Many of the nearest white dwarf stars (e.g., Sirius B and Procyon B) are in such binaries and would have remained undiscovered if they were even a little bit further away. White dwarfs which are sufficiently hot (T(eff) > 10,000 K) would, if present in binary systems with a relatively cool (F, G, K, or M) main–sequence secondary, be visible in IUE images as a hot companion to the main sequence star. We systematically examined 318 IUE images of 280 different G, K, and M stars which had been observed for other purposes. No previously undiscovered white dwarf stars were found.

In 1984 a program has been set up for a systematic search of double degenerates (DD) among spectroscopically confirmed white dwarfs. For each WD at least two CCD spectra are obtained, which are then cross-correlated in order to get a radial velocity difference, if any. For this purpose we have used the ESO Faint Object Spectrograph and Camera (EFOSC) attached at the ESO 3.6m telescope. Exposure times are typically in the range 5–15 min, and the typical error in the radial velocity difference (Δυx) is ∼ 20 km s−1.

Until now we had two observing runs, respectively in September 1985 and January 1988, for a total of 5 usable nights. For 20 WDs we have obtained more than one spectrum. Among them we have singled out two DD candidates. The first object (WD0957–666) has shown Δυr values up to ∼ 220 km s−1 and three other spectra were kindly taken for us in April 1988 by Stefano Cristiani. These also showed Δυr,’s up to ∼ 140 km s−1. Another object (WD0954–710) exhibits a Δυr ≃ 70 km s−1. The two DD candidates will be observed again in the next observing run in order to confirm their binarity and in case determine the periods.

Far ultraviolet (FUV) fluxes have been used for determining the effective temperatures of a number of DA white dwarfs hotter than 20,000 K. The spectra were obtained with the International Ultraviolet Explorer (IUE). The analysis consisted of comparing the observed FUV fluxes with model fluxes scaled to the V-band flux. After suitable corrections were performed for the time-dependent sensitivity degradation of the IUE, it was found that the available flux calibrations for the IUE were insufficiently accurate for precise temperature determinations. Accordingly, we used seven white dwarfs for which accurate, independent temperature determinations have been made from line profile analyses to improve the accuracy of the IUE flux calibration. The correction to the original calibration was as great as 20% in individual 5-Å wavelength bins, while the average over the IUE wavelength range was 5%. We present both our IUE flux correction and the temperatures obtained for the hot white dwarfs.

While the observed number of hot, helium-rich degenerates is noticeably larger than that of their hydrogen-rich counterparts, the calibration of their effective temperatures has been comparatively much less trustworthy. The spectroscopic classification scheme introduced three years ago by Wesemael, Green, and Liebert (1985, hereafter WGL), and the crude temperature domains associated with each class remain, to this date, the only comprehensive effort at defining a temperature scale for DO stars. The current uncertainty in this is perhaps best epitomized by two objects, HD149499B and PG1034+001. The former belongs to a binary system which also contains a KO V primary, 2” away. The temperature determined for the degenerate secondary ranges from 85,000±15,000 K (Wray, Parsons, and Henize 1979) to 55,000±5000 K (Sion, Guinan, and Wesemael 1982, hereafter SGW). PG1034+001, on the other hand, is the prototype of the so-called hot DO spectroscopic class; WGL assign an uncertain temperature of 80,000±20,000 K to this object.

In the past ten years, more than 775 low resolution spectra of white dwarfs have been taken with the International Ultraviolet Explorer satellite (IUE). This wealth of information has yielded many new discoveries in the field of white dwarf research; a few of which include: the λ1400 and λ1600 quasi-molecular features discovered in hydrogen rich DA white dwarfs (Greenstein 1980; Wegner 1982, 1984; Nelan and Wegner 1985; and Koester et. al. 1985), strong C I lines in some DQ white dwarfs (Koester, Weidemann, and Vauclair 1980; Wegner 1981a,b), and the absence of these same lines in hotter DB white dwarfs by Wegner and Nelan (1987) which may indicate convective mixing (Pelletier et al. 1986).

It is shown that a combination of the observed luminosity function of the local white dwarfs and the theoretical cooling rates of a typical white dwarf suggests an approximately constant rate of formation of the white dwarfs. This rate is found to be about a factor of three lower than the observed birthrate of their immediate progenitors. This discrepancy is here interpreted as a three-fold increase in the scaleheight of the white dwarfs due to dynamical interaction with stars, molecular clouds; an average white dwarf being much more aged than an average progenitor. Since the low mass stars on an average are even slightly more long-lived than these white dwarfs, one can place a lower bound on the scaleheights of the low mass stars to be given by the required scaleheights of the white dwarfs, which is, according to the present work, 660 pc in the solar neighbourhood.

The West German astronomy satellite ROSAT comprises two imaging telescopes - an X-ray instrument covering the energy band 0.15-2keV and an EUV instrument (provided by the UK) covering the range 0.02-0.21keV. A primary aim of the mission is to perform a sensitive all-sky survey with both telescopes (two ‘colours’ in the EUV) in the first 6 months. This will be followed by a least one year of pointed observations at specific targets. This paper briefly describes the ROSAT instrumentation and discusses the likely impact of the survey on white dwarf studies.

320 blue stars of the Giclas and Luyten catalogues with positions δ 15° and magnitudes have been observed photometrically in UBVRI and Stromgren colours with the ESO lm telescope at La Silla in order to increase the number of close-by white dwarfs. From their Strömgren colours more than 120 stars belong to the white dwarf region. But, as outlined by Rupprecht and Bues (1983), a combination of the photometric systems and the combined two-colour diagrams (R−I)/(U−V) and (R−I) / (u−b) provide additional information on binary components. Fig.1 shows a sample in the (R−I)/(u−b)diagram. For (u−b) <.2 and (R−I) <.2 a second component is present.

This poster reports the results of a search for variable radial velocities in 29 DA white dwarfs. The survey was sensitive to periods between 1h and 66d, although non-ideal sampling limited the longest practically detectable period to 2d. Three stars were discovered to have radial velocity shifts at above the 3σ level. The discovery of these stars, along with that of Saffer, et al. (1988) can put only a lower limit on the space density of close binary white dwarfs, as this survey was less than 100% efficient in detecting radial velocity variations, and its efficiency depended strongly on period.

The CCD/Transit Instrument (CTI) is a 1.8 m, f/2.2 meridian-pointing telescope located on Kitt Peak. It has no moving parts, but utilizes two RCA charge-coupled devices (CCDs) aligned with columns in the east-west direction and operated in the “time-delay and integrate” (TDI) mode at the apparent sidereal rate to form an image of the sky as it transits (McGraw et al. 1986). The strip is 8.25 arcminutes wide north-south with length determined by the length of the night. Thus, each night the CTI surveys about 15 square degrees of the sky, or about 45 square degrees per year, to a nightly limiting magnitude of mv ≥ 20. One of the two CCDs in the focal plane always observes through a V filter while the other utilizes one of (U), B, R, or I, depending upon the sky brightness. These data are searched to find every detectable object and photometric parameters are calculated for each. These data become part of a Master List which contains the best estimate for each parameter and a History List which maintains a light curve for every detected object. The vital statistics of the project are listed in Table 1.

The existence of a gap in the distribution of helium atmosphere white dwarfs at the effective temperature interval 30,000 K ≤ Te ≤ 45,000 K is well documented (Wesemael, Green and Liebert 1985; Liebert et al 1986; Green and Liebert 1987).

To explain the presence of this gap and other variations of the non-DA to DA ratio, it has been proposed (Fontaine and Wesemael 1987; Liebert, Fontaine and Wesemael 1987) that the helium-rich PG 1159 stars are the progenitors of essentially all white dwarfs. Some minute quantity of hydrogen of total mass, MH, is assumed to be mixed in the outer helium envelope, and settle upward as the star cools from the PG 1159 stage. Eventually enough hydrogen is at the surface to make the star appear a DA. The star has to become DA before it cools down to ≈ 45,000 K and cools to ≈ 30,000 K as a DA. At this temperature, depending on MH, the underlying helium convection zone may break into the hydrogen layer, diluting it and making the star a DB.

While the efficiency of gravitational settling to produce chemically pure atmospheres in white dwarf stars was outlined for the first time 30 years ago (Schatzman 1958), the competing role of the radiation flux in the hot white dwarfs was considered only 10 years ago (Fontaine and Michaud 1979; Vauclair, Vauclair and Greenstein 1979). At that time, there was more motivation to understand how metals could reappear in the long lived cool non DA white dwarfs, where diffusion time scales are shorter by orders of magnitude than evolutionary time scales. Various processes were invoked to help restore some metal content in the white dwarf atmospheres: convection mixing and dredge up, accretion of interstellar matter. In cool white dwarfs, the radiative acceleration is negligeable in the diffusion process; this is not the case at the hot end of the sequence where radiation may balance gravity. The short lived hot white dwarfs just started to become exciting with the contemporary discoveries that i) some show metallic lines in their spectra, both hydrogen rich and hydrogen poor; ii) some of these are pulsating. In the following years, the number of hot white dwarfs revealing trace abundance of metals has increased, mainly owing to IUE observations.

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.

We present first preliminary results from an exploratory spectral analysis of PG1159-035. An effective temperature of 120000 K (±20000 K) and a surface gravity of about log g=7 are derived from optical and ultraviolet profiles of carbon and helium lines. NLTE model atmospheres are used which are composed of H (1%), He (19%), C (40%) and O (40%, mass fractions). The abundances adopted are in accordance with predictions of stellar pulsation theory. A direct spectroscopic determination is under way. The analysis of optical CIV lines is rendered difficult due to the lack of a reliable line broadening theory which would have to account for a gradual change to broadening by linear Stark effect. Due to the complexity of the spectral analysis, reliable abundance ratios can only be derived from a systematic investigation employing a large grid of models.

We present the observations of the photospheric X-ray spectra of hot DA white dwarfs, obtained with the 500 lines mm−1 Transmission Grating Spectrometer on EXOSAT. These spectra cover the full soft X-ray band, at high wavelength resolution and statistical quality. They allow us to do an accurate measurement of the photospheric parameters, particularly of effective temperature and chemical composition of the atmosphere.

We consider the case of HZ 43 in some detail. Model atmospheric spectra that satisfy all measured absolute optical, UV and X-ray fluxes turn out not to fit the shape of the measured X-ray spectrum. However, from a comparison of model spectra calculated with different model atmospheres codes we infer the existence of a 15% systematic uncertainty in the model fluxes at the shortest wavelengths (λ < 100 Å) in current model calculations. This can explain the fitting problem. Since the systematic uncertainty in the models is larger than the statistical uncertainty in the shape of the measured X-ray spectrum of HZ 43, we cannot at present use this measured shape to derive the effective temperature and gravity. We revert to broad band photometry, using the measured integrated soft X-ray flux and the optical flux, to determine Te = 45,000 – 54,000K, R/R⊙ = 0.0140 – 0.0165. From the absence of the He II Ly edge at 227 Å in the measured spectrum, we set a upper limit on the photospheric helium abundance of He/H = 1.0 × 10−5; this upper limit is independent of the uncertainties in the model calculations mentioned above.

Predicted soft X-ray fluxes for model atmospheres containing varying concentrations of CNO metals are compared with those observed by EXOSAT for the planetary nebula nucleus K1-16. An effective temperature in the range ≈ 125000 − 180000K is determined for K1-16 and a limit on the concentration of CNO in the atmosphere (between 0.02 and 20 ×solar relative to He) obtained. Some comments on the application of the models to the apparently metal rich star H1504+65 are included.

The theory of gravitational separation of elements under the combined influence of gravity and electric fields (Schatzman 1958) has been very successful in explaining the general mono-elemental composition observed in most hot white dwarf atmospheres. Today the exceptions to the rule - DO and DAO with mixed compositions (Wesemael et al.1985), DA with traces of helium (Kahn et al. 1984, Petre et al. 1986, Jordan et al.1987, Paerels 1987), DAB (Liebert et al. 1984), and DBA (Shipman et al. 1987) - pose a problem, because they seem to demand a mechanism that counteracts gravitational separation. Such a mechanism that satisfies the observational constraints is very hard to find, and this has led in recent years to the growing conviction that we indeed observe the equilibrium state of diffusion, but in white dwarfs with extremely thin hydrogen layers that remain transparent (at least in the EUV) in many objects (Jordan and Koester 1986, Liebert et al.1987, Vennes et al. 1987, Fontaine et al.1988, Vennes et al.1988).

Understanding the origin of DA and non-DA stars — the bifurcation of the white dwarf cooling sequence — is one of oldest and perniciously difficult problems in the white dwarf research field. Processes which could play a role have been discussed for years. For example, the idea of diffusive element separation was suggested by Schatzman (1958). The suggestion of convective mixing goes back at least as far as Strittmatter and Wickramasinghe (1971) and Shipman (1972), and this process was actively discussed by a number of authors in the 1970s (see the excellent resume by D’Antona and Mazzitelli 1979).

In recent years cool white dwarfs have been studied for various aspects ( see e.g.Winget et al.,1987 Winget and van Horn, 1987, Koester, 1987, Llebert, 1980) and much effort has been Invested in attempts to interpret the energy distributions of these stars ( Greenstein, 1984, Zeldler-K.T. et al, 1986, Llebert et al., 1987, and others). However, it seems that in spite of these efforts the spectra in particular of the very cool objects with effective temperatures below about 6000 K are not yet fully under-stood, since they are extremely diverse and each objects needs special consideration. In addition, the analyses are extremely difficult because the principal constiuents of the atmospheres ( H, He ) and elements, which may donate the majority of electrons, are essentially invisible. Since usually only one ionlsatlon stage of an element is present, this implies that the gas pressure Pg is high ( compared e.g. to the solar photosphere ), the accurate value of Pg, however, cannot be determined reliably.

It has already been shown that most, and probably all, of the DA white dwarfs become variable in a narrow temperature range as they cool down (Fontaine et al. 1982). Optical photometry and spectrophotometry has led to several determinations of the boundaries of this instability strip. The strip has been found to cover the range 10300 - 13600 K (McGraw 1979), 10400 - 12100 K (Greenstein 1982), 10000 - 13000 K (Weidemann and Koester 1984) and 11000 - 13000 K (Fontaine et al. 1985). Theoretical calculations show that the location of the blue edge is very sensitive to the efficiency of convection used in the unpertubed models (Winget et al. 1982; Winget and Fontaine 1982; Fontaine, Tassoul, and Wesemael 1984). Also, the sharpness of this boundary depends on the range of stellar mass and thickness of the hydrogen envelope found in ZZ Ceti stars. Recently, Wesemael, Lamontagne, and Fontaine (1986) and Lamontagne, Wesemael, and Fontaine (1987) have obtained and compared ultraviolet observations of several DA white dwarfs, in or near the instability strip, with published model calculations from Nelan and Wegner (1985), hereafter NW, and Koester et al. (1985), hereafter KWZV. They determined the boundaries of the variability region at 11400 - 12500 K or 11700 - 13000 K depending on which grid was used. We present here a reanalysis of these IUE observations with an improved grid of model atmospheres in order to define more precisely the location of the blue edge.