bluuebunny/crossref_metadata_2025_split huggingface dataset:
Simultaneous Optical/Gamma-ray Observations of GRBs
Authors: Greiner J. , Wensel W. , Hudec R. , Varady M. , Štěpán P. , Spruný P. , Florián J. , Moskalenko E.I. , Barabanov A.V. , Ziener R. , Birkle K. , Bade N. , Tritton S.B. , Ichikawa T. , Fishman G.J. , Kouveliotou C. , Meegan C.A. , Paciesas W.S. , Wilson R.B. 
Published: July 2016

AbstractThis status report presents details on the project to search for serendipitous time-correlated optical photographic observations of γ-ray bursters. The ongoing photographic observations at nine observatories are used to look for plates which have been exposed simultaneously with a γ-ray burst detected by BATSE and contain the burst position. The results for the third year of BATSE operation are presented.
Time Variability Studies with ROSAT
Authors: Kürster Martin 
Published: July 2016

AbstractAn overview over the mission of the X-ray astronomy satellite ROSAT (Röntgensatellit) is given with an emphasis on the first soft X-ray all-sky survey carried out with an imaging telescope. Several examples of time variability studies made possible by the all-sky survey are shown for active late-type stars. Two exceptional long-duration stellar flares are presented.
After Dinner Address
Authors: Richter G.A. 
Published: July 2016

It was exactly 22 days before Cuno Hoffmeister’s 65th birthday when I began my work at Sonneberg Observatory, following my studies at the University of Jena. In those days, all Sonneberg astronomers were busy preparing scientific papers which they were going to dedicate to their leader, with the exception of myself. I was not able to write a scientific paper in such a short time.And I was quite ashamed when on the 2nd of February 1957 I had to offer my birthday congratulations empty-handed. But I distinctly remember how extremely happy he looked. Hoffmeister was near the zenith of his scientific career. New buildings and domes were planned or under construction, and the Zeiss company of Jena had promised to loan a 40 cm astrograph in compensation for the loss of a telescope to the Soviet Union as part of the reparations agreement of 1945.
Post-Eruptive Flare Energy Release as Detected on AU Mic by EUVE
Authors: Katsova M.M. , Drake J.J. , Livshits M.A. 
Published: July 2016

The long-duration emission arising after the impulsive rise and decay in a flaring event observed by the Extreme Ultraviolet Explorer on the red dwarf star AU Mic is discussed. The decay of the intensity in the Deep Survey 65–190 Å band and in the Fe XVIII line during this prolonged event is 10 times slower than the time of radiative cooling of coronal loops with the typical for the flare plasma density. The temporal behavior of the emission measure is determined for both the 65–190 Å band and the Fe XVIII line fluxes. The total energy emitted in the 1–2000 Å region over nearly 12 hrs is 3 · 1035 ergs. We first point out some difficulties with earlier explanations proposed for this event; we then propose the following physical model: the source of the prolonged emission is a system of high coronal loops, the size of which is more than the active region scale, but less than the stellar radius. Such systems are observed in soft X-rays during large solar flares after coronal mass ejections. Some additional post-flare energy input into this high coronal loop system can be caused by reconnection in a vertical current sheet, and this post-eruptive energy release provides prolonged and intensive EUV emission.Apparently, we are faced here with new kind of the surface activity on late-type stars which is intermediate between impulsive flares on red dwarfs and long-duration, powerful events the subgiants components of the RS CVn binaries.
ALEXIS Observations of the Diffuse Cosmic Background in the Extreme Ultraviolet
Authors: Smith Barham W. , Pfafman T.E. , Bloch J.J. , Edwards B.C. 
Published: July 2016

Abstract.We present preliminary results of ALEXIS satellite observations of the extreme ultraviolet diffuse sky. ALEXIS was designed to be a half-sky monitor in three narrow wavelength bands between 13.0 and 19.0 nm. In our band centered at 172 Å we find a clear signal from the diffuse sky that is about 20 counts per second above the signal when looking at the dark earth from the same part of an orbit. This difference corresponds to an upper limit on the true cosmic diffuse background signal in this narrow band. When estimates of the geocoronal contributions, both in and out of band, are removed, our upper limit is reduced.
The Arcetri Spectral Code for Optically-Thin Plasmas
Authors: Fossi Brunella C. Monsignori , Landini Massimo 
Published: July 2016

The Arcetri-95 spectral code for optically thin plasmas computes the continuum and line emission of the ions of the most abundant elements. It includes the most updated atomic models and the main atomic processes for ions from Fe IX to Fe XXIII and for ions of the Belike, C-like, and N-like isoelectronic sequences. The power emitted per unit emission measure is produced as a function of temperature and density.Comparison with observations requires the knowledge of the differential emission measure (DEM) as a function of temperature. A numerical code evaluates the best DEM distribution that satisfies observations and theoretical predictions. The spectral code together with the DEM code allows to compute synthetic spectra for any specified temperature distribution model of the plasma.
Probing the Extraordinary Ends of Ordinary Stars: White Dwarf Seismology with the Whole Earth Telescope
Authors: Kawaler Steven D. 
Published: July 2016

AbstractDuring the final evolution of most stars, they shed their outer skin and expose their core of the hot ashes of nuclear burning. As these hot and very dense cores cool into white dwarf stars, they go through episodes of multiperiodic, nonradial g-mode pulsation. The tools of stellar seismology allow us to use the pulsation spectra as powerful probes into the deep interiors of these stars. Progress in white dwarf seismology has required significant international cooperation, since another consequence of the complex pulsations of these stars is decoding the true pulsation frequencies requires a coordinated global effort involving high-speed photometric observations. Through one such effort, the Whole Earth Telescope project, we have located subsurface composition changes, detected differential rotation and magnetic fields, and measured fundamental quantities such as stellar mass, luminosity, and distance to extraordinary accuracy.
More on Solar-like Oscillations in η Boo
Authors: Bedding Timothy R. , Kjeldsen Hans 
Published: July 2016

AbstractWe recently reported strong evidence for solar-like oscillations in the G0 IV star η Boo (Kjeldsen et al. 1995, AJ 109, 1313). We measured small temperature fluctuations produced by oscillations through their effect on the equivalent widths of the Balmer lines. Here we address several issues that were raised at this conference.
Quadratic and Cubic Couplings of Oscillation Modes of Stars
Authors: Van Hoolst T. 
Published: July 2016

The strength of nonlinear interactions of oscillation modes of stars is determined by the amplitudes as well as by the eigenfunctions of the oscillation modes. The intrinsic couplings of modes through their eigenfunctions can be described by coupling coefficients. Here, we concentrate on quadratic and cubic coupling coefficients that describe the nonlinear coupling of modes with itself and are called self-coupling coefficients.We considered radial and nonradial oscillation modes of polytropic models with degrees of central condensation that correspond to central condensations of main sequence stars to highly condensed evolved stars. We study the influence of the radial order and the degree of the oscillation mode on the self- coupling coefficients.
Uncertainties in the Position of the β Cephei Instability Strip in the HR Diagram
Authors: Pamyatnykh A.A. , Dziembowski W.A. , Mikołaj P. 
Published: July 2016

AbstractWe discuss the sensitivity of the theoretical B star instability domains to the heavy element abundance Z, the adopted metal mixture, the assumed overshooting from stellar convective cores and the choice of the opacity data.
A Search for Variable B Stars in the Northern Open Clusters
Authors: Pigulski Andrzej , Jerzykiewicz Mikolaj , Kopacki Grzegorz 
Published: July 2016

AbstractIn April 1994 we started a program of searching for variable B-type stars in young open clusters. The equipment consisted of a 60-cm reflecting telescope and a CCD camera with a Johnson’s B filter. Up to date we obtained nearly 1500 CCD frames of the central regions of four clusters: IC4996, NGC7235, NGC7510, and NGC654. In the range from B 0 to B 5, the photometric accuracy we achieve allows detection of short-period variability with amplitudes of about 5 mmag or greater. In this note we present the short-period variable stars we found in NGC 7235 and NGC 7510.
Eta Orionis
Authors: De Mey K. , Aerts C. , Van Winckel H. , Waelkens C. 
Published: July 2016

Eta Orionis is an interesting multiple system of early type stars. It consists of a visual triple system (A, B, C), a spectroscopic triple system (Aabc) and an eclipsing binary Aab. A is a triple system consisting of a 7.989 day eclipsing binary Aab and a gravitationally bound third component Ac with orbital period of about 9.5 years. All components of the pair AB have spectral types in the range 09-B3 (C. Waelkens & P. Lampens 1988). The radial velocity curve is used to constrain the orbital elements and properties of the binary Aab.The second component Ab is a B-type star that displays rapid absorption line profile variability. Regular patterns are easily visible in the data.
Radial pulsation among δ Sct stars
Authors: Rodríguez E. , Rolland A. , Coca P. López de , Martín S. 
Published: July 2016

AbstractIt has been pointed out by earlier authors that radial pulsation seems to be the only one type present in high amplitude δ Set stars, at least for monoperiodic stars. In order to confirm or not this point, we have collected all the multicolour data available for these stars in the Strömgren and Johnson photometric systems. Then, the type of pulsation has been analysed on the basis of the phase shifts and amplitude ratios between observed light and colour variations. The results indicate that all the stars analysed are radial pulsators.
New WET Observations of the δ Scuti Star CD-24 7599: amplitude variability and discovery of 13 pulsation modes
Authors: Handler G. , O’Donoghue D. , Buckley D. A. H. , Giovannini O. , Kepler S. O. , Provencal J. L. , Wood M. A. , Clemens J. C. , O’Brien M. S. , Kleinman S. J. , Watson T. K. , Nather R. E. , Winget D. E. , Kanaan A. , Nitta A. , Montgomery M. H. , Klumpe E. , Bradley P. A. , Sullivan D. J. , Wu K. , Leibowitz E. 
Published: July 2016

The variability of CD-24 7599 (V=11.48 mag) was discovered by JCC during observing run XCOV7 of the Whole Earth Telescope (WET, Nather et al. 1990) network in February, 1992. The star was observed as an additional target and 117 hours of high-quality temporal spectroscopic observations were obtained.Our analysis of these data revealed the presence of 7 independent pulsation modes between 27.0 and 38.1 cycles per day (313 – 441 μHz) with semiamplitudes of 2.1 – 10.2 milli-modulation amplitudes (mma). We showed that peaks at linear combination frequencies detected in the power spectra were not due to eigenmodes excited to visible amplitude by resonant mode coupling.
The Chemical Composition of S-Cepheids and Double-Mode Cepheids
Authors: Usenko I.A. , Kovtjukh V.V. , Andrievsky S.M. , Klochkova V.G. , Galazutdinov G.A. 
Published: July 2016

The main aim of this work is to consider the evolutionary status of s-Cepheids (SC), - small-amplitude Cepheids with smooth sinusoidal light, colour and radial velocity curves. The unusual character of these objects could be explained in two ways: (1) SCs are crossing the Cepheid instability strip for the first time and they are first (or high) overtone pulsators (Efremov 1970); (2) SCs have close early-type companions, which affect gravitationally the Cepheid He II zone. Such an effect can decrease the Cepheid pulsational activity (Lloyd Evans, 1968).According to theoretical calculations, low helium abundance in Cepheid atmospheres, which are crossing the instability strip for the first time, could produce small-amplitude oscillations. On the other hand, double-mode Cepheids (DMC) show oscillations with two (or three) periods: fundamental with large-amplitude asymmetrical light, colour and radial velocity curves, and first overtone with smooth sinusoidal small-amplitude ones. Nevertheless, detailed spectral and photometric investigations permit the detection of companions for the brightest SCs, and support the second hypothesis.
Type II? Cepheid Radii and TX Del
Authors: Laney C.D. 
Published: July 2016

There is often some dispute in the literature as to whether a particular Cepheid (S Vul, for example) belongs to Type I or Type II. JHK photometry has been used to calculate Baade-Wesselink radii for a number of stars always, sometimes or occasionally stated to be Type II Cepheids. Radial velocities have been taken from the literature.The (K, J – K) radius of SW Tau is 8.30 ± 0.34. This is about half of the expected radius for a Type I Cepheid at this period and normal for Type II.For V553 Cen, the relative phasing of radial velocities and infrared photometry has to be done by choosing a relative phasing where the phase shift between the radius displacement curve integrated from the radial velocities and the photometric radius displacements is minimized, and where the surface brightness coefficient A (Balona 1977) is normal for a star with the colour indices of V553 Cen. Uncertainty in the reddening has a negligible effect on the radius solution obtained in this way. I have also considered the degree to which the K, J – K and K, V – K solutions give similar answers, as the relative phasing of the optical photometry and the radial velocities was more straightforward. The (K, J 73x2013; K) radius is 11.2 ± 0.4, normal for a Type II Cepheid.
Turbulence Variations in Cepheids
Authors: Bersier David , Burki Gilbert 
Published: July 2016

From radial velocity measurements obtained with a cross-correlation technique, the variation of turbulence during the pulsation cycle is studied for a sample of 40 Cepheids. We will propose a new way to separate classical and s-Cepheids. More complete results will appear in a forthcoming paper (Bersier & Burki 1995)The radial velocities have been measured with the spectrometer CORAVEL (Baranne et al. 1979), whose cross-correlation function (CCF) is fitted with a Gaussian, giving the radial velocity Vr, the width σobs the depth H and the continuum, normalised to 1. The pulsation broadens the lines and thus also the CCF. In the Gaussian approximation one can writewhere σobs is the observed width, σinst is the instrumental width, σpuls is the additional width caused by the pulsational velocity field and σres contains all the other effects (turbulence, rotation, magnetic field, etc.). To be less affected by the noise in the data, a Fourier series has been fitted to each curve of σobs. With numerical simulations, one is able to synthesise the additional Doppler width due to pulsation, with a high accuracy. The instrumental width being well known for CORAVEL, the computation of σres is then straightforward. One then has a curve in phase for σres. From this curve, we determined the maximum residual broadening σmax (observed at or very close to minimum radius), and the width σo that the star would have if it did not pulsate. As shown by Bersier & Burki (1995), σo is slightly higher than the mean value of σres.
Mode Switching in Red Semiregular Variable Stars?
Authors: Gál J. , Szatmáry K. 
Published: July 2016

AbstractLight curve analysis of the carbon variable RY Dra and the oxygen-rich variables TX Dra and AF Cyg is summarised. In each case, two shorter periods were found (which may be connected with the radial pulsation of the stars) and also a long term variation in the mean brightness. In the case of RY Dra, the amplitudes of the shorter periods both decreased while the amplitude of the long secondary variation was found to be increasing. The light curves and frequency spectra of TX Dra and AF Cyg can be interpreted as a result of mode switching. These stars pulsate with two different periods and there are time intervals when the longer period disappears and the shorter one remains dominant, and it lasts for many cycles. We concluded that the double and multi-periodic red semiregular variables may be very important from both theoretical and observational point of view, because, based on the ratio of the periods, we can identify the pulsational modes in these variables.
Pulsations in the Nucleus of Planetary Nebula IC 418?
Authors: Kuczawska E. , Zola S. 
Published: July 2016

AbstractIC 418 belongs to the group of about a dozen planetary nebulae with central stars that show irregular spectroscopic and photometric variations. Recent theoretical results provide arguments that this group may constitute a new class of pulsators. We present new photometric observations of the nucleus of IC 418 showing variations with a timescale of about 1/4 day. We discuss two possible explanations of the observed phenomena: pulsational instability and an unstable wind.
Decomposing Interacting Binary Light Curves: The Eclipses of the Mean Light, Secular Variability and Flickering in RW Tri
Authors: Bennie Paul J. , Hilditch R. W. , Horne Keith 
Published: July 2016

AbstractWe describe a new method of orbital light curve decomposition which is applicable to long-term photometry of interacting binaries. This method determines the orbital dependence (including eclipses) of the mean light, secular variability and RMS flickering. We identify the true line of centres of RW Tri and find that the accretion disc is a location of secular variability and a source of flickering.
The Temperatures of White Dwarfs in Dwarf Novae
Authors: Hassall B. J. M. , La Dous C. 
Published: July 2016

AbstractWe attempt to measure the temperature of all the quiescent dwarf novae that have been observed with IUE, by fitting the observed flux distribution of white dwarfs (WDs) with known temperatures. This approach gives results similar to those using much more sophisticated modelling, but allows us to investigate the relationship with system parameters and subclasses.
Inner Edge Drag by an Asynchronous Primary and Accretion Disc Structure in Close Binaries
Authors: Lanzafame G. , Belvedere G. , Molteni D. 
Published: July 2016

In this work a 3–D ‘Smoothed Particle Hydrodynamics’ ([1]; [4]; [5]) accretion disc is simulated where particles at its inner edge are dragged by a fast spinning compact central star, as in the case of the intermediate polars. The angular velocity of the central star is twice the orbital angular velocity ωo. This drag can be attributed mainly to viscous interaction in the dense compact star atmosphere, although magnetic coupling may also play a role.Our simulations have been performed for a system with a primary (a white dwarf) mass M1 = 1.3 M⊙, a secondary mass M2 = 2.2 M⊙, a separation between the components (centre to centre) d12 = 6.33 1011 cm and an orbital period = 1.7 d, the integration time being as long as ≃ 75 d. As in [2] and [3], we have considered quasi-polytropic structures with γ = 1.01. We adopt the term quasi-polytropic since we have in fact solved also an energy equation.
Thermal-Tidal Instability Model of Dwarf Novae Below the Period Gap: A Unification Theory
Authors: Osaki Y. 
Published: July 2016

AbstractA unification theory is proposed in which almost all of outburst behavior in dwarf novae may be understood in a unified way within the basic framework of the disk instability model. In particular, a rich variety of outburst behavior of cataclysmic variables below the period gap may be understood by the thermal-tidal instability model in which the coupling of the two intrinsic instabilities in the accretion disk plays a unique role.
Model Spectra for the Helium Cataclysmic Variable: AM CVn
Authors: Bard S. 
Published: July 2016

AbstractAccretion through Roche lobe overflow in the helium cataclysmic binary AM CVn is considered and its emitted spectrum is computed using model stellar atmospheres. The accretion disc in AM CVn is helium-rich and such discs have hitherto not been constructed. We have computed the models by numerically solving the radiative transfer equation for helium opacities. The method adopted is that of treating the accretion disc as a set of optically thick and geometrically thin concentric annuli. Each annulus is treated individually as a blackbody or as a stellar atmosphere. We find that our description of AM CVn is consistent with both photometric and spectroscopic measurements of the system which have been made using the Nordic Optical Telescope (NOT) and the International Ultraviolet Explorer (IUE). We find that the stellar model synthetic spectrum provides a better fit to the observed spectrum than the Planckian disc spectrum. Also, for a given accretion rate, the synthetic spectrum composed of the sum of a series of stellar atmospheres is flatter than the corresponding blackbody sum.
Accretion Disc Formation in Intermediate Polars
Authors: Wynn G. A. , King A. R. 
Published: July 2016

The large-scale accretion flow in the intermediate polars (IPs) is still a matter of vigorous debate. It is known that the magnetic field of the white dwarf (WD) controls the accretion flow close to the surface, channeling the plasma onto the polecaps and giving rise to X-ray emission modulated at the WD spin period (Pspin). After their discovery it was assumed that IPs were the WD analogues of the pulsing X-ray binaries, where a magnetic neutron star accretes from a disrupted accretion disc. However, a number of authors have pointed out that the criteria for disc formation in IPs are less certain than those for the X-ray binaries.The simplest possible criterion for disc formation in a binary is that the accretion flow should be able to orbit freely about the primary star (see Frank, King & Raine 1991 for a review). In non-magnetic systems this is merely the condition that the minimum approach distance of the free stream (Rmin) should exceed the radius of the primary. The situation in magnetic systems is more complex, as the magnetic field of the primary presents an obstacle to the infalling accretion stream. In many treatments of IPs it is assumed that the plasma stream is able to orbit freely about the WD until the ram pressure of the stream is of the same order as the magnetic pressureρv2~B2/8π, whereρis the stream density,vthe stream velocity andBthe local magnetic field strength. This condition fixes the magnetospheric radius,Rmag, inside which the magnetic field is assumed to thread the stream material and direct the accretion flow along the fieldlines.
HST FOS Observations of YY Dra
Authors: Haswell Carole A. , Patterson Joseph 
Published: July 2016

AbstractThe UV continuum of YY Dra exhibits a sinusoidal modulation with 15.5% semi-amplitude and period 264.7 s, commensurate with one half the white dwarf spin period. The Civ line profile varies with spin phase, implying accretion onto, and visibility of, both magnetic poles. The line width is greatest at the phases of continuum pulsation maximum, when the wings extend out to ±3000 km s−1. This implies a minimum white dwarf mass of 0.62 M⊙, and constrains the angle between the line of sight and the magnetic axis.
Magnetic Fields and Accretion Streams in Polars
Authors: Schwope Axel D. 
Published: July 2016

AbstractThe flow of matter, the accretion geometry and the energy release in polars (AM Her binaries) are to a large extent dominated by the strong magnetic field of the accreting white dwarf. In this review I summarize the main tools used to investigate the magnetic fields and the accretion streams, using optical and X-ray spectroscopy and photometry.