bluuebunny/crossref_metadata_2025 huggingface dataset:
Flares in T Tauri Stars
Authors: Gahm Gösta F.
Published: July 2016

This review concerns rapid variability, on time-scales of a few hours or less, occurring on T Tauri stars (TTS). There are several recent reviews on observed properties of TTS (e.g. Appenzeller & Mundt 1989, Bertout 1989, Gahm 1990a, Kuhi & Cram 1989), some with more emphasis on “flare-like” activity (Feigelson et al. 1991, Gahm 1990b, Montmerle 1991, Montmerle et al. 1993), and the ambition below has been to summarize more recent work. We will also give a compilation of published works on observations of optical variability on these time-scales and a related statistical overview. Some early, very interesting and extensive studies of this kind were made here at the Sonneberg Observatory, where for instance Götz & Wenzel (1967) concluded that the light-curve of RW Aurigae contains several components: quasi-periodic fluctuations over days, ‘waves’ over several hours, rapid outbursts with symmetric light-curves, rapid outbursts with asymmetric (flare-like) light-curves (see also Fürtig & Wenzel 1964) and small fluctuations with very small amplitudes, which they postulated could be due to changes in the emission line fluxes. Modern photometric results of higher sensitivity and time-resolution confirm the existence of these different types of rapid variations in classical TTS (CTTS), having strong emission (lines and continuous, so called veiling) superimposed on the photospheric absorption line spectrum. As described in the subsequent chapters there are new concepts for the interpretation of the short-lived fluctuations. Concerning the long-term quasi-periodic variations we can now usually relate them to the stellar rotational period (bright or dark spots), let be that RW Aur still is an uncertain case. There are other slow changes most likely related to variable circumstellar extinction in circumstellar dust in the line-of-sight to the star. The long-term optical changes were most recently discussed by Herbst et al. (1994).
X-ray Flare Rate of T Tauri Stars in Taurus
Authors: Neuhäuser R. , Preibisch Th.
Published: July 2016

We study the X-ray emission of several hundred (young, low-mass, late-type, pre-main sequence) T Tauri stars (TTS) in the Taurus T association, a nearby well-studied region of ongoing star formation. We report on X-ray emission variability of TTS as observed with the flux-limited ROSAT All-Sky Survey (RASS). Since RASS observations are spatially unbiased, we can investigate the X-ray flare rate of TTS on a large sample. We find that large flares are very rare (once per year), while medium-size flares can occur once in ∼ 40 days.
Polarimetric Outbursts in Young Herbig Ae/Be Stars and in Peculiar B[e] Stars
Authors: Yudin R.V.
Published: July 2016

Quasi-simultaneous optical and IR photometric (BVRIJHK) and optical polarimetric (BVRI) observations were carried out at the 1 m telescope of Assy Observatory (Institute of Astrophysics of the Kazakhstan Academy of Sciences). The photometer-polarimeter of the Pulkovo Observatory (Bergner et al. 1988) was used for the study. All our measurements were done with apertures of 26" and 52".

HDE 259431. Our data show a strong polarimetric variability Δp = 2% – 3% and ΔΘ 30° (especially in UV and optical region). Although the general nature of polarization is connected with the dust disk around the star (Dzhakusheva et al. 1988), the polarimetric variability probably has an another origin. This is due to the fact that we have observed a strong polarimetric variability on a time-scale of up to several hours (Dzhakusheva et al. 1988). Incidentally, this rapid polarimetric variability has no correspondence in photometry.
Photometry of CH Cygni During 1991-1994
Authors: Panov K.P. , Stegert J.S.W. , Hildebrandt G.
Published: July 2016

CH Cygni has been observed since about one decade at the Bulgarian National Astronomical Observatory Rozhen. UBV photometry was obtained with the 60 cm telescope. The computer controlled photometer is equipped with an EMI 9789 QB multiplier, a set of u,b,v filters and a diaphragm of 28". The reduction includes corrections for dead-time and background, differential extinction and transformation to the standard UBV system.

Observations between 1991 and 1994 are displayed in Fig. 1 and show the new outburst, which is still going on. The previous major outburst lasted until 1984, and CH Cyg declined slowly between 1985 and 1988. In 1988, it reached its minimum in brightness and activity. The cause for the activity is probably mass transfer from the M giant component onto the hot companion. In this model, only the M star is seen during activity minimum. In July 1988, the mean brightness was V = 8.50, B - V = 1.39 and U - B = 1.48. Since 1989, some erratic activities appeared, including a hot variable continuum, broad and variable emissions in the Balmer lines and rapid light variations (Leedjarv 1990, Tomov and Mikolajewski 1992, Kuczawska et al. 1992, Panov and Ivanova 1992). Since 1991, a trend of increasing activity is observed, which persisted also in 1994. The brightening of CH Cyg of about 5m in U in 1994 with respect to 1991 shows that it undergoes another major outburst. Moreover, it exhibits light variations with different other time scales, minutes (flickering), days and weeks, probably resulting from the mass-transfer. Fig. 2 showns an example of a short time variation on October 1, 1994. The flickering of CH Cyg was very pronounced in the U, B and V filters and the amplitudes reached several 0m.1 in about half an hour. More observations of this interesting system are planned.
Extension of the Radio Spectrum of AE Aqr to the Sub-millimetric Range
Authors: Abada-Simon Meil , Bastian Tim S. , Bookbinder Jay A. , Aubier Monique , Bromage Gordon , Dulk George A. , Lecacheux Alain
Published: July 2016

AE Aquarii is a magnetic cataclysmic variable containing a white dwarf and a K3-K7 star which lies slightly above the main sequence. The white dwarf is the most rapidly rotating known (Prot ≃ 33.08 s, Patterson 1979), and it is the most strongly asynchronous with its revolution (Porb = 9.88 hr). The white dwarf accretes matter from the K star, which approximately fills the Roche lobe. AE Aqr exhibits flares in the soft X-rays, the ultra-violet, and almost continuously in the visible and the radio regimes. Rapid optical and TeV γ-ray bursts have also been discovered, which are modulated with the period of the white dwarf and at half of this period (de Jager & Meintjes 1993). This modulation, also found in X-rays, is interpreted as the accretion of matter onto the white dwarf’s magnetic poles. The strength of the white dwarf’s magnetic field is not well-determined, it is estimated to be ∼ 6.104 - 105 G (Lamb & Patterson 1983, Cropper 1986) at the white dwarf’s surface. Eracleous et al. (1994) recently suggested that the magnetic dipole axis lies close to the equatorial plane (∼ 20°). De Jager et al. (1994) discovered a rapid spin down of the white dwarf leading to a spin down power which exceeds the accretion power. They suggest that a significant fraction of the spin down power may be converted to the acceleration of particles, which may explain the radio and the γ-ray emissions. Both the characteristics of the optical flares and the existence of TeV γ-rays suggest a relation with the non-thermal radio flares.
The Origin of Flaring Activity in AE Aquarii
Authors: Ikhsanov Nazar
Published: July 2016

AE Aqr is a close non-eclipsing binary system with an orbital period Porb ≈ 9.88 hr, eccentricity of the orbit e = 0.02 (Chincarini & Walker 1981) and a mass ratio q = 1.14 (Reinsch & Beuermann 1994), situated at the distance of ∼ 84 pc (Van Paradijs et al. 1989). The secondary is K3 red dwarf on or close to the main sequence (Bruch 1991). The primary cannot be observed directly. However, the stable photometric oscillations at a period of 33 s (and at half this period) in the optical (Patterson 1979), UV (Eracleous et al. 1994), X-rays (Patterson et al. 1980) and, probably, VHE γ-rays (Meintjes et al. 1992, Bowden et al. 1992) leave no doubt that the primary is a rapidly rotating magnetized compact object. The observed rate of deceleration of the primary rotation is Ṗ = 5.64 × 10−14 ss−1 (De Jager et al. 1994), and its mass is m1sin3i (0.56 ± 0.03) M⊙ (Reinsch & Beuermann 1994). The lack of eclipses allows to put a lower limit to the mass of the primary m1 ≥ 0.62M⊙.

AE Aqr emits detectable radiation in all parts of the electromagnetic spectrum. In radio and γ-rays it is a powerful non-thermal source. However, in the optical, UV and X-rays the radiation is likely to be thermal and can be well explained by gas accretion onto a compact star (Patterson 1979, Patterson et al. 1980). The observed impulse profile of the pulsating component (Eracleous et al. 1994) looks similar to that of the accreting X-ray pulsars.
The Benefit of Archival Research in Investigating Dwarf Novae
Authors: la Dous C.
Published: July 2016

Dwarf novae and nova-like stars are a sub-group of the class of cataclysmic variables. Most of these stars show high and low brightness states (maxima or outbursts and minima or quiescent states, respectively). They all are short-period interacting binaries consisting of a white dwarf primary, a late-type Roche-lobe filling secondary, and an accretion disk around the white dwarf. For details on observations of cataclysmic variables and their theoretical explanations and modelling, the interested reader is referred to recent reviews.

The main problem when trying to understand these systems is the wealth of patterns presented by the whole group: no two objects are nearly identical and it is hard to say which the ‘typical’ features are. As a result of this, from an individual observation, or from observations of a few systems only, it is not possible to decide which of the observed features are characteristic of the entire class, and which are specific of individual members. The only way is to investigate many different members of the same class, arrange them according to their different physical parameters, and try to determine what the general characteristics are.
Flares in Dwarf Novae
Authors: Bateson Frank M.
Published: July 2016

This report provides a summary of flares observed visually in the dwarf novae V442 Cen, OY Car, TU Men and Z Cha. The flares precede an outburst by an average of around ten days.
Eclipse Mapping the Flickering in HT Cas
Authors: Welsh William F. , Wood J.H.
Published: July 2016

Flickering is a well-known, though poorly understood, signature of accretion in cataclysmic variables. HT Cas is an excellent system in which to study flickering because its well determined characteristics allow a calibration of the experimental system (see Horne et al. 1991).

Patterson (1981) argues that the flickering in HT Cas originates from regions very near the white dwarf. In U Gem, the flickering appears to be associated with the bright spot (Warner & Nather 1971). In RW Tri, the flickering occurs throughout the disk but is centrally concentrated (Horne & Stiening 1985); the same holds true for V2051 Oph (Warner & Cropper 1983).

HT Cas is a 16m.4 eclipsing dwarf nova with a binary period of 106 min and an inclination of ∼ 81°. The entire eclipse last about 15 min with the white dwarf totally eclipsed for ∼ 5 min. The mass ratio is 0.15 with the white dwarf having mass ∼ 0.6M⊙ (Horne et al. 1991).

We obtained 20 eclipses of HT Cas between 1982 and 1991 in U, B and V at Mt. Palomar and also McDonald Observatory with 0.5 – 2 sec integrations. These data were folded on the ephemeris given by Horne et al. (1991) and corrected for the 30 sec drift in the phasing as noted by Wood et al. (1995).
Multiple Time Scales in Cataclysmic Variables: The Examples
Authors: Andronov Ivan L.
Published: July 2016

Decades of patrol observations of cataclysmic variables (CVs) show variations of the accretion rate also at a time scale of a few years which may be explained by a solar-type activity of the secondary and/or by orientation changes of the magnetic axis of the white dwarf in polars. Secondary photometric periods seen in TT Ari and possibly some other CVs need theoretical explanation. Irregular variability of some objects is characterized by power-law power spectra with a power index depending on the luminosity state of the system.
Irregular X-ray Variation of LMC X-3
Authors: Pan H.C. , Skinner G.K. , Sunyaev R.A. , Alexandrovich N.L.
Published: July 2016

LMC X-3 is an X-ray binary in the Large Magellanic Cloud. It was discovered by UHURU and observations with various satellites showed that the X-ray source was variable by a factor of up to 100 (e.g. Traves et al. 1988). Using the GINGA and HEAO-1 observations, Cowley et al. (1991) found a strong ∼ 198 (or possibly ∼ 99) day modulation in the X-ray luminosities of LMC X-3. They suggested that this modulation may be due to an accretion disc precession, or periodic variations in the mass transfer rate, or a combination of both.

We observed LMC X-3 with the TTM in 1988-1990. The present paper gives some results from the analysis of the data obtained.

The TTM is a coded mask telescope on board the MIR space station. It is capable of producing images in the energy range 2-30 keV with a spectral resolution of about 18% at 6 keV (Brinkman et al. 1985).
Observations of High-energy Gamma-ray Bursts with EGRET
Authors: Merck M. , Bertsch D.L. , Dingus B.L. , Fichtel C.E. , Hartman R.C. , Hunter S.D. , Kanbach G. , Kniffen D.A. , Lin Y.C. , Mayer-Hasselwander H.A. , Michelson P.F. , von Montignty C. , Mukherjee R. , Nolan P.L. , Pohl M. , Sreekumar P. , Schneid E.J. , Thompson D.J.
Published: July 2016

Gamma Ray Bursts (GRBs) have puzzled astronomers since their discovery more than 20 years ago. As no counterparts at wavelengths other than X- and γ-rays have yet been found the identification of the sources is still missing. Theoretical explanations range from colliding comets (1993) and merging neutron stars (1982) to more exotic objects, such as superconducting cosmic strings (1988). Data accumulated until now still do not discriminate between these models, although results from the BATSE (Burst and Transient Source Experiment) instrument aboard the Compton Gamma Ray Observatory (CGRO) strongly favor extragalactic models.

The Energetic Gamma Ray Experiment Telescope (EGRET) aboard CGRO has s ofar detected photons from 5 GRBs with its spark chamber. These are the highest energy γ-rays associated with GRBs to date. In this work we review previously published data and summarize the properties of these events. Elsewhere we present possible constraints from the data on the models proposed to explain GRBs.
ETC Observations of the Gamma-ray Burst GRB 941014
Authors: Vanderspek Roland , Richer George R.
Published: July 2016

The Explosive Transient Camera (ETC) is a dedicated wide-field sky monitor for short-duration optical transients. The ETC operates completely automatically, searching for transients in the night sky without the need for a human observer on site. Data products from the ETC include data from any transients detected, as well as CCD images of the fields-of-view observed. We report here the imaging by the ETC of the 2.5σ error region of GRB 941014 90 seconds after the end of the burst.
EUV Astrophysics with ALEXIS: The Wide View
Authors: Bloch Jeffrey
Published: July 2016

The Array of Low Energy X-ray Imaging Sensors (ALEXIS) satellite is Los Alamos’ pathfinding small space mission achieving low cost and rapid development time for its technology demonstration and science goals. The ALEXIS satellite contains the ALEXIS telescope array, which consists of six EUV/ultrasoft X-ray telescopes utilizing normal incidence multilayer mirrors, microchannel plate detectors, and thin UV rejecting filters. Each telescope is tuned to a relatively narrow bandpass centered at either 130, 171, or 186 angstroms. Each telescope has a 33° field-of-view, and a resolution of ~ 0.25°. With each 50 s rotation of the satellite, the telescopes scan most of the anti-solar hemisphere of the sky. The spacecraft is controlled exclusively from a ground station located at Los Alamos.

This paper discusses the characteristics and performance of the ALEXIS telescopes and the results from the mission in spite of the damage incurred to the spacecraft at launch.
Active Galactic Nuclei in the Extreme Ultraviolet
Authors: Königl Arieh
Published: July 2016

EUV observations could provide valuable and unique information about the nature of active galactic nuclei. I discuss their potential usefulness and review some of the main results obtained so far with the ROSAT WFC and the EUVE instruments. About three dozen sources, mostly Seyfert galaxies and BL Lacertae objects, have already been identified, and spectroscopic measurements of several of them have yielded a number of surprises. However, more observations are clearly needed to confirm some of the reported spectral features and to validate their interpretation.
Line Emission from Warm Material in NGC 5548
Authors: Kaastra J.S. , Mewe R. , Roos N.
Published: July 2016

EUVE observations of the Seyfert 1 galaxy NGC 5548 have shown the presence of line emission features identified as a Ne VII/Ne VIII blend at 88 Å and Si VII emission at 70 Å. The lines show significant broadening (FWHM 3800 km s−1) placing the emitting region at the same distance as the inner broad-line region. A fit to a thermal plasma yields a temperature of 6 × 105 K. The line emission can be attributed to the warm absorbing material discovered before from oxygen and iron absorption features, which appears to be an optically thin (for the Lyman continuum), highly ionized phase of the broad-line region.
Continued Analysis of EUVE and Optical Observations of a Flare on AD Leonis
Authors: Cully S.L. , Fisher, G.H. , Hawley, S.L. , Simon T.
Published: July 2016

The flare star AD Leo (dM3.5e, 4.9 pc) was observed by EUVE from 1993 March 1-March 3 UT. A flare was detected by the EUVE DS/S and seen in optical photometry on 1993 March 2 UT. We summarize an analysis of the flare’s physical parameters, and present differential emission measure (DEM) curves calculated for the quiescent, flare peak and flare decay phases of the observation.
A Catalogue of Ultraviolet Observations of Chromospherically Active Binary Stars
Authors: Dous C. La , Giménez Alvaro
Published: July 2016

We present the recently-published IUE-ULDA Access Guide on Chromospherically Active Binary Stars which should be of interest for the interpretation and analysis of EU V data obtained for this type of objects. We provide background information on both high and low resolution IUE spectra of chromospherically active late-type binary stars that have been taken until the end of 1992. Physical information on all systems, arranged by variable star name, together with characteristics of the individual exposures, the position of the observation in the orbital light curves, and an average low resolution ultraviolet spectrum are given.
A Spectroscopic Survey in the EUV of the “Coolest” Hot DA Stars
Authors: Dupuis Jean , Vennes Stéphane
Published: July 2016

We present an analysis of the extreme ultraviolet (EUV) spectroscopy of a sample of 10 DA white dwarfs observed by the Extreme Ultraviolet Explorer (EUVE). We have selected white dwarfs cooler than about 50,000 K and with presumably low heavy element abundances. The goal of this study is to determine the fundamental atmospheric parameters, namely the effective temperature and chemical composition, of these stars by fitting their continua with synthetic spectra computed from pure hydrogen LTE/line-blanketed model atmospheres. The question of the presence (or absence) of trace elements is explored by comparing EUV-determined effective temperatures to the one obtained from a fit of hydrogen balmer lines. It is found that the majority of the DA in the sample are consistent with having a pure hydrogen atmosphere. One of the star, MCT0027-634, is another possible example of a HZ 43-type white dwarf, having an effective temperature above 50000 K and a low heavy element abundance, i.e., much lower than predicted by diffusion theory.
Guide to Modeling the Interstellar Medium
Authors: Cox Donald P.
Published: July 2016

We are in a period that is rich with data and ideas, but the collection of ideas is fundamentally incomplete, leaving us with no useful model for the general characteristics of the interstellar medium (Cox 1995). At the same time, we wrestle with tentative models at various levels, and concepts that such synthesis modeling must involve. In the list and commentary that comprise this paper I offer to future explorers what limited wisdom I can.
The Morphology and Physics of the Local Interstellar Medium
Authors: Bruhweiler Fredrick C.
Published: July 2016

We are finally on the threshold of obtaining a coherent morphological and physical picture for the local interstellar medium (LISM), especially the region within 300 pc of the Sun. The EUVE is playing a special role in revealing this picture. This instrument can provide direct measurements of the the radiation field that photoionizes both hydrogen and helium. It also can yield direct measurements of the column densities of hydrogen, but especially He I and He II toward nearby white dwarfs. These observations suggest that the ionization in the Local Cloud, the cloud in which the Sun is embedded, is not in equilibrium, but in a recombination phase. Heuristic calculations imply that the the present ionization is due to the passage of shocks, at times greater than 3 × 106 years ago. The origin of these shocks are probably linked to the supernova which was responsible for the expanding nebular complex of clouds know as the Loop I supernova remnant, of which the Local Cloud is a part, extreme- UV radiation field, that which ionizes both hydrogen and helium in the LISM. Of the ISM within 300 pc, the volume appears to be predominantly filled by hot (106 K) coronal gas. This gas is laced with six largescale shell structures with diameters ~100−150 pc including the long-recognized radio loops, Loop I−IV, as well as the Orion-Eridanus and Gum Nebulae are identified. An idea that has evolved in the literature for over two decades is that the kinematically-linked OB associations representing Gould’s Belt, plus the gas and dust of Lindblad’s Ring, require that previous supernova activity and stellar winds carved out a 400–600 pc diameter cavity some 3 to 6 × 107 yr ago. This activity produced a pre-existing low density region, into which the present young loop structures have expanded. The outer boundaries of the identified expanding loop structures, inside this preexisting cavity, delineate the periphery of the the mis-named “local interstellar bubble.” Thus, this picture naturally explains some of the problems often associated with the presence of this low density region exterior to Loop I.
ALEXIS Lunar Observations
Authors: Edwards B.C. , Bloch J.J. , Roussel-Dupré D. , Pfafman T.E. , Ryan Sean
Published: July 2016

The ALEXIS small satellite was designed as a large area monitor operating at extreme ultraviolet wavelengths (130 − 190 Å). At these energies, the moon is the brightest object in the night sky and was the first source identified in the ALEXIS data. Due to the design of ALEXIS and the lunar orbit, the moon is observed for two weeks of every month. Since lunar emissions in the extreme ultraviolet are primarily reflected solar radiation these observations may be useful as a solar monitor in the extreme ultraviolet. The data show distinct temporal and spectral variations indicating similar changes in the solar spectrum. We will present a preliminary dataset of lunar observations and discussions covering the variations observed and how they relate to the solar spectrum.
RE J1255+266—Detection of an Extremely Bright EUV Transient
Authors: Dahlem Michael
Published: July 2016

During a pointed ROSAT observation in the direction of the Coma cluster of galaxies an exceptionally bright EUV source, RE J1255+266, was detected serendipitously. The source is located close to the Galactic North pole, at bII ~ 89°. Its observed EUV flux (62-110 eV) at the time of the detection was of order 7 × 10−9 ergs s−1 cm−2, making RE J1255+266 temporarily one of the brightest EUV sources on the sky.

The EUV flare of RE J1255+266 has a light curve with a decay time of about 0.86 days. With respect to earlier non-detections, the source brightened by a factor of > 7000. Such a behavior has not been observed before. Thus, it is unclear what type of source RE J1255+266 might be. The most likely optical counterpart is a faint (V ~ 18.5 mag) object with a blue spectrum (taken from an objective-prism Schmidt plate). For more details on the optical identification see the paper by J. Pye (this conference).

Simultaneous observations with CGRO/BATSE resulted in non-detections of the source in the 8-50 keV energy range.
The EUVE Guest Investigator Science Program
Authors: Anderson Ken , Stroozas Brett
Published: July 2016

In order to promote research using data from NASA’s Extreme Ultraviolet Explorer (EUVE) satellite, the Center for EUV Astrophysics has implemented a Guest Investigator (GI) Science Program. The purpose of the GI Program is to provide researchers with information, services, and training in the use of public EUVE data sets; in effect, it offers to the research community the technical experience and intricate knowledge of the EUVE data sets resident at CEA. All interested researchers are encouraged to participate as GIs.
Astrophysical Applications of Delta Scuti Stars
Authors: Breger Michel
Published: July 2016

The δ Scuti stars pulsate with a large number of low-order radial and nonradial p-modes. There also exists some evidence for the presence of g-modes.

The pulsational variability outside the classical δ Scuti star instability strip (A1V - F0V on the main sequence) is examined. New measurements do not confirm the short-period variability of the hot B9p star ET And. However, a number of cool, early-F stars shows variability around one day: the γ Dor variables. An explanation involving nonradial g modes appears the most promising.

An examination of available information on the multi-periodicity of δ Scuti stars shows no systematic behaviour in which pulsation modes axe excited to an observable level with luminosity and temperature. The asteroseismological potential of δ Scuti vaxiables is briefly examined.

So far, the period changes of five luminous Pop. I δ Scuti variables have been studied. While stellar evolution predicts period increases, decreases have been observed and reported for all five stars. The statistical significance of this result and alternative explanations axe briefly examined.
Helioseismology
Authors: Gilliland Ronald L.
Published: July 2016

Observations of solar p-mode oscillations over the last two decades have provided a fundamentally new and unique means of probing the Sun’s interior structure. The history of understanding solar oscillations as trapped normal-mode pulsations will be briefly discussed. The large number of excited modes seen in the Sun have been used to constrain the solar core structure, determine the convection zone depth and to fix the variation of angular velocity with depth and latitude. Standard stellar structure and evolution theory does remarkably well in matching the helioseismic results. Further work promises to reveal the solar helium abundance. Upcoming network and space observations should provide the stability and resolution necessary to return detailed information on near-surface structures associated with solar activity.
Radio Observations of Mira Variables, OH/IR Stars and M-Supergiants
Authors: Chapman Jessica M. , Habing H. J. , Killeen N. E. B.
Published: July 2016

We review radio maser observations of the circumstellar envelopes of oxygen-rich Mira variables, OH/IR stars and M-type supergiants.
Long Term Non Linear Thermal Effects in the Pulsation of Mira Variables
Authors: Yaari A. , Tuchman Y.
Published: July 2016

The main conclusions that can be drawn from most of previous non-linear simulations of LPV (Keeley 1970, Rose & Smith 1972, Wood 1974, Tuchman et al. 1978, 1979) are the following:

In all cases where the fundamental mode is predicted, by the linear analysis, to be the dominant, the pulsations are seen to diverge and lead to a rather violent mass-loss process (Wood 1974, Tuchman, Sack & Barkat 1979).

A steady pulsation with (roughly) a limit cycle does set in only in those cases where the linear analysis predicts that the dominant mode (with the largest value of the growth rate) is the first overtone (or higher). In all these cases the actual limit cycle is found to be in a good agreement with the linear predictions, with regard to the pulsation mode and period (Wood 1974, Tuchman, Sack & Barkat 1979). However, it is important to note that periods, for these overtone pulsators, are always found to be smaller than 300 days (Wood 1974, Tuchman, Sack & Barkat 1979). Increasing the luminosity of these models, in order to increase their periods, results in a switch to the fundamental mode, followed by the violent relaxation oscillations, mentioned above.
Classical Cepheids and Galactic Structure
Authors: Fernie J.D.
Published: July 2016

In this review I consider recent results on the space distribution of classical Cepheids, first dealing with their Z-distribution normal to the galactic plane, and second considering their X,Y components in the plane. In doing so, use is made of a new database of 505 stars known to be or likely to be classical Cepheids which has recently been compiled by Fernie et al. (1995) and which is available electronically on the World Wide Web at the URL http://ddo.astro.utoronto.ca/cepheids.html, and by anonymous ftp at perseus.astro.utoronto.ca (128.100.77.18) in the directory pub/cepheids.

Following the discussion of space distribution, a review of recent work on Cepheid kinematics and the resulting determination of the distance to the galactic centre will be given.
Theoretical Pulsations of Luminous Blue Variables
Authors: Cox Arthur N. , Guzik Joyce A. , Soukup Michael S. , Morgan Siobahn M.
Published: July 2016

Both radial and low degree and order g-mode nonradial pulsations are predicted for luminous blue variables that occur in the blue supergiant region of the Hertzsprung-Russell diagram. It is found that the radial strange modes have very large growth rates due to helium ionization in models at surface effective temperatures between 10,000 and 20,000 K.
Rotation Curve and Metallicity of the Outer Disc from Cepheids
Authors: Pont F. , Queloz D. , Mayor M. , Bratschi P.
Published: July 2016

A group of faint, remote Cepheids in the outer disc of the Galaxy has been observed in spectroscopy and photometry. These Cepheids are used as tracers to measure the rotation curve of the disc beyond R=11 kpc, and to study the metallicity gradient in the outer disc.
Properties of the SX Phœnecis Stars in the Metal-Poor Globular Cluster NGC 5053, and Pulsation Modes of Three of the Newly-Discovered SX Phe Stars in 47 Tuc
Authors: Nemec James M.
Published: July 2016

The properties of variable blue stragglers (i.e., SX Phe stars) in the extremely metal-poor globular cluster NGC 5053 are reviewed. Pulsation modes are derived for three of the SX Phe stars recently discovered in the central regions of 47 Tuc by Gilliland et al. (1995a,b).
EROS: Short Period Cepheids in the Bar of the LMC
Authors: Beaulieu J.P.
Published: July 2016

We present Fourier analysis at 490 nm for 87 Cepheid light curves in the bar of the Large Magellanic Cloud. The photometry has been obtained through the EROS project whose main purpose is the search for baryonic dark matter in the Galactic halo. The very high quality of the photometry and the good phase coverage enable us to obtain accurate Fourier decomposition of the light curves. As a consequence we confirm that the so-called s-Cepheids are overtone pulsators and represent about 30 % of our sample, whereas the Classical Cepheids are fundamental pulsators. We have identified a number of features in plots of amplitude ratios and phase differences versus period. These features are usually related to resonances between different pulsation modes. If we accept that Fourier decomposition techniques applied to light curves yield positions for the centres of resonances, then it seems that resonances occur in LMC Cepheids at the same period as in Galactic Cepheids for fundamental pulsators, whereas they appear to be at shorter periods for overtone pulsators.
SUSI’s Potential for Cepheid Studies
Authors: Booth A.J. , Davis J. , Shobbrook R.R.
Published: July 2016

The Sydney University Stellar Interferometer (SUSI) is the first instrument with sufficient resolving power to measure the angular sizes of a large number of stars over the full range of spectral classes. It can also resolve the orbits of spectroscopic binaries, allowing masses and distances to be determined directly. One of the major programs envisaged for the instrument is a study of pulsating stars, in particular Cepheids.
Rapid Photometric and Spectroscopic Variability of the Be Star DX Eri
Authors: Štefl S. , Balona L. A.
Published: July 2016

Short-period variations of high-resolution He I 667.8 nm line profiles obtained on 5 nights are analysed and compared to extensiveuvbyphotometry obtained during 1986 – 1993. The separation of short-period and medium-term light variations was done for 4 seasons. All prewhitened magnitudes can be phased with a single period of 1.2564 d. The radial velocity and the first moment of the He I 667.8 line follows the photometric period with a semi-amplitude of 42 km s−1. The large amplitude is very difficult to understand in terms of current models of non-radial pulsation or star spots.
The Line Profile Variations of 60 Cyg (HD 200310)
Authors: Koubský P. , Ballereau D. , Chauville J. , Horn J.
Published: July 2016

Results of the preliminary investigation of the monitoring of line-profile variations of the Be star 60 Cyg are reported. At least three apparent absorption or emission bumps moving from blue to red were detected.
Two Modes of Pulsation in the Rapidly Rotating O9.5 Dwarf, HD 93521
Authors: Reid A.H.N. , Bohlender D. , Bolton C.T. , Fullerton A.W. , Howarth I.D. , Manz C. , Smith K.C. , Townsend R.H.D.
Published: July 2016

We present preliminary results of a time-series analysis on the He II λ4541Å, Si III λλ4552, 4567, 4575Å and Hβ λ486lÅ lines of the late O-type dwarf, HD 93521. The spectra show line-profile variations (lpv) − ‘bumps’ and ‘dips’ − of amplitude ∼ 1% of the continuum, moving systematically redwards through the photospheric absorption lines. The analysis implies non-radial pulsation (NRP) as the origin for the lpv.
Fine Analysis of Pulsating CP Stars
Authors: Kupka F. , Gelbmann M. , Heiter U. , Kuschnig R. , Weiss W. W. , Ryabchikova T. A.
Published: July 2016

We present fine analysis results for roAp and λ Boo stars.
Asteroseismology in Open Clusters
Authors: Frandsen S. , Viskum M. , Hernandéz M.M. , Belmonte J.A.
Published: July 2016

Within two groups or networks we are trying to build the observational fundamentals for better tests of stellar evolution. The first group is STACC (Small Telescope Array with CCD Cameras) and the second the EU HCM program ANTENA (involving institutes in Spain, France and Denmark).

The first information needed is photometric data. Colour-Luminosity diagrams and time series photometry must be obtained. The time series require extensive campaigns in order to give the high quality data needed for asteroseismology. Ideally one should have time series of several quantities (see Balona & Stobie 1979, Mantegazza et al. 1994).

The second type of observations is of spectroscopic nature. It is needed to ascertain membership of the stars in a given cluster and to check binarity. Also rotational velocities must be observed to help the attempt to identify the modes in 6 Scuti stars. Finally some information about metallicity and peculiarity is required.
Insights into the Pulsational Behaviour of δ Sct Stars
Authors: Mantegazza L. , Poretti E.
Published: July 2016

Extensive photometric monitoring of bright δ Scuti stars, made in the last years, allowed us to get reliable frequency determinations also in very complicated cases. Table 1 gives a full description of our photometric targets. Recently, we realized that combining photometry with simultaneous high resolution spectroscopy, in order to study line profile variations, offers the possibility to perform a much more reliable mode identification. Hence, the photometric runs of FG Vir, X Cae, HD 2724 were paired with spectroscopic ones obtained at the Coudé Auxiliary Telescope (ESO, La Silla, Chile). The stellar spectra are centred at 4508 Å and they cover a range of 37.6 Å; the resolving power is about 50000-60000 and the linear dispersion is 2.4 Å/mm.
Simultaneous Photometric and Spectroscopic Observations of the unusual Variable F-type stars HD 224638 and HD 224945.
Authors: Mantegazza L. , Poretti E. , Bossi M. , Zerbi F.M.
Published: July 2016

The stars HD 224638 and HD 224945 belong to the new class of F0 V spectral type stars which show an unusual kind of variability for that region of the HR diagram (Mantegazza et al. 1993).

The variability of these two stars has been discovered and studied by Mantegazza, Poretti & Zerbi (1994). They show small amplitude light variations with characteristic time scales of the order of one day. The light curves are not periodic and it is difficult to satisfactorily fit them even with several periodic terms.

In order to explain our 1991 B colour observations we tentatively suggested that in both stars two close periodic terms were present with a characteristic frequency of about 0.8 c/d, and that these terms had a double wave shape for HD 224638 and a triple wave one for HD 224945.
The Cepheid Data Base
Authors: Berdnikov L. N.
Published: July 2016

Since 1981, we have been regularly carrying out photoelectric observations of Cepheids. By now nearly 20450 observations in UBVRI have been acquired. These observations are the basis of our Cepheid data base; presently we have collected on magnetic media near 88000 photoelectric as well as near 18000 radial velocity measurements. We shall continue to append to this Cepheid data base our new observations as well as published data.
Models of the Double-mode Cepheids in the Large Magellanic Cloud
Authors: Petersen J. O. , Christensen-Dalsgaard J.
Published: July 2016

Very recently the MACHO Collaboration published detailed data for 45 double-mode Cepheids in the Large Magellanic Cloud. Here we compare these data with new stellar pulsation models. We conclude that the new period ratios are in good agreement with a metal content Z = 0.005 – 0.01 in the Large Magellanic Cloud.
Uncertainties in the Galactic Cepheid Distance Scale
Authors: Barnes Thomas G. , Moffett Thomas J.
Published: July 2016

We have examined the uncertainties involved in using the visual surface brightness technique on Galactic Cepheid variables. The random error in a single Cepheid distance measurement is well determined to be ±8%. An upper limit to the systematic uncertainty is shown to be ±6% in distance. These combine for a single Cepheid to a typical uncertainty of ±10% and, for samples larger than ten Cepheids, to a typical uncertainty of less than ±6% in distance.
Baade-Wesselink Analysis of Type II Cepheids
Authors: Vinkó J. , Balog Z.
Published: July 2016

New physical parameters of selected Cepheid variables classified as CW or CEP (Type II) in GCVS are presented. The analysis is based on published light and radial velocity curves. Two of the stars turned out to be classical Cepheids.
The Centre-of-Mass Velocity of a Radially Pulsating Star: Insights from NLTE Models
Authors: Sasselov Dimitar D.
Published: July 2016

The problem of deriving the centre-of-mass velocity of a radially pulsating star is reexamined. New observations of line asymmetry and Non-LTE radiation hydrodynamics point to a systematic effect of about 1 km s−1 in Cepheids.
Accurate CCD Light Curves of Cepheids in the Large Magellanic Cloud
Authors: Poretti E. , Mantegazza L. , Antonello E.
Published: July 2016

In the past years, in a series of papers (Antonello & Poretti 1986, Antonello et al. 1990, Mantegazza & Poretti 1992, Poretti 1994) the Fourier decomposition was successfully applied to the light curves of galactic Cepheids with P < 8 d. The separation of these Cepheids into two groups was evident. The first, more numerous (195 stars after the latest inclusion) group is constituted by the classical Cepheids, i.e., the Cepheids which follow the Hertzsprung progression: they occupy a narrow region in the ϕ21 – P plane and are also characterised by a R21 ratio larger than 0.20. The second group, containing 30 objects, is constituted by Cepheids which deviate from the Hertzsprung progression, describing a ‘Z’ crossing the classical sequence in the ϕ21 – P plane; their R21 values are smaller than 0.20. The relation between stars of the upper and lower sequence of the ‘Z’ is confirmed by the ϕ31 – P plane, where these stars describe an unique sequence. As first suggested by Antonello & Poretti (1986), the splitting of Cepheids into two groups can be explained by a different pulsation mode: while the classical Cepheids are pulsating in the fundamental radial mode, the other Cepheids are pulsating in the first overtone radial mode.
On the Variability of Supergiant Stars
Authors: Burki G. , Kienzle F.
Published: July 2016

The la-type supergiants of spectral type B to G are probably all variable in radial velocity and luminosity; in addition, on the average, the amplitude of the variations increases with increasing intrinsic luminosity and are larger for the earliest and latest spectral types (Abt, 1957; Maeder & Rufener, 1972; Burki et al., 1978; Grenon, 1993). The variations are not strictly periodic, but can be frequently described by a characteristic time Tc, i.e., some kind of ‘most probable periodș (e.g., Sterken, 1977; Rufener et al., 1978; Percy et al., 1979; van Genderen et al., 1992).

It is possible to define a Tc-luminosity-colour relation for the supergiant stars (Maeder & Rufener, 1972; Burki, 1978). However, this relation exhibits a large dispersion, due to the imprecise luminosity and colour determinations for these stars and, essentially, to the fact that each supergiant can show very different Tc values. Good determinations of >Tc require very long-term, continuous, photometric monitorings.
Pulsation Properties of a Sample of Mira and OH/IR Stars
Authors: Gaylard M. J. , West M. E.
Published: July 2016

Well-sampled light curves have been obtained for 1612-MHz hydroxyl masers in Mira and OH/IR stars monitored with the Hartebeesthoek radio telescope. Pulsation modes are investigated via the ratios of the observed periods in the maser variations.
Light Curve Analysis of Long Period Pulsating Stars with the Wavelet Method: The semiregular star V Boo
Authors: Szatmáry K. , Gál J. , Kiss L. L.
Published: July 2016

We have collected long data series of some SR and Mira stars from data banks. In many cases the light curves show strong fluctuation or variation in the period, phase and amplitude. The detection of the significant changes of these parameters is important from the point of view of pulsation theory and stellar evolution. We present here the analysis of V Boo. The used method is the Fourier and wavelet analysis in order to point out the modulations in the frequency and amplitude.
The AAVSO and its Variable Star Databank on Pulsating Stars
Authors: Mattei Janet A.
Published: July 2016

The American Association of Variable Stars Observers is the world’s largest organization of variable star observers. Its services to the astronomical community and database on pulsating variables are described.
Pulsating Degenerate Stars in the EC Survey
Authors: Stobie R. S. , Koen C. , Kilkenny D. , O’Donoghue D.
Published: July 2016

For asteroseismological reasons the discovery of new pulsating degenerate stars is important. Bradley (1993) lists the number of known pulsating white dwarfs at that time - 23 known DAV (ZZ Ceti) stars, 7 known DBV stars and 5 known DOV (PG1159) stars. Of these degenerate pulsators 80% (including all DBVs) are in the northern hemisphere. This illustrates the great incompleteness in the search for such objects in the southern hemisphere.

The Edinburgh-Cape Blue Object Survey is a potential source of candidate degenerate pulsators in the southern hemisphere (Stobie et al. 1992). The blue stellar objects are identified from COSMOS scans of U and B plates taken with the UK Schmidt Telescope. Follow-up photometry and spectroscopy are being obtained at the South African Astronomical Observatory (SAAO) to classify and determine the nature of each blue stellar object. Currently, over 190 DA white dwarfs have been identified, of which the majority are not in existing catalogues (O’Donoghue et al. 1993). From these white dwarfs, we have selected DA stars with B-V colours near or within the range 0.15 ≤ B-V ≤ 0.25 together with all the non-DA degenerate stars (DO, DB, etc.) to monitor for variability.
The Effect of Proper Motion on in Pulsating Stars
Authors: Pajdosz G.
Published: July 2016

We show that the proper motion of a pulsating star results in a secular increase of the radial velocity of the star and consequently contributes to the observed . We found that this mechanism is very important in studying period changes in pulsating DA white dwarfs, where its effect is comparable with the expected evolutionary period change in these stars due to their cooling.
Applications of Indirect Accretion Disc Imaging to Orbital Solutions for Primary Stars
Authors: Still Martin
Published: July 2016

The general inability of optical observations of low mass interacting binaries to provide accurate radial velocity measures for the stellar components is a problem for which there is no universally applicable solution. Velocity measurements are handicapped because the accretion disc about the primary often dominates the optical spectrum. Therefore spectroscopists resort to measuring line profile symmetry in individual spectra of the accretion disc in order to provide information on the primary’s motion. However, discs are not axisymmetric structures and the standard approach generally fails because the asymmetry-free inner disc emission is usually dominated by statistical noise.
Distances and Absolute Magnitudes of a Sample of Faint Cataclysmic Variables
Authors: Sproats L. N. , Howell S. B. , Mason K. O.
Published: July 2016

Our current knowledge of the basic characteristics of CVs, such as their distance and luminosity, are largely based on observations of optically bright (V < 16 mag) systems. This has tended to favour the more luminous members (Mv ~ 7 … 10 mag) lying nearby in the Galactic plane (z < 400 pc;). In order to alleviate this bias, Howell & Szkody (1990) compiled a list of ~90 CVs that were faint enough and were at high galactic latitude to potentially place them well outside the Galactic plane, in the halo. We have obtained infrared J and K band images of a number of the CVs listed by Howell & Szkody (1990) in order to establish whether these systems are truely at large distances, or are instead a population of intrinsically faint objects lying relatively nearby. The distances were determined using Bailey’s method (1982) and incorporated the recent SK calibrations by Ramseyer (1994).
Studies in Flickering
Authors: Welsh W. F. , Wood Janet H. , Horne K.
Published: July 2016

Flickering is a poorly understood phenomenon associated with accretion processes, but this does not suffice to make it interesting. Why then should we bother studying this ‘noise’? Three reasons come to mind: (i) flickering is a fundamental signature of accretion, to the point of being a necessary characteristic (if it doesn’t flicker, it’s not a CV); (ii) energetically, flickering is not a small effect and can contribute up to a few tens of percent of the total luminosity of the system (hence it is often more luminous than the entire secondary star); (iii) flickering is inherently a time-dependent phenomenon and it is hoped that it can provide clues to the nature of the disc viscosity, something that time-independent theory cannot provide.