bluuebunny/crossref_metadata_2025 huggingface dataset:
Flares: the Solar-Stellar Connection
Authors: Byrne P. Brendan
Published: July 2016

The systematic study of stellar flares in now more than 30 years old. Throughout that time the solar flare paradigm has been used, highly successfully, to understand various aspects of the stellar phenomenon. In this introductory overview I will attempt to justify the use of the solar model in understanding the stellar equivalent. But I will also point out differences between the two which may require extension of the basic solar model and speculate briefly on what these extensions might be.

There are many excellent previous reviews of stellar flares in the literature (Haisch 1989, Byrne 1983, 1992), to which the reader is referred, along with the other reviews in this volume, for further reading.
Radio Emission from Flares in Single Late-type Stars
Authors: Benz Arnold O.
Published: July 2016

Radio observations provide the most direct information on non-thermal electrons in stellar flares and in the coronae of late-type stars. Radio emissions of single main-sequence F, G, and of many K stars have recently been discovered, in addition to the well-known dwarf M stars. Their long-duration radio flares with low circular polarization, slow variations and broad bandwidth can be attributed to gyrosynchrotron emission of mildly relativistic electrons. The same holds for the low-level (‘quiescent’) radio emission. On the other hand, highly polarized radio flares of M stars have been interpreted by coherent emissions from loss-cone instabilities of magnetically trapped electrons. These conjectures are consistent with recent VLBI observations. The identification of the radio emission process allows to estimate the high-energy component of the flare and compare it to the total flare energy. The weakly polarized radio emission may serve as a proxy for hard X-ray signatures of relativistic electrons. The fraction of primary energy released into energetic electrons then appears to be large and similar to solar flares.
The Red Dwarf Star Population in the Galaxy
Authors: Mirzoyan L.V.
Published: July 2016

Red dwarf stars in the Galaxy occur in three forms: as flare stars, T Tau stars and red dwarf stars of constant brightness. Haro (1957) suggested that all these stars present regular evolutionary stages of red dwarf stars.

The space distribution of UV Cet flare stars in the solar vicinity indicates that these low luminosity stars belong to the disk population of the Galaxy. Therefore one can suppose that all red dwarf stars have a space distribution which is similar to that of the flare stars (Mirzoyan et al. 1988a). Only a small part of them is found in star clusters and associations. The T Tau stars are an exception: because of their very short lifetime, all of them are found in stellar associations.

The space distribution of red dwarf stars is determined by the fact that all of them are formed in star clusters and associations and are finally lost from their stellar systems and merge into the general galactic field (Mirzoyan 1995).
New EUV Selected Flare Stars
Authors: Ball Bill , Bromage Gordon
Published: July 2016

The ROSAT Wide Field Camera all-sky EUV survey of 1990-91 identified over 200 active late-type stars (Pounds et al. 1993). Most of these were not known to be active prior to the survey and follow-up optical spectroscopy has been successful in selecting the most active dMe stars. These stars are characterized by their Balmer line emission which is direct evidence for an active chromosphere. We present photometric monitoring data for 4 new dMe stars, including the discovery of flaring activity on all 4 stars.

The observations were made at the South African Astronomical Observatory at Sutherland near Capetown in October 1994, using the 0.5m and 1.9m telescopes for photometry and spectrometry, respectively. The 0.5m telescope is equipped with an automated, pulse-counting photometer. Flare monitoring of all the stars was done in the U-band with either 5 or 10 s integration times. All the stars monitored were between magnitude V = 11 and 13. A total of 15 flares were detected, including at least one on each star (Table 1). Comparison stars were measured in standard UBVRI photometric bands for each star.
Spectral and Photometrical Investigation of EV Lac in Different States with High Time Resolution
Authors: Beskin G.M. , Mitronova S.N. , Panferova I.P.
Published: July 2016

Spectroscopic and photometric monitoring of the flare star EV Lac was carried out with the 6-m telescope of the Special Astrophysical Observatory on the following 3 nights: 1993 September 10 (18:54-20:16 UT), 11 (17:46-20:03 and 21:25-22:16 UT) and 12 (19:46-20:08 and 21:23-22:21 UT).

We used the observation method described in detail by Berdyugin et al. (1994) and Alekseev et al. (1994). Observations were carried out with the 1024 channel TV scanner (Drabek et al. 1986) installed on the spectrograph SP-124 at the Nasmyth focus which covered a wavelength range 3480-5530Å with a spectral resolution 2Å per channel. Exposure times were 3 — 5 minutes, the signal/noise ratio was 20 at 3500Å and 40 at 5500Å and dead time between exposures was 10 — 15 sec. 92 spectra were obtained in total.
Photoelectric Observations of AD Leo: 1989-1994
Authors: Konstantinova-Antova R. , Antov A.P.
Published: July 2016

An investigation of the activity of the flare star AD Leo was made using U-band patrol observations. 60 cm computer-controlled telescopes with identical single channel photon-counting photometers at the National Astronomical Observatory at Rozhen and at the Belogradchik Observatory were used. The integration time was 1 sec. Differential photometry was carried out, the AD Leo measurements were made relative to BD+20°2475. The data was reduced with the program package APR (Kirov et al. 1991).

Two simultaneous observing runs were made. The results of the first have already been published (Antov etal. 1991). The other was part of the observational campaign in May, 1991 with the ROSAT satellite. In Bulgaria, the observations were carried out at Rozhen in the U-band and at Belogradchik in the B-band. Four flares were detected by both.
Flares Detected on Some Late Giants: Are They Real?
Authors: Konstantinova-Antova R. , Ivanov M.M. , Antov A.P.
Published: July 2016

Recently, some apparently single red giants with evidence of chromospheric activity were reported. In this work we observed those single late giants with reported flares, referenced by Pettersen (1989) and Schaefer (1989), as part of a program for investigation of the chromospheric activity in G and K giants.

According to Simon & Drake (1989), active single G and K giants may be first crossing stars that have just entered the phase of rapid spin-up as their convection zones advance upon their more rapidly rotation radiative cores. These stars are situated on the base of the red giant branch on the HR diagram. As a result they may possess dynamo driven activity at this stage of evolution.
A Long-Duration Pre-flare Dip in the Light Curve of V1054 Oph
Authors: Ventura R. , Pagano I. , Peres G. , Rodonò M.
Published: July 2016

Photometric observations of V1054 Oph (Wolf 630 AB, mv = 9.69) and of the comparison star HD 152678 (mv = 8.1) were carried out in the UBV bands at Catania Astrophysical Observatory on 1994, June 15, using the 91 cm Cassegrain telescope equipped with an improved version of the twin-beam photometer URSULA (De Biase et al. 1988). We have detected a relatively low-intensity flare event preceded by a pre-flare dip of low amplitude and the exceptionally long duration of 36 min. The integration times were 10, 5 and 5 sec in the U, B and V bands, respectively. The variable and the comparison stars were observed simultaneously in the two independent channels of the twin-beam photometer. The sky background was subtracted from the photon count rates of V1054 Oph and its comparison star in each spectral band, and the flux of the variable was normalized to that of the comparison star.
CCD Monitoring of Flare Stars in Stellar Aggregates
Authors: Kurtanidze Omar M.
Published: July 2016

After Haro’s fundamental discovery of flare stars in stellar associations and young clusters, their importance was fully recognized. The flare star system provides one of the most important records of the stellar aggregate’s history. This record can be used to establish the chronology of these systems and to test theories of star and aggregate formation.

Unfortunately, the observational material contains an unavoidable strong selection with respect to the statistics and the physical characteristics of flare stars in aggregates. This explains the difficulties which arise when one tries to compare physical and statistical characteristics of flare stars in aggregates and in the solar neighborhood, since the latter objects are studied almost exclusively by photoelectric methods. The flare frequency in aggregates is more than one order of magnitude smaller than that of flare stars in the solar neighbourhood. This is probably a selection effect, since the photographic observations in aggregates have been carried out with exposure times 5-10 min. Small amplitude flares cannot be recorded at all. Flare stars in aggregates are usually objects whose recorded flares have amplitudes > 0m.6 − 0m.7, and which last for at least 5 min. The long exposure masks the true amplitude of the flare.
Flare Stars Database: Present Status
Authors: Tsvetkova K.P. , Tsvetkov M.K. , Stavrev K.Y.
Published: July 2016

An updated data collection for UV Cet type stars and their registered flare events in stellar aggregates and in the solar neighbourhood during more than 9580 hours observing time is described. The Flare Stars Database (FSDB) presently includes 3127 entries for 1622 stars.
Optical Observations on Milli-, Micro-, and Nanosecond Timescales
Authors: Dravins D. , Lindegren L. , Mezey E.
Published: July 2016

Instrumentation and observing methods are developed for optical high-speed astrophysics, aiming at exploring milli-, micro-, and nanosecond variability. Such rapid fluctuations can be expected from instabilities in accretion flows, and in the fine structure of photon emission. For the optical, we have constructed a dedicated instrument, whose first version was tested on La Palma to study atmospheric scintillation on very short timescales. A second version is now under development, using photon-counting avalanche photodiodes as detectors.
Physical Processes During Impulsive Solar and Stellar Flares
Authors: Katsova Maria , Livshits Moissei
Published: July 2016

Investigations of impulsive flares on both the Sun and red dwarf stais during more than 30 years allow us to arrive at quite definite conclusions. Here we will consider impulsive events; on the Sun the impulsive phase of a flare is observed as a hard X-ray burst with the emission of photons with energies E > 30 keV up to the γ-ray range. At the same time microwave radio bursts, and sometimes UV and optical continuum bursts are registered. Typical durations of these processes are ∼l-3 min. In this time interval other kinds of flare emission like soft X-ray (2-10 keV) emission, meter radio bursts and Balmer line emission begin to rise, but their maxima occur later on, in the gradual (thermal) phase of the flare.

Impulsive stellar flares are often observed as a significant increase in optical continuum, especially in the U-band, of similar duration (1-3 min), and this time interval is, like in the solar case, the rise phase of the soft X-ray emission.

Modern observations demonstrate that both the impulsive phase of a flare or an impulsive flare develops in low-lying loops. Earlier only indirect evidence existed in optical and radio data. Recently, however, the heights of the hard X-ray sources in impulsive solar events were determined directly from YOHKOH’s HXT (Kosugi 1994, Masuda 1994) (Fig. 1a). Statistically, the height of the hard X-ray source in the 14-23 keV range is 9700 ± 2000 km above the photosphere, and this height reduces to 6500 km in the 53-93 keV range. Besides two hard X-ray sources in the loop footpoints, a third hard X-ray source exists at the top of the loop at least in some cases. The authors of this experiment suppose that the appearance of this loop-top source is due to reconnection in the impulsive phase. Note that the reconnection begins close to the apex of the loop, when this loop is filled by hot plasma that evaporated from both footpoints.
Actvity on T Tauri Stars
Authors: Gullbring Erik , Gahm Gösia F. , Barwig Heinz , Chen Peicheng
Published: July 2016

We have made a detailed investigation of the short-term variability of the classical T Tauri star BP Tauri in UBVRI. Data were collected from the Wendelstein Observatory in 1991, 1992 and 1993 with time resolutions down to 1 sec. The 0.8m telescope was equipped with a fiber-fed fifteen channel high-speed photometer (Barwig et al. 1987). Observations (in UBV) were also collected in China at the Yunnan and Shanghai Observatories to get a long base line in time. To search for differences in the properties between the brightness variations of classical T Tauri stars (CTTS) and T Tauri stars with weak emission lines (WTTS) we performed simultaneous photometry (in the UBV and Strömgren systems) and spectroscopy of 6 young stars during two observing periods at ESO La Silla. The study concerned mainly short-term variability on time-scales of minutes to a few hours. The sample contained two CTTS, SY Ori and VW Cha; three WTTS, San 1, SZ Cha and ADA 481 and one post-T Tauri candidate, HD 70309B.
A Very Active T Tauri Star in NGC 7129
Authors: Semkov Evgeni
Published: July 2016

The star forming region NGC 7129 was first studied by Herbig (1960), who discovered two bright (BD+65° 1637 and LkHα234) and some weaker Hα emission stars. Subsequent investigations in this region led to the detection of five Herbig-Haro objects (Strom et al. 1974, Gyulbudaghian et al. 1978, Eiroa et al. 1992) and one T Tauri star, V350 Cep (Gyulbudaghian & Sarkissian 1977). Some non-stable stars in NGC 7129 are surrounded by reflection and cometary nebulae.

Our Hα-emission survey (Semkov &: Tsvetkov 1986) revealed an interesting irregular variable star in the dark clouds near the emission nebulae. On objective prism plates the star shows very strong Hα-emission and was included in our list of such stars as No. 7. An identification chart and the coordinates can be found in Semkov (1993a). The observations reported here include long-time photographic photometry and CCD spectroscopic and photometric observations.
Flares and Flickering in the Cataclysmic Variable AE Aquarii
Authors: Welsh William F. , Horne Keith , Gomer Richard
Published: July 2016

Detected from the radio to the γ-ray bands, AE Aquarii is an extraordinary cataclysmic variable. It is a relatively bright (V ∼11.4), non-eclipsing binary with an orbital period of 9h.9. The mass-donor secondary star is of approximate spectral type K5V and contributes about ∼ 80% of the total flux in the optical. While the mass ratio is well determined at q = M2/M1 = 0.646, the individual stellar masses are poorly known since the inclination is unknown. AE Aqr’s flickering behavior is unique among CVs, alternating from periods of very violent activity to near total quiescence. Large amplitude flares have been detected in the radio, optical and UV. Previous work has shown the optical flare spectrum to be similar to stellar flare spectra.

Four nights of exactly simultaneous Hα spectroscopy and photometry were obtained on the 2.5 and 1.5m telescopes at Mt. Wilson on 27-30 July 1982. The simultaneity allowed us to renomalize the spectroscopy to agree with the photometry, giving us spectrophotometric data (spectral resolution ∼ 50 km s−, time resolution ∼ 68). A total of ∼ 17h of data were obtained, resulting in 10,098 spectra.
Multi-wavelength Flashes from GRBs
Authors: Hartmann Dieter H.
Published: July 2016

The mystery of the nature and origin of gamma-ray bursts (GRBs) has persisted over two decades of intensive research, observational and theoretical. The basic problem is the absence of quiescent or transient counterparts at other wavelengths, resulting in distance scale uncertainties of more than 10 orders of magnitude. Searches for counterparts are hampered by the lack of sufficiently accurate localizations and by a lack of criteria that uniquely identify the expected appearance of counterparts (although see Meszaros &: Rees 1993, Meszaros et al. 1994). The detection of archival optical transients (OT’s) inside γ-ray burst error boxes suggested burst recurrence and generated hope for a breakthrough in the γ-ray burst counterpart search (e.g., Schaefer 1990). However, counterparts of these OT’s have never been identified, and the reality of several archival images was questioned (Zytkow 1990, but see Schaefer 1990). The search for multi-wavelengths transients has recently been extended to neutrino- and gravitational wave emission. The status of multiwavelength surveys for quiescent or transient counterparts prior to 1988 was reviewed by Hartmann & Woosley (1988). Here we update that review and discuss “optical” emission in view of the current debate on the burster distance scale. Other recent reviews are by Schaefer (1994) and Greiner (1995).
Simultaneous Optical/Gamma-ray Observations of GRBs
Authors: Greiner J. , Wensel W. , Hudec R. , Varady M. , Štěpán P. , Spruný P. , Florián J. , Moskalenko E.I. , Barabanov A.V. , Ziener R. , Birkle K. , Bade N. , Tritton S.B. , Ichikawa T. , Fishman G.J. , Kouveliotou C. , Meegan C.A. , Paciesas W.S. , Wilson R.B.
Published: July 2016

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

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

It was exactly 22 days before Cuno Hoffmeister’s 65th birthday when I began my work at Sonneberg Observatory, following my studies at the University of Jena. In those days, all Sonneberg astronomers were busy preparing scientific papers which they were going to dedicate to their leader, with the exception of myself. I was not able to write a scientific paper in such a short time.

And I was quite ashamed when on the 2nd of February 1957 I had to offer my birthday congratulations empty-handed. But I distinctly remember how extremely happy he looked. Hoffmeister was near the zenith of his scientific career. New buildings and domes were planned or under construction, and the Zeiss company of Jena had promised to loan a 40 cm astrograph in compensation for the loss of a telescope to the Soviet Union as part of the reparations agreement of 1945.
EUV and Soft X-ray Evidence for Partially Ionized Gas in Active Galactic Nuclei
Authors: Mittaz J.P.D. , Lieu R. , Bowyer S. , Hwang C.-Y. , Lewis J.
Published: July 2016

We present a synoptic study of active galactic nuclei (AGN) detected by EUVE. We also present complementary ROSAT PSPC spectra for these sources and for other AGN in directions of low galactic absorption. It is found that the best-fit power-law photon indices of the X-ray spectra at 0.1–2.4 keV are anti-correlated with their galactic hydrogen columns. The indices for the 0.9–2.4 keV range do not show such a correlation, and are considerably smaller (i.e. flatter). We discuss a number of possible interpretations of this correlation but only one of these, the presence of a partially ionized absorbing gas in the AGN, explains the observations satisfactorily. The ubiquity of this effect suggests that this component be may very common in AGN.
Extreme Ultraviolet Spectroscopy of the Seyfert 1 Galaxy Markarian 478
Authors: Liedahl D.A. , Paerels F. , Hur M.Y. , Kahn S.M. , Fruscione A. , Bowyer S.
Published: July 2016

The Seyfert 1 galaxy Mrk 478, observed during the EUVE all-sky survey, is the brightest EUV source among its class. The SW spectrum of this object shows evidence of discrete emission, although this interpretation is tentative, since the source spectrum must be extracted against a bright background. If the EUV flux is, in fact, composed partly of line emission, we consider the implications if this is the result of emission from a collision-driven plasma at temperatures ≳ 106 K. In this context, we discuss some of the constraints imposed on the emission-line region by this observation.
EUVE Observations of the Seyfert Galaxy MRK 279
Authors: Hwang C.-Y. , Bowyer S. , Lampton M.
Published: July 2016

We report EUVE spectral and photometric data of the Seyfert 1 galaxy MRK 279. The photometric data show large amplitude variations over time scales less than 10,000 s. The spectrum is characterized by several features between 80 and 100 Å. We compare the observed data with several models. We can rule out the possibility that the EUV emission is from a diffuse corona or intercloud medium. Models that assume the soft X-ray/EUV emission results from reprocessing in an optical BLR region are also inconsistent with the data. A collisional excitation model is consistent with the observations but requires a cloud density ≥ 1011 cm−3.
Cool Stars in the EUV: Spectral and Structural Variability
Authors: Brown Alexander
Published: July 2016

EUV observations of RS CVn binaries and M dwarf flare stars are used to illustrate the variability of coronal emission seen in EUVE spectra and photometry. The EUVE emission line spectra show that the quiescent coronae of active stars are extremely hot (≥10 million K) and flares, which are even hotter, are common. Spectral sequences showing the evolution of flare temperature and density are presented. Very high coronal electron densities (log Ne ~ 13) have been detected during M dwarf flares. EUVE data for the RS CVn binary HR1099 are compared with those from simultaneous observations in other spectral regions, including X-ray (ASCA), ultraviolet (IUE), and radio (VLA, AT) data. These combined data show that the peak coronal temperature is above the sensitivity of EUVE and that the time of maximum flare brightness is spectral region dependent.
High Temperature Structure in Cool Binary Stars
Authors: Dupree A.K. , Brickhouse N.S. , Hanson G.J.
Published: July 2016

Strong high temperature emission lines in the EUVE spectra of binary stars containing cool components (Alpha Aur [Capella], 44ι Boo, Lambda And, and VY Ari) provide the basis to define reliably the differential emission measure of hot plasma. The emission measure distributions for the short-period (P ≤ 13 d) binary systems show a high temperature enhancement over a relatively narrow temperature region similar to that originally found in Capella (Dupree et al. 1993). The emission measure distributions of rapidly rotating single stars 31 Com and AB Dor also contain a local enhancement of the emission measure although at different temperatures and width from Capella, suggesting that the enhancement in these objects may be characteristic of rapid rotation of a stellar corona. This feature might be identified with a (polar) active region, although its density and absolute size are unknown; in the binaries Capella and VY Ari, the feature is narrow and it may arise from an interaction region between the components.
A Search for Rotational Modulation in the EUV Emission from AB Doradus
Authors: White S.M. , Lim J. , Rucinski S.M. , Roberts G. , Kilkenny D. , Ryan S.G. , Prado P. , Kundu M.R.
Published: July 2016

One of the goals of the EUVE observation of AB Doradus was to search for rotational modulation of the EUV emissions. In support of this goal we carried out optical photometry in Chile, Australia and South Africa and radio observations in Australia. In addition, an ASCA observation of AB Dor was scheduled to occur during the campaign. Several spectacular X-ray and optical flares with accompanying brightening in the EUV were seen, but no rotational modulation of the EUV emission was evident, except for a dip in the He II 304 Å line coincident with the optical minimum. An X-ray flare was seen with no accompanying EUV flare.
Post-Eruptive Flare Energy Release as Detected on AU Mic by EUVE
Authors: Katsova M.M. , Drake J.J. , Livshits M.A.
Published: July 2016

The long-duration emission arising after the impulsive rise and decay in a flaring event observed by the Extreme Ultraviolet Explorer on the red dwarf star AU Mic is discussed. The decay of the intensity in the Deep Survey 65–190 Å band and in the Fe XVIII line during this prolonged event is 10 times slower than the time of radiative cooling of coronal loops with the typical for the flare plasma density. The temporal behavior of the emission measure is determined for both the 65–190 Å band and the Fe XVIII line fluxes. The total energy emitted in the 1–2000 Å region over nearly 12 hrs is 3 · 1035 ergs. We first point out some difficulties with earlier explanations proposed for this event; we then propose the following physical model: the source of the prolonged emission is a system of high coronal loops, the size of which is more than the active region scale, but less than the stellar radius. Such systems are observed in soft X-rays during large solar flares after coronal mass ejections. Some additional post-flare energy input into this high coronal loop system can be caused by reconnection in a vertical current sheet, and this post-eruptive energy release provides prolonged and intensive EUV emission.

Apparently, we are faced here with new kind of the surface activity on late-type stars which is intermediate between impulsive flares on red dwarfs and long-duration, powerful events the subgiants components of the RS CVn binaries.
The EUV Flux of Chromospherically Active Binary Stars
Authors: Giménez Alvaro , Dous Constanze La
Published: July 2016

We have compared EUVE and ROSAT/WFC photometric measurements of chromospherically active binary systems at around 100 Å. Long term variations have found for some 20% of the systems as well as a tight linear correlation between the EUV and soft X-ray fluxes.
Equilibrium Abundances of Heavy Elements Supported by Radiative Levitation in the Atmospheres of Hot DA White Dwarfs
Authors: Chayer P. , Vennes S. , Pradhan A.K. , Thejll P. , Beauchamp A. , Fontaine G. , Wesemael F.
Published: July 2016

We present revised estimates of the equilibrium abundances of heavy elements supported by radiative levitation in the atmospheres of hot DA white dwarfs. We emphasize, in particular, the role of trace pollutants that may be present in the background plasma, an effect which has been heretofore neglected. We take advantage of the availability of a table of detailed monochromatic opacities calculated for a plasma made of H containing small amounts of C, N, O, and Fe to illustrate how the equilibrium abundances of levitating elements react to the flux redistribution caused by the addition of these small traces of opaque material. We also consider two other improvements: a more sophisticated treatment of the momentum redistribution process and ion experiences following a photoexcitation, and use of an upgraded value for the line profile width associated with pressure broadening.
Confining the Edges of the GW Vir Instability Strip
Authors: Werner Klaus , Dreizler Stefan , Heber Ulrich , Rauch Thomas
Published: July 2016

We report on our NLTE model atmosphere analyses of PG 1159 stars. The results enable us to confine the location of the GWVir instability region in the Hertzsprung-Russell diagram. The analysis of a spectrum of the non-pulsator PG 1520+525 taken with the EUVE satellite in comparison with HST data of the pulsating protoype PG 1159-035 (=GW Vir) locates the blue edge of the instability strip near Teff=140 000 K for stars in the respective luminosity range.
New Insights on the Interstellar Medium from EUV Observations
Authors: Bowyer Stuart
Published: July 2016

Observations in the EUV band have provided new insights into the interstellar medium. In the following I discuss two areas in which EUV observations are providing unique information: the ionization state of the ISM, and the pressure of the hot phase of the local interstellar medium and the bearing of this work on the McKee-Ostriker model of the ISM.
ALEXIS Observations of the Diffuse Cosmic Background in the Extreme Ultraviolet
Authors: Smith Barham W. , Pfafman T.E. , Bloch J.J. , Edwards B.C.
Published: July 2016

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

In the standard theory of dwarf novae in outburst, the boundary layer region between the inner edge of the accretion disk and the white dwarf surface radiates primarily in the extreme ultraviolet. Using EUVE, observers have been able to obtain spectra with sufficient spectral resolution to characterize accurately the emission from several dwarf novae in outburst, including U Geminorum and SS Cygni. I present an overview of the observations and early analyses of the dwarf nova observations. The spectra obtained of dwarf novae are complex compared to the EUV spectra of magnetic cataclysmic variables and single white dwarfs. Detailed spectral modeling of an expanding atmosphere will most like be required to fully understand the spectra. Nevertheless, we already know there were significant differences in the effective temperatures and other properties of the EUV emissions. If we assume the EUV emission arises primarily from the boundary layer and parameterize the EUV spectrum in terms of a blackbody, then for U Gem the derived boundary luminosity is comparable to the disk luminosity, consistent with the standard theory, and the minimum size of the emitting region is about that of the white dwarf surface. The count rates from U Gem were modulated strongly with the orbital period; differences in the shape of the spectrum in eclipse and out of eclipse suggest that while the bulk of the emission arose from the vicinity of the white dwarf, there was an extended source of emission as well. For SS Cyg, however, the derived boundary layer luminosity was a small fraction of the disk luminosity. In U Gem, the effective temperature dropped during the decline from outburst. In contrast, in SS Cyg, the effective temperature remained constant as the count rate rose by a factor of 100 and the effective size increased. Thus while the observations of U Gem seem broadly consistent with the standard theory for the boundary layer emission from dwarf novae, SS Cyg appears to present fundamental challenges to that theory.
Model Spectra for Accretion Disks Truncated at the Inner Edge
Authors: Wade Richard A. , Diaz Marcos , Hubeny Ivan
Published: July 2016

Attempts to model the FUV spectra of accretion disks in cataclysmic variables have suggested that in some cases the innermost part of the disk is “missing”. We present computed spectra of accretion disks with various amounts of the inner disk removed arbitrarily, to illustrate whether the absence of the inner disk can be detected via the profiles of strong lines. Results are presented for lines in the UV and FUV for selected disk parameters and inclinations. UV/FUV line diagnostics can in principle establish whether the hot inner disk is present as expected from standard steady-state disk theory. The presence or absence of the inner disk has direct impact on the interpretation of the EUV spectra of cataclysmic variables, in particular the boundary layer emission.
The Hot Winds of Novae
Authors: Hauschildt Peter H. , Starrfield S. , Baron E. , Allard F.
Published: July 2016

We discuss the physical effects that are important for the formation of the late wind spectra of novae. Nova atmospheres are optically thick, rapidly expanding shells with almost flat density profiles, leading to geometrically very extended atmospheres. We show how the properties of nova spectra can be interpreted in terms of this basic model and discuss some important effects that influence the structure and the emitted spectrum of nova atmospheres, e.g., line blanketing, NLTE effects, and the velocity field. Most of the radiation from hot nova winds is emitted in the spectral range of the EUVE satellite. Therefore, we present predicted EUVE spectra for the later stages of nova outbursts. Observations of novae with EUVE could be used to test our models for the nova outburst.
EUV Constraints on Models of Low Mass X-Ray Binaries
Authors: Christian D.J. , Edelstein J.E. , Mathioudakis M. , McDonald K. , Sirk M.M.
Published: July 2016

We present EUVE survey results for moderate column directions containing known low-mass X-ray binaries (LMXB). We derive Lexan band (100 Å) count rates and upper limits for nearly 40 LMXB chosen generally with EB-V ≤ 0.3. Detections include Sco X-1, Her X-1, and the GRO transient CJ0422+32. Super soft sources in the LMC yield 3 σ upper limits of ≤ 10 counts ks−1. The extrapolation of two component spectral models (such as blackbody plus thermal bremsstrahlung), are in agreement with the survey upper limits. Contemporary LMXB spectral models, which involve Comptonization in an inner disk corona, predict a large flux of EUV photons. If the above model is correct in the EUV, such a component could be detected in source with low column densities. We argue that additional intrasystem column hampers its detection.
An EUVE Detection of a Low-Mass X-ray Binary? AC211 in M15
Authors: Callanan Paul J. , Bookbinder Jay , Drake Jeremy J. , Fruscione Antonella
Published: July 2016

We have observed M15 with EUVE, in an attempt to detect AC211, the well know globular cluster Low Mass X-ray Binary (LMXB). Our observations are part of an attempt to characterize the EUVE properties of LMXBs by looking at those towards which the extinction is extremely low (as it is for M15): these observations are impossible for the vast majority of Galactic LMXBs. EUVE successfully detected this cluster, at a countrate of 0.002 counts s−1. In this paper we discuss the association of this source with AC211, and alternative scenarios for EUVE emission from M15.
EUV Studies of Solar System Objects: A Status Report
Authors: Chakrabarti Supriya , Gladstone G. Randall
Published: July 2016

EUV studies have contributed substantially to our understanding of the physical and chemical properties the Sun, planets, and their satellites. Although the spectroscopic data set is limited to Venera 11/12, Voyager 1/2, Astro 1/2, EUVE, Galileo, and a handful of sounding rocket experiments, these data have provided important insights regarding the atmospheres and surfaces of several planets and satellites to the point where rudimentary comparative planetology can be conducted. In this paper we highlight some of these results.
An Optical Study of the Field of EUVE J1027+323: Discovery of a QSO and a Hidden Hot White Dwarf
Authors: Génova Ricardo , Bowyer Stuart , Vennes Stéphane , Lieu Richard , Henry J. Patrick , Beckman John E. , Gioia Isabella
Published: July 2016

We have carried out optical and Far UV studies of the field around the EUV source EUVE J1027+323. We find two sources which contribute to this flux which are spatially unresolvable with EUVE. One is a non-cataloged QSO and one is a “hidden” hot white dwarf. Reasonable scenarios ascribe the majority of the flux to the white dwarf.
The Arcetri Spectral Code for Optically-Thin Plasmas
Authors: Fossi Brunella C. Monsignori , Landini Massimo
Published: July 2016

The Arcetri-95 spectral code for optically thin plasmas computes the continuum and line emission of the ions of the most abundant elements. It includes the most updated atomic models and the main atomic processes for ions from Fe IX to Fe XXIII and for ions of the Belike, C-like, and N-like isoelectronic sequences. The power emitted per unit emission measure is produced as a function of temperature and density.

Comparison with observations requires the knowledge of the differential emission measure (DEM) as a function of temperature. A numerical code evaluates the best DEM distribution that satisfies observations and theoretical predictions. The spectral code together with the DEM code allows to compute synthetic spectra for any specified temperature distribution model of the plasma.
DEM Analyses with the Utrecht Codes
Authors: Mewe R. , van den Oord G.H.J. , Schrijver C.J. , Kaastra J.S.
Published: July 2016

We address the inversion problem of deriving the differential emission measure (DEM) distribution D(T) = nenHdV/d log T from the spectrum of an optically thin plasma. In the past we have applied the iterative Withbroe-Sylwester technique and the Polynomial technique to the analysis of EXOSAT spectra of cool stars, but recently we have applied the inversion technique discussed by Craig & Brown (1986) and Press et al. (1992) in the analysis of EUVE spectra of cool stars. The inversion problem-a Fredholm equation of the first kind-is ill-posed and solutions tend to show large, unphysical oscillations. We therefore apply a second-order regularization, i.e., we select the specific DEM for which the second derivative is as smooth as is statistically allowed by the data. We demonstrate the importance of fitting lines and continuum simultaneously, discuss the effect on the DEM of continuum emission at temperatures where no line diagnostics are available, and address possible ways to check various model assumptions such as abundances and photon destruction induced by resonant scattering.
The Berkeley Spectrometer for ORFEUS: Laboratory and In-Flight Performance
Authors: Hurwitz Mark , Bowyer Stuart
Published: July 2016

The Berkeley spectrometer aboard the ORFEUS payload achieved a variety of “firsts” during its inaugural mission in September 1993. The instrument utilizes spherical gratings with mechanically ruled varied line-spacing, and curved microchannel plate detectors with delay-line anode readout systems, to cover the 390 – 1200 Å band at a resolution of λ/5000. The instrument will be discussed, and its performance illustrated with calibration and in-flight spectra. Science highlights from the ORFEUS-I mission will be presented (oral presentation only). The payload will be available for use by guest investigators during the ORFEUS-II mission currently scheduled for late 1996.
Population I Stellar Structure and Evolution: facing the lingering difficulties to make a step forward
Authors: Maeder André
Published: July 2016

The understanding of stellar structure, evolution and pulsations is a key piece of the proper understanding of the Universe. The interpretation of the luminosity and spectra of galaxies, the properties of star populations and of starbursts, the calibration of distances, the nucleosynthetic prescriptions etc... are all requiring good models of stellar structure and evolution. The many interesting and new results in the field have been presented in several recent reviews by Maeder (1991), Chiosi et al. (1992), Maeder & Conti (1994). Table 1 below presents a list of the recent grids of stellar models. The various columns give the author’s name, then the mass range, the metallicity Z range, the kind of convective assumption adopted, where “Schw.” means Schwarzschild’s criterion, “oversh” means with convective overshooting, “all” means that different assumptions have been used. In the last column, a few remarks are given. It is interesting to note that the models mentioned above often arrive to very different conclusions about the kind of mixing supported by the observations. Authors of models with overshooting generally conclude that overshooting is best supported, authors of models without overshooting favour the absence of overshooting, while authors who consider various convective assumptions generally favour semiconvection (Stothers & Chin 1991; Mowlawi & Forestini 1994).
Probing the Extraordinary Ends of Ordinary Stars: White Dwarf Seismology with the Whole Earth Telescope
Authors: Kawaler Steven D.
Published: July 2016

During the final evolution of most stars, they shed their outer skin and expose their core of the hot ashes of nuclear burning. As these hot and very dense cores cool into white dwarf stars, they go through episodes of multiperiodic, nonradial g-mode pulsation. The tools of stellar seismology allow us to use the pulsation spectra as powerful probes into the deep interiors of these stars. Progress in white dwarf seismology has required significant international cooperation, since another consequence of the complex pulsations of these stars is decoding the true pulsation frequencies requires a coordinated global effort involving high-speed photometric observations. Through one such effort, the Whole Earth Telescope project, we have located subsurface composition changes, detected differential rotation and magnetic fields, and measured fundamental quantities such as stellar mass, luminosity, and distance to extraordinary accuracy.
Confining the Edges of the GW Vir Instability Strip
Authors: Werner K. , Rauch T. , Dreizler S. , Heber U.
Published: July 2016

Ground based optical spectroscopy as well as EUVE and HST observations are used for model atmosphere analyses of PG1159 stars in order to confine the GW Vir instability strip.
More on Solar-like Oscillations in η Boo
Authors: Bedding Timothy R. , Kjeldsen Hans
Published: July 2016

We recently reported strong evidence for solar-like oscillations in the G0 IV star η Boo (Kjeldsen et al. 1995, AJ 109, 1313). We measured small temperature fluctuations produced by oscillations through their effect on the equivalent widths of the Balmer lines. Here we address several issues that were raised at this conference.
Detection of Weak Periodic Signals from Irregularly Spaced Observations
Authors: Pfleiderer Jörg , Mössner Martin
Published: July 2016

We perform time series analysis on irregularly spaced data with large gaps. Our algorithm is based on multidimensional nonlinear optimisation. Particular emphasis is given to the selection and deletion of frequencies. The final result is checked by various statistical tests.
Quadratic and Cubic Couplings of Oscillation Modes of Stars
Authors: Van Hoolst T.
Published: July 2016

The strength of nonlinear interactions of oscillation modes of stars is determined by the amplitudes as well as by the eigenfunctions of the oscillation modes. The intrinsic couplings of modes through their eigenfunctions can be described by coupling coefficients. Here, we concentrate on quadratic and cubic coupling coefficients that describe the nonlinear coupling of modes with itself and are called self-coupling coefficients.

We considered radial and nonradial oscillation modes of polytropic models with degrees of central condensation that correspond to central condensations of main sequence stars to highly condensed evolved stars. We study the influence of the radial order and the degree of the oscillation mode on the self- coupling coefficients.
Asteroseismology of the β Cephei Variable 16 (EN) Lacertae
Authors: Dziembowski W.A. , Jerzykiewicz M.
Published: July 2016

Various observational data and model calculations are used to identify four pulsation modes detected in this star and to infer its properties. It is concluded that the star’s interior must rotate faster than the outer layers.
Uncertainties in the Position of the β Cephei Instability Strip in the HR Diagram
Authors: Pamyatnykh A.A. , Dziembowski W.A. , Mikołaj P.
Published: July 2016

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

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

Eta Orionis is an interesting multiple system of early type stars. It consists of a visual triple system (A, B, C), a spectroscopic triple system (Aabc) and an eclipsing binary Aab. A is a triple system consisting of a 7.989 day eclipsing binary Aab and a gravitationally bound third component Ac with orbital period of about 9.5 years. All components of the pair AB have spectral types in the range 09-B3 (C. Waelkens & P. Lampens 1988). The radial velocity curve is used to constrain the orbital elements and properties of the binary Aab.

The second component Ab is a B-type star that displays rapid absorption line profile variability. Regular patterns are easily visible in the data.
Spatial Integration Grids in Line Profile Modelling of Nonradially Pulsating Magnetic Stars
Authors: Fensl R.M. , Könighofer G. , Stift M.J.
Published: July 2016

For an optimum grid we postulate a fractional change of the emergent Stokes parameters between two adjacent surface points not to exceed some fixed value (say 20 %) at any frequency point. The contribution from a local Stokes IQUV profile to the observed integrated profile is a complicated non-linear function of rotational and pulsational Doppler shifts respectively, of Zeeman splitting, magnetic field direction relative to the line-of-sight, of the magnetic field azimuth in the observer’s frame and of limb darkening. The run of IQUV with visible disk coordinates (x, y) can be represented by curved surfaces f(x,y); these are different for each Stokes parameter and frequency point. The optimum integration grid then has to ensure that the maximum change δf(x,y) over all frequencies and Stokes parameters does not exceed the above-mentioned limit.
Radial pulsation among δ Sct stars
Authors: Rodríguez E. , Rolland A. , Coca P. López de , Martín S.
Published: July 2016

It has been pointed out by earlier authors that radial pulsation seems to be the only one type present in high amplitude δ Set stars, at least for monoperiodic stars. In order to confirm or not this point, we have collected all the multicolour data available for these stars in the Strömgren and Johnson photometric systems. Then, the type of pulsation has been analysed on the basis of the phase shifts and amplitude ratios between observed light and colour variations. The results indicate that all the stars analysed are radial pulsators.