bluuebunny/crossref_metadata_2025 huggingface dataset:
X-ray Observations of the dMe Star EQ1839.6+8002 in 1980-1993
Authors: Pan H.C. , Jordan C. , Makishima K. , Stern R.A. , Hayashida K. , Inda-Koide M.
Published: July 2016

During Ginga observations of galaxy 3C390.3 on 1991 February 14, a large impulsive flare was detected between 22:30-23:00 UT (Inda et al. 1994). Inda et al. (1994) suggested that the flare was associated with EQ1839.6+8002, an M4Ve star at a distance of 15.2 pc (Fleming et al. 1988). The star has been observed previously with the Einstein IPC and EXOSAT LE (Gioia et al. 1990; Grommi et al. 1991), and more recently with the ROSAT PSPC.

In this paper we present the results of an analysis of the observations of EQ1839.6+8002 with Ginga and of the data obtained with the Einstein IPC (between 1980 January and May), the EXOSAT LE (in 1984-1986), and the ROSAT PSPC instruments (in 1991 March, 1992 April and 1993 April).
Magnetic Field Reconnection as a Possible Source of Nonthermal Processes in Accreting Relativistic Systems
Authors: Pustil’nik L.A.
Published: July 2016

Recent observations of the EGRET-telescope aboard the Compton Gamma Ray Observatory (CGRO), results from Čerenkov shower telescopes, and fast variability studies have demonstrated the dominant role of nonther mal processes for some accreting relativistic objects. This is perhaps an unex pected result, because the basic assumption of the standard accretion models (Shakura & Sunyaev 1973, Sunyaev & Titarchuk 1980, Galeev et al. 1979) is the thermal nature of all processes that determine the energy output. At the same time, the recent observations argue that the source of the emission is not the thermal plasma as a whole, but a very small fraction of charged particles accelerated to very high energy (ε/mc2 > (30-1000) with a “universal” energy spectrum nε ∣ ε−α. These observational results suggest a revison of the global approach to the accretion process to find any new (or forgotten “old”) energy conversion mechanisms with predominately nonthermal output (Lynden-Bell 1969, Ikhsanov & Pustil’nik 1994).

The nature of the energy source and mechanisms of energy conversion to either plasma heating (“thermal” mode) or to acceleration of charged particles (“nonthermal” mode) is the main question for accretion theories.
Properties of X-ray Flares on Young Stars
Authors: Preibisch Th. , Neuhäuser R.
Published: July 2016

The interest in the giant flares found on young stars results from the fact that their energetics is vastly different from solar flares. The recent ROSAT observations of young clusters yield a good basis for a systematic study of these flares.

We have investigated 18 very deep pointed observations of star forming regions in IC348, Chamaeleon, ρ Ophiuchi, and Orion. To study the age dependence of the flare properties we also investigated the observations of the young clusters IC2391, Pleiades, and Hyades. In this way we have evaluated the X-ray emission of several hundred young stars with ages ranging from about 106 years to 7 ń 108 years. We have found 36 large flares on identified cluster members and have determined the flare properties with a simple flare model.

The flares show the same relation between the total X-ray flare energy and the quiescent X-ray luminosity as the dM star flares. The total X-ray flare energies decrease smoothly from the very high values found for very young stars to solar values with increasing cluster age. Our results indicate that the large flares on young stars are similar to scaled-up solar-like activity rather than to accretion processes.
Hydrogen and Helium Flashes in Cataclysmic Variables
Authors: Kato Mariko
Published: July 2016

Light curve analysis for the decay phase of novae gives estimates of the white dwaif mass, the luminosity (or distance) and the chemical composition of ejecta. Differences in these parameters among nova subclass, i.e. fast, slow and recurrent, are briefly summarized to speculate on their cause. Helium shell flashes occur in mass-increasing white dwarfs, their theoretical light curves are given in UV and visual wavelength bands. An evolutional scenario for fast, slow and recurrent, and slow novae through a helium nova/EUV source-stage toward a type Ia supernova/neutron star formation-stage is given.
Detection of Nonthermal Optical Flashes with 10−3-10−1 s Duration from Some LMXBs
Authors: Beskin G.M. , Neizvestny S.I. , Mitronova S.N. , Plokhotnichenko V.L. , Popova M. Yu. , Benvenuto O.G. , Feinstein C. , Mendez M. , Bartolini C. , Guarnieri A. , Minarini R. , Piccioni A.
Published: July 2016

In order to study very rapid optical variability of astrophysical objects on time scales between 10−7 s and 102 s (Shvartsman 1977), at SAO the MANIA (Multichannel Analysis of Nanosecond Intensity Alterations) experiment is being used. A special photometric registration system and software has been developed (Beskin et al. 1982’ Plokhotnichenko 1983, Zhuravkov et al. 1994).

One of the applications of these tools is the detection of radiation from accreted or ejected plasma near compact objects. A choice between two models of accretion onto compact objects in binary systems - hydrodynamic flow or magnetc flaring (Shakura & Sunyaev 1973, Pustil’nik & Shvartsman 1974) has not yet been made. Recent optical and X-ray observations of X-ray binaries provide information on their (fast) variability, their nonthermal radio emission, generation of very high energy particles, and nonthermal processes in rapid optical flares (Bartolini et al. 1994, Beskin et al. 1994). However, the data are not fully described by classical hydrodynamical models.
Population of Short γ-ray Bursters — Result of Gravitational lensing of “Quiet” γ-ray Sources in Remote Galaxies
Authors: Beskin G.M.
Published: July 2016

We consider the hypothesis of the existence of a class of short (10−2-10−3s) and weak (10−6-10−7erg/s· cm2) γ-bursts of cosmological origin. These events arise from gravitational microlensing of bright, quiet γ-ray sources (with luminosity about 3·1038erg/s) located in distant (z ≥ 1) galaxies by stars inside foreground galaxies. If the size of the γ-ray sources is 104-106cm, motion with respect to the observer or the intermediate galaxy may bring them close to the neigborhood of cusps along critical curves generated by individual stars (corresponding to caustics in the observer’s plane). In these cases the observed fluxes increase by a factor 1011-1013, the temporal variation of intensity being due to the brightness distribution in the source and to the distance from the cusp (in the source plane). Using standard assumptions for the number and sizes of “quiet” γ-ray sources, the number and characteristics of galaxies, we estimate the observed burst rate to be between 1 and 10 bursts per year (see below). Their temporal and spectral properties are determined by the nature of sources (ejecting pulsar, accreting neutron star with a weak or strong magnetic field, black hole) and by their velocity relative to the cusp. Similar effects have been discussed in the context of flux variability and structure studies of quasars (e.g. Schneider & Weiss 1986, Grieger et al. 1988, Refsdal & Surdej 1994). However, in these cases the source sizes were larger than 1014cm3 and amplification did not exceed 102.
Deep Search for Optical Counterparts of Gamma-ray Bursters
Authors: Sokolov V.V. , Kopylov A.I. , Kurt V.G. , Zharykov S.V.
Published: July 2016

Preliminary results are presented of the first (searching) stage of our observational program of the search and study of weak blue star-like objects in the two small γ-ray burst boxes GB 790418 and GB 790613. The basic search purposes are (1) the selection of star-like objects weaker than 23m inside these boxes, (2) the selection of the bluest ones in B — V among these objects. The first result is the fact that there are no blue (with B — V < 0) star-like objects brighter than 24m in both boxes. But in each of these GRB error boxes one blue object with smaller flux was selected for further study to elucidate their nature. In the smallest of the archival GRB error boxes (GB 790613) obtained by three satellite triangulation, the bluest star-like object with V = 24.56(±0.16) and B — V = −0.20(±0.19) is near the center (at the distance < 10") of the GRB box. In the GB 790418 error box, the bluest star-like object with B = 24.40(±0.20), B — V = −0.40(±0.40) is found at ≈50" from the center.
Wide-field X-ray Optics
Authors: Hudec René , Inneman Adolf , Pina Ladislav , Řehák Petr , Gorenstein Paul
Published: July 2016

Classical X-ray mirror optics usually has a limited Field of View (FoV) of ∼ 1°. Wide-field imaging can be achieved by lobster-eye type reflecting X-ray optics. We summarise several different approaches and suggest an innovative technology for the production of X-ray reflecting flats and cells necessary to develop one- or two-dimensional wide-field X-ray optics. The technology is based on double-sided, replicated reflecting foils produced by electroforming and CF/composite technologies.
Results from the ROSAT EUV Wide Field Camera
Authors: Pye John P.
Published: July 2016

I review some of the major achievements of the ROSAT extreme-ultraviolet all-sky survey, including the results of the recently completed 2RE source catalogue and associated study of temporal variability in EUV sources.
Variability and Spectra of AGN in the EUV and the Relation to Other Bands
Authors: Marshall Herman L.
Published: July 2016

Data from several collaborations will be shown which demonstrate the utility of EUVE observations. For Mk 478, a Seyfert 1 galaxy, the rapidly variable EUV flux is shown to have a steep, featureless continuum. The EUV data are combined with UV and optical data to form an overall spectrum that is consistent with an accretion disk model; slight temperature variations in the innermost regions could cause the large EUV flux changes. EUVE data for other sources are presented: NGC 5548, which shows significant variations and has an EUV spectrum that shows no emission lines, contrary to a previous report; 3C 273, which did not vary much; and PKS 2155-304, which was observed simultaneously with ASCA and IUE when a hard X-ray flare was detected as an EUV polarization measurement was being attempted.
Hot Times in the Hertzsprung Gap
Authors: Ayres Thomas R.
Published: July 2016

Moderate-mass giants represent a touchstone for probing the mechanisms of magnetic activity among fast-rotating convective stars. Extreme ultraviolet and soft X-ray observations of such stars detect generally hot coronae: the Hertzsprung-gap giants (F5–G2), in particular, have remarkable high-excitation peaks (107 K, or hotter) in their emission-measure distributions. While the high-temperature coronal plasmas are reminiscent of violent solar flares, the high-energy spectra of the Hertzsprung-gap giants appear to be quite steady over time; in contrast to other hot-corona objects whose optical light curves carry the stamps of starspots, and whose high-excitation emissions are sporadically—and dramatically—variable. The constancy of the Hertzsprung-gap stars is particularly puzzling in light of high-dispersion FUV spectroscopy that reveals supersonic flows in their 105 K subcoronal “transition zones.”
The Metallicity of Hot DA White Dwarfs as Inferred from EUV Photometry
Authors: Finley David S.
Published: July 2016

Photometric EUV observations have shown that the hotter DA white dwarfs tend to show significant excess opacity relative to hydrogen. EUV and high-resolution FUV spectroscopy have conclusively demonstrated that the excess opacity is due to the presence of trace heavy elements in the white dwarf photospheres. In the past, the general abundance distribution in hot DA has been studied as a function of temperature, assuming that He was the trace absorber. We present here the first determination of the variation of relative total heavy element abundances in DA as a function of both temperature and gravity using realistic models that include metals. We compare the observational results with theoretical calculations of equilibrium abundances due to radiative acceleration.
EUVEandORFEUSObservations of the Cool DO White Dwarf HD 149499 B
Authors: Napiwotzki R. , Jordan S. , Bowyer S. , Hurwitz M. , Koester D. , Rauch T. , Weidemann V.
Published: July 2016

We present the results of a recent spectroscopic investigation of the cool DO white dwarf HD 149499 B in the EUV and FUV ranges. Observations were performed with the spectrograph of theEUVEsatellite and the Berkeley EUV/FUV spectrometer of theORFEUSspace experiment. The analysis of theORFEUSspectrum, performed with a grid of LTE model atmospheres, yielded the basic parametersTeff= 49500 ± 500 K and logg= 7.97 ± 0.08. This result is confirmed by theEUVEspectra. The photospheric hydrogen Lyman lines in the FUV spectrum indicate the presence of hydrogen: lognH/nHe= −0.65 ± 0.12. The implications of this finding for the spectral evolution of white dwarfs are discussed. A check of the LTE assumption was performed by a comparison with NLTE atmospheres calculated for appropriate parameters. The interstellar hydrogen column towards the HD 149499 system amounts toNH= (7 ± 2) · 1018cm−2.
Observations of Diffuse Emission from the Hot ISM
Authors: Sanders W.T. , Edgar R.J.
Published: July 2016

The Diffuse X-ray Spectrometer (DXS) is a Bragg-crystal spectrometer designed to obtain spectra of the diffuse x-ray background in the 83–44 Å (150–284 eV) range, with ~ 3 Å spectral resolution (10–25 eV), and ~ 15° angular resolution. It was flown successfully as an attached Shuttle payload on the STS 54 mission of NASA’s Space Shuttle Endeavour in January 1993, and spectra were obtained from the diffuse background along an arc extending roughly along the galactic plane from longitude 150° to longitude 300°. The primary conclusions so far from the analysis of the DXS data are: (1) The spectra of the diffuse background in the 83–44 Å range show emission lines or emission-line blends, indicating that the emission is thermal. Although most models of this emission have assumed that it is of thermal origin, this is the first detection of lines in the diffuse background in this wavelength range. (2) The detected spectra do not resemble the model spectra of cosmic abundance equilibrium plasmas at any temperature in the 105 – 107 K range. This is independent of the particular plasma model used, Raymond & Smith, Mewe & Kaastra, or Monsignori-Fossi & Landini. (3) The detected spectra do not resemble the model spectra of depleted abundance equilibrium plasmas at any temperature in the 105 – 107 K range, for a variety of assumed elemental depletions and the same emission models. This aspect of the analysis is not completed. (4) Tentative line identifications can be made, but other lines predicted to arise from the same ions must be of consistent strength in both the DXS and EUVE data sets.
ϵ Canis Majoris and the Ionization of the Local Cloud
Authors: Vallerga J.V. , Welsh B.Y.
Published: July 2016

Using the EUV (70 – 730 Å) spectrum of the brightest EUV source, ϵ CMa taken with the Extreme Ultraviolet Explorer Satellite (EUVE) and simple models that extrapolate this spectrum to the Lyman edge at 912 Å, we have determined the local interstellar hydrogen photoionization parameter, Г, solely from this B2 II star to be 1.1 × 10−15 s−1. This figure is a factor of 7 greater than previous estimates of Г calculated for all nearby stars combined (Bruhweiler & Cheng 1988). Using measured values of the density and temperature of neutral interstellar hydrogen gas in the Local Cloud, we derive a particle density of ionized hydrogen, n(H+), and electrons, ne, of 0.015 – 0.019 cm−3 assuming ionization equilibrium and a helium ionization fraction of less than 20%. These values correspond to a hydrogen ionization fraction, XH from 19% to 15%, respectively.
EUVE Spectrophotometry of QS Tel: The Second Pole Becomes Active
Authors: Rosen S.R. , Mittaz J.P.D. , Buckley D.A.H. , Layden A. , McCain C. , Osborne J.P. , Watson M.G.
Published: July 2016

We present results of EUVE spectrophotometry of the EUV luminous polar, QS Tel (RE1938-461), together with contemporaneous optical photometry and spectroscopy. In marked contrast to the ROSAT survey observations, the EUVE light curve shows two flux maxima per orbital cycle, implying that both magnetic poles were active. A deep, narrow dip is observed during one of the two flux maxima, exhibiting a complex morphology which includes pronounced flickering behaviour. Although this feature is probably caused by stream occultation of the emission region, the apparent lack of spectral hardening at this time disfavours photoelectric absorption by cold gas as the dominant source of opacity. Whilst the overall EUVE spectrum can be characterized by a low temperature (~15eV) blackbody, implying a large soft/hard component flux ratio (~50), tentative evidence of an absorption edge from NeVI at 85A and lines due to NeVIII and NeVII at 98Å and 116Å respectively indicate that more sophisticated models must be employed. Quasi-simultaneous optical photometry shows a substantial change in the light curve over an interval of just 3 days and little evidence of correlated behaviour with the EUV flux. We consider the implications of these results on the accretion geometry and the structure of the accretion flow.
Ionization in the Winds of Early-B Stars: Constraints Imposed by EUVE
Authors: MacFarlane J.J. , Cohen D.H. , Cassinelli J.P. , Wang P.
Published: July 2016

Extreme Ultraviolet Explorer (EUVE) spectral observations of ε CMa (B2 II) provide significant new information about the ionization dynamics of its wind, its mass loss rate, and the interaction of its wind with the soft X-ray/EUV radiation field. We present results from wind ionization calculations which show how EUV radiation, emitted predominantly in the form of Fe IX-XVI lines from plasma with T ~ 1 – 3 × 106 K, significantly alters the ionization state of its wind. EUV photons from the hot plasma photoionize He II in the cool portion of ε CMa’s wind, producing anomalously high abundances of He III. The subsequent recombination from He III results in He II Lα (304 Å) and Lβ (256 Å) lines which are observed by EUVE. Also observed is O III λλ 374, which results from the O III λλ 304 multiplet being pumped by He II Lα — the Bowen mechanism (Cassinelli et al. 1995). We report on initial results from numerical simulations which show the effect of the Bowen mechanism on the O III 374 Å line emission.
The ALEXIS Point Source Detection Effort
Authors: Roussel-Dupré Diane , Bloch Jeff , Ryan Sean , Edwards Bradley , Pfafman Timothy , Ramsey Keri , Stem Steve
Published: July 2016

Los Alamos National Laboratory’s ALEXIS satellite (a wide area EUV monitoring instrument) was launched April 25, 1993. Due to the damage sustained at launch by the satellite, the ALEXIS project team has had to spend over a year devising new methods to determine spacecraft attitude knowledge, essential for putting photons back on the sky correctly. These efforts have been successful and currently the ALEXIS attitude solutions are precise to better than 0.5 degree close to the original 0.25 degree pre-flight specification. This paper will discuss the number and types of point sources that have been revealed in the ALEXIS data to date. We will also discuss ALEXIS observations of the June, 1994 super outburst of the Cataclysmic Variable VW Hyi, a program to look for simultaneous EUV emission from Gamma Ray Bursts, as well as an effort to detect EUV transients with a 12 – 24 hour response time.
Recent Advances in EUV Solar Astronomy
Authors: Doschek G.A.
Published: July 2016

I discuss recent advances in EUV solar astronomy. The new work is primarily a result of current solar space missions such as the Yohkoh high energy solar physics mission, as well as upcoming space missions such as the ESA Solar and Heliospheric Observatory (SOHO). I discuss spectroscopic and atomic physics work, and new results concerning solar flares that are directly relevant to stellar research.
Looking for the FIP Effect in EUV Spectra: Examining the Solar Case
Authors: Haisch Bernhard , Saba Julia L. R. , Meyer Jean-Paul
Published: July 2016

Systematic differences between elemental abundances in the corona and in the photosphere have been found in the Sun. The abundance anomalies are correlated with the first ionization potentials (FIP) of the elements. The overall pattern is that low-FIP elements are preferentially enhanced relative to high-FIP elements by about a factor of four; the transition occurs at about 10 eV. This phenomenon has been measured in the solar wind and solar energetic particle composition, and in EUV and X-ray spectra of the corona and flares. The FIP effect should eventually offer valuable clues into the process of heating, ionization and injection of material into coronal and flaring loops for the Sun and other stars. The situation for the Sun is remarkably complex: substantial abundance differences occur between different types of coronal structures, and variations occur over time in the same region and from flare to flare. Anomalies such as enhanced Ne/O ratios, distinctly at odds with the basic FIP pattern, have been reported for some flares. Are the high-FIP elements underabundant or the low-FIP elements overabundant with respect to hydrogen? This issue, which has a significant impact in physical interpretation of coronal spectra, is still a subject of controversy and an area of vigorous research.
Solar EUV Rocket Telescope and Spectrograph (SERTS) Observations of Fe XII Emission Lines
Authors: Keenan F.P. , Thomas R.J. , Neupert W.M. , Foster V.J. , Greer C.J. , Tayal S.S.
Published: July 2016

Theoretical electron density sensitive emission line ratios involving transitions in the 186–383 Å wavelength range are compared with observational data for a solar active region and a subflare, obtained by the Solar EUV Rocket Telescope and Spectrograph (SERTS). Electron densities derived from the majority of the ratios are consistent with one another, and are also in good agreement with the values of density estimated from diagnostic lines in other species formed at similar temperatures to Fe XII. These results provide observational support for the general accuracy of the diagnostic calculations. In addition, our analysis indicates that a line at 283.70 Å in the active region spectrum is the 3s23p32D3/2−3s3p42P1/2 transition in Fe XII, the first time (to the best of our knowledge) that this line has been identified in the solar spectrum. Several of the line ratios considered are predicted to be relatively insensitive to the adopted electron temperature and density, and the generally good agreement found between theory and observation for these provides evidence for the reliability of the SERTS instrument calibration. The application of the Fe XII diagnostics to EUVE observations of the F5 subgiant Procyon is briefly discussed.
Mode Identification of the β Cephei star BW Vulpeculae
Authors: Aerts C. , Van Hoolst T. , Mathias P.
Published: July 2016

We identify the pulsation mode of BW Vul by means of both the moment method and the method of photometric amplitudes and find a radial pulsation. We briefly study the non-linear behaviour of BW Vul.
Dust Formation and the Doubly-Periodic Variable Stars
Authors: Evans T. Lloyd
Published: July 2016

The doubly-periodic semiregular variable V Hydrae became redder and developed resonance emission lines and bands during the 1992-94 deep minimum, which therefore resulted from the production of a circumstellar dust cloud. This removes the need to postulate a secondary pulsation period, ten times as long as the main pulsation period, in the stellar as opposed to circumstellar envelope.
Cepheids in the Magellanic Clouds
Authors: Welch D.L. , Alcock C. , Bennett D.P. , Cook K.H. , Allsman R.A. , Axelrod T.S. , Freeman K.C. , Peterson B.A. , Quinn P.J. , Rodgers A.W. , Griest K. , Marshall S.L. , Pratt M.R. , Stubbs C.W. , Sutherland W.
Published: July 2016

In the past few years, the Magellanic Clouds have been the targets for several major variable star surveys. The results of these surveys are now becoming available and it is clear that a Renaissance in LMC and SMC variable star research will result. In this review, I will describe the results of such surveys and review the questions that are likely to be answered by further work.

With respect to results, I will concentrate on LMC MACHO Project data, including beat Cepheids, discovery statistics, mode identification, Fourier decomposition of lightcurves, and the differences between the LMC and galactic sample.
Cepheid Radii and Distances
Authors: Laney C. D. , Stobie R. S.
Published: July 2016

Laney & Stobie (1995) (LS95) have determined Baade-Wesselink radii for 49 galactic Cepheids using BVRI and JHK photometry. Four of the possible magnitude-colour combinations were studied in detail — namely, (V,B–V), (V,V–Ic), (K,V–K) and (K,J–K). In that paper and in the present note we have used the maximum likelihood method of Balona (1977) as recently modified to allow for the nonlinear effects of the non-negligible radius excursions of Cepheids.

One major advantage of infrared photometry is that the light variation at K is dominated by the change in surface area, unlike the light variation at V which is dominated by temperature changes. Another is that (as shown both empirically and by model atmospheres) one may use a J–K or V–K colour index to calculate the surface brightness at K without having to worry very much about the effects of varying surface gravity or Doppler broadening velocity, while the same is definitely not true when optical colour indices are used to calculate the surface brightness at V.
The H0 Key Project: Cepheids in NGC925, M101 and M100
Authors: Hughes S. M. G.
Published: July 2016

As part of the Extragalactic Distance Scale Key Project, the Hubble Space Telescope has been used to identify Cepheids in M100, M101 and NGC925, and to measure distances derived from the Cepheid PL relation. For M100, the distance of 17.1 ± 1.8 Mpc has been used to infer a preliminary value for H0 of ~ 80 km/s/Mpc, which brings the age of the Universe derived from the standard model of the Big Bang into conflict with the ages of the oldest stars.
Interferometry of Cool Pulsating Stars
Authors: Haniff Chris
Published: July 2016

The drive for high-angular resolution from the ground has led to the development of optical and near infrared interferometric telescopes analogous to radio synthesis arrays. These now provide a diffraction-limited imaging capability well suited to studies of nearby cool giants and supergiants. In this review I summarise the current status of these instruments, their scientific potential for studies of cool pulsating variables (LPVs), and present some recent results that highlight the promise that such methods hold.
A New Asymptotic Treatment of g-modes of a Star
Authors: Smeyers P. , Van Hoolst T. , Boeck I. De , Decock L.
Published: July 2016

An asymptotic representation of low-frequency, linear, isentropic g-modes of a star is developed without the usual neglect of the Eulerian perturbation of the gravitational potential. Our asymptotic representation is based on the use of asymptotic expansions adequate for solutions of singular perturbation problems (see, e.g., Kevorkian & Cole 1981).

Linear, isentropic oscillation modes with frequency different from zero are governed by a fourth-order system of linear, homogeneous differential equations in the radial parts of the radial displacement ξ(r) and the divergence α(r). These equations take the form

The symbols have their usual meaning. N2 is the square of the frequency of Brunt-Väisälä. The functions K1 (r), K2 (r), K3 (r), K4 (r), depend on the equilibrium model, e.g.,

We introduce the small expansion parameter

and assume, for the sake of simplification, N2 to be positive everywhere in the star so that the star is everywhere convectively stable.
Second-Order Rotational Effects on Observations of β Cephei Stars
Authors: Engelbrecht C. A.
Published: July 2016

Practically all published models of β Cephei (β Cep) stars assume a spherical profile, despite the fact that many actual β Cep stars display high rotation velocities. As a result of the centrifugally altered profile of the surface of a rapidly rotating star, the surface area as well as the surface normal of an arbitrary element on the stellar surface will be modified. Numerical integration over the observed surface elements produces the observed luminosity and radial velocity of such a star. The effect of including the centrifugal distortion of a rotating model β Cep star on its observed characteristics was studied for inclination angles (i) of 0°, 22°.5, 45°, 67°.5 and 90° and equatorial rotation speeds of 0, 80, 160, 240, 320 and 400 km/s, for pulsation modes of order l = 0 to 4.
Asteroseismology in Praesepe: the case of EP Cnc
Authors: Belmonte J.A. , Claret A. , Hernández F. Pérez
Published: July 2016

On the 1992 STEPHI IV campaign, the multi-periodic δ Scuti stars BN Cnc and BU Cnc of the Praesepe cluster were monitored during a three-week, three-continental run (Belmonte et al. 1994). Five and six frequency peaks, respectively, were recorded as a result of the campaign. In a subsequent paper (Pérez Hernández et al. 1995, hereafter Paper I), an attempt to identify these frequency peaks as pulsation modes was done. An important point is that both stars belong to the same cluster and, hence, more severe constraints than for isolated stars can be imposed. Recently, Breger et al. (1994) have reported results on EP Cnc, another δ Scuti in Praesepe. This star is also multi-periodic, showing three frequency peaks above the noise level. In the present work we extend our analysis to this star.
Slowly Variable Early F-type Stars
Authors: Balona L.A. , Krisciunas K.
Published: July 2016

A third periodicity is found in the early F-type star γ Dor which probably rules out an interpretation in terms of differential rotation of a spotted star. One of the two periods in 9 Aur is not visible in the radial velocity. This is not explained by the star spot model, but may indicate that the two periodicities are nonradial pulsations of different spherical harmonic degrees.
Velocity Variations of the roAp Star γ Equulei: Probing a puzzling pulsator
Authors: Matthews Jaymie M. , Scott Sandra
Published: July 2016

We present a pulsational RV curve for γ Equ whose amplitude is about 200 m/s and period is roughly 11 minutes. These data, combined with rapid photometry and line-profile variations, are being used to identify the nonradial mode(s) and study mode growth/decay in this unusual star.
The Use of Maximum Light in Cepheid Distance Estimators
Authors: Kanbur S.M. , Hendry M.A.
Published: July 2016

Cepheid PLC relations are usually constructed using mean light. We show that the use of maximum and mean light leads to a distance estimator for the LMC with 30% less scatter than existing relations.
Cepheid Distances to Nearby Galaxies
Authors: Gieren Wolfgang P. , Candia Pablo , Fouqué Pascal , Hertling Gisela , Brieva Eduardo
Published: July 2016

We have determined new Cepheid distances on a homogeneous system to nine nearby galaxies with Cepheid observations in the V bandpass, using the extensive calibration of the galactic Cepheid PL(V) relation of Gieren, Barnes & Moffett (1993). Absorption corrections to the observed distance moduli were taken from the RC3 catalog (de Vaucouleurs et al. 1991). For most of the galaxies, the galactic PL(V) relation of GBM gives a very good fit to the Cepheid data.
On the Nature of M stars with a 60 microns Excess
Authors: Loup C. , Waters L.B.F.M. , Kerschbaum F. , Hron J. , Josselin E. , Kester D. , Bontekoe Tj.R. , Zijlstra A.
Published: July 2016

Some years ago, Willems & de Jong (1988) noticed that many carbon stars display an excess of emission at 60 µm and explained it by the presence of a fossil dust shell, containing only cold dust. This detached dust shell would be the result of an interruption of the mass loss, consequence of a thermal pulse. Detached shells around C stars have actually been mapped in the CO lines (Olofsson et al. 1992), and at 60µm (Waters et al. 1994). In 1992, Zijlstra et al. found about 100 M stars displaying an excess of emission at 60 µm, and proposed that interruptions of the mass loss due to thermal pulses is a general phenomenon on the AGB. This assumption is now supported by the theoretical calculations of Vassiliadis & Wood (1993). Here we present a detailed study of the 100 M stars of Zijlstra et al. in order to test the previous assumption.
Thermonuclear Excitation of R-modes in Neutron Stars
Authors: Lee Umin , Strohmayer Tod E.
Published: July 2016

Neutron stars display phenomena which result from the burning of accreted nuclear fuel in their surface layers. In addition to producing X-ray bursts and other temporal phenomema, such burning can excite oscillation modes of the neutron star. Here we investigate the stability of R-modes in uniformly rotating neutron stars using nonadiabatic pulsation calculations of steady burning envelopes on accreting neutron stars. We find that the fundamental R-modes are unstable for a large range of accretion rates, and thus might manifest themselves observationally in the power spectra of accreting sources above about 1 kHz.
Near-surface Effects in Modelling Oscillations of η Boo
Authors: Christensen-Dalsgaard Jørgen , Bedding Timothy R. , Houdek Günter , Kjeldsen Hans , Rosenthal Colin , Trampedach Regner , Monteiro Mario J.P.F.G. , Nordlund Åke
Published: July 2016

Following the report of solar-like oscillations in the G0 V star η Boo (Kjeldsen et al. 1995), a first attempt to model the observed frequencies was made by Christensen-Dalsgaard et al. (1995). This attempt succeeded in reproducing the observed frequency separations Δv and δv02, although there remained a difference of ∼ 10 μHz between observed and computed frequencies. In those models, the near-surface region of the star was treated rather crudely: convection was described by means of a local mixing-length theory neglecting turbulent pressure, and the oscillations were assumed to be adiabatic. These approximations are likely to affect both Δv and absolute frequencies.
Temperature Structure of the Disk in V1315 AQL
Authors: Moreno C. , Rutten R. G. M. , Dhillon V. S.
Published: July 2016

We present a progress report on the analysis of time-resolved spectrophotometric observations of the cataclysmic variable V1315 Aql. The spectral range of the data covers the entire Balmer series up to and including the Balmer jump. We aim to use these data to test the steady-state accretion disk model.
New Observational Frontiers of ER UMa (= PG 0943+521) Type Dwarf Novae
Authors: Nogami D. , Kato T. , Masuda S. , Hirata R. , Matsumoto K. , Tanabe K. , Yokoo T.
Published: July 2016

The ER UMa-type, including ER UMa, V1159 Ori and RZ LMi, is a subgroup of SU UMa-type dwarf novae. Outbursts of these stars are characterized by: (i) the extremely short recurrence time of the superoutburst (~ 40 d in ER UMa and V1159 Ori, ~ 20 d in RZ LMi), (ii) short outbursts with short recurrence times (~ 4 d) between the bright states, and (iii) extremely long duty cycles (~ 0.5). Assuming that the mass transfer rate from the secondary is ten times higher than that of ordinary SU UMa stars, which was invoked by Kato & Kunjaya (1995), and a weak tidal torque in the case of RZ LMi, Osaki (1996) showed that the light curves of these stars can be reproduced by the disk instability model, which does not require mass transfer bursts. These indicate that ER UMa, V1159 Ori and RZ LMi are not on the natural extension of SU UMa-type dwarf novae.
Is There Observational Evidence for the Evaporation of the Inner Accretion Disk in Dwarf Novae at Quiescence?
Authors: La Dous C. , Meyer-Hofmeister E. , Meyer F.
Published: July 2016

We present the preliminary results of a comparison of observed general patterns of quiescent dwarf novae and theoretical predictions of the disk evaporation model. We demonstrate that, even though details still have to be clarified, on the whole there is strong evidence that observations and theory are in agreement.
Simultaneous UBVRIJK Photometric and Polarimetric Observations of PQ Gem
Authors: Potter Stephen , Mason K. O. , Cropper M. S. , Bailey J. A. , Hough J. H.
Published: July 2016

We present simultaneous multiwavelength observations of the intermediate polar PQ Gem (Mason et al. 1992) obtained on 1993 February 18 and 19 using the Hatfield Polarimeter on UKIRT.

The data are folded on the 13.9 m spin period in Fig. 1. The light curves are double peaked at long wavelengths, with dips at phase ~ 0.15 and phase ~0.65, but almost sinusoidal in the U and B bands where the phase ~0.65 dip is absent. The percentage of circular polarisation also varies with the spin cycle, most notably in the I band, with both positive and negative excursions. The peaks in the positive and negative polarisation occur at phase ~ 0.15 and phase ~ 0.65 respectively, approximately coincident with the two intensity dips.
Magnetic Fields in AM Her Binaries
Authors: Wu Kinwah , Mason Paul A.
Published: July 2016

There is growing evidence of complex white dwarf magnetic fields in CVs (e.g. Meggitt & Wickramasinghe 1989; Robinson & Cordova 1994). The properties of systems with non-dipolar white dwarf magnetic fields have been investigated, and it was found that a non-negligible quadrupole component is essential for the magnetic locking of AM Her binaries (AM Hers) (Wu & Wickramasinghe 1993). In a recent study the ratio of the quadrupole to dipole component of the surface field of white dwarfs was shown to grow by a factor of 10 during the white dwarf cooling lifetime (Muslimov, van Horn & Wood 1995), thus the quadrupole field component is long lived and significant.
Soft X-Ray Flickering of AM Her
Authors: Wheatley Peter J.
Published: July 2016

I present preliminary results from an analysis of the flickering in the only off-axis, high-state observation of AM Her with the ROSAT PSPC. At high Ṁ, the ROSAT count rates are dominated by optically-thick emission from the white-dwarf surface, which is heated directly by the accretion flow. This emission is very soft, predominantly below 0.4keV, and exhibits strong flickering.
QPOS of By Cam in Low and High States
Authors: Pavlenko E. P. , Shugarov S. Yu. , Antipin S. V. , Sokolov D. A. , Borisov G. , Budzinovskaya I.
Published: July 2016

We present the analysis of BV photometry of BY Cam based on observations carried out in Crimea. The long QPOs in brightness were detected in both high and low brightness states. Their typical periods, equal to 6, 12 and 24 m, are found. The amplitude of the QPOs depends on the phase of Pbeat.
High Speed UV Photometry of the AM Her Systems AM Her, VV Pup And V834 Cen, with the Hubble Space Telescope
Authors: Schaefer K. , Bond H. , Chanmugam G.
Published: July 2016

We have used the High Speed Photometer (HSP) aboard the Hubble Space Telescope to observe the magnetic cataclysmic variables VV Pup, AM Her, and V834 Cen in the UV (1400…3300 Å) with 0.01 s time resolution. We detected low frequency flickering in all three systems, and compare the time-scales with the predictions of King (1989). At higher frequencies we searched for shock oscillations from the accretion column(s) in these systems. The data were analyzed using the Gabor transform wavelet-like technique (Heil & Walnut 1989) to search for frequency evolution throughout each observation. Preliminary analysis suggests the detection of rapid UV quasi-periodic oscillations (QPOs) in VV Pup at 0.74 Hz, and at 4.4 Hz in V834 Cen. As in ground based observations, our observations failed to yield any rapid QPOs in AM Her itself.
Far Ultraviolet Observations of Dwarf Novae made with the Hopkins Ultraviolet Telescope
Authors: Long Knox S.
Published: July 2016

Observations with the Hopkins Ultraviolet Telescope on the Astro-1 and Astro-2 space shuttle missions have provided the first set of moderate (3 Å) resolution far ultraviolet (FUV) spectra of dwarf novae to include the wavelength range between Lyα and the Lyman limit. Important lines which are detected in the HUT spectra of dwarf novae in this wavelength range include S VI λλ933,945, C III λ977, O VI λλ1032,1038, P V λλ1118,1128 and C III λ1176, as well as the higher order Lyman lines. The observations confirm earlier IUE observations that two dwarf novae – U Gem and VW Hyi – have quiescent FUV spectra dominated by the white dwarf, but suggest that the quiescent FUV emission from three other dwarf novae – SS Cyg, WX Hyi and YZ Cnc – are dominated by emission from a hot portion of the disk or a disk corona. The spectra obtained of the dwarf novae Z Cam and EM Cyg in outburst and also of the nova-like variable IX Vel can be modeled reasonably successfully in terms of steady state disks constructed by adding appropriately-weighted stellar model spectra.
Euve Observations of VW Hyi in Superoutburst
Authors: Mauche Christopher W.
Published: July 2016

EUVE observed the SU UMa-type dwarf nova VW Hyi in superoutburst for an interval of nearly 2 d in 1994 June and produced EUV light curves and the first EUV spectrum of this important CV.
ASCA Observations of Cataclysmic Variables
Authors: Ishida Manabu , Fujimoto Ryuichi , Matsuzaki Keiichi
Published: July 2016

ASCA has observed ~20 cataclysmic variables since launch. In this review, we present the current status of ASCA observations of CVs and scientific results. In Sect. 2, we introduce the scientific instruments of ASCA, and show the list of CVs observed so far. Data analysis and results on magnetic and non-magnetic CVs are described in Sect. 3 and 4, respectively. In Sect. 5, we make a brief summary.
Are Novae Standard Candles?
Authors: Pistinner Shlomi , Shaviv Giora , Hauschildt Peter H. , Starrfield Sumner
Published: July 2016

Novae in outburst can be used as distance indicators. The distance measurement process is considerably simplified if novae could be regarded as standard candles. The term standard candles in the context of this paper means that the absolute magnitude at maximum for different novae is, to a good approximation, constant. The t3 – Mb diagram (Schmidet 1957) implies that this is the case for M31 and LMC novae (Della Valle & Livio 1995). However, this conclusion does not necessarily apply to non Local Group novae (Livio 1992; Della Valle & Livio 1995); for example the Virgo novae are not similar to Local Group novae (Della Valle & Livio 1995). Here we focus on a question of principle, namely: Does the fact that novae have similar spectra imply that they have similar luminosities?
Evolution of an Accretion Disk in the Symbiotic Binary CI Cyg
Authors: Mikołajewska Joanna
Published: July 2016

We have combined IUE spectra with optical spectroscopic and photometric data collected over a few orbits of the symbiotic binary CI Cyg to follow the evolution of a disk and boundary layer during a full activity cycle. Our results indicate an extended optically thin disk during quiescent periods, which evolves into an optically thick state in an eruption; a classical boundary layer at the inner edge of the disk ionizes a surrounding H II region in quiescence, and this emission fades during the rise to visual maximum in outburst. This evolution of Ṁ rivals that observed in classical CVs and pre-main sequence stars, so symbiotic systems like CI Cyg represent another opportunity to study the physics of disk accretion.
An Archival Study of HST Observations of Her X-1/HZ Her
Authors: Cheng F. H. , Vrtilek S. D. , Raymond J. C.
Published: July 2016

Her X-1 is an X-ray pulsar with a rotation period of 1.24 s and a binary period of 1.7d (Tananbaum et al. 1972). The 1.7 d variations in optical and ultraviolet flux are attributed to X-ray heating of the companion star and disk (e.g. Howarth & Wilson 1983, hereafter HW83). The system displays a 35 d period, attributed to the effects of a tilted, precessing, accretion disk. Optical and ultraviolet flux variations continue unchanged throughout. This work is motivated by the following reasons:

• The observed IUE spectra have significantly flatter slopes than those predicted by previous models (e.g. HW83).

• The observed strength of the Balmer jump is anomalously low compared to that expected for a normal B star (Anderson et al. 1994).

• HST observations obtained by Anderson et al. (1994) in order to study emission lines have yielded high quality spectra of the continuum emission from HZ Her, enabling for the first time detailed model fitting efforts.
Doppler Imaging of V404 Cyg in Quiescence
Authors: Casares Jorge
Published: July 2016

We present Doppler images of the accretion disc in the black hole X-ray transient V404 Cyg during its quiescent phase. The Hα map shows evidence for a hot spot, indicating that mass transfer is continuing long after the 1989 outburst. Both the Hα and the IR Ca II emission maps exhibit intense bright spots at the position of the secondary star, which we take as a sign of chromospheric activity. The Ca II emission arises from the outer parts of the accretion disc, and its surface brightness distribution is consistent with dwarf novae discs (I∝R−3/2).
Investigation of the Optical Variability of Relativistic Objects with High Time Resolution
Authors: Mitronova S. N. , Beskin G. M. , Neizvestny S. I. , Plokhotnichenko V. L. , Popova M. Yu. , Zharikov S. , Benvenuto O. G. , Feinstein C. , Mendez M. , Boriakoff V. , Dossa D.
Published: July 2016

1. Detection of nonthermal optical flares in low mass X-ray binaries. Eleven low-mass X-ray binaries have been observed with the MANIA (Multichannel Analysis of Nanoseconds Intensity Alteration) complex; X0420+32, A0620-00, X1728–169, X1813–14, X1957+11 in 1986 at the 6 m telescope of SAO [7], and 2S0921–630, 4U1543–475, 4U1636–536, 4U1559–487, MXB1735–44, 4U1822–371 in 1991 at the 2.15 m telescope of CASLEO (Argentina) [1]. Analysis of the data using the special y2 and d2 function formalism [4], [6] has shown no brightness variations on time-scales of 10−7... 10 s for all objects except A0620–00 and MXB1735–44.

A0620–00. The five millisecond flashes were detected on 1986 February 13 (Fig. 1). The first two flares have durations of 3 ms and 5 ms and their rise times were 1...2 ms. The other three events lasted 0.4…0.5 ms with rise times of about 0.1 ms. The lower limit of the flare amplitudes is 40 counts ms−1 and the corresponding brightness temperatures are 5 109 K for the first two and 5 1011 K for the others.
Rosat and Optical Observations of Supersoft X-Ray Sources
Authors: Reinsch K. , van Teeseling A. , Beuermann K. , Thomas H.-C. , Abbott T. M. C.
Published: July 2016

We present X-ray, UV and optical observations of the Galactic supersoft X-ray source (SSS) RX J0019.8+2156 and first results of our study of the X-ray and optical long-term variability of bright SSS.
Visual Light from the Eclipsing Supersoft X-Ray Source Cal 87
Authors: Schandl S. , Meyer-Hofmeister E. , Meyer F.
Published: July 2016

We calculate the visual light curve of CAL 87 based on the assumption that an accreting, steadily burning white dwarf irradiates the accretion disk and the secondary star, as suggested by van den Heuvel et al. (1992). The shape of the eclipse light curve and the absolute luminosities put strong constraints on the contributions of the various elements in the system. We find a reasonable fit is obtained with the following sources of visual light: a Roche lobe filling secondary star where the energy from irradiation is spread out over the surface, and an accretion disk with an optically thick, cold, clumpy spray which is caused by the massive accretion stream impinging on the disk (hot spot). This spray moving around the disk and illuminated by the white dwarf is found to be the brightest source of visual light in the binary and can account for the asymmetry in the light curve.
High-Resolution X-Ray Spectroscopy with AXAF
Authors: Fabbiano G.
Published: July 2016

This paper is a very short description of the Advanced X-ray Astronomy Facility (AXAF) and its spectral capabilities. A more detailed description can be found at the WWW location http://hea-www.harvard.edu/asc/SIN/SIN.html.