outbursts. As to WZ Sge itself, the outburst cycle is over 10000 days in average. The outburst amplitude is 8 magnitude. Its magnitude is around 15th in quiescence.
The Radio X-Ray Correlation in Cygnus X-3 and Other Galactic Microquasars
Authors: Choudhury M. , Rao A. R.
Published: June 2019

Cygnus X-3, a Galactic X-ray binary, shows the presence of outflows in the form of radio jets. The SED in the X-ray band shows a complicated structure and evolution. We review some recent results of the long-term correlation of the radio and X-ray emissions, chiefly in the low (hard) states of the source. Comparing the results with those of other Galactic microquasars, we attempt to provide a consistent picture of the accretion — ejection mechanism in these sources.
Photometric Periodicities in Magellanic Cloud Be/Neutron Star Binaries
Authors: Schmidtke P. C. , Cowley A. P. , Levenson Lance , Sweet Katie
Published: June 2019

We are investigating the photometric properties of Be/neutron star systems in the Magellanic Clouds using V, R, and I data from the MACHO and OGLE-II surveys. Here we give results for 6 systems, 4 of which are X-ray pulsars.

The long-term light curves for each of these systems were searched for periodic variations. We first pre-whitened the data by fitting an appropriate polynomial to “flatten” lengthy portions of the light curves. We then used the method described by Home & Baliunas (1986) to find periodicities in the relative magnitudes.
Long-Term Variability in the Quiescent X-Ray Transient V404 CYG
Authors: Zurita C. , Casares J. , Charles P. A. , Hynes R. I. , Pavlenko E. P. , Shahbaz T.
Published: June 2019

We present the results of optical and infrared photometry of the quiescent X-ray transient V404 Cyg during the period 1992-2003.

It is largely known that the Soft X-ray Transient (SXT) V404 Cyg exhibits short-term (sub-orbital) variations during quiescence (see eg. Pavlenko et al. 1996). It seems that fast variability during quiescence is not a peculiarity of V404 Cyg but a fingerprint of quiescent SXTs, irrespective of the nature of the compact object (Zurita, Casares & Shahbaz, 2003). Although these variations seem to originate in the accretion disc, the underlaying physical mechanism is not yet understood. Since V404 Cyg is the brightest SXT, and hence accessible with 1-m type telescopes, we embarked on a study of the fast variability over the period 1992-2003.
New Clues on the Model of YY Her After Two Seasons
Authors: Hric L. , Gális R. , Dobrotka A. , Śimon V. , Petrík K. , Niarchos P. , Velić Z. , Gazeas K. , Šmelcer L. , Hájek P. , Sobotka P. , Koss K. , Brát L. , Lomoz F.
Published: June 2019

During two observational seasons of international campaign of YY Her we secured photometric data which covered the primary as well as the secondary minima and revealed unexpected outburst activity of this system.

YY Her belongs to the classical symbiotic binaries with nova-like outbursts. Recently Mikolajewska et al. (2002) explained the light variability of YY Her by combining the ellipsoidal changes and sinusoidal variations of the nebular continuum and line emission. In 2001 (Hric et al. 2001) we discovered in the binary system YY Her the presence of the secondary minimum that was indicated, however, by only three photometric observations from one observatory.
Dramatic Spectral Evolution of WZ Sge during the 2001 Superoutburst
Authors: Nogami D. , Iijima T.
Published: June 2019

We spectroscopically observed WZ SGE in 11 nights during the 2001 superoutburst. The feature of the spectra dramatically varied from at the outburst maximum through during the rebrightening phase.

WZ SGE is the prototypical star of WZ SGE-type stars, a small class of SU UMa-type dwarf novae. WZ SGE stars have unusual characteristics of quite large outburst amplitudes up to ~8 mag, extraordinary long recurrence cycles of the outbursts (≥10 years), no (or only a few) normal outbursts.
Understanding CV Secondaries and the Energetics of Dwarf Novae Outbursts Via Parallaxes
Authors: Johnson Joni J. , Harrison Thomas E. , Osborne Heather L.
Published: June 2019

We combine HST and ground-based parallaxes for cataclysmic variables (CVs) to examine their outburst energetics, as well as the nature of their secondary stars. Harrison et al. (2004) have recently published parallaxes for WZ Sge, YZ Cnc, and RU Peg, and have reanalyzed the HST parallaxes for U Gem, SS Aur, and SS Cyg. Combined with existing HST parallaxes for RW Tri (MeArthur et al. 1999), TV Col (MeArthur et al. 2001), EX Hya and V1223 Sgr (Beuermann et al. 2004a,b), ten high-quality parallaxes are available to constrain the various types of outbursts of CVs. In addition, Thorstensen (2003) has published ground-based parallaxes for fourteen CV systems.
Peculiar Abundances in CV Secondary Stars: Revising their Evolutionary History?
Authors: Harrison Thomas E. , Howell Steve B. , Johnson Joni J. , Osborne Heather L. , Homeier Derek
Published: June 2019

We have recently completed an infrared (K-band) spectroscopic survey of two dozen cataclysmic variables. The systems range in orbital period from Рorb = 1-35 hr (EF Eri), to Рorb = 11.0 hr (V442 Cen). We detect the secondary star in every system.

Nearly all of the CVs in which we can clearly see the secondary have weak CO features. Most show evidence for enhanced levels of 13CO. In addition, we see apparent enhancements and/or deficits of such species as sodium, calcium, titanium, magnesium, silicon, and iron. In addition, we find a few lines that we have been unable to identify (such as the strong feature we call “cataclysmicum” at 2.2767 μm seen in the spectrum of EF Eri, and two other very short period CVs).
Constraints on Stellar Evolution from Observations of Compact Object Binaries
Authors: Bulik T. , Moderski R. , Belczyński K.
Published: June 2019

The masses of compact object (black hole, neutron star) binaries depend strongly on the parameters describing stellar evolution. Such masses or their functions can be measured using gravitational waves or through microlensing searches. We analyze an example of the varying common envelope efficiency and show the dependence of distributions of the measured chirp masses in gravitational waves mass ratios through microlensing taking into account the relevant selection effects.
Time-Resolved Spectroscopy and Doppler Tomography of UX UMa
Authors: Suleimanov V. F. , Neustroev V. V. , Borisov N. V. , Fioktistova I. S.
Published: June 2019

Medium resolution spectroscopy of nova-like cataclysmic variable UX UMA was performed using the 6-m telescope SAO RAS in April 1999. Obtained spectra cover the total orbital period including eclipse phases and allow us to reproduce the radial velocity curve. The radial-velocity variations of the Hβ emission line are found to have semi-amplitude of about 100 km/s.
HS 2237+8154: A New Pre-CV just above the Period Gap
Authors: Gänsicke B. T. , Araujo-Betancor S. , Hagen H.-J. , Harlaftis E. T. , Kitsionas S. , Dreizler S. , Engels D.
Published: June 2019

We report follow-up observations of the new pre-cataclysmic variable HS 2237+8154, identified from the Hamburg Quasar Survey. The orbital period derived from ellipsoidal modulation observed in the iR-band as well as from the Hα radial velocity variation is orb = 178.10 ± 0.08 min. We briefly discuss the evolutionary state of this system.
Analysis of Ulysses Dust Measurements within the Asteroid Belt
Authors: Mann I. , Wilck M. , Grün E.
Published: June 2019

Results of Ulysses impact measurements are compared to Zodiacal cloud models. We discuss the influence of radiation pressure forces on particles of different size and composition and show its relevance for the understanding of impact measurements, and give an upper estimate for the dust density in an IRAS dust band.