bluuebunny/crossref_metadata_2025 huggingface dataset:
Middleterm Flares in T Tau Stars of the Tau — Aur Complex
Authors: Hojaev Alisher
Published: July 2016

Light variations of different kinds appear to be one of the characteristic features of T Tau stars. They are mainly caused by changes in the continuum radiation. The availability of large observational databases makes it possible to investigate different patterns of variability such as stochastic, quasiperiodic and quasicyclic ones with typical time scales from days to years, as well as sporadic flare and fuorlike outbursts. Recent attempts in classification of the photometric variability of PMS-stars were made by Herbst (1994). In this paper, we study the observational results on flares in T Tau and related stars belonging to the Taurus-Auriga complex, which resemble that of ordinary flare stars.

A census of T Tau stars with homogeneous observations of short and middleterm flares is presented in Table 1. Our selection criteria were: pre- and postilare levels of brightness ought to be quasiquiescent; the amplitude has to be more than 3 times the value of standard error; the duration of brightening does not exceed a few hundred minutes; the lightcurve shape should be similar to that of a typical flare. For convenience, all observational data in Table 1 are divided into 3 subgroups.
Observations of Short-term Dips in the Orion Variables DD Serpentis and AB Aurigae
Authors: Verlyuk I.A.
Published: July 2016

Short-term fadings (dips) in the range of a few seconds with amplitudes from 0m.3 to 1m.2 were discovered in the two T Orions type variables DD Ser and AB Aur. Similar features were observed in T Tau and R CrB by other observers.
Space Densities of Cataclysmic Variables and Nova Recurrence Times
Authors: Duerbeck H.W. , Covarrubias R.
Published: July 2016

Space densities of novae, dwarf novae and novalike stars are derived on the basis of the most recent catalogue of cataclysmic variables and improved values of absolute magnitudes. Assuming that these objects represent different stages of the same type of variable, changing over short-term cycles, we derive an average time interval of 10000 years between two subsequent nova eruptions. During this time, the cataclysmic variable remains almost 2000 years in the novalike stage and the remaining time in the dwarf nova stage, in which a total of nearly 3 · 10−5M⊙ is accreted on the white dwarf primary. This amount is comparable to the average mass of ejected nova shells.
Time Dependence of the UV Resonance Lines in the Cataclysmic Variable Star VW Hyi
Authors: Özkan M.T. , Ak T. , Saygac A.T. , Esenoglu H.H. , Guler S.
Published: July 2016

In this work, we study orbital variations in the ultraviolet resonance lines of the SU UMa type dwarf nova VW Hyi examining spectra in the data archive of the International Ultraviolet Explorer (IUE). We show that VW Hyi exhibits an orbital and long term variability in terms of UV line profiles and continuum flux at 1450 Å during superoutburst.
Do We See Magnetic Effects in Dwarf Nova Outbursts?
Authors: Vogt N. , Meyer-Hofmeister E. , Meyer F.
Published: July 2016

Many observations indicate that fast rotating late type stars show magnetic activity. We therefore argue that some of the secondary stars in cataclysmic binaries might also have magnetic fields. Such magnetic fields would reach over the accretion disk around the white dwarf primary. We investigate their effect on dwarf nova outbursts. The magnetic field lines will penetrate the disk and remove angular momentum. This shifts the accumulation of mass towards the inner disk, closer to the white dwarf, and therefore leads to a different outburst behaviour, which can be recognized in observed light curves of dwarf novae. If a magnetic field of the order 50 - 100 gauss is acting on the accretion disk, we expect narrower and more frequent outbursts as compared to the non-magnetic case. Outburst records of three dwarf novae above the period gap (P > 3h), whose long-term light curves are well covered, were used to search for traces of magnetic activity. All three cases display a pronounced bimodality in the distribution of their outburst width, i.e. either narrow or wide outbursts occur. We found evidence for a cyclic behaviour in one case, SS Aur: possible “magnetic episodes” repeating every 18 years reveal epochs with abnormally frequent narrow outbursts and nearly or totally missing wide ones. There are also indications for a similar behaviour of SS Cyg with a 7-years cycle, but with less pronounced periodicity. The third case, U Gem, does not show clear evidence of magnetic activity although we found some indications for a transitory oscillation of the width of wide outbursts after 1926, with a period of 13.6 years. The behaviour of SS Aur and SS Cyg resembles the theoretical predictions, there is, however, an important difference: Throughout the entire cycle, marked by the “magnetic episodes”, neither wide nor narrow outbursts alter their mean light curves: magnetic fields seem to affect only the observed proportion in the frequency of both outburst types without altering the light curves of individual outbursts. Remarkably, most of the “anomalous outbursts” (which are characterized by an abnormally slow rising branch to an outburst) occur also at or near the “magnetic episodes”.
Flickering in Cataclysmic Variables: Inventory and Perspectives
Authors: Bruch Albert
Published: July 2016

Stochastic variations on short time scales called flickering are a characteristic feature in the light curves of CVs. Their amplitudes indicate that their origin must be related to one of the principle light sources of these systems. But the physical mechanism behind the flickering is unknown. New attempts to quantify the properties of flickering are presented, and a consistent scenario for its origin is outlined.
Strong X-ray Bursts with Expansion: Oscillations due to Sound Waves in Atmospheres
Authors: Lapidus Iosif , Nobili Luciano , Turolla Roberto
Published: July 2016

Photospheric expansion has been observed during several powerful type I X-ray bursts, and it is associated with a supersonic outflow driven by the super-Eddington energy release in the thermonuclear He-burning at the base of the envelope (Lewin et al. 1993). Such expanded atmospheres represent perfect background for the propagation of sound, and we believe there is a number of arguments in favor of the reality of these phenomena.

We have suggested an acoustic origin for the second range photospheric oscillations detected in the X-ray bursters 1608-522 and 2127+119 (Lapidus et al. 1994). On the basis of our models of radiation driven winds from neutron stars during strongest bursts (Nobili et al. 1994), we have shown that purely radial standing sound waves in expanded spherical envelopes can have periods very close to the observed ones. Both the photosphere and the sonic surface may act as a reflecting boundary; this is the reason of quite different periods in the two sources (0.65 s and ∼ 7 s, respectively).
Quasi-periodic Oscillations and Noise in Accreting Black Holes and Low-magnetic Field Neutron Stars
Authors: van der Klis M.
Published: July 2016

The X-ray spectrum and the rapid X-ray variability of accreting compact objects have a common origin, and their properties can therefore be expected to be coupled. Indeed, it turns out that the X-ray spectrum and the power spectrum show correlated variations, which occur as a function of variations in the mass fluxx Ṁ.

Stellar mass black holes and neutron stars have similar mass and size, and therefore their accretion phenomena may be expected to show similarities. Indeed, similarities exist that indicate that a unified description may be possible. If a particular phenomenon is seen in both neutron star and black-hole candidate systems this shows immediately that it cannot be due to any property that is unique to either neutron stars or black holes, such as the presence or absence of a surface, or of a strong non-aligned magnetic field. The quest for characteristics that are unique to black holes continues - I shall mention a few candidates below.
Observations of γ-ray Bursts and Solar Flares with Granat
Authors: Terekhov O. , Sunyaev R.A. , Denisenko D. , Tkachenko A. , Barat C. , Dezalay J.-P. , Talon R. , Lund N. , Brandt S. , Castro-Tirado A.J.
Published: July 2016

The Granat observatory was launched into a high apogee orbit on 1989 December 1 by a PROTON launcher. The initial Granat orbit had an apogee close to 200000 km and a perigee of 2000 km with an orbital period of 4 days. The satellite is entering the radiation belts for a few hours every orbit. Due to orbital evolution the perigee increased and the apogee decreased with time. After about 1.5 years of operation the perigee increased up to 20000 km. This has put the satellite completely outside of the proton radiation belts which makes the detector activation during the perigee passage negligible. Moreover, the satellite orbit is outside of the magnetosphere during the most parts of the mission. This makes such an orbit very attractive for high energy astrophysics missions which require low background level in order to achieve high sensitivity, and makes it essential for the investigation of the high energy transient phenomena such as cosmic γ-ray bursts and solar flares.
CCD Sky Monitoring and Burst Alert
Authors: Hudec René , Soldán Jan
Published: July 2016

Two proposals for detection of optical flashes related to γ-ray bursts are briefly described. The aim of the first proposal is to develop a reliable monitoring service at optical wavelengths, and the second proposal deals with an automatically working telescope with full remote control via Internet network.
The Plate Collection at the Harvard College Observatory
Authors: Hazen Martha L.
Published: July 2016

This review presents an overview of the large and important photographic plate collection at the Harvard College Observatory, including its history, uniqueness, contents, storage, uses and successes.
Optical Sky Monitoring: Past and Future
Authors: Bräuer H.-J. , Vogt N.
Published: July 2016

After outlining the achievements of photographic sky monitoring at Sonneberg Observatory we describe a concept of a future CCD-based sky patrol. We suggest a world-wide network of 6 identical observing stations which cover the entire sky down to a limiting magnitude of ∼ 18TO and with a time resolution of between 8 and 24 hours. A preliminary technical design is proposed, and the astrophysical and general impact of such a project is briefly discussed.
Wide-Field Plate Database: A Progress Report
Authors: Tsvetkov M.K. , Stavrev K.Y. , Tsvetkova K.P. , Mutafov A.S.
Published: July 2016

The current status of a database containing information on wide-field observations stored in plate archives all over the world is presented. The database will permit on-line access to information on nearly 2 · 106 wide-field (⪆ 1 degree) plates and films, obtained over more than a century with about 200 telescopes and cameras.
GCVS Problems with Flare Stars
Authors: Kazarovets Elena V. , Samus Nikolai N.
Published: July 2016

The astrometric aspect of cataloguing variable stars becomes more and more important. During the past thirty years there has been approximately an three-fold increase of the number of known variables; now the rate of new discoveries becomes extremely high, and we may expect a new twofold increase in a couple of years due to discoveries made with space-borne instruments. To avoid confusion, we should continue our policy of naming new variables, and this is impossible without checking identifications with known variable stars, requiring good coordinates and/or finding charts. Operation of automatic telescopes also needs excellent coordinates just for pointing. For all these reasons, we are now working on improving considerably the accuracy standard of variable star coordinates, providing at least reasonably good coordinates for extragalactic variables (GCVS, 4th Edition, Vol. V, now in press, gives coordinates mainly to a second of arc) and variables in globular clusters, identifying variables, including those with rough published positions, with positional catalogues, etc.

Flare stars are very important variables; potentially they are one of the most abundant types of variable stars. Their identification in minimum light may be rather complicated, especially for dense regions of star clusters and associations. Most regretfully, there are many cases of papers on flare stars introducing additional confusion connected with wrong designations, wrong coordinates, missing finding charts. Thus, there appear problems preventing us from incorporating some flare stars in the GCVS. Many definite variables remain in the status of “suspects”, being positionally close to known variables, and no finding chart being available either for the earlier known variable or for the new one. Sometimes we do include such a “pretender” into the GCVS, and years after a finding chart or good coordinates appear, and it turns out that we have two entries for the same star (see, for instance, numerous cases of duplicated names for flare stars in Taurus in the GCVS Vol. III).
Wide-field Cameras for GRB Observations in X-rays and EUV
Authors: Moskalenko Eugene
Published: July 2016

Recent observations of the ASCA satellite resulted in the first identification of a GB source (Murakami et al. 1994). This success confirmed the importance of simultaneous observations in different wavelength bands for GB studies. Besides the ASCA results, there were several observations of GBs in X-ray band with the Ginga (Yoshida et al,.1989), V 78/1 (Laros et al. 1984) and other satellites. It became clear that GBs emit 4 - 8% of their energy in the 2 - 10 keV range. The main task now is to have an equipment which will be able to monitor the sky in X-rays in a mode similar to that of GRO observations, i.e. the telescope should have an all-sky field-of-view (FoV) and should work continuously.

A telescope with these features but operating at soft X-ray energies may directly determine the GB distance scale, due to interstellar absorption of the photons with energies less than 2 keV, as was pointed out first by Schaefer (1993). Flaring sources similar to GBs in time scale may be found also in the EUV (hundreds of angstroms) with the help of very wide-field cameras. Of course each such device - in X-ray, soft X-ray and EUV bands - will discover many transient objects, flaring events, will study time variability of bright “stationary” sources etc. In this paper we describe several instrumental approaches in these fields.
EUV Observation with Normal Incidence Multilayer Telescopes
Authors: Kunieda Hideyo , Yamashita Koujun , Yamazaki Takashi , Ikeda Kazuaki , Misaki Kazutami , Takizawa Yoshiyuki , Nakamura Masato , Yoshikawa Ichiro , Yamaguchi Asami
Published: July 2016

The EUV emission from hot interstellar plasmas is observed by normal incidence telescopes on board a sounding rocket. It was performed on January 29, 1995, to observe the sky area around the HZ 43 close to the north galactic pole. The wave bands (δλ ~ 10 Å) are provided at 130 and 170 Å by the multilayer coating on the reflectors of 20 cm in diameter and of 30 cm in focal length. The focal plane images are detected by Csl coated MCP’s. The observed flux of HZ 43 is 1.5 counts s−1 at 130 Å and 3 counts s−1 at 170 Å. The diffuse emission is 27 c/s/deg2 at 130 Å and 20 c/s/deg2 at 170 Å. Those preliminary numbers are subjects to change along with the data analysis.
Discovery of Warm Gas in the Virgo Cluster
Authors: Lieu R. , Mittaz J.P.D. , Bowyer S. , Schmitt J.H.M.M. , Lewis J.
Published: July 2016

During the EUVE sky survey of the Virgo region, a central source positionally coincident with the X-ray emitting galaxy M87, and a surrounding halo of extended emission, were detected in the 0.065–0.248 keV band. A detailed comparison of these data with the ROSAT PSPC data of M87 revealed an excess flux at energies < 0.4 keV within the central 30’ radius which cannot be associated with the well-known cluster gas at X-ray temperatures (kT ≥ a few keV). Instead, it is necessary to introduce a second gas component, of temperature T ~ 5 × 106K (kT ~ 0.1 keV). The resulting two-component model (warm + hot) can account for all the data. The origin and stability of the warm component, with a temperature near the peak of the thermal plasma cooling curve, is unclear. Both the temperature and spatial extent argue against cooling flow as the primary process responsible for its production. Other mechanisms, such as a galactic wind and heating by galaxy motions, must be considered.
Dissecting the EUV Spectrum of Capella
Authors: Brickhouse Nancy S.
Published: July 2016

Extreme ultraviolet spectra of Capella, obtained at various orbital phases over the past two years by the EUVE satellite, show strong emission lines from a continuous distribution of temperatures (~ 105 − 107.3 K). In addition to the strong He II λ303.8, the spectra are dominated by emission lines of highly ionized iron. Strong lines of Fe IX, XV, XVI, and XVIII–XXIV are used to construct emission measure distributions for the individual pointings, which show several striking features, including a minimum near 106 K and a local maximum at 106.8 K. Furthermore, intensities of the highest temperature lines (Te > 107 K) show variations (factors of 2–3) at different orbital phases, while the lower temperature Fe lines show variations of about 30% or less. The low variability of most of the strong low temperature features motivates a detailed analysis of the summed spectrum. With ~ 280 ks of total exposure time, we have measured over 200 emission features with S/N ≥ 3.0 in the summed spectrum. We report here initial results from the analysis of this spectrum. We can now identify lines of Fe VIII and X–XIV, as well as a number of electron density and abundance diagnostic lines.

We also report here the first direct measurement of the continuum flux around ~ 100 Å in a cool star atmosphere with EUVE. The continuum flux can be predicted from the emission measure model based on Fe line emission, and demonstrates that the Fe/H abundance ratio is close to the solar photospheric value.
Are Some Stellar Coronae Optically Thick?
Authors: Schrijver C.J. , van den Oord G.H.J. , Mewe R. , Kaastra J.S.
Published: July 2016

We discuss the coronal spectra of a sample of cool stars observed with the spectrometers of the Extreme Ultraviolet Explorer (EUVE). The emission measure distributions show (a) a relatively weak component between 0.1 MK and 1 MK, (b) a dominant component somewhere between 2 MK and 10 MK, and (c) in all cases but one a component in the formal solution at temperatures exceeding ≈ 20 MK. Where this hot tail is not associated with a real hot component, it is a spurious result reflecting a lowered line-to-continuum ratio, which, for instance, may be the result of a low abundance of heavy elements or of resonant scattering in some of the strongest coronal lines. We suggest that in Procyon’s corona photons in the strongest lines formed around a few million Kelvin undergo resonant scattering in a circumstellar medium, possibly a stellar wind. The flare spectrum of AU Mic suggests that resonant scattering may also occur in dense, hot flare plasmas. The electron densities of the 5–15 MK component are some three orders of magnitude higher than typical of the solar-like component around 2 MK; the volume filling factors of the hot components are therefore expected to be relatively small.
A Re-Analysis of the X-ray Spectrum of HR 1099 and Its Implications for Coronal Abundances
Authors: Drake Stephen A. , Singh Kulinder P. , White Nicholas E.
Published: July 2016

We have analyzed archival X-ray spectra of the RS CVn binary system HR 1099 in an attempt to see if we can obtain a consistent picture of the state of the X-ray emitting plasma. We have modeled six spectra obtained by the Exosat ME and LE telescopes, two spectra obtained by the Einstein SSS and MPC instruments, as well as a more recent ROSAT PSPC spectrum. We find that these spectra are in general poorly fitted by solar-abundance Raymond and Smith or Mewe and Kaastra thermal plasma models, and that no simple combination of these models significantly improves these fits: the observed continuum is too strong relative to the line features. We find acceptable fits for thermal models with two or more components in which the heavy elements are depleted by a factor of 2 to 4 relative to their solar photospheric values. These results are consistent with those obtained from the analysis of higher-resolution EUVE and ASCA spectra of active binary and single stars. We discuss the implications of these findings on ongoing analyses of the EUV spectra of HR 1099 and other RS CVn binaries.
New Developments in Hot White Dwarf Models
Authors: Koester Detlev
Published: July 2016

A short history of EUV observations of white dwarfs is presented, and how they have forced us to use increasingly sophisticated theoretical models—starting from blackbody spectra up to the present generation of NLTE models which include the blanketing effect of millions of atomic lines.
Features of the Soft X-ray Background and Implications for the EUV Background
Authors: Barber C.R. , Warwick R.S.
Published: July 2016

A series of eight overlapping ROSAT PSPC observations situated in the Lockman hole region have been analysed in the 0.1 – 0.4 keV band. The total exposure of ~ 137 ks and the ultra-low interstellar column density make the data extremely suitable for studies of the diffuse signal. After excluding the contribution of bright discrete sources the remaining signal was examined on a scale of 20′. As expected the 0.1 – 0.4 keV signal exhibits a strong anticorrelation with column density. However, there is also evidence for significant excess fluctuations superimposed on this anticorrelation which may be attributable to some form of cloud structures within the hot ISM.

The diffuse signal expected in the ROSAT WFC all-sky survey for this region is predicted and shown to be far in excess of that observed. The origin of the fluctuations in the soft X-ray background and their possible relevance to the Extreme-Ultraviolet background are discussed.
The Interstellar Gas in the Line of Sight to ϵ Canis Majoris
Authors: Gry C. , Lemonon L. , Vidal-Madjar A. , Lemoine M. , Ferlet R.
Published: July 2016

We analyse Hubble Space Telescope GHRS observations of the interstellar medium in the direction to ϵ CMa, the strongest EUV source in the sky located 200 pc away in a region deficient in neutral gas. We show that the neutral gas density is the lowest yet measured in a galactic sight-line. The line of sight contains three main components among which the Local Cloud, and we derive their column densities, their velocity their temperature and their turbulence velocity. We discuss the ionization of the Local Cloud and we show that we detect the conductive interface between diffuse local cloud and the hot local bubble.
EUVE Photometry of SS Cygni: Dwarf Nova Outbursts and Oscillations
Authors: Mauche Christopher W.
Published: July 2016

I present EUVE Deep Survey photometry and AAVSO optical measurements of the 1993 August and 1994 June/July outbursts of the dwarf nova SS Cygni. The EUV and optical light curves are used to illustrate the different response of the accretion disk to outbursts which begin at the inner edge and propagate outward, and those which begin at the outer edge and propagate inward. Furthermore, we describe the properties of the quasi-coherent 7-9 s sinusoidal oscillations in the EUV flux detected during the rise and plateau stages of these outbursts.
EUV Radiation from B Stars: The Broad Implications for Stellar and Interstellar Astronomy
Authors: Cassinelli Joseph P.
Published: July 2016

Observations made with the Extreme Ultraviolet Explorer (EUVE) of the two bright stars ϵ CMa (B2 II) and β CMa (Bl II-III) are discussed. The photospheres show excess EUV radiation. The wind of ϵ CMa exhibits the Bowen Fluorescence mechanism, along with high ionization stages that help explain the nature of the wind shocks. The pulsation and beat phenomena exhibited by the variable star β CMa suggest that deposition of residual pulsation energy might heat and modify the structure of the atmospheres of early-type stars near the β Cephei strip. The possibility that many other B stars show a large excess Lyman continuum radiation is considered as a possible source of the ionization of the warm ionized medium (WIM) in the galactic ISM.
Photospheric Variability in EUVE Observations of β Canis Majoris (B1 II-III)
Authors: Cohen David H. , MacFarlane Joseph J. , Cassinelli Joseph P.
Published: July 2016

The only pulsating early-type star observed by EUVE, the β Cephei variable β CMa, displays periodic variability in its Lyman continuum which is basically consistent with the long-known optical and UV variability. The amplitude of the primary pulsation component is significantly larger in the EUV than in the optical or UV. This is consistent with a temperature change being the explanation for the variability. It is notable that the pulsations have been detected in the Lyman continuum because this part of the spectrum is formed in a much higher layer of the photosphere than either the UV or optical continua.
Modelling the Soft X-Ray and EUV Emission in Classical Novae: EUVE and ROSAT Observations of V1974 Cygni
Authors: Stringfellow Guy S. , Bowyer Stuart
Published: July 2016

We have conducted an extensive analysis of the observability of Classical Novae with the EUVE Lex/B and Al/Ti/C detectors. Predicted count rates have been computed using optically thin, isothermal plasma models for solar and metal-rich compositions, and hot ONeMg white dwarf model atmospheres. We find EUVE to be quite sensitive to both the EUV and soft X-ray emission emitted by the underlying hot white dwarf during novae outbursts, except for the coolest temperatures with very high intervening hydrogen column density. These results are used to interpret the emission detected during the EUVE all-sky survey of Nova Cygni 1992 (≡ V1974 Cyg), 279–290 days after visual maximum. We find the best fit to the observed emission from V1974 Cyg arises from a hot ONeMg white dwarf with surface temperature ~ 4 × 105 K and a mass of ~1.2 M⊙, and derive an interstellar hydrogen column density of ~ 3 × 1021 cm−2. Virtually all this emission arises from supersoft X-rays rather than the EUV. We also report the detection of V1974 Cyg with the EUVE Deep Survey detector at 549 days after visual maximum. This observation is compatible with the above properties, indicating that the mechanism responsible for the soft X-ray emission, connected with the underlying white dwarf, had not yet entirely turned off. We also present analysis of a ROSAT PSPC observation which is contemporaneous with the EUVE survey observations; this independently confirms the high column density we derived from the EUVE survey observation. Light curves for the EUVE and ROSAT observations are presented. Statistical tests for variability show that all of these observations are indeed highly variable over various time scales. The EUVE survey data shows one day variations, the EUVE DS data show ~30 minute fluctuations, while the ROSAT data vary rapidly on time scales of seconds. The EUVE data shows no periodic variability on any time scale. The implications of the rapid variability are briefly discussed.
EUV/Soft X-ray Spectra for Low B Neutron Stars
Authors: Romani Roger W. , Rajagopal Mohan , Rogers Forrest J. , Iglesias Carlos A.
Published: July 2016

Recent ROSAT and EUVE detections of spin-powered neutron stars suggest that many emit ‘thermal’ radiation, peaking in the EUV/soft X-ray band. These data constrain the neutron stars’ thermal history, but interpretation requires comparison with model atmosphere computations, since emergent spectra depend strongly on the surface composition and magnetic field. As recent opacity computations show substantial change to absorption cross sections at neutron star photospheric conditions, we report here on new model atmosphere computations employing such data. The results are compared with magnetic atmosphere models and applied to PSR J0437−4715, a low field neutron star.
The Secrets of EUVE J2056-17.1
Authors: Mathioudakis M. , Drake J.J. , Craig N. , Kilkenny D. , Doyle J.G. , Sirk M. , Dupuis J. , Fruscione A. , Christian C.A. , Abbott M.J.
Published: July 2016

EUVE J2056-17.1 is one of the brightest sources in the First EUVE Source Catalog with 0.24 counts s−1 in the Deep Survey Lexan/B band. We present optical and EUV results that show this source is one of the most active late-type dwarfs. EUVE observed a large flare with energy in excess of 1035 ergs in its Lexan/B band. The quiescent optical spectrum of the source reveals strong hydrogen Balmer and Ca II H and K emission. A strong Li I 6707 Å line is also present in the spectrum. We have estimated a Li abundance of log N(Li) = 2.5±0.4. The high Li abundance and the high flare activity favors an interpretation where the enhanced Li is sustained by spallation reactions.
Searching EUVE Data for Transient/Flaring Extreme Ultraviolet Sources
Authors: Lewis J. , Bowyer S. , Lampton M. , Wu X. , Mathioudakis M.
Published: July 2016

The Extreme Ultraviolet Explorer (EUVE), because of its sky survey strategy, performed two observations of each point along a 180° by 1° strip of the ecliptic during the initial survey phase of the mission. One observation used the deep survey telescope, and another, 90 days earlier or later, used the all sky scanner telescopes, two of which have a nearly identical passband to that of the ecliptic deep survey. Since the completion of the initial sky survey, EUVE has been used to carry out deep, pointed observations of selected targets. Many areas of the sky have therefore been observed two or more times, allowing us to compare count rates for some objects over a long temporal baseline. Objects with significantly varying count rates for widely separated times are of particular astrophysical interest.

With this technique, we have discovered one such object, which appears in the First EUVE Source Catalog as EUVE J2056-171. We present upper and lower limits on how frequently other highly-variable objects will be detected by EUVE in future observations.
The Iron Project: Atomic Data for Fe I-Fe VI
Authors: Bautista Manuel A. , Nahar Sultana N. , Peng Jianfang , Pradhan Anil K. , Zhang Hong Lin
Published: July 2016

Recent progress in large scale computations of photoionization cross sections, radiative transitions probabilities, and collision strengths for the lowest ionization stages of Iron, i.e., Fe I-VI, are reported. These results are part of an international collaboration called the IRON Project. The present results exhibit large differences with respect to data currently in use and is expected to be of importance in the study of astrophysical objects in the UV and the EUV.
Rapid Pulsations in Ap Stars
Authors: Martinez Peter , Kurtz D.W.
Published: July 2016

The rapidly oscillating Ap (roAp) stars are upper mainsequence stars that pulsate in non-radial p modes of high overtone. We present an observer’s overview of the roAp phenomenon and discuss significant developments since the last major observational reviews of Kurtz (1990a) and Matthews (1991).
Simulated Power Spectra of Solar-Type Oscillations Driven by an Elementary Excitation Source
Authors: Roxburgh I. W. , Vorontsov S. V.
Published: July 2016

We consider the synthetic power spectra of multi-mode solar-type acoustic oscillations in stars, computed within a general framework of the linear response of the dynamical system to an arbitrary excitation force which is decomposed in spherical harmonics in space and harmonic functions in time. We discuss possible signatures in the observational data which might be used as diagnostics of the excitation source.
Yellow Supergiants and R Coronae Borealis Stars
Authors: Cottrell P.L.
Published: July 2016

The study of luminous variable stars has important consequences both with respect to the scale of the Universe and for our understanding of the later stages of stellar evolution.

This review will look at a number of groups of these stars, in particular the RV Tauri stars, the hydrogen-deficient Carbon (HdC) stars, including the R Coronae Borealis (RCB) stars and some aspects of Cepheid variables.

Both photometric and spectroscopic observations will be presented to show the amount of progress that has been made in this area over the last several years. Links will be made to various models which are being used to understand the observational data.

The most important conclusion is that there is still enormous scope for the acquisition and detailed analysis of data on the relatively bright members of these groups of stars. These can be used to provide further understanding of the more distant objects of each group and of the wider problems in astrophysics.
Pulsating Cool Stars and Galactic Structure
Authors: Whitelock Patricia
Published: July 2016

The review concerns the use of Miras and OH/IR stars as probes of Galactic structure and concentrates on groups of stars with both kinematic and luminosity data. The reliability of distance determinations for these stars is covered in some depth. Kinematic and scale height data for local Miras suggest that those with pulsation periods of around 200 day are members of the thick disk or inner halo. Longer-period stars are kinematically cooler and have smaller scale heights. Stars with moderate dust shells, which may be more evolved, seem to belong to the same population as thin-shelled sources with the same pulsation period. Our current knowledge of the structure of the Galactic Bulge is reviewed, as is our understanding of its rotation as derived from observations of SiO Masers from Mira variables. Infrared and radio surveys, in progress or planned, offer us the potential to extend the study of low-mass metal-rich stars well beyond the solar neighbourhood.
Pulsating Stars and the Distance Scale - I
Authors: Walker Alistair R.
Published: July 2016

A summary is given of recent measurements, by various methods, of the absolute magnitudes for RR Lyrae stars. Calibrations within our galaxy, using the Baade-Wesselink method and by Statistical Parallaxes, give a result that is some 0.2–0.3 mag fainter than is obtained by all other methods considered. The resolution of this conundrum will be likely in the near future when new and improved datasets become available.
Pulsating Stars and the Distance Scale - II
Authors: Feast Michael
Published: July 2016

The current Cepheid zero-point is equivalent to an LMC distance modulus of 18.57 ± 0.10. The zero-point from corrected Baade-Wesselink data is probably not significantly different from this. A reexamination of the Baade-Wesselink data for RR Lyrae variables leads to an LMC modulus of 18.51, an age difference between β- and α-group galactic globular clusters of +1.46±0.70 Gyr, and an Mv - [Fe/H] slope in agreement with theory. Other questions discussed include; Avoiding bias in using the Cepheid PL relation; Metallicity spread amongst Cepheids; Cepheids and Ho.
Common Misconceptions about the Galactic Cluster Calibration of the Cepheid PLC Relation
Authors: Turner David G.
Published: July 2016

Comments in the literature in recent years reveal a variety of misconceptions about the role and value of galactic clusters for calibrating the Cepheid period-luminosity-colour (PLC) relation. This paper attempts to clarify some of these points using personal experiences gained from studies of the sparse galactic clusters and groups in which some classical Cepheids are located.

Reddening is often blamed for problems originating from other causes. Three features of reddening are sometimes overlooked: (i) no universal reddening law applies to all fields, (ii) mean reddening laws are not applicable to samples of dispersed objects, and (iii) patchy extinction is the norm.
Light Variability of 28 Aql
Authors: Wehlau William , Wehlau Amelia
Published: July 2016

28 Aquilae (HR 7331, V1208 Aql) was first announced as a δ Scuti type variable by Breger in 1969 with a period of about 0.15 day. It was clear that the light variability could not be adequately represented as a sinusoidal variation with just one frequency. Since then several observers have provided additional light curves and new analyses of the light variation. In this paper we discuss observations which we have made and an analysis of the data available since 1966.

Our objective was to find the frequencies of components which would represent the light variation of 28 Aql over the 28 years for which the star has been observed.

Using the 30 cm reflector of the Hume Cronyn Memorial Observatory we observed 28 Aql in 1979, 1980, 1981, and 1993 in blue light. The light curves for JD 2444140 and 2449190 are shown below. Comparison stars were HD 180868 and HD 181383. Magnitude differences are referred to HD 181383. Nights on which the scatter in the observations, due to poor transparency, appeared to be excessive were discarded. Observations on 21 nights were retained for analysis.
The Variable Star Population in the Center of 47 Tuc as denned by HST Observations
Authors: Gilliland Ronald L. , Edmonds Peter D. , Petro Larry , Saha Abhijit , Shara Michael
Published: July 2016

The high stellar densities at the centre of some globular clusters provide prolific breeding grounds for exotic stars, e.g., the 11 msec pulsars in 47 Tuc and numerous blue stragglers. The same high density of stars as viewed on the sky makes ground-based photometry problematic for variable star detection. Even a very few tight binaries can be of fundamental importance for the dynamic evolution of the cluster core. We present results of a continuous 40-hour sequence of U-band CCD exposures acquired with WF/PC on HST. The high spatial resolution coupled with a stable PSF allows nearly Poisson limited time series on some 20,000 stars. The 1000 second sampling and long coverage provide excellent sensitivity to W UMa and δ Scuti variations. Light curves are shown for selected variables and the relative frequencies of binary and variable stars are discussed.
Estimating Cepheid Metallicities: a new approach
Authors: Zsoldos E.
Published: July 2016

A relation between the Fourier decomposition parameters of Cepheid light curves and metallicities in the form [Fe/H]=f({Ri1, ρi1}) is presented. It gives [Fe/H] values with prediction errors 0.16−0.22 dex depending on the number of Fourier parameters used in the relation.
Comparison of the Galactic Cepheid Open Cluster and Surface Brightness Distance Scales
Authors: Gieren Wolfgang P. , Fouqué Pascal
Published: July 2016

We compare the best determined Baade-Wesselink (BW) period-luminosity (PL) relation for 100 galactic Cepheids to the PL relation derived for 32 open cluster and association Cepheids by the ZAMS-fitting method. Eighteen stars in common lead to the conclusion that BW and ZAMS-fitting distances can agree to better than 0.1 mag, after proper allowance of systematic effects in both methods.
The Long-term Variation in the RV Tauri Star U Mon
Authors: Pollard Karen R. , Cottrell P. L.
Published: July 2016

The RV Tauri stars are semiregular pulsating variables located in the brightest part of the Cepheid II instability strip. They have a characteristic light curve of alternating deep and shallow minima. A subset of the RV Tauri stars (the RVb subclass) exhibit long-term (500 to 2600 day) light and radial velocity variations. Although it is well established that the short-term variations are due to pulsations, the long-term behaviour is not well understood.

BVRI photometry and high-resolution spectra of U Mon (the brightest member of the RVb subclass) were obtained at the Mt John University Observatory (MJUO) between 1990 Aug and 1994 May. The light and colour curves obtained clearly show the long-term variation in U Mon (Fig. 1(a) and (b)). The reddest colours occur slightly later than the long-term minimum in the light curve. The short-term light and colour variations are ‘damped’ at the long-term minimum.
Mid-IR Imaging of AGB Stars and Circumstellar Modelling
Authors: Busso M. , Origlia L. , Silvestro G. , Marengo M. , Persi P. , Ferrari-Toniolo M.
Published: July 2016

The evolution of low and intermediate mass (1-8 M⊙) stars along the Asymptotic Giant Branch (AGB) is ruled by processes of mass loss, causing the whole convective envelope to be gradually ejected into space. If the stellar mass is sufficiently high (M ≥ 1.5 M⊙) the envelope itself becomes enriched in nucleosynthesis products (carbon and s-process nuclei) and the star evolves into a C-rich phase. AGB stars are hence surrounded by O-rich or C-rich envelopes, opaque at optical wavelengths, which are best studied through direct imaging in the infrared (IR).
Pulsations and Dust Formation in R Coronae Borealis Stars
Authors: Clayton Geoffrey C. , Whitney Barbara A. , Lawson Warrick A.
Published: July 2016

The observed correlation between pulsational phase in RCB stars and the timing of their declines shows empirically that the stellar pulsations and dust formation are intimately connected (e.g., Lawson et al. 1992). However, the nature of this relationship and the process of dust formation itself are not well understood. We have shown that it is likely that dust is forming in close proximity (< 2 stellar radii) to the RCB star photosphere, based on time scales for acceleration of the dust, eclipse of the chromospheric region, and dispersal of the dust (Clayton et al. 1992, 1993; Whitney, Balm & Clayton 1993; Whitney et al. 1993). The temperature at which amorphous carbon forms can be as high as 4000 K, and can occur in conditions far removed from thermodynamic equilibrium, as long as a mechanism exists to contain carbon atoms within a given volume. A likely form of carbon condensate is fullerenes such as C60 (Whitney, Balm & Clayton 1993). Shocks in the stellar atmosphere due to the pulsations may provide such a mechanism for containing the carbon. However, no spectroscopic signature related to the dust formation has been seen. We have searched unsuccessfully for IR emission bands of C60 (Clayton et al. 1995a).
An Analytic Theory of Adiabatic p-modes in the Atmospheres of the Sun and Sun-like Stars
Authors: Dzhalilov Namig S. , Staude Jürgen
Published: July 2016

An analytic theory of resonant transmission has been used to study adiabatic, non-vertical oscillations in a realistic model of the solar or a stellar atmosphere and subphotosphere. Basic observed features of solar p-modes can be explained in this way.
On Expected Constraints on Stellar Transport Processes from Space Seismic Missions: EVRIS, COROT
Authors: Goupil M.J. , Michel E. , Cassisi S. , Dziembowski W.A. , Lebreton Y. , Baglin A. , Auvergne M. , Catala C.
Published: July 2016

The first space seismological experiment, EVRIS (Baglin 1991) will be launched in 1996 and will observe ∼ 10 stars over ∼ 20 days each. A second one, COROT (Catala et al., 1994), should follow soon after (1999-2000), which should monitore ∼ 5 stars over ∼ 5 months each. The observed stars will be main sequence stars. For an optimum scientific outcome of EVRIS mission, one must select the best possible target stars. The first step of this selection is outlined. What would be further gained with COROT project is discussed.

Specifications of both missions imply that among possible target stars,δScuti stars should present a large number of self excited modes, likely many more than presently observed from ground based networks.
Variability Studies and Asteroseismology with the XMM Optical Monitor
Authors: Horner S. , Antonello E.
Published: July 2016

It will be possible to make photometric variability studies from space of stars serendipitously falling in the field of view of the Optical Monitor, and to search for asteroseismological phenomena in bright target stars.
Time Resolved Optical Spectroscopy of V795 Her
Authors: Dickinson R. J. , Prinja R. K. , Rosen S. R. , Horne K.
Published: July 2016

We present high resolution optical spectroscopy of the cataclysmic variable (CV) V795 Her. The time- and phase- dependent behaviour of the emission source is examined using two-dimensional image analysis and Doppler reconstruction techniques on the orbital profiles. A clear S-wave modulation of a weak emission feature in the Hβ and Hγ line wings (extending to ~1800 km s−1) is present, with complex asymmetric properties.
Dwarf Nova Outbursts and Superoutbursts
Authors: Smak J.
Published: July 2016

Dwarf nova outbursts provide an almost unique opportunity of getting an insight into the nature of viscosity in accretion disks or, within the α- disk approach, of putting some constraints on α. In particular, the strong dependence of the viscous time-scale on viscosity itself permits us to estimate a almost directly from the observed time-scales. In the case of the hot branch of the ∑ — Te relation, the most reliable estimates (αhot) are based on the rate of decline following the dwarf nova outburst. From a comparison with model light curves calculated with different αs one gets: αhot ≈ 0.2(e.g. Smak 1984b). An independent, but much cruder, estimate can be obtained from the widths of normal outbursts, by assuming that the duration of an outburst represents the travel time of an instability wave across the disk. The result is similar: αhot ≈ 0.2 (Gicger 1987).
Eclipse Observations of IP Peg During Outburst
Authors: Worraker W. J. , Naylor T. , Poyner G. , Robinson S. J.
Published: July 2016

IP Peg is a U Gem-type dwarf nova with a very high orbital inclination such that the secondary star eclipses the white dwarf primary, accretion disc and hot spot in each orbit. An observing project was set up by ‘The Astronomer’ (TA) magazine and the British Astronomical Association Variable Star Section to make visual and CCD eclipse observations of IP Peg during outbursts in the 1994/95 season.
Slingshot Prominences During Dwarf Nova Outbursts?
Authors: Steeghs D. , Horne K. , Marsh T. R.
Published: July 2016

We present phase-resolved spectroscopy of two dwarf novae, IP Peg and SS Cyg, observed in outburst with the WHT/ISIS spectropolarimeter. Doppler tomograms of several emission lines show peculiar narrow, stationary emission components.
SPH Simulations of Tidally Unstable Accretion Disks
Authors: Murray James
Published: July 2016

We use two dimensional Smoothed Particle Hydrodynamics simulations to investigate the the Tidal Resonance Instability (TRI) model for superhumps in SU UMa stars.

In the TRI model, superhumps are the signature of an eccentric disk being periodically stressed by the binary system’s rotating tidal field. A slow prograde motion of the disk in the inertial frame is then responsible for the superhump period slightly exceeding the binary orbital period. The disk eccentricity is caused by a 3:1 resonance between orbits in the disk and the orbit of the binary itself. The development of the TRI model is due mainly to Whitehurst (1988), Hirose & Osaki (1990) and Lubow (1991a). Lubow in particular carried out a nonlinear mode analysis of a fluid disk’s response to a weak tidal field and identified the feedback mechanism responsible for disk eccentricity growth.
The Intermediate Polars
Authors: Hellier Coel
Published: July 2016

Recently discovered intermediate polars extend the limits of known behaviour. They now fill the range between, and overlap with, nonmagnetic CVs and AM Hers. They have a wide range of asynchronism, a full range of field strengths, and several different accretion modes and pulsation mechanisms. I highlight (i) a puzzle over the symmetry between magnetic poles, (ii) the X-ray spectra, (iii) the similarity with polars, and (iv) the mass of EX Hya.
The Hard X-Ray Spectra of EF Eri and Other CVs
Authors: Done C. , Osborne J. P. , Beardmore A. P.
Published: July 2016

Reflection of the X-ray emission from the surface of the white dwarf should be present in all hard X-ray emitting CV systems. It is clearly identified in the Ginga 2… 20 keV spectra of the polars AM Her and EF Eri, and also in the quiescent spectra of the dwarf novae SS Cyg. The inclusion of the hard reflection spectrum lowers the derived continuum temperature, so resolving a long standing problem of the mismatch of the iron line and continuum properties, and, with more realistic multi-temperature continuua, the derived shock temperatures are close to those expected from theoretical models of a strong shock.