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White dwarfs
Ads. Space Res. Vol. ii, No. 11, pp. (11)47—(11)56, 1991 Printed in Great Britain. Allsights reserved.
0273—i 177/91 $0.00 +.50 Copyright © 1991 COSPAR
WHITE DWARFS M. A. Barstow X-ray Astronomy Group, PhysicsDepartment, University ofLeicester, University Road, Leicester, LE1 7RH, U.K
ABSTRACT Astronomical observations at EUV (~1O0-1O0OA)and FUV (~l00O-20OOA)wavelengths have revolutionised our knowledge of white dwarfs and evolutionarily related objects such as subdwarfs and the nuclei of planetary nebulae. Data obtained principally with the IUE and EXOSAT observatories have revealed the presence of trace elements within white dwarf atmospheres and dramatically improved estimates of their photospheric temperatures. With these results it is now possible to begin to understand the various physical processes that determine the observed characteristics of white dwarfs, gaining a deeper insight into this late stage of stellar evolution. This review aims to summarise recent progress in this field. Attention will be paid to the role of computer generated atmospheric models in interpreting the data and understanding the results, considering their limitations and outlining areas where future work may need to be concentrated. The launch of new instrumentation will invigorate further an already exciting area of EUV and FUV astronomy. In particular, EUV sky surveys will discover many new white dwarfs expanding the statistical basis of evolutionary studies dramatically. INTRODUCTION Approximately 1500 white dwarfs are known /1/. With such a large sample of objects, astronomers are well placed to study the evolution of white dwarfs. However, there are several pieces of detailed information about each white dwarf that we need to find out for this work — the star’s temperature, surface gravity and atmospheric composition — principally from study of the stellar spectra. Unlike most other astronomical objects the flux at any wavelength in the spectrum of a white dwarf arises from a single physical source and mechanism, the transfer of radiation from the hot core through the nondegenerate atmosphere of the star. Therefore, in principle, observations in any waveband could acquire the information needed, measurements in the visible waveband being the easiest. However, visual data have many limitations in certain circumstances. Ifthe white dwarfis very hot, most ofthe total luminosity emerges in the far ultraviolet (FUV, ~10OO-30OoA) and extreme ultraviolet (EUV, ~l00-1000A) regions of the electromagnetic spectrum. Data from longer wavelengths cannot constrain the effective temperature very accurately. EUV and/or FUV observations are then essential. Often transitions from important ionisation states of elements in the white dwarf atmosphere lie in the FUV or EUV. Consequently, during the last 15 years, EUV and FUV observations have become increasingly important in improving our understanding of white dwarfs. CLASSIFICATION AND EVOLUTION OF WHITE DWARFS White dwarfs are considered to be divided into two main groups (figure 1). In one the main atmospheric constituent is hydrogen and in the second it is helium. Hydrogen rich stars were all classified as ‘DA’. The helium rich objects are divided into two subgroups. The hottest are the ‘DO’ types, showing Hell lines, with temperatures greater than ~45,00OK. Below ~30,0O0K, the helium rich white dwarfs are classified as ‘DB’ and exhibit Hel features. Where intermediate types exist the letters denoting composition are combined with the first giving the dominant component, as in the DAO stars which are mostly hydrogen but have weak helium features in their optical spectra /2/. The very coolest white dwarfs are classified as DC, DK and DZ stars having features such as molecular H 2 bands and weak Call lines. However, these are rather too cool to be of interest to FUV and EUV astronomers and will not be discussed further in this paper. This system of white dwarf classification is based entirely on features observed in optical spectra. It is still used but has been refined somewhat in the intervening years through work in the FUV and EUV wavebands. This is mainly as a resrilt of the detection of traces of other materials in what were previously thought to be pure H or He atmospheres (ie. He,C,N,O,Si in DAs and H,C,N,O, etc. in DOs/DBs). Important parts of these developments will be discussed later.
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M.A. Barstow DA
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Figure 1. Schematic diagram of H-rich and He-rich white dwarf evolutionary paths, starting from several possible progenitor channels. It is clear that, when compared with normal main sequence stars, the composition of all white dwarfs is rather peculiar. Since white dwarfs form when these stars come to the end of their nuclear fuel, determining how the atmospheric composition of the stars change and which objects are at an intermediate stage of evolution between the main sequence and white dwarfcooling track is of great interest. Since the upper limit to the white dwarf mass is ~1.4M®, and, in fact, the mass of the majority of white dwarfs is around 0.6M®, these intermediate phases of evolution must involve loss of stellar material. Several possible routes for white dwarf production have been studied. These follow on from the red giant phase of the star’s life where evolution occurs at constant luminosity, the temperature increasing as the star contracts. Mass loss can take place in two ways, either as a stellar wind or as a shell of material blown away by reignition of nuclear burning, such as a helium shell flash. The paths followed can be rather complex, especially if there are several occurrences of shell burning. However, in broad terms the star will eventually become a subdwarf and/or a planetary nebula nucleus (PNN). The precise relationship between these groups of objects, if indeed they are separate, is not well understood. However, they do seem to be the immediate progenitors of white dwarfs. There are two main theories concerning the origin of the H-rich and He-rich branches, which have been neatly summarised by Shipman /3/. The primordial view considers that the distinction between DA and non-DA occurs at the pre-white dwarf stage of stellar evolution. There is some evidence for this in signs of bifurcation in PNN also. The converse idea is that both DA and non-DA stars form by a single route from the PNN, possibly via the very hot DO type PG1159-035 objects /4/ which only
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have a little hydrogen. The DA stars then arise by gravitational settling of the heavier elements in the DO atmospheres. Two important features support this. No DAs have temperatures above 75,000K and so it is difficult to establish a direct DA-progenitor link /5/. Also, there are no DOs or DBs lying in the temperature range 30,000-45,000K. Hence, there must be some interchange of objects between the two groups via processes that redistribute elements in the stellar envelope (eg. gravitational settling, convection, radiative support etc.), by accretion or by mass loss (eg. winds, PN ejecta). To answer the question as to which view of white dwarf evolution is correct detailed and accurate measurements of atmospheric compositions are required for a large number of objects throughout the cooling sequence (and including PNN and subdwarfs). OBSERVATIONS OF DA WHITE DWARFS
Results from EXOSAT The low energy telescopes of the EXOSAT observatory /6/ had a response extending from the soft X-ray well into the EUV, a total range of 6-400A. These telescopes were operated in two modes; spectroscopic, where a transmission grating was inserted into the optical system, and photometric, where different EUV ‘colours’ analagous to UBVRI bands in optical work could be defined by thin film filters. Extraction of photometric filter data from EXOSAT images has been discussed several times, most recently by Barstow and Tweedy /7/. During the lifetime of EXOSAT around 20 DA white dwarfs were detected by the LE telescopes. Spectroscopic data were obtained for three of these — HZ43, Feige 24 and Sirius B. A number of important and interesting results were obtained from the EXOSAT spectra. The spectrum of HZ43 was used to determine an important upper limit to the abundance of He (relative to H) in its atmosphere (
M.A.Barstow
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to°
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Figure 2. EXOSAT transmission grating spectrum of Feige 24 compared with the predictions of several model atmosphere compositions — pure H, H+He and H+He+metals.
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Figure 3. 99% confidence areas for T~and He/H for the DA white dwarfs observed by EXOSAT. Data for the stars observed by Einstein are shown by the points with error bars.
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White Dwarfs
He/H ranges from i0~ to i0~ and there is no correlation with effective temperature. However, above 3. Paerels and Heise suggest that at the lower temperatures the 45,000K all but HZ43 have He/H>10 He/H abundance is determined by accretion from the ISM and that the scatter in the observed values reflects density fluctuations in the ISM itself. At the higher temperatures accretion cannot explain the He/H abundance and radiative levitation is suggested as a possible mechanism. Recently, it has been demonstrated /18/ that the amount of He that could be supported radiatively is much too small, by two orders of magnitude, to account for the -above results. In the absence of • forces counteracting the effect of gravity the photospheric composition would become stratified with H floating on top of a layer of He. A transition zone would exist, determined by the equations of diffusive equilibrium. A measured He abundance could then be interpreted as the vertical extent of the He diffusion tail in the atmosphere. This would depend upon the temperature of the star and the overlying mass of H. Koester /19/ has performed a self consistent analysis of 11 DAs in the EXOSAT sample using a set of stratified model atmospheres. He found that the EXOSAT DA data could be explained in terms of stratified atmospheres with H layer masses in the range 3 x 10 16_5 x 10’4M®. No single valued relation for MH with respect to temperature was found but a tendency for thinner H layers in the hotter DAs and thicker layers in cooler objects was noted. Some problems still remain with this interpretation. Large values for the interstellar column densities are obtained and in the case of HZ43 and GD153 are in serious disagreement with the very low values required by Voyager data /20/. However, use of the stratified models could allow significant He in HZ43 since the Hell 228A edge can be smoothed out by overlapping of high series members of the HeIlE lyman series (figure 4). I
I
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Lambda (A I Figure 4. Theoretical EUV flux for two stratified H=He models (centre curves) with the same spectra (bottom and top) smoothed to a resolution similar to the EXOSAT TGS (6A). Spectroscopic observations with IUE and Voyager Most of the EXOSAT observations were photometric and the few spectra that were obtained were of modest, though useful, resolution (~6A).In the FUV and long wavelength end of the EUV the spectroscopic capabilities of IUE and Voyager add an extra dimension to white dwarf studies. Information is provided in two ways. First, temperatures can be determined from the shape of the continuum. Secondly,
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M.A. Barstow
detailed study of absorption features (eg -figure 5) can tell us about the atmospheric compositions, the presence of winds and surface gravities besides effective temperature. 1.6
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Figure 5. Example of an IUE low dispersion short wavelength spectrum of a white dwarf, in this case the very hot object PG1159-035. The positions of interesting features and their identifications are marked. Two important pieces of work, by different groups, have concentrated on determining surface gravities and effective temperatures by analysis of low dispersion IEJE short wavelength spectra. Holberg et al /12/ have used the lyman a profiles of 12 DA white dwarfs to determine their effective temperature and surface gravity. The data were analysed using pure hydrogen LTE blanketed model atmospheres, each model containing a synthetic lyman a profile. Table I summarises the results of this work. It is notable that the errors in the temperature determination are at least as good as those for the EXOSAT results, and often superior. The results do not provide a sensitive determination of the surface gravity but a few significant conclusions can be drawn. Most of the results are clustered around log g=8.0. It may be expected that the hottest stars have lower gravity and this is true for 3 out of 4 stars studied. However, the obvious exceptional object (as in the EXOSAT study) is HZ43 which, with log g>8.35, has the highest gravity in the whole sample. This may explain why HZ43 appears to have minimal He since higher gravitational field would cause this element to settle out to greater depths in the atmosphere. Table 1. DA Temperature and surface gravity determinations from Lyman a line profile measurements. DA white dwarf T 6 (K) log g 40 En B 16, 325 ±325 7.65 ±0.20 Wolf 1346 20, 680 ±210 8.08 ±0.15 GD14O 21,375 ±395 8.45 ±0.15 CoD-38°10980 24, 500 ±140 8.08 ±0.15 GD71
33,300 ±830
7.78 ±0.20
GD394 GD659 GD153 GD246 Feige 24 HZ43 G191-B2B
36, 125 ±940 36,850 ±1,390 42, 375 ±1,480 53, 600 ±2, 940 55, 000 ±5, 360 57, 500 ±3,310 62,250 ±3,520
8.13 ±0.25 8.17 ±0.30 8.23 ±0.30 7.64 ±0.45 7.23 ±0.23 > 8.35 7.55 ±0.35
•
-
A complementary study of DA white dwarf temperatures performed by Finley et al /21/ uses the UV
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continuum slope. Although no information on surface gravity can be derived-, there are two advantages in this technique. First, it is not necessary to assume that the line of sight column density is neglible. In the lyman a profile study, absorption by neutral H along the line of sight can contribute to the observed profile, if the intrinsic stellar profile is weak and NH is large. The procedure used by Holberg et al ignores this effect. Secondly, fitting to a continuum can be performed for faint objects where the signal to noise in the lyman a profile is too poor•. Since the continuum technique is independent of the lyman a profile the results of each serve as a check on the other. Results obtained for objects common to both samples are in good agreement. An important atmospheric constituent, the abundance of which was addressed by the EXOSAT observations, is He. A second wa.y of measuring the He abundance is to observe the several Hell and Hel transitions that can be found in DA white dwarf spectra. One of these, Hell 1640A, lies in the FUV and the rest (Hell 4686A, Jiel 4471A) are optical features. Holberg et al /5/ have taken the following approach. Effective temperature and log g for a sample of white dwarfs were determined by detailed fitting of the H balmer proffles to a grid of pure H model atmospheres. He abundances were then estimated from from a related grid of models with He/H ratios from io—~to 3 x ~o 2 and homogeneous composition. Measurements of the He abundance were made for 8 very hot DA white dwarfs (table 2). The pattern of abundances with temperature is different to that seen in the EXOSAT results, suggesting that DA He abundances at high temperatures can have a very wide range, extending from less than i0~ for HZ43 to 10—1.5 for DAOs such as PGI21O+533. Table 2. Spectroscopically determined helium abundances in DA white dwarfs. DA white dwarf T 0 (K) log g log (He/H) PG0134+181 72,500 ±5,000 7.0 ±0.5 <—3.3 PG0823+317 62,700±4,200 7.25 ±0.25 —2.5 ±0.25 PG0846+249 63, 700 ±5, 500 7.0 ±0.35 —2.6 ±0.25 PG0950+139 70, 500 ±5,000 7.15 ±0.5 <—2.2 PG11O8+325 64, 300 ±5, 000 7.75 ±0.35 < -3.5 PG1305-017 53,300 ±3, 500 7.25 ±0.35 ~ —1.0 PGJ21O+533 50,000 8.0 ~ —1.0 G191-B2B 62, 250 ±3, 520 7.6 ±0.4 < —3.58 Studies of metal abundances in DA white dwarfs using ThE data are interesting. For example, there are no detectable metal features in the spectrum of HZ43 /22/. In contrast, C, N and Si can all be identified in the spectrum of Feige 24 /13/. Qualitatively this is consistent with more rapid gravitational settling of trace elements due to the higher surface -gravity of HZ43. However, the abundances determined for C,N and Si in Feige 24 (log(C/H)=-6.4±O.6,log(N/H)-5.3±1.0 and log(Si/H)=-6.3±0.9) can only be explained by the influence of selective radiative forces counteracting the gravitational potential /23/. It is interesting to recollect that C,N and Si alone cannot explain the EXOSAT spectrum, additional trace aborbers being required. In some cooler DA stars, such as Wolf 1346 /22/ and CoD-38°10980 /24/, only Si can be seen in the TUE spectra. Abundances of N and Si predicted by diffusion theory /25/ are in keeping with the observations. However, the abundance of C at the temperature of Feige 24 is predicted to be higher than observed.
OBSERVATIONS OF THE DO WHITE DWARFS DO white dwarfs and the related very hot PG1159 stars are a much less populous group of objects than the DA white dwarfs. Only 20-30 such stars are known. It appears that the lowest temperature at which these stars occur is l2~45,000Kand the temperatures of the very hottest of the PG1159 stars are certainly in excess of 100,000K and could be as hot as 180,000K or 200,000K. Except for the coolest objects (<100,000K) the FUV spectral slope is insensitive to temperature and it is then necessary to rely on more complex methods of temperature measurement such as EUV photometry or line profile fitting. Since the stars are expected to have atmospheres that are predominantly composed of He there will only be significant EUV fluxes at temperatures above-80,000K. As a result of these limitations data from a variety of wavebands have simultaneously been brought to bear on the study of individual stars.
Multiwaveband observations of PG1159 stars -
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Sion et al /4/ have proposed that the PG1159 stars represent an evolutionary link between He-rich PNN and DO white dwarfs proper, on the basis of similarities between their FUV, UV and optical spectra. Typical features are a trough of Hell 4686A blended with transitions of CIV, CIII and NIh in the optical, and OVI in the UV at 3434A and FUV 1036A. It is possible, though by no means certain, that a significant number of DAs could evolve via this route, rather than directly from H-rich progenitors. Hence, a detailed
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M.A. Barstow
understanding of the evolution of the stellar abundances - with temperature for the PG1 159 stars is very important. Some of the PG1159 group are known to be non-radial pulsators, including PG1159-035 (the prototype object) /26/ arid K1-16, a PG1159-like PNN. At these high temperatures the driving mechanism is thought to be partial ionisatioñ of C and 0 in a C-O rich envelope /27/. Six PG1159 related objects were observed by EXOSAT and five were detected. The results of these observations have been summarised by Barstow and Holberg /28/. These data were analysed using a grid of homogeneous, LTE spectra covering a large range of abundances and including H,He,C,N and 0 /29/. At high temperatures and low gravities the assumption of LTE will become invalid but no NLTE models with CNO were-then available with which to carry out the study. In order to reduce the number of free parameters considered, IUE lyman a profiles were used to obtain measurements of the interstellar column density. Since these objects do not have H features in- their spectra there is either no H at all or it is too highly ionized to contribute to the opacity. Either way, all the observed lyman a absorption in the IUE spectra can be attributed to interstellar I-I. Further information was also obtained from the IUE data in the form of the spectral slope, which c~ou1dbe used to assign a rough temperature order to the objects. Including all this information in the analysis a range of temperatures and abundances was determined for each object, as summarised in table 3. H1504+65 is the hottest object in the group at ~180,000Kand KPDOO05+5106 the coolest. There appears to be a weak tendency for the atmospheric He and metal abundances to decrease as a- function of decreasing temperature. Table 3. Effective temperature, log NH and abundance determined PG1159 star data. Most likely values are in bold type. 2) Object Temperature (K) NH (log cm H1504+65 150000-180000-185000 19.65-19.70 PG1159-035 123000-124000 19.93-19.96 -PG1144+005 93000-124000 19.70-19.95 K1-16 103000-110000 19.89-20.12 KPD0005+5106 83000-97500-100000 19.64-20.05
from the LTE analysis of EXOSAT H:He 0-1-b®
CNO:He
1® 1-b® 0.5®-1® i®-hO®
0.02®-® 0.05®-0.1® 0. 1®-®
Subsequently, Werner et a! /30/ have performed a detailed analysis of C,N and 0 line profiles using NLTE model atmospheres, concentrating on the analysis of optical data and not considering EUV or FUV fluxes. The results obtained for PG1159-035, the only object common to both samples are in serious disagreement with the EXOSAT analysis. They obtained a similar temperature, 130,000-140,000K compared to 124,000K, but very different estimates of the atmospheric abundances. They find that little or no H is present (He/H~4.75)and, - the C & 0 abundances are (relative to H) ~3.33 and ~0.625 respectively. On the face of it the discrepancy isprobably due to the use of an improved physical model, namely the inclusion of NLTE effects. Recent work /31/ has looked at this problem in detail, establishing that the cause of the discrepancy is a combination of the inadequacy of the LTE approximation and insufficient detail in the oxygen model atom used in the LTE calculations. Furthermore, the soft X-ray fluxes predicted by NLTE models with high He and CNO abundances, as favoured by the optical data, are found to be consistent with the EXOSAT observations. Using some new grids of NLTE model atmospheres the EXOSAT data is being reanalysed. Early results /31/ indicate that the metal abundances in these stars do decrease to lower temperatures. The effect is greater than seen in the LTE analysis and the abundances determined for KPD0005+5b06, the coolest object studied, remain unchanged. Hence, the PG1159 route seems a plausible one fOr producing both DO and DA white dwarfs. Some FUV spectroscopy has been performed on PG1159 stars and DO white dwarfs. Unfortunately, not many of the objects are bright enough for high dispersion IUE spectra to be obtained. Downes et al /32/ have obtained such spectra for KPD0005+5106 and used this data to obtain limits on the N abundance. The EXOSAT NLTE results are in good agreement with this. High dispersion TUE spectra have also been obtained for the fainter PG1159-035/33/ and K1-16 by exposing the SWP camera for two contiguous low background shifts. Many features such as NV 1240A, NIV 1270A, OV 1371A and CIV 1550A have been identified in the PG1159-035 spectrum. Poulin et al /34/ have studied the FUV energy distributions of PGb0344~001and HD1449499B with Voyager and IUE. This data was used to obtain effective temperature estimates (~80,000K~ and 54,000K respectively) by fitting the spectra to pure H model flux distributions. — -
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OBSERVATIONS OF DB WHITE DWARFS DB white dwarfs are unlikely to be EUV sources. Their combination of low effective temperature (<30,000K) relative to the DOs, and He-rich atmospheres result in a high predicted EUV photospheric opacity. Unless their atmospheres contain significant traces of, as yet undetected, H predicted EUV fluxes are extremely low. In addition, the hottest objects in this group, the pulsators, seem to be all at large distances and high column density. Consequently, the emergent flux will be very highly absorbed by the ISM. Much useful work has, however, been carried out with FUV observations by IUE. Recent studies (eg. /35/) have improved temperature estimates, particularly for the pulsating stars. Establishing an accurate temperature scale is particularly important in attempting to understand the problem of the DO-DB gap /36/. DBA stars contain traces of U in their mostly He atmospheres at the i~~-i~—~ level /37/. Lyman a observations would allow much better estimates of the H abundances to be made. Unfortunately these are at the limit of IUE, since the stars are somewhat faint and cool. In principle it should be possible to search for trace metal features in the FUV using high resolution spectra. This has been possible for only one object, the prototype pulsating DB GD358 /38/. In fact, two IUE SWP echelle images were recorded and then coadded. In addition to the expected Hell 1640A absorption feature a CII (1336A) line was discovered. CONCLUSION: THE FUTURE This review has dicussed the current state of our knowledge of white dwarf morphology and evolution, highlighting in particular the contributions of EUV and FUV astronomy. An attempt has been made to cover as wide an area as possible but inevitably it is impossible to do complete justice to all the very many contributions made. The recently launched ROSAT mission, carrying both soft X-ray and EUV telescopes is likely to discover several thousand new white dwarfs /39/. At the same time it will aquire photometric data of similar quality to that of EXOSAT, but for at least two orders of magnitude more objects, dramatically increasing the size of the white- dwarf sample available for more detailed study. It is readily apparent that interpretation of any of the data discussed in this review or obtained by current or future missions relies very heavily on the availability of appropriate model atmospheres and the flux distributions and line profiles which these predict. The current problems that need addressing centre on their physical validity. For example, the need for NLTE effects to be included in the models for hot or low gravity objects. Much progress has been made in recent years. NLTE codes that are able to treat a large number of line transitions are now being used and useful models generated. In parallel a significant amount of work has been going on developing stratified LTE H+He models as well as studies into radiative support of elements. The next step will be to integrate these approaches to produce an evolutionary set of models where material abundances and the degree of stratification changes as the temperature decreases. ACKNOWLEDGMENTS MAB acknowledges the support of the SERC through the ROSAT WFC programme and the Royal Society for travel funds to attend the COSPAR meeting. REFERENCES 1. G.P.McCook and E.M.Sion, Ap.J.SuppL, 65, 603 (1987). 2. E.M.Sion, J.L.Greenstein, J.D.Landstreet, J.Liebert, H.L.Shipman and G.A.Wegner, Astrophys.J., 269, 253 (1983). 3. H.L.Shipman, in White Dwarfs, ed. G.Weguer, Springer-Verlag, 220, (1989) 4. E.M.Sion, J.Liebert, and S.G.Starrfield, Astrophys.J., 292, 471 (1985). 5. J.B.Holberg, K.Kidder, J.Liebert, F.Wesemael, en White Dwarfs, ed. G.Wegner, Springer-Verlag, 188 (1989). 6. P.A.J.de Korte, et a!., Spac.Sci.Rev., 30, 495, (1981). 7. M.A.Barstow and R.W.Tweedy, Mon.Not.R.astr.Soc., 242, 484 (1990). 8. J.Heise, F.B.S.Paerels, J.A.M.Bleeker and A.C.Brinkman, Ap.J, 334; 959 (1988). 9. J.B.Holberg, B.R.Sandel, W.T.Forrester, A.L.Broadfoot, H.L.Shipman and J.L.Barry, Ap..!., 242, L119 (1980). 10. F.B.S.Paerels, J.A.M.Bleeker, A.C.Brinkman and J.Heise, Ap.J., 329, 849 (1988). 11. F.B.S.Paerels, J.A.M.Bleeker, A.C.Brinkman and J.Heise, Ap.J., 309, L33 (1986). 12. J.B.Holberg, F.Wesemael and J.Basile, Ap.J., 306, 629 (1986). 13. F.Wesemael, R.C.B.Henry and H.L.Shipman, Api., 287, 868 (1984).
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14. S.Vennes, G.Fontaine, F.Wesemael, in White Dwarfs, ed. G.Wegner, Springer-Verlag, 363 (1989). 15. S.M.Kahn, F.Wesemael, J.Liebert, J.C.Raymond, J.E.Steiner, and H.L.Shipman, Ap.J., 278, 255 (1984). 16. R.Petre, H.L.Shipman and C.Canizares, Ap.J., 304, 356 (1986). 17. F.B.S.Paerels and J.Heise, Ap.J., 339, 1000 (1989). 18. S.Vennes, C.Pelletier, G.Fontaine and F.Wesemael, Ap.J., 331, 876 (1988). 19. D.Koester, Api., 342, 999 (1989). 20. J.B.Holberg, in The second conference on faint blue stars, eds. A.G.Davis Philip, D.S.Hayes and J.Liebert, L.Davis press, Schenectady, 285 (1987). 21. D.S.Finley, G.Basri and S.Bowyer, in White Dwarfs, ed. G.Wegner, Springer-Verlag, 139 (1989). 22. F.Bruhweiler and Y.Kondo, Ap.J., 269, 657 (1983). 23. P.Chayer, G.Fontaine, F.Wesemael and G.Michaud, in The second conference on faint blue stars, eds. A.G.Davis Philip, D.S.Hayes and J.Liebert, L.Davis press, Schenectady, 653 (1987). 24. J.B.Holberg, F.Wesemael, G.Wegner and F.C.Bruhweiler, Ap.J., 293, 294 (1985). 25. G.Vauclair, in White Dwarfs, ed. G.Wegner, Springer-Verlag, 176 (1989). 26. M.A.Barstow, J.B.Holberg, A.D.Grauer and D.E.Wiuget, Ap.J., 306, L25 (1986). 27. S.Starrfield, A.N.Cox, R.B.Kidman and W.D.Pesnell, Ap.J., 281, 800 (1984). 28. M.A.Barstow and J.B.Holberg, Mon.Not.R.astr.Soc., in press. 29. M.A.Barstow, Mon.Not.R.astr.Soc., 243, 182 (1990). 30. K.Werner, U.Heber and K.Hunger, submitted to Astron.Astr.. 31. M.A.Barstow, R.W.Tweedy and K.Werner, in Proceedings of the 7th European White Dwarf Workshop, ed. G.Vauclair, in press. 32. R.A.Downes, E.M.Sion, J.Liebert and J.B.Holberg, Ap.J., 321, 943 (1987). 33. J.L.Liebert, F.Wesemael, D.Husfeld, R.Wehrse, S.G.Starrfield and E.M.Sion, Astron.J., 97, 1440, 1989. 34. E.Poulin, F.Wesemael, J.B.Holberg and G.Fontaine, in White Dwarfs, ed. G.Wegner, Springer-Verlag, 144 (1989). 35. J.Liebert, F.Wesemael, C.J.Hansen, G.Fontaine, H.L.Shipman, E.M.Sion, D.E.Winget and R.F.Green, Api., 309, 241 (1986). 36. J.Liebert, in Hydrogen deficient stars and related objects, eds. K.Hunger, D.Schonberner and }C.Rao, 367 (1986). 37. H.L.Shipman, J.Liebert and R.F.Green, Ap.J., 315, 239 (1987). 38. E.M.Sion, J.Liebert, G.Vauclair and G.Wegner, Villanova University preprint (1989). 39. M.A.Barstow, in White Dwarfs, ed. G.Wegner, Springer-Verlag, 156 (1989).
0273—i 177/91 $0.00 +.50 Copyright © 1991 COSPAR
WHITE DWARFS M. A. Barstow X-ray Astronomy Group, PhysicsDepartment, University ofLeicester, University Road, Leicester, LE1 7RH, U.K
ABSTRACT Astronomical observations at EUV (~1O0-1O0OA)and FUV (~l00O-20OOA)wavelengths have revolutionised our knowledge of white dwarfs and evolutionarily related objects such as subdwarfs and the nuclei of planetary nebulae. Data obtained principally with the IUE and EXOSAT observatories have revealed the presence of trace elements within white dwarf atmospheres and dramatically improved estimates of their photospheric temperatures. With these results it is now possible to begin to understand the various physical processes that determine the observed characteristics of white dwarfs, gaining a deeper insight into this late stage of stellar evolution. This review aims to summarise recent progress in this field. Attention will be paid to the role of computer generated atmospheric models in interpreting the data and understanding the results, considering their limitations and outlining areas where future work may need to be concentrated. The launch of new instrumentation will invigorate further an already exciting area of EUV and FUV astronomy. In particular, EUV sky surveys will discover many new white dwarfs expanding the statistical basis of evolutionary studies dramatically. INTRODUCTION Approximately 1500 white dwarfs are known /1/. With such a large sample of objects, astronomers are well placed to study the evolution of white dwarfs. However, there are several pieces of detailed information about each white dwarf that we need to find out for this work — the star’s temperature, surface gravity and atmospheric composition — principally from study of the stellar spectra. Unlike most other astronomical objects the flux at any wavelength in the spectrum of a white dwarf arises from a single physical source and mechanism, the transfer of radiation from the hot core through the nondegenerate atmosphere of the star. Therefore, in principle, observations in any waveband could acquire the information needed, measurements in the visible waveband being the easiest. However, visual data have many limitations in certain circumstances. Ifthe white dwarfis very hot, most ofthe total luminosity emerges in the far ultraviolet (FUV, ~10OO-30OoA) and extreme ultraviolet (EUV, ~l00-1000A) regions of the electromagnetic spectrum. Data from longer wavelengths cannot constrain the effective temperature very accurately. EUV and/or FUV observations are then essential. Often transitions from important ionisation states of elements in the white dwarf atmosphere lie in the FUV or EUV. Consequently, during the last 15 years, EUV and FUV observations have become increasingly important in improving our understanding of white dwarfs. CLASSIFICATION AND EVOLUTION OF WHITE DWARFS White dwarfs are considered to be divided into two main groups (figure 1). In one the main atmospheric constituent is hydrogen and in the second it is helium. Hydrogen rich stars were all classified as ‘DA’. The helium rich objects are divided into two subgroups. The hottest are the ‘DO’ types, showing Hell lines, with temperatures greater than ~45,00OK. Below ~30,0O0K, the helium rich white dwarfs are classified as ‘DB’ and exhibit Hel features. Where intermediate types exist the letters denoting composition are combined with the first giving the dominant component, as in the DAO stars which are mostly hydrogen but have weak helium features in their optical spectra /2/. The very coolest white dwarfs are classified as DC, DK and DZ stars having features such as molecular H 2 bands and weak Call lines. However, these are rather too cool to be of interest to FUV and EUV astronomers and will not be discussed further in this paper. This system of white dwarf classification is based entirely on features observed in optical spectra. It is still used but has been refined somewhat in the intervening years through work in the FUV and EUV wavebands. This is mainly as a resrilt of the detection of traces of other materials in what were previously thought to be pure H or He atmospheres (ie. He,C,N,O,Si in DAs and H,C,N,O, etc. in DOs/DBs). Important parts of these developments will be discussed later.
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M.A. Barstow DA
Non.DA
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Helium Dominated
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~N1TORS-He-ri~l
~
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Di Polsattoos 10000K
3000(tK
t
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Figure 1. Schematic diagram of H-rich and He-rich white dwarf evolutionary paths, starting from several possible progenitor channels. It is clear that, when compared with normal main sequence stars, the composition of all white dwarfs is rather peculiar. Since white dwarfs form when these stars come to the end of their nuclear fuel, determining how the atmospheric composition of the stars change and which objects are at an intermediate stage of evolution between the main sequence and white dwarfcooling track is of great interest. Since the upper limit to the white dwarf mass is ~1.4M®, and, in fact, the mass of the majority of white dwarfs is around 0.6M®, these intermediate phases of evolution must involve loss of stellar material. Several possible routes for white dwarf production have been studied. These follow on from the red giant phase of the star’s life where evolution occurs at constant luminosity, the temperature increasing as the star contracts. Mass loss can take place in two ways, either as a stellar wind or as a shell of material blown away by reignition of nuclear burning, such as a helium shell flash. The paths followed can be rather complex, especially if there are several occurrences of shell burning. However, in broad terms the star will eventually become a subdwarf and/or a planetary nebula nucleus (PNN). The precise relationship between these groups of objects, if indeed they are separate, is not well understood. However, they do seem to be the immediate progenitors of white dwarfs. There are two main theories concerning the origin of the H-rich and He-rich branches, which have been neatly summarised by Shipman /3/. The primordial view considers that the distinction between DA and non-DA occurs at the pre-white dwarf stage of stellar evolution. There is some evidence for this in signs of bifurcation in PNN also. The converse idea is that both DA and non-DA stars form by a single route from the PNN, possibly via the very hot DO type PG1159-035 objects /4/ which only
WhiteDwarfs
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have a little hydrogen. The DA stars then arise by gravitational settling of the heavier elements in the DO atmospheres. Two important features support this. No DAs have temperatures above 75,000K and so it is difficult to establish a direct DA-progenitor link /5/. Also, there are no DOs or DBs lying in the temperature range 30,000-45,000K. Hence, there must be some interchange of objects between the two groups via processes that redistribute elements in the stellar envelope (eg. gravitational settling, convection, radiative support etc.), by accretion or by mass loss (eg. winds, PN ejecta). To answer the question as to which view of white dwarf evolution is correct detailed and accurate measurements of atmospheric compositions are required for a large number of objects throughout the cooling sequence (and including PNN and subdwarfs). OBSERVATIONS OF DA WHITE DWARFS
Results from EXOSAT The low energy telescopes of the EXOSAT observatory /6/ had a response extending from the soft X-ray well into the EUV, a total range of 6-400A. These telescopes were operated in two modes; spectroscopic, where a transmission grating was inserted into the optical system, and photometric, where different EUV ‘colours’ analagous to UBVRI bands in optical work could be defined by thin film filters. Extraction of photometric filter data from EXOSAT images has been discussed several times, most recently by Barstow and Tweedy /7/. During the lifetime of EXOSAT around 20 DA white dwarfs were detected by the LE telescopes. Spectroscopic data were obtained for three of these — HZ43, Feige 24 and Sirius B. A number of important and interesting results were obtained from the EXOSAT spectra. The spectrum of HZ43 was used to determine an important upper limit to the abundance of He (relative to H) in its atmosphere (
M.A.Barstow
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to°
,
--l—rr-r~T—r
-r--r i-- -r —r-~i—r—i--—r —r1
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EXOSAT 500 tines/mm
-
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160
200
I
240
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I
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Figure 2. EXOSAT transmission grating spectrum of Feige 24 compared with the predictions of several model atmosphere compositions — pure H, H+He and H+He+metals.
EXOSAT LE/OA MOOELS
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Figure 3. 99% confidence areas for T~and He/H for the DA white dwarfs observed by EXOSAT. Data for the stars observed by Einstein are shown by the points with error bars.
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White Dwarfs
He/H ranges from i0~ to i0~ and there is no correlation with effective temperature. However, above 3. Paerels and Heise suggest that at the lower temperatures the 45,000K all but HZ43 have He/H>10 He/H abundance is determined by accretion from the ISM and that the scatter in the observed values reflects density fluctuations in the ISM itself. At the higher temperatures accretion cannot explain the He/H abundance and radiative levitation is suggested as a possible mechanism. Recently, it has been demonstrated /18/ that the amount of He that could be supported radiatively is much too small, by two orders of magnitude, to account for the -above results. In the absence of • forces counteracting the effect of gravity the photospheric composition would become stratified with H floating on top of a layer of He. A transition zone would exist, determined by the equations of diffusive equilibrium. A measured He abundance could then be interpreted as the vertical extent of the He diffusion tail in the atmosphere. This would depend upon the temperature of the star and the overlying mass of H. Koester /19/ has performed a self consistent analysis of 11 DAs in the EXOSAT sample using a set of stratified model atmospheres. He found that the EXOSAT DA data could be explained in terms of stratified atmospheres with H layer masses in the range 3 x 10 16_5 x 10’4M®. No single valued relation for MH with respect to temperature was found but a tendency for thinner H layers in the hotter DAs and thicker layers in cooler objects was noted. Some problems still remain with this interpretation. Large values for the interstellar column densities are obtained and in the case of HZ43 and GD153 are in serious disagreement with the very low values required by Voyager data /20/. However, use of the stratified models could allow significant He in HZ43 since the Hell 228A edge can be smoothed out by overlapping of high series members of the HeIlE lyman series (figure 4). I
I
I
I
60000 K
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190.0
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=
=
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Lambda (A I Figure 4. Theoretical EUV flux for two stratified H=He models (centre curves) with the same spectra (bottom and top) smoothed to a resolution similar to the EXOSAT TGS (6A). Spectroscopic observations with IUE and Voyager Most of the EXOSAT observations were photometric and the few spectra that were obtained were of modest, though useful, resolution (~6A).In the FUV and long wavelength end of the EUV the spectroscopic capabilities of IUE and Voyager add an extra dimension to white dwarf studies. Information is provided in two ways. First, temperatures can be determined from the shape of the continuum. Secondly,
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M.A. Barstow
detailed study of absorption features (eg -figure 5) can tell us about the atmospheric compositions, the presence of winds and surface gravities besides effective temperature. 1.6
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Figure 5. Example of an IUE low dispersion short wavelength spectrum of a white dwarf, in this case the very hot object PG1159-035. The positions of interesting features and their identifications are marked. Two important pieces of work, by different groups, have concentrated on determining surface gravities and effective temperatures by analysis of low dispersion IEJE short wavelength spectra. Holberg et al /12/ have used the lyman a profiles of 12 DA white dwarfs to determine their effective temperature and surface gravity. The data were analysed using pure hydrogen LTE blanketed model atmospheres, each model containing a synthetic lyman a profile. Table I summarises the results of this work. It is notable that the errors in the temperature determination are at least as good as those for the EXOSAT results, and often superior. The results do not provide a sensitive determination of the surface gravity but a few significant conclusions can be drawn. Most of the results are clustered around log g=8.0. It may be expected that the hottest stars have lower gravity and this is true for 3 out of 4 stars studied. However, the obvious exceptional object (as in the EXOSAT study) is HZ43 which, with log g>8.35, has the highest gravity in the whole sample. This may explain why HZ43 appears to have minimal He since higher gravitational field would cause this element to settle out to greater depths in the atmosphere. Table 1. DA Temperature and surface gravity determinations from Lyman a line profile measurements. DA white dwarf T 6 (K) log g 40 En B 16, 325 ±325 7.65 ±0.20 Wolf 1346 20, 680 ±210 8.08 ±0.15 GD14O 21,375 ±395 8.45 ±0.15 CoD-38°10980 24, 500 ±140 8.08 ±0.15 GD71
33,300 ±830
7.78 ±0.20
GD394 GD659 GD153 GD246 Feige 24 HZ43 G191-B2B
36, 125 ±940 36,850 ±1,390 42, 375 ±1,480 53, 600 ±2, 940 55, 000 ±5, 360 57, 500 ±3,310 62,250 ±3,520
8.13 ±0.25 8.17 ±0.30 8.23 ±0.30 7.64 ±0.45 7.23 ±0.23 > 8.35 7.55 ±0.35
•
-
A complementary study of DA white dwarf temperatures performed by Finley et al /21/ uses the UV
White-Dwarfs
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continuum slope. Although no information on surface gravity can be derived-, there are two advantages in this technique. First, it is not necessary to assume that the line of sight column density is neglible. In the lyman a profile study, absorption by neutral H along the line of sight can contribute to the observed profile, if the intrinsic stellar profile is weak and NH is large. The procedure used by Holberg et al ignores this effect. Secondly, fitting to a continuum can be performed for faint objects where the signal to noise in the lyman a profile is too poor•. Since the continuum technique is independent of the lyman a profile the results of each serve as a check on the other. Results obtained for objects common to both samples are in good agreement. An important atmospheric constituent, the abundance of which was addressed by the EXOSAT observations, is He. A second wa.y of measuring the He abundance is to observe the several Hell and Hel transitions that can be found in DA white dwarf spectra. One of these, Hell 1640A, lies in the FUV and the rest (Hell 4686A, Jiel 4471A) are optical features. Holberg et al /5/ have taken the following approach. Effective temperature and log g for a sample of white dwarfs were determined by detailed fitting of the H balmer proffles to a grid of pure H model atmospheres. He abundances were then estimated from from a related grid of models with He/H ratios from io—~to 3 x ~o 2 and homogeneous composition. Measurements of the He abundance were made for 8 very hot DA white dwarfs (table 2). The pattern of abundances with temperature is different to that seen in the EXOSAT results, suggesting that DA He abundances at high temperatures can have a very wide range, extending from less than i0~ for HZ43 to 10—1.5 for DAOs such as PGI21O+533. Table 2. Spectroscopically determined helium abundances in DA white dwarfs. DA white dwarf T 0 (K) log g log (He/H) PG0134+181 72,500 ±5,000 7.0 ±0.5 <—3.3 PG0823+317 62,700±4,200 7.25 ±0.25 —2.5 ±0.25 PG0846+249 63, 700 ±5, 500 7.0 ±0.35 —2.6 ±0.25 PG0950+139 70, 500 ±5,000 7.15 ±0.5 <—2.2 PG11O8+325 64, 300 ±5, 000 7.75 ±0.35 < -3.5 PG1305-017 53,300 ±3, 500 7.25 ±0.35 ~ —1.0 PGJ21O+533 50,000 8.0 ~ —1.0 G191-B2B 62, 250 ±3, 520 7.6 ±0.4 < —3.58 Studies of metal abundances in DA white dwarfs using ThE data are interesting. For example, there are no detectable metal features in the spectrum of HZ43 /22/. In contrast, C, N and Si can all be identified in the spectrum of Feige 24 /13/. Qualitatively this is consistent with more rapid gravitational settling of trace elements due to the higher surface -gravity of HZ43. However, the abundances determined for C,N and Si in Feige 24 (log(C/H)=-6.4±O.6,log(N/H)-5.3±1.0 and log(Si/H)=-6.3±0.9) can only be explained by the influence of selective radiative forces counteracting the gravitational potential /23/. It is interesting to recollect that C,N and Si alone cannot explain the EXOSAT spectrum, additional trace aborbers being required. In some cooler DA stars, such as Wolf 1346 /22/ and CoD-38°10980 /24/, only Si can be seen in the TUE spectra. Abundances of N and Si predicted by diffusion theory /25/ are in keeping with the observations. However, the abundance of C at the temperature of Feige 24 is predicted to be higher than observed.
OBSERVATIONS OF THE DO WHITE DWARFS DO white dwarfs and the related very hot PG1159 stars are a much less populous group of objects than the DA white dwarfs. Only 20-30 such stars are known. It appears that the lowest temperature at which these stars occur is l2~45,000Kand the temperatures of the very hottest of the PG1159 stars are certainly in excess of 100,000K and could be as hot as 180,000K or 200,000K. Except for the coolest objects (<100,000K) the FUV spectral slope is insensitive to temperature and it is then necessary to rely on more complex methods of temperature measurement such as EUV photometry or line profile fitting. Since the stars are expected to have atmospheres that are predominantly composed of He there will only be significant EUV fluxes at temperatures above-80,000K. As a result of these limitations data from a variety of wavebands have simultaneously been brought to bear on the study of individual stars.
Multiwaveband observations of PG1159 stars -
-
Sion et al /4/ have proposed that the PG1159 stars represent an evolutionary link between He-rich PNN and DO white dwarfs proper, on the basis of similarities between their FUV, UV and optical spectra. Typical features are a trough of Hell 4686A blended with transitions of CIV, CIII and NIh in the optical, and OVI in the UV at 3434A and FUV 1036A. It is possible, though by no means certain, that a significant number of DAs could evolve via this route, rather than directly from H-rich progenitors. Hence, a detailed
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M.A. Barstow
understanding of the evolution of the stellar abundances - with temperature for the PG1 159 stars is very important. Some of the PG1159 group are known to be non-radial pulsators, including PG1159-035 (the prototype object) /26/ arid K1-16, a PG1159-like PNN. At these high temperatures the driving mechanism is thought to be partial ionisatioñ of C and 0 in a C-O rich envelope /27/. Six PG1159 related objects were observed by EXOSAT and five were detected. The results of these observations have been summarised by Barstow and Holberg /28/. These data were analysed using a grid of homogeneous, LTE spectra covering a large range of abundances and including H,He,C,N and 0 /29/. At high temperatures and low gravities the assumption of LTE will become invalid but no NLTE models with CNO were-then available with which to carry out the study. In order to reduce the number of free parameters considered, IUE lyman a profiles were used to obtain measurements of the interstellar column density. Since these objects do not have H features in- their spectra there is either no H at all or it is too highly ionized to contribute to the opacity. Either way, all the observed lyman a absorption in the IUE spectra can be attributed to interstellar I-I. Further information was also obtained from the IUE data in the form of the spectral slope, which c~ou1dbe used to assign a rough temperature order to the objects. Including all this information in the analysis a range of temperatures and abundances was determined for each object, as summarised in table 3. H1504+65 is the hottest object in the group at ~180,000Kand KPDOO05+5106 the coolest. There appears to be a weak tendency for the atmospheric He and metal abundances to decrease as a- function of decreasing temperature. Table 3. Effective temperature, log NH and abundance determined PG1159 star data. Most likely values are in bold type. 2) Object Temperature (K) NH (log cm H1504+65 150000-180000-185000 19.65-19.70 PG1159-035 123000-124000 19.93-19.96 -PG1144+005 93000-124000 19.70-19.95 K1-16 103000-110000 19.89-20.12 KPD0005+5106 83000-97500-100000 19.64-20.05
from the LTE analysis of EXOSAT H:He 0-1-b®
CNO:He
1® 1-b® 0.5®-1® i®-hO®
0.02®-® 0.05®-0.1® 0. 1®-®
Subsequently, Werner et a! /30/ have performed a detailed analysis of C,N and 0 line profiles using NLTE model atmospheres, concentrating on the analysis of optical data and not considering EUV or FUV fluxes. The results obtained for PG1159-035, the only object common to both samples are in serious disagreement with the EXOSAT analysis. They obtained a similar temperature, 130,000-140,000K compared to 124,000K, but very different estimates of the atmospheric abundances. They find that little or no H is present (He/H~4.75)and, - the C & 0 abundances are (relative to H) ~3.33 and ~0.625 respectively. On the face of it the discrepancy isprobably due to the use of an improved physical model, namely the inclusion of NLTE effects. Recent work /31/ has looked at this problem in detail, establishing that the cause of the discrepancy is a combination of the inadequacy of the LTE approximation and insufficient detail in the oxygen model atom used in the LTE calculations. Furthermore, the soft X-ray fluxes predicted by NLTE models with high He and CNO abundances, as favoured by the optical data, are found to be consistent with the EXOSAT observations. Using some new grids of NLTE model atmospheres the EXOSAT data is being reanalysed. Early results /31/ indicate that the metal abundances in these stars do decrease to lower temperatures. The effect is greater than seen in the LTE analysis and the abundances determined for KPD0005+5b06, the coolest object studied, remain unchanged. Hence, the PG1159 route seems a plausible one fOr producing both DO and DA white dwarfs. Some FUV spectroscopy has been performed on PG1159 stars and DO white dwarfs. Unfortunately, not many of the objects are bright enough for high dispersion IUE spectra to be obtained. Downes et al /32/ have obtained such spectra for KPD0005+5106 and used this data to obtain limits on the N abundance. The EXOSAT NLTE results are in good agreement with this. High dispersion TUE spectra have also been obtained for the fainter PG1159-035/33/ and K1-16 by exposing the SWP camera for two contiguous low background shifts. Many features such as NV 1240A, NIV 1270A, OV 1371A and CIV 1550A have been identified in the PG1159-035 spectrum. Poulin et al /34/ have studied the FUV energy distributions of PGb0344~001and HD1449499B with Voyager and IUE. This data was used to obtain effective temperature estimates (~80,000K~ and 54,000K respectively) by fitting the spectra to pure H model flux distributions. — -
White Dwarfs -
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OBSERVATIONS OF DB WHITE DWARFS DB white dwarfs are unlikely to be EUV sources. Their combination of low effective temperature (<30,000K) relative to the DOs, and He-rich atmospheres result in a high predicted EUV photospheric opacity. Unless their atmospheres contain significant traces of, as yet undetected, H predicted EUV fluxes are extremely low. In addition, the hottest objects in this group, the pulsators, seem to be all at large distances and high column density. Consequently, the emergent flux will be very highly absorbed by the ISM. Much useful work has, however, been carried out with FUV observations by IUE. Recent studies (eg. /35/) have improved temperature estimates, particularly for the pulsating stars. Establishing an accurate temperature scale is particularly important in attempting to understand the problem of the DO-DB gap /36/. DBA stars contain traces of U in their mostly He atmospheres at the i~~-i~—~ level /37/. Lyman a observations would allow much better estimates of the H abundances to be made. Unfortunately these are at the limit of IUE, since the stars are somewhat faint and cool. In principle it should be possible to search for trace metal features in the FUV using high resolution spectra. This has been possible for only one object, the prototype pulsating DB GD358 /38/. In fact, two IUE SWP echelle images were recorded and then coadded. In addition to the expected Hell 1640A absorption feature a CII (1336A) line was discovered. CONCLUSION: THE FUTURE This review has dicussed the current state of our knowledge of white dwarf morphology and evolution, highlighting in particular the contributions of EUV and FUV astronomy. An attempt has been made to cover as wide an area as possible but inevitably it is impossible to do complete justice to all the very many contributions made. The recently launched ROSAT mission, carrying both soft X-ray and EUV telescopes is likely to discover several thousand new white dwarfs /39/. At the same time it will aquire photometric data of similar quality to that of EXOSAT, but for at least two orders of magnitude more objects, dramatically increasing the size of the white- dwarf sample available for more detailed study. It is readily apparent that interpretation of any of the data discussed in this review or obtained by current or future missions relies very heavily on the availability of appropriate model atmospheres and the flux distributions and line profiles which these predict. The current problems that need addressing centre on their physical validity. For example, the need for NLTE effects to be included in the models for hot or low gravity objects. Much progress has been made in recent years. NLTE codes that are able to treat a large number of line transitions are now being used and useful models generated. In parallel a significant amount of work has been going on developing stratified LTE H+He models as well as studies into radiative support of elements. The next step will be to integrate these approaches to produce an evolutionary set of models where material abundances and the degree of stratification changes as the temperature decreases. ACKNOWLEDGMENTS MAB acknowledges the support of the SERC through the ROSAT WFC programme and the Royal Society for travel funds to attend the COSPAR meeting. REFERENCES 1. G.P.McCook and E.M.Sion, Ap.J.SuppL, 65, 603 (1987). 2. E.M.Sion, J.L.Greenstein, J.D.Landstreet, J.Liebert, H.L.Shipman and G.A.Wegner, Astrophys.J., 269, 253 (1983). 3. H.L.Shipman, in White Dwarfs, ed. G.Weguer, Springer-Verlag, 220, (1989) 4. E.M.Sion, J.Liebert, and S.G.Starrfield, Astrophys.J., 292, 471 (1985). 5. J.B.Holberg, K.Kidder, J.Liebert, F.Wesemael, en White Dwarfs, ed. G.Wegner, Springer-Verlag, 188 (1989). 6. P.A.J.de Korte, et a!., Spac.Sci.Rev., 30, 495, (1981). 7. M.A.Barstow and R.W.Tweedy, Mon.Not.R.astr.Soc., 242, 484 (1990). 8. J.Heise, F.B.S.Paerels, J.A.M.Bleeker and A.C.Brinkman, Ap.J, 334; 959 (1988). 9. J.B.Holberg, B.R.Sandel, W.T.Forrester, A.L.Broadfoot, H.L.Shipman and J.L.Barry, Ap..!., 242, L119 (1980). 10. F.B.S.Paerels, J.A.M.Bleeker, A.C.Brinkman and J.Heise, Ap.J., 329, 849 (1988). 11. F.B.S.Paerels, J.A.M.Bleeker, A.C.Brinkman and J.Heise, Ap.J., 309, L33 (1986). 12. J.B.Holberg, F.Wesemael and J.Basile, Ap.J., 306, 629 (1986). 13. F.Wesemael, R.C.B.Henry and H.L.Shipman, Api., 287, 868 (1984).
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