Search

We report the results of our final search for hot white dwarfs in unresolved, Sirius-type, binary systems with IUE. One new system, RE J0500-364 (DA+F6/7V), has been identified. This star appears to lie at a distance of between 500-1000pc, making it one of the most distant white dwarfs, if not the most distant, to be detected in the EUV surveys. The very low line-of-sight neutral hydrogen volume density to this object could place a lower limit on the length of the Beta CMa interstellar tunnel of diffuse gas, which stretches away from the Local Bubble in a similar direction to RE J0500-364., 1 LaTex file plus 15 figures; accepted for publication in Monthly Notices of the Royal Astronomical Society

ABSTRA C T Using the latest non-local thermodynamic equilibrium (non-LTE) synthetic spectra and stellar model calculations, we have evaluated the potential effect of the presence of heavy elements in the photospheres of hot H-rich DA white dwarfs. In particular, we have examined their influence on the effective temperature and surface gravity perceived from analysis of the Balmer line profiles. It is apparent that both the inclusion of non-LTE effects in the models and significant quantities of heavy elements act independently to lower the value of Teff determined from a particular spectrum. Hence, the true effective temperatures of the heavy element-rich DA white dwarfs, currently estimated to be above 55 000 K, are apparently lower than previously reported from pure-H LTE analyses, by some 4000‐7000 K. We do not see any similar influence on measurements of log g. This work concentrates on a group of relatively bright well-studied objects, for which heavy element abundances are known. As a consequence of this, establishment of correct temperatures for all other hot white dwarfs will require a programme of far-UV spectroscopy in order to obtain the essential compositional information. Since only stars with effective temperatures lying notionally in the range from

Long-standing questions regarding the temperature, gravity, mass, and radius of the well-known white dwarf Sirius B are considered in light of new data. Recently obtained Extreme Ultraviolet Explorer (EUVE) observations and reprocessed IUE NEWSIPS data have produced a new, well-defined effective temperature of 24,790 ± 100 K and a surface gravity of log g = 8.57 ± 0.06 for Sirius B. A new Hipparcos parallax for the Sirius system of π = 037921 ± 000158 is used in conjunction with the above spectroscopic results and the previously published gravitational redshift to yield a mass of 0.984 ± 0.074 M☉ and a radius of R = 0.0084 ± 0.00025R☉ for the white dwarf. Combining these results with the existing astrometric mass for Sirius B gives a refined mass estimate of M = 1.034 ± 0.026 M☉. The mass and radius for Sirius B are found to be consistent with the theoretical mass-radius relation for a carbon-core white dwarf. The EUVE spectrum is also used to determine a firm upper limit of He/H = 1.8 × 10-5 for the helium mixing ratio in the photosphere of Sirius B. IUE echelle spectra of Sirius B provide an estimate of log NH I = 17.72 ± 0.1 for the interstellar H I column to this star.

We present the fractional distribution of spectroscopic subtypes, range and distribution of surface temperatures, and kinematical properties of the white dwarfs within 25pc of the sun. There is no convincing evidence of halo white dwarfs in the total 25 pc sample of 224 white dwarfs. There is also little to suggest the presence of genuine thick disk subcomponent members within 25 parsecs. It appears that the entire 25 pc sample likely belong to the thin disk. We also find no significant kinematic differences with respect to spectroscopic subtypes. The total DA to non-DA ratio of the 25 pc sample is 1.8, a manifestation of deepening envelope convection which transforms DA stars with sufficiently thin H surface layers into non-DAs. We compare this ratio with the results of other studies. We find that at least 11% of the white dwarfs within 25 parsecs of the sun (the DAZ and DZ stars) have photospheric metals that likely originate from accretion of circumstellar material (debris disks) around them. If this interpretation is correct, then it suggests the possibility that a similar percentage have planets, asteroid-like bodies or debris disks orbiting them. Our volume-limited sample reveals a pileup of DC white dwarfs at the well-known cutoff in DQ white dwarfs at Tef about 6000K. Mindful of small number statistics, we speculate on its possible evolutionary significance. We find that the incidence of magnetic white dwarfs in the 25 pc sample is at least 8%, in our volume-limited sample, dominated by cool white dwarfs. We derive approximate formation rates of DB and DQ degenerates and present a preliminary test of the evolutionary scenario that all cooling DB stars become DQ white dwarfs via helium convective dredge-up with the diffusion tail of carbon extending upward from their cores., Comment: Accepted for publication in The Astronomical Journal

We have obtained Hubble Space Telescope high-dispersion spectroscopy of two hot white dwarfs using the Goddard High-Resolution Spectrograph. The stars were selected on the basis of EUV flux as examples of H-rich DA stars containing high and low levels of short-wavelength opacity. The high-opacity star, REJ 1614-085, possesses strong features due to C IV, Si IV, and N V. Carbon and silicon in this star are underabundant with respect to the predictions of radiative levitation; nitrogen, however, is superabundant. This is in sharp contrast to the apparently similar DA star GD 394, where silicon is in extreme overabundance. The low-opacity star PG 1057+719, as expected, shows no clear evidence of these or any other heavy ions. We find evidence of weak blueshifted components in the REJ 1614-085 C IV and Si IV lines, which may indicate the star is experiencing ongoing mass loss.

We present the final calibration of the Hopkins Ultraviolet Telescope (HUT) for its flight during the Astro-1 space shuttle mission in 1990 December. The calibration is defined by a comparison between an observation obtained in flight of the hot DA white dwarf G191-B2B and the model atmosphere for this star calculated by P. Bergeron. We also present the postflight laboratory calibration of the HUT sensitivity, which we find to be in excellent agreement with the white dwarf calibration. The ratio of the white dwarf to the laboratory effective area calibration has a mean value of 1.0044 over the wavelength range 912-1840 A, a slope of 7.61 × 10-6 A-1 (which corresponds to a relative difference of only 0.35% at either end of the spectrum), and fluctuations about the mean value with an rms amplitude of 3%. The uncertainties in the laboratory calibration are 4% for the overall normalization, 5% relative to the mean on wavelength scales of 50 to several hundred A, and an uncertainty in the overall slope that corresponds to 1% at 1840 A and that increases smoothly to 3% at 912 A. These results validate the use of white dwarf models as absolute flux standards in the far-ultraviolet, especially for wavelengths shortward of Lyα, where previous experiments often disagreed by large factors. In particular, the normalization and wavelength dependence of the sensitivity calibration adopted by the Hubble Space Telescope Faint Object Spectrograph is confirmed to within the uncertainties of our laboratory calibration, while the normalization of the IUE final calibration is found to be low by an amount just exceeding our 1 σ uncertainty. Fits to the spectra of both G191-B2B and the hot DA white dwarf HZ 43 yield effective temperatures and surface gravities in good agreement with values derived from Balmer line profiles. We also present a comparison with Voyager spectra for the stars G191-B2B, HZ 43, BD +28°4211, π Aqr, and HD 25443. When averaged over broad bands (912-1050 A, 1050-1180 A), fluxes from the two instruments typically agree to within 10%.

One of the most important results of the ROSAT All-Sky X-ray and EUY Surveys has been the detection of a large number of white dwarfs, allowing a detailed study of the general properties of DA white dwarf atmospheres to be carried out. However, this work relies on a knowledge of the effective temperatures, surface gravities and visual magnitudes. We present analyses of optical data from our follow-up programme, including the values of Teff, logg, mv and stellar mass obtained. We also list the PSPC and WFC count rates for each star. The distribution of masses and surface gravities derived from the optical work also provides important information about the sample of white dwarfs and gives some indication of possible selection biases. Comparing the ROSAT sample of 89 stars with the cooler optically selected sample of Bergeron, Saffer & Liebert reveals a statistically significant excess (~3 times the number found by them) of hot, massive DAs, which may represent a population of coalesced binary white dwarfs. As the optical and EUY samples do not cover the same range of white dwarf temperatures, the high­ mass excess may partly arise from differences in the cooling rates of 'normal' (~O.6-M0) and massive (> 1.0-M0) stars. Consequently, this feature is not necessarily the result of selection on the basis of EUY flux, and it might also be present in an optically selected sample covering an appropriate temperature range.

We present a detailed analysis of the extreme-ultraviolet (EUV) spectra of 13 hydrogen-rich DA white dwarfs, observed by the Extreme Ultraviolet Explorer (EUVE) satellite, paying attention to the possible sources of absorbing material along the lines of sight both in the local interstellar medium (ISM) and in the photospheres of the stars themselves. The range of interstellar column densities seen are consistent with our previous understanding of the local distribution of material. Absorption from interstellar Heii is found in the direction of five stars, allowing us to measure directly the He ionization fraction and estimate, indirectly, that of H. The weighted mean ionization fractions along these lines of sight are 0.27+/-0.04 and 0.35+/-0.1 respectively. Where Heii is directly detected, the observed ionization fractions are not correlated with direction or with the volume/column density of material along the line of sight. Furthermore, the limits on the amount of Heii established in all other directions completely encompass the range of observed values. Indeed, all the data can be consistent with more or less constant He and H ionization fractions throughout the local ISM. It is clear that there is little photospheric opacity, from either He or heavier elements, in the majority of the stars we have studied. This poses further difficulties in explaining the observed division of white dwarfs into H- and He-rich groups, the temperature gap in the He-rich sequence and the detailed spectral evolution of the H-rich DA white dwarfs as they cool. A striking observational result is that our spectroscopic evidence indicates that radiative levitation effects are only important at temperatures above 50000K, rather than the 40000K suggested by broad-band photometry. There is clearly an urgent need for further theoretical work on the mechanisms that determine the photospheric composition of white dwarf stars.

Recent optical spectra of the DA3 white dwarf EG 102 clearly show a feature due to Mg II λ4481. This is the first spectroscopic detection of Mg in a DA star at a temperature of 20,000 K. The presence of this ion in the stellar photosphere poses a severe dilemma for the theory of radiative levitation, which predicts a vanishingly small equilibrium abundance of Mg in a star such as EG 102. Ongoing accretion must to be the source of this ion. An accretion rate of 1.6 × 10-16 M☉ yr-1 is implied by the short timescale (3.3 days) for the diffusive loss of Mg from the photosphere. Such a rate could be supplied by the interstellar medium, if EG 102 is embedded in a diffuse interstellar cloud. Alternately, EG 102 may possess a very low mass companion, which is transferring material onto the white dwarf. Finally, we use an IUE echelle spectrum to demonstrate the presence of photospheric Si in this star. We find respective abundances of log (Mg/H) = -5.3 and log (Si/H) = -7.0 for EG 102.

This paper describes a revised photometric calibration of the \emph{Galaxy Evolution Explorer} magnitudes, based on measurements of DA white dwarfs. The photometric magnitudes of white dwarfs measured by \emph{GALEX} are compared to predicted magnitudes based on independent spectroscopic data (108 stars) and alternately to \emph{IUE} UV fluxes of the white dwarfs (218 stars). The results demonstrate a significant non-linear correlation and small offset between archived \emph{GALEX} fluxes and observed and predicted UV fluxes for our sample. The primary source of non-linearity may be due to detector dead time corrections for brighter stars, but it should be noted that there was a predicted non-linearity in the fainter stars as well. Sample expressions are derived which 'correct' observed \emph{GALEX} magnitudes to an absolute magnitude scale that is linear with respect, and directly related, to the \emph{Hubble Space Telescope} photometric scale. These corrections should be valid for stars dimmer than magnitudes 9.3 and 10.5 in the NUV and FUV respectively, and brighter than magnitude 17.5 in both, Will be published in MNRAS. 13 pages, 6 figures, 4 tables, two of the tables are abridges; full versions available at MNRAS

The Extreme Ultraviolet wavelength range was one of the final windows to be opened up to astronomy. Nevertheless, it provides very important diagnostic tools for a range of astronomical objects, although the opacity of the interstellar medium restricts the majority of observations to sources in our own galaxy. This review gives a historical overview of EUV astronomy, describes current instrumental capabilities and examines the prospects for future facilities on small and medium-class satellite platforms., Published in Advances in Space Research

The star G191-B2B is one of a number of hot DA white dwarfs whose atmospheres have been found to contain significant quantities of heavy elements, including C, N, O, Si, Fe, and Ni. Several earlier studies have measured their abundances using IUE echelle data in conjunction with synthetic spectra derived from theoretical model atmosphere calculations of varying degrees of sophistication. However, predictions of the EUV spectrum based on these observations failed completely to match either its shape or absolute flux level. We present here the results of new non-LTE model calculations including the effects of line blanketing from more than 9 million (mainly Fe and Ni) transitions. For the first time, we are able to find an effective temperature and composition that can consistently match the optical, FUV and EUV data. However, to maintain this agreement below the He ii Λ228 Lyman limit, it is necessary to incorporate additional He ii opacity in the form of photospheric, circumstellar or interstellar material.

Deep Hubble Space Telescope (HST) observations with WFPC2 of the nearby globular cluster NGC 6752 have allowed us to obtain accurate photometry for the cluster white dwarfs (WDs). A sample of local WDs of known trigonometric parallax and mass close to that of the cluster WDs have also been observed with WFPC2. Matching the cluster and the local WD sequences provides a direct measure of the distance to the cluster: (m - M)0 = 13.05, with an uncertainty less than ±0.1 mag, which allows a substantial reduction in the uncertainty in the age of the cluster. Indeed, coupling this value of the cluster distance to the cluster metallicity, helium abundance, and α-element enhancement [α/Fe] = 0.5 yields an age of 15.5 and 14.5 Gyr using evolutionary models that do not include or do include helium diffusion, respectively. The uncertainty affecting these age determinations is ~10%. The majority of the cluster WDs appear to be of the DA variety, while the color-magnitude location of two WDs is consistent with the DB type. This suggests a cluster DB/DA ratio similar to that of WDs in the solar neighborhood.

The ROSAT and EUVE all-sky surveys have resulted in an important change in our understanding of the general composition of hydrogen-rich DA white dwarf atmospheres, with the photospheric opacity dominated by heavy elements rather than helium in the hottest stars (T > 40, 000 K). Most stars cooler than 40,000 K have more or less pure H atmospheres. However, one question, which has not been resolved, concerned the specific nature of the heavy elements and the role of helium in the hottest white dwarfs. One view of white dwarf evolution requires that H-rich DA stars form by gravitational settling of He from either DAO or He-rich central stars of planetary nebulae. In this case, the youngest (hottest) DA white dwarfs may still contain visible traces of He. Spectroscopic observations now available with EUVE provide a crucial test of these ideas. Analysis of data from the EUVE Guest Observer programme and EUVE public archive allows quantitative consideration of the sources of EUV opacity and places limits on the abundance of He which may be present.

We propose a new probe of the dependence of the fine structure constant, alpha, on a strong gravitational field using metal lines in the spectra of white dwarf stars. Comparison of laboratory spectra with far-UV astronomical spectra from the white dwarf star G191-B2B recorded by the Hubble Space Telescope Imaging Spectrograph gives limits on the fractional variation of alpha of (Delta alpha/alpha)=(4.2 +- 1.6)x10^(-5) and (-6.1 +- 5.8)x10^(-5) from Fe V and Ni V spectra, respectively, at a dimensionless gravitational potential relative to Earth of (Delta phi) ~ 5x10^(-5). With better determinations of the laboratory wavelengths of the lines employed these results could be improved by up to two orders of magnitude., Comment: 8 pages, 2 figures. v2: minor corrections

Approximately 70 percent of the nearby white dwarfs appear to be single stars, with the remainder being members of binary or multiple star systems. The most numerous and most easily identifiable systems are those in which the main sequence companion is an M star, since even if the systems are unresolved the white dwarf either dominates or is at least competitive with the luminosity of the companion at optical wavelengths. Harder to identify are systems where the non-degenerate component has a spectral type earlier than M0 and the white dwarf becomes the less luminous component. Taking Sirius as the prototype, these latter systems are referred to here as Sirius-Like. There are currently 98 known Sirius-Like systems. Studies of the local white dwarf population within 20 parsecs indicate that approximately 8 per cent of all white dwarfs are members of Sirius-Like systems, yet beyond 20 parsecs the frequency of known Sirius-Like systems declines to between 1 and 2 per cent, indicating that many more of these systems remain to be found. Estimates are provided for the local space density of Sirius- Like systems and their relative frequency among both the local white dwarf population and the local population of A to K main sequence stars. The great majority of currently unidentified Sirius-Like systems will likely turn out to be closely separated and unresolved binaries. Ways to observationally detect and study these systems are discussed., Comment: 20 pages, 2 figures

CNES, Universite Pierre et Marie Curie (UPMC); Centre National de la Recherche Scientifique (CNRS) in France

The ROSAT and EUVE all-sky surveys have resulted in an important change in our understanding of the general composition of hydrogen-rich DA white dwarf atmospheres, with the photospheric opacity dominated by heavy elements rather than helium in the hottest stars (T > 40, 000 K). Most stars cooler than 40,000 K have more or less pure H atmospheres. However, one question, which has not been resolved, concerned the specific nature of the heavy elements and the role of helium in the hottest white dwarfs. One view of white dwarf evolution requires that H-rich DA stars form by gravitational settling of He from either DAO or He-rich central stars of planetary nebulae. In this case, the youngest (hottest) DA white dwarfs may still contain visible traces of He. Spectroscopic observations now available with EUVE provide a crucial test of these ideas. Analysis of data from the EUVE Guest Observer programme and EUVE public archive allows quantitative consideration of the sources of EUV opacity and places limits on the abundance of He which may be present.

We report the discovery of a hot DO white dwarf, RE 0503−289, which lies in a region of very low interstellar H I density. The star was found independently by the ROSAT EUV and Edinburgh-Cape surveys. It lies within 1° of the hot DA white dwarf RE 0457−281 which we have reported as having the lowest directly measured H I column. A follow-up observation made using the Voyager 2 ultraviolet spectrometer reveals a strong continuum shortward of the 912-A Lyman limit, and hence a similarly low hydrogen column density of 1.3×10 17 atom cm −2 . Optical and UV spectroscopy gives a temperature in the range 60000-80000 K and a carbon abundance ≃1 per cent

We report the discovery of the hot DA white dwarf RE 0457−281 which has the lowest line-of-sight neutral hydrogen column density yet measured. The star was found independently by the ROSAT EUV, Montreal-Cambridge-Tololo and Edinburgh-Cape surveys. With an effective temperature of 60 700 K and very soft EUV spectrum, this white dwarf resembles the well-studied hot DA white dwarf G 191−B2B. A follow-up observation made using the Voyager 2 UV spectrometer reveals a strong continuum shortward of the 912-A Lyman limit from which we deduce that the neutral hydrogen column density is 1.3×10 17 atom cm −2

We report the discovery of two hot white dwarfs which have the lowest line-of-sight neutral hydrogen column densities yet measured. The stars were found independently by the ROSAT EUV, Montreal-Cambridge-Tololo, and Edinburgh-Cape surveys. Follow-up observations made using the Voyager 2 ultraviolet spectrometer reveal strong continua shortward of the 912A Lyman limit from which we deduce that the neutral hydrogen column densities are 1.3 x 10(exp 17) and 2.0 x 10(exp 17) atoms/sq cm.

We have made a detailed study of a sample of 28 hot DA white dwarfs detected in the ROSAT EUV and soft X-ray all-sky-survey.

The ultraviolet spectrometers on the Voyager spacecraft are radiometrically stable and presently capable of making long-term, precise measurements of the solar spectral irradiance between 50 nm and 170 nm. If the detection efficiencies of these spectrometers were confirmed through a dedicated program of underflights, and if a campaign of regular Voyager observations of the Sun was instituted, some of the needs for timely, long-term solar EUV flux data could be efficiently met in this decade

Million degree plasmas are ubiquitous in the Universe, and examples include the atmospheres of white dwarfs (WDs); accretion phenomena in young stars (classical T-Tauri), cataclysmic variables (CVs) and active galactic nuclei (AGN); the coronae of stars; and the interstellar medium (ISM) of our own galaxy and of others. Understanding their nature is fundamental to astrophysics. This White Paper addresses primarily two Astro2010 Thematic Science Areas, SSE and GAN, and has important implications for the other three areas.

A search for positional coincidences of catalogued hot subdwarfs with sources detected in the ROSAT wide-field camera (WFC) EUV survey have been made. Six such coincidences were found. Optical follow-up observations reveal four of these stars to be hot DA white dwarfs (plus one other, less certain DA), previously misclassified as subdwarfs. These five are all within the 90 per cent confidence levels of the WFC source positions, and are therefore reliable identifications with a class of sources known to be detectable EUV emitters. The remaining star is confirmed to be a hot, helium-rich subdwarf. This star lies just outside the 90 per cent confidence level of the weak WFC source position. The possibility that this could be the first detection of EUV emission from a hot subdwarf is discussed

We present absolute far UV (900 to 1200 A) spectrophotometry of two hot subluminous stars, BD +28°4211, an sdO, and G191-B2B, a DA white dwarf. These observations, obtained with the Voyager ultraviolet spectrometers, are combined with other data to produce complete absolute energy distributions extending from the Lyman limit at 912 A to 1 μm. For BD +28°4211, this energy distribution is closely approximated by a single power law. Alternately, the energy distribution of G191-B2B is well matched by a single pure H model atmosphere whose effective temperature and gravity are independently determined from spectroscopy. These two distinct results indicate that simple models provide good agreement with observed energy distributions, even at the shortest wavelengths. Comparisons of the Voyager absolute calibration with previous rocket results are discussed, including the recent rocket observations of BD +28°4211 and G191-B2B.

We have studied the EUV spectra of 13 DA white dwarfs, observed by the EUVE satellite, paying attention to the possible sources of absorbing material along the lines-of-sight in both the local interstellar medium and in the photospheres of the stars themselves. The range of interstellar column densities seen are consistent with our previous understanding of the local distribution of material. Absorption from interstellar He ii is found in the direction of five stars, allowing us to measure directly the He ionization fraction and estimate, indirectly, that of H. The weighted mean ionization fractions along these lines-of-sight are 0.27 ± 0.04 and 0.35 ± 0.1 respectively. Where He ii is directly detected, the observed ionization fractions are not correlated with direction or with the volume/column density of material along the line-of-sight. Furthermore, the limits on the amount of He ii established in all other directions completely encompass the range of observed values. Indeed, all the data can be consistent with more or less constant He and H ionization fractions throughout the local ISM. However, observation of very hot DA stars, indicating higher He ii columns, might contradict this picture if the material is not photospheric or circumstellar.

We have used IUE archive observations to obtain a new and independent estimate of the effective temperature of Sirius B. In this effort we have modeled the observed Lyman alpha profile of Sirius B as a function of effective temperature and find; Teff = 24,500 ± 500 K. This temperature is in good agreement with a recent result obtained from analysis of EXOSAT soft X-ray spectra of Sirius B. In addition to providing confirmation of the soft X-ray result, this temperature implies that Sirius B is significantly cooler and larger in radius than most previous estimates. The radius, R = 0.090 ± 0.005 R⊙, implied by the lower temperature is well in excess of the radius of fully degenerate star having the mass of Sirius B. This investigation employed a unique set of SWP archive spectra, acquired with Sirius B in the small aperture, which have proved to be free of scattered light contamination from Sirius A.

It has long been predicted that Extreme Ultraviolet (EUV) observations of white dwarfs would be one of the principal keys to understanding the composition and structure of white dwarf atmospheres. Indeed, the ROSAT X-ray and EUV sky survey has revealed that most H-rich white dwarfs hotter than 40,000K contain significant quantities of heavy elements in their atmospheres. However, the ROSAT data are unable to say much about the role of helium in the hottest white dwarfs, which remains an important question. Spectroscopic observations with the Extreme Ultraviolet Explorer allow a direct and sensitive search for Hell absorption to be performed. Furthermore, they potentially reveal the detailed nature of the sources of EUV opacity. In most cases examined no trace of He is found but HZ43 and G191-B2B show tantalising evidence for the presence of He, which would have important implications for our views of white dwarf evolution.

The pulsating 77 Ceti storm cover a narrow range of effective temperatures along the cooling sequence of DR white dwarfs ~see. eg., Vinget and Fontoine 1982). Fast-photometric searches for pulsating stars in that class hove provided strong evidence that the 77 Ceti phase is on evolutionary phase through which ol_./I cooling DR stars will eventually go through (Fontoine e~ a/. 1982}. Recent investigations, based on optical or ultraviolet photometry and spectrophotometry, hove set the boundaries of the instabi l i ty strip at temperatures near 10,000-11,000 K and 12,000-13,000 K, respectively (McGraw 1979; Greenstein 1982; Veidemonn and Koester 1984; Fontaine et e/. 1985; Vesemoel, Lomontogne, and Fontoine 1986; Lomontogne, Vesemoel, and Fontoine 1987, 1988}.

We have used optical and UV spectroscopy to determine He abundances and upper limits to He abundances in the photospheres of a selected sample of very hot hydrogen-rich white Dwarfs He abundances in the range log(He/H) −3 to −1.5 are observed in several of these DAs and upper limits of −3 determined for the remainder. In apparent contradiction to the relatively large He abundances inferred from soft X-ray observations for the hot DA G191 B2B, we find no evidence of He in the optical and UV.

Because of the similarity of the primary star of HS 1136+6646 to the planetary nebula central star BE Ursae Majoris, we did wide field imaging of the former with an H-alpha filter. No nebulosity was detected. On the other hand, the point spread function of the star appeared extended. A partially-resolved red component is detected in the image with the five-band Sloan Digital Sky Survey. Most importantly, a companion is easily resolved in the HST acquisition image for the published STIS observation. A companion to the pre-cataclysmic binary is present at a separation of 1.349'' at position angle 54.4 deg. Evidence indicates that it is likely of K spectral type. We cannot demonstrate conclusively that this component has common proper motion with the close binary. However, the similar apparent z magnitudes and spectral types of HS 1136+6646B and the resolved component make it likely that we have in reality a hierarchial triple system. In any case, the presence of this component needs to be taken into account in future ground-based studies., 4 pages, 5 figures, requires emulateapj.sty. Accepted for publication in PASP. Figure 1 reduced in resolution

The Sirius AB binary system has masses that are well determined from many decades of astrometric measurements. Because of the well-measured radius and luminosity of Sirius A, we employed the TYCHO stellar evolution code to determine the age of the Sirius A,B binary system accurately, at 225-250 Myr. Note that this fit requires the assumption of solar abundance, and the use of the new Asplund et al. primordial solar metallicity. No fit to Sirius A's position is possible using the old Grevesse & Sauval scale. Because the Sirius B white dwarf parameters have also been determined accurately from space observations, the cooling age could be determined from recent calculations by Fontaine et al. or Wood to be 124 +/- 10 Myr. The difference of the two ages yields the nuclear lifetime and mass of the original primary star, 5.056 +0.374/-0.276 solar masses. This result yields in principle the most accurate data point at relatively high masses for the initial-final mass relation. However, the analysis relies on the assumption that the primordial abundance of the Sirius stars was solar, based on membership in the Sirius supercluster. A recent study suggests that its membership in the group is by no means certain., 12 pages, one figure (reduced resolution JPG posted), accepted for publication in the ApJ Letters. Replaced with revised version correcting a typo in the text (the derived age of Sirius A on page 5); conclusions unchanged

We report the detection, from FUSE data, of phosphorus in the atmospheres of GD71 and two similar DA white dwarfs. This is the first detection of a trace metal in the photosphere of the spectrophotometric standard star GD71. Collectively, these objects represent the coolest DA white dwarfs in which photospheric phosphorus has been observed. We use a grid of homogeneous non-LTE synthetic spectra to measure abundances of[P/H]=-8.57 +0.09 -0.13, -8.70 +0.23 -0.37 and -8.36 +0.14 -0.19 in GD71, RE J1918+595 and RE J0605-482 respectively. At the observed level we find phosphorus has no significant impact on the overall energy distribution of GD71. We explore possible mechanisms responsible for the presence of this element in these stars, concluding that the most likely is an interplay between radiative levitation and gravitational settling but possibly modified by weak mass loss., Comment: Accepted for publication in MNRAS Letters

We present heavy element abundance measurements for 16 DAO white dwarfs, determined from Far-Ultraviolet Spectroscopic Explorer {FUSE} spectra. Evidence of absorption by heavy elements was found in the spectra of all the objects. Measurements were made using models that adopted the temperatures, gravities and helium abundances determined from both optical and FUSE data by Good et al. (2004). Despite the different evolutionary paths that the types of DAO white dwarfs are thought to evolve through, their abundances were not found to vary significantly, apart from for the silicon abundances. Abundances measured when the FUSE derived values of temperature, gravity and helium abundance were adopted were, in general, a factor 1-10 higher than those determined when the optical measure of those parameters was used. Satisfactory fits to the absorption lines were achieved in approximately equal number. The models that used the FUSE determined parameters seemed better at reproducing the strength of the nitrogen and iron lines, while for oxygen, the optical parameters were better., Comment: 15 pages, 24 figures. Accepted for publication in MNRAS

Just before earth passed through Saturn's ring plane on 10 August 1995, the Hubble Space Telescope Faint Object Spectrograph detected ultraviolet fluorescent emissions from a tenuous atmosphere of OH molecules enveloping the rings. Brightnesses decrease with increasing distance above the rings, implying a scale height of about 0.45 Saturn radii (Rs). A spatial scan 0.28Rs above the A and B rings indicates OH column densities of about 10(13) cm(-2) and number densities of up to 700 cm(-3). Saturn's rings must produce roughly 10(25) to 10(29) OH molecules per second to maintain the observed OH distribution.

We have undertaken a detailed near-IR spectroscopic analysis of eight notable white dwarfs, predominantly of southern declination. In each case the spectrum failed to reveal compelling evidence for the presence of a spatially unresolved, cool, late-type companion. Therefore, we have placed an approximate limit on the spectral-type of a putative companion to each degenerate. From these limits we conclude that if GD659, GD50, GD71 or WD2359-434 possesses an unresolved companion then most probably it is substellar in nature (M, Accepted for publication in MNRAS 9-12-04

APEX is a proposed mission for a Small Explorer (SMEX) satellite. APEX will investigate the density, temperature, composition, magnetic field, structure, and dynamics of hot astrophysical plasmas (log T = ~5-7), which emit the bulk of their radiation at EUV wavelengths and produce critical spectral diagnostics not found at other wavelengths. APEX addresses basic questions of stellar evolution and galactic structure through high-resolution spectroscopy of white dwarf stars, cataclysmic variables, the local interstellar medium, and stellar coronae. Thus APEX complements the Chandra , Newton-XMM , FUSE , and CHIPS missions. The instrument is a suite of 8 near-normal incidence spectrometers (~90-275 Angstroms, resolving power ~10,000, effective area 30-50 cm 2 ) each of which employs a multilayer-coated ion-etched blazed diffraction grating and a microchannel plate detector of high quantum efficiency and high spatial resolution. The instrument is mounted on a 3-axis stabilized commercial spacecraft bus with a precision pointing system. The spacecraft is launched by a Taurus vehicle, and payload size and weight fit comfortably within limits for the 2210 fairing. Of order 100 targets will be observed over the baseline mission of 2 years. These are selected carefully to maximize scientific return, and all were detected in the EUVE and the ROSAT WFC surveys.

This text describes the development of astronomy in the Extreme Ultraviolet (EUV) wavelength range, from the first rocket-based experiments in the late 1960s through to later satellite missions. Discussions of the results from important space projects are followed by an analysis of the contributions made by EUV astronomy to the study of specific groups of astronomical objects. Within this framework, the book provides detailed material on the tools of EUV astronomy, dealing with the instrumentation, observational techniques, and modelling tools for the interpretation of data. Prospects for future EUV missions are discussed, and a catalogue of the known EUV sources is included. This book will be of great value to graduate students and researchers. It gives a complete overview of Extreme Ultraviolet astronomy.