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.

Observations with the ROSAT satellite have already indicated that metal absorbers must be present in the atmosphere of the hot DA white dwarf PG 1234+482. This is now confirmed by strong absorption features found in the short and medium wavelength EUVE spectrum of the star. With fully blanked model atmospheres, taking into account several million lines of heavy elements, we could attribute the strongest features to absorption by FeVI and FeVII. Since the spectrum has not been dithered during the observation other elements could not be identified with the same level of confidence, but upper limits could be determined. These are in general lower than predicted by models, which attempt to explain the presence of the metals by theoretical calculations of radiative forces in hot DA white dwarf atmospheres.

We present the results of a recent spectroscopic investigation of the cool DO white dwarf HD 149499 B in the EUV and FUV ranges. Observations were performed with the spectrograph of the EUVE satellite and the Berkeley EUV/FUV spectrometer of the ORFEUS space experiment. The analysis of the ORFEUS spectrum, performed with a grid of LTE model atmospheres, yielded the basic parameters Teff = 49500 ± 500 K and log g = 7.97 ± 0.08. This result is confirmed by the EUVE spectra. The photospheric hydrogen Lyman lines in the FUV spectrum indicate the presence of hydrogen: log nH/nHe = −0.65 ± 0.12. The implications of this finding for the spectral evolution of white dwarfs are discussed. A check of the LTE assumption was performed by a comparison with NLTE atmospheres calculated for appropriate parameters. The interstellar hydrogen column towards the HD 149499 system amounts to NH = (7 ± 2) · 1018 cm−2.

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.

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.

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.

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

Using the EUV (70 – 730 Å) spectrum of the brightest EUV source, ϵ CMa taken with the Extreme Ultraviolet Explorer Satellite (EUVE) and simple models that extrapolate this spectrum to the Lyman edge at 912 Å, we have determined the local interstellar hydrogen photoionization parameter, Г, solely from this B2 II star to be 1.1 × 10−15 s−1. This figure is a factor of 7 greater than previous estimates of Г calculated for all nearby stars combined (Bruhweiler & Cheng 1988). Using measured values of the density and temperature of neutral interstellar hydrogen gas in the Local Cloud, we derive a particle density of ionized hydrogen, n(H+), and electrons, ne, of 0.015 – 0.019 cm−3 assuming ionization equilibrium and a helium ionization fraction of less than 20%. These values correspond to a hydrogen ionization fraction, XH from 19% to 15%, respectively.

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.

We present the results of an attempt to locate the signature of the diffuse soft X-ray background in the ROSAT Wide-Field Camera (WFC) all-sky survey. After removal of non-cosmic background sources (eg. energetic charged particles), the field-of-view integrated count rate in the WFC S1a filter (90–185 eV) shows no consistent variation with Galactic latitude or longitude. We place limits on the signal from the soft X-ray background (SXRB) in the WFC, and show that these limits conflict with the observations of the Wisconsin Sky Survey if the SXRB in this energy range is assumed to be produced by a thermal plasma of cosmic abundance and a temperature T ~ 106 K within d ~ 100 pc of the Sun.

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

The diffuse signal expected in the ROSAT WFC all-sky survey for this region is predicted and shown to be far in excess of that observed. The origin of the fluctuations in the soft X-ray background and their possible relevance to the Extreme-Ultraviolet background are discussed.

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

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.

A longstanding problem in the interpretation of the X-ray and extreme ultraviolet emission from strongly magnetic cataclysmic variables can be addressed definitively with high resolution EUV spectroscopy. A detailed photospheric spectrum of the accretion-heated polar cap of the white dwarf is sensitive in principle to the temperature structure of the atmosphere. This may allow us to determine where and how the bulk of the accretion energy is thermalized. The EUVE data on AM Herculis and EF Eridani are presented and discussed in this context.

I present EUVE Deep Survey photometry and AAVSO optical measurements of the 1993 August and 1994 June/July outbursts of the dwarf nova SS Cygni. The EUV and optical light curves are used to illustrate the different response of the accretion disk to outbursts which begin at the inner edge and propagate outward, and those which begin at the outer edge and propagate inward. Furthermore, we describe the properties of the quasi-coherent 7-9 s sinusoidal oscillations in the EUV flux detected during the rise and plateau stages of these outbursts.

We present preliminary analysis of EUVE pointed data of 8 magnetic cataclysmic variables. Blackbody temperatures, luminosities, and interstellar columns have been better constrained. Using these luminosities we look for correlations between the EUV excess (over optical and hard X-rays) and various system parameters. While it appears there is no correlation between the EUV excess and the system inclination and orbital period, correlations are suggested between the EUV excess and the longitude of the accretion spot, the colatitude of the accretion spot, the white dwarf magnetic field, and the magnetic capture radius.

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

Six non-magnetic cataclysmic variables were detected during the ROSAT WFC survey; four dwarf novae and two nova-like variables. In two dwarf novae (VW Hyi & SS Cyg) the flux evolution through outburst was followed across a broad wavelength range. The two other detections (Z Cam & RX J0640−24) also suggest the presence of distinct luminous EUV emission components; supporting the view that such components are a ubiquitous feature of dwarf nova outbursts. Two bright nova-like variables were detected, but these detections are found to be consistent with the soft tail of the X-ray emission.

We present EUV light curves for a number of AM Her systems observed either as guest observer targets or with the EUVE Right Angle Program. We have formed light curves for eight AM Her stars and show in our presentation the similarities and differences present. We draw some conclusions by grouping the systems by inclination, magnetic field strength, and accretion region geometry. In order to understand the physical structures responsible for the EUV emissions, we have developed a software model to generate synthetic light curves. We find that the EUV accretion regions in the systems UZ For, VV Pup, and AM Her cannot be fit with a flat spot confined to the white dwarf surface, regardless of its shape or brightness profile. Rather, a small, symmetric, raised spot is the only shape consistent with the data. The light curves for systems EF Eri, RE1149+28, AN UMa, and V834 Cen show evidence for additional structure that precedes the primary accretion region in phase. Our model indicates that a large portion of the light curve in each system is seen in absorption. Finally, in three systems, we detect a very gradual rise and fall in the EUV flux (<5% of the peak flux) at phases when the spot is completely obscured behind the white dwarf (WD) limb. We attribute this detection to emission from the accretion column that decreases exponentially with distance from the WD and detect emission at heights up to 15% of the WD radius.

We present observations of the eclipsing binary V471 Tauri by the Extreme Ultraviolet Explorer (EUVE) and the Very Large Array (VLA). The EUV spectrum is dominated by the continuum of the hot white dwarf and the time-averaged spectrum is fitted by a 33.1 ± 0.5 × 103 K pure hydrogen white dwarf atmosphere assuming log g = 8.5. An ISM hydrogen column density of 1.5 ± 0.4 × 1018 cm−2 is required to explain the attenuation of the white dwarf spectrum thus setting the HI column in the line of sight of the Hyades cluster. The He II λ304 Å line is in emission and varies over the orbital period of V471 Tauri following a sinusoidal modulation with the maximum reached when the K star is at inferior conjunction. Transient dips are detected at orbital phase −0.12 in the SW and MW spectrometers integrated lightcurves but are notably absent in the LW lightcurve indicating the occulting material is ionized. The VLA observation suggest the presence of a K star coronal magnetic loop between the two stars reconnecting with the white dwarf magnetic field. Such a structure could be the occulting source needed to explain the dips seen in the lightcurves of V471 Tauri in the EUV.