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.

International Ultraviolet Explorer observations of the ROSATWFC-discovered AM Hers RE2107–05, RE1938–46, RE1833–74, RE1149+28, and RE0751+14 reveal UV spectra (1200 – 2000 Å) which are unique when compared with previously studied systems. Specifically, the intensities of individual emission lines due to CIV, CII, N V, HeII, and Si IV, are 2 – 4 times stronger then typically seen. However, the equivalent width ratios of these same emission lines, (CII/CIV, N V/CIV, HeII/CIV, and Si IV/CIV), are similar to those in typical AM Hers. These results indicate that the EUV-discovered AM Hers must have larger UV emitting volumes. Using additional observations of these stars made by EUVE, we speculate on the cause of their larger UV line fluxes.

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

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

The only stars other than white dwarfs whose photospheric extreme ultraviolet radiation has been detected are ϵ and β CMa. It is therefore of considerable theoretical interest to compare the EUVE observations of these two giant B stars to predicted spectra. However, both LTE and non-LTE very sophisticated line blanketed model atmospheres fail to match the observed flux. This failure leaves the stellar photosphere theory, even for seemingly “simple” objects as normal B giants were believed to be, in a rather dubious position. This paper briefly summarizes possible reasons for the failure of existing models to describe the EUVE observations of hot stars. In particular, we discuss the effects of uncertainties in the line blanketing, and the effects of the photosphere-wind interaction.

The only pulsating early-type star observed by EUVE, the β Cephei variable β CMa, displays periodic variability in its Lyman continuum which is basically consistent with the long-known optical and UV variability. The amplitude of the primary pulsation component is significantly larger in the EUV than in the optical or UV. This is consistent with a temperature change being the explanation for the variability. It is notable that the pulsations have been detected in the Lyman continuum because this part of the spectrum is formed in a much higher layer of the photosphere than either the UV or optical continua.

Multiwavelength observations have shown that, after optical decline, the stellar remnants of classical nova outbursts evolve at constant, near-Eddington, bolometric luminosity to high effective temperature (> 2 105 K), before turning off. Here we briefly review the observations of classical novae in this phase of evolution and show that, in principle, EUV and soft X-ray observations can be used to determine the mass of the underlying white dwarf and place limits on the rate of mass loss by stellar winds and the rate of mass gain due to accretion from the stellar companion. We also describe our model atmosphere calculations of EUV/Soft X-ray emission from hot, high gravity stars and their application to the EUVE all-sky survey detection of Nova Cygni 1992 (V1974 Cyg).

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

I present theoretical light curves of novae and helium novae in EUV and visible bands derived from optically thick wind theory. The EUV light curves are very useful in determining the white dwarf mass and the distance to the star. Helium novae are bright EUV sources but much less luminous in the optical. Identification of helium novae will be observational evidence for the growth of the white dwarf, which will eventually become a Type la supernova. A semi-detached system may be a new progenitor model of Type la supernovae, which is a very bright EUV source with a faint optical counterpart.

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.

Both GQ Mus and V1974 Cyg were observed to turnoff in X-rays by ROSAT. The turnoff of V1974 Cyg was also observed with EUVE. GQ Mus was observed near the beginning of its outburst with EXOSAT and then 7 years later by ROSAT in the all-sky survey. Later ROSAT PSPC observations showed that its X-ray intensity was slowly declining with time and it was not detected in the last pointing that occurred in August 1993. We observed GQ Mus with IUE over the entire active phase of its outburst and found a change in the slope of the UV continuum around the time that the X-rays turned off. V1974 Cyg was observed by ROSAT throughout its entire active phase in X-rays which lasted about 18 months. V1974 Cyg was detected in the EUVE all-sky survey, but not in pointed observations that occurred in August 1993 (and June and November 1994). We use the measured times of the active phases to determine important properties of these two novae. For example, for V1974 Cyg we predict that more than 10−5M⊙ of helium rich material was left on the white dwarf when it returned to quiescence. For GQ Mus, ~ 10−4M⊙ was left on the white dwarf. These results imply that a significant amount of the helium seen in nova ejecta was produced in outbursts prior to the one that was just observed. They also imply that the mechanism which mixes core material into the ejecta must be efficient.

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.

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.

We summarize the detections of extreme ultraviolet (EUV) emission from neutron stars. Three firm detections have been made of spin-powered pulsars: the aged millisecond pulsar PSR J0437−4715, the middle-aged X-ray pulsar Geminga, and the radio pulsar PSR B0656+14. These observations allow us to evaluate both power-law and thermal-law emission models as the source of the EUV flux. For the case of PSR B0656+14 the lack of flux modulation with pulse period argues that the EUV radiation originates from the cooling neutron star surface rather than from a hot polar cap. If the emission is from a thermalized neutron star surface, then limits can be placed on the surface temperature. For the case of Geminga we can explain the observed EUV flux using thermal models that are consistent with standard neutron cooling scenarios. We also have a weak indication that the EUV emission from Geminga is pulsed in a manner consistent with the lowest energy channel observed with Rosat. For the case of the millisecond pulsar PSR J0437−4715 standard neutron star cooling models require surface re-heating. We compare different heating models to the data on this object. We rule out re-heating by crust-core friction, and find that models for the accretion from the interstellar medium, accretion from the white dwarf companion and a particle-wind nebula do not account for the EUV luminosity. Models of pulsar re-heating by magnetic monopole catalysis of nucleon decay are used to establish new limits to the flux of monopoles in the Galaxy. A single power-law source with properties derived from X-ray data cannot explain the EUV flux from PSR J0437−4715. The strongest model for explaining the EUV emission consists of a large ~ 3 km2 polar cap heated from particle production in the pulsar magnetic field. We consider the prospects for detecting other neutron stars in the extreme ultraviolet.

Recent ROSAT and EUVE detections of spin-powered neutron stars suggest that many emit ‘thermal’ radiation, peaking in the EUV/soft X-ray band. These data constrain the neutron stars’ thermal history, but interpretation requires comparison with model atmosphere computations, since emergent spectra depend strongly on the surface composition and magnetic field. As recent opacity computations show substantial change to absorption cross sections at neutron star photospheric conditions, we report here on new model atmosphere computations employing such data. The results are compared with magnetic atmosphere models and applied to PSR J0437−4715, a low field neutron star.

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.

First results from a 3-D model of EUVE observations of the Io Plasma Torus are reported. The semi-empirical model calculations follow a method previously used to describe visible and near-UV emissions. The extension to EUV wavelengths is described. Several EUV emissions have been successfully modelled although some discrepancies remain at this stage. Most EUV emissions peak at a jovicentric distance of ≈ 5.8 RJ. The observed dawn-dusk asymmetry of the torus was well fitted with a shift parameter (ϵ) of 0.03. The modelling also indicates that optical depth effects need to be considered for several EUV emission lines.

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.

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.

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