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We report on recent results from a high-resolution spectroscopic survey of hot DA white dwarfs, based on IUE, FUSE and HST observations. For the first time, we address the measurement of element abundances in a completely objective manner with a spectroscopic model fitting technique, which allows us to consider formally the limits that can be placed on abundances in stars where no heavy elements are detected. We also include our latest analysis of the high resolution EUV spectrum of G191-B2B recorded by J-PEX.

We present results from a survey for low-redshift CIV absorbers in the sight lines to nine QSOs. Using echelle spectra obtained with the Space Telescope Imaging Spectrograph (STIS) at 7 km s-1 resolution, we robustly identify six CIV systems. All systems show at least the CIV doublet and HI Lyα; Si III, CIII, O VI, and higher Lyman series lines are also detected in several of these absorbers in the STIS data and complementary Far Ultraviolet Spectroscopic Explorer (FUSE) spectra. We calculate the number of CIV systems per unit redshift (dN/dz) and the cosmological mass density of the CIV ions (ΩCIV) We find dN/dz = 5.2 ± 2.7 for absorbers with a rest-frame equivalent width greater than 38 mA. Comparison of our estimate of ΩCIV at z > 0.1 to analogous measurements at z < 2 provides a preliminary indication that the metallicity of the IGM is increasing with time. These results may have implications for understanding the effectiveness of observed galactic-scale outflows at high-z at enriching the IGM.

We report on the analysis of optical and FUV spectra of a sample of DAO white dwarfs. Optical spectra obtained at Kitt Peak were used to determine their temperature and gravity and compared with results from Lyman series observations made by the FUSE satellite. Significant differences were found between the Balmer and Lyman line techniques. Most of the objects in the sample were found to consist of low gravity, high temperature objects and hence have low mass, with one DAO already known to be the result of common envelope evolution. Overall the results from the DAO stars can now be compared with the larger population of DA white dwarfs.

After 2 years of operation onboard the Spanish satellite MINISAT, EURD has achieved an unprecedented success in the observation of the terrestrial nightglow in the EUV, covering a range of ∼ 350 – 1100 A. EURD has provided a total of more than 543 hours of integration in the long wavelength spectrometer and more than 898 hours in the short wavelength one, allowing the achievement of the most detailed atlas of the terrestrial EUV nightglow ever obtained. We present here the spectra obtained, together with the identification of the lines, some of them detected for the first time in the nightglow. These spectra represent an improvement in sensitivity of several orders of magnitude with respect to previous observations. It has been possible, for the first time, to identify the complete Lyman series of atomic hydrogen, resolving up to Lyman-e. It has also been possible to identify the helium Lyman-β line at 537 A and to detect other lines of the blended Lyman series of helium, at 515 and 522 A. The spectra clearly show the presence of the OII lines at 617, 644, 673, and 718 A, previously observed in the dayglow but seen here for the first time in the nightglow. In addition to the recombination continuum of the atomic oxygen at 911 A, two features of OI have been detected in the nightglow: the 3s′ 3D o transition at 989 A, previously observed by (1984) and the 2p34s3S o transition at 1040 A, partially overlapped with Lyman β, but clearly distinguishable from it. This feature has been seen for the first time in the terrestrial nightglow. The radiative recombination continuum of atomic oxygen at 911 A, that was absent in the observations of the first year of operation of EURD, is now clearly visible. The reasons of the absence of the OI feature during the first year of operation are still unknown. (1976) also noticed a strong variation with time of this 911 A emission.

During the extremely successful Astro-2 mission flown in March 1995, high-quality spectra were obtained for 9 DA white dwarfs hotter than about 20,000 K with the Hopkins Ultraviolet Telescope. The spectra covered the range of 820–1840 A and were thus ideal for our purpose of investigating Stark broadening in the Lyman series. The objects observed included GD 50, one of the most massive known DA; RE 0512–004, the least massive known DA in this temperature range; RE 1738+665, the hottest known field DA; the HST standards G191-B2B, GD 153, GD 71, and HZ 43; and GD 394 and Wolf 1346. Our major conclusion is that the wings of the Lyman lines are in reality significantly weaker than predicted using the standard (Vidal-Cooper-Smith) Stark broadening theory. We also discovered satellite features in the Lyman s profile of Wolf 1346; these have now been successfully modeled using the same techniques that were previously applied to Ly α satellites.

Since the first soft X-ray and EUV studies of hot DA white dwarfs it has been apparent that the fluxes of white dwarfs in these wavelength ranges are significantly lower than predicted (e.g. Kahn et al. 1984; Petre, Shipman & Canizares 1986; Jordan et al. 1987; Paerels & Heise 1989; Koester 1989). All this work made the simplifying assumption that helium was the sole photospheric opacity source, although the He in these stars was never identified spectroscopically. Moreover, the shape of the EXOSAT spectrum of Feige 24 could only be matched by models including trace abundances of elements heavier than H or He (Vennes et al. 1989). More recent studies combining the soft X-ray and EUV data available from the ROSAT sky survey show that He alone cannot be responsible for absorption in these bands and that most of the hottest stars (T eff > 40, 000K) must have heavier elements in their atmospheres to explain the observed fluxes (Barstow et al. 1993; Wolff et al; 1996, Marsh et al. 1995). In parallel, observational evidence has been acquired by the International Ultraviolet Explorer (IUE) revealing high ionization features attributed to C, N, and Si (e.g. Bruhweiler & Kondo 1981 & 1983). Subsequently, O, Fe and Ni were also directly.

This paper reviews a number of numerical techniques for computing static stellar atmospheres and emergent flux distributions accounting for the blanketing and blending effects of all atomic transitions in full statistical equilibrium. The framework for calculation is the complete linearization of angle and frequency moments of the transfer equations; the so-called multi-frequency / multi-gray approach. Here I discuss the algorithms, setting up the problem, and computational strategies.

This paper presents some preliminary results from analysis of our high- resolution (30–35 km/s FWHM) spectra of the Lyman a forest region in the z = 3.78 QSO 2000-330. These spectra were obtained at the Anglo-Australian Telescope over several observing seasons and have been analysed by fitting multiple-cloud Voigt profiles to Lyman series and heavy-element absorption lines. Two specific issues are addressed here: (i) The distribution of column densities, N(H I), and velocity dispersion, b, for hydrodgen clouds in the interval zabs = 3.43–3.78; (ii) Heavy-element abundances in a system at zabs = 3.1723.

There is now a wealth of observational data on the emission lines of H seen in the atmosphere. The α and β lines of the solar Lyman series at 1216 and 1026 A excite the extensive H in the terrestrial atmosphere causing it to re-emit them; they are then seen from space above the absorbing lower atmosphere. Scattering of the solar Ly-β line by the atmospheric H also produces the Balmer series Hα line (6563 A) seen from the ground. The interpretation of early Ly-α observations has been summarized (Mange, 1972); discussion is limited here to contributions from 1970 on. Auroral H emission and its excitation are neglected.