My title is so inclusive that it is not possible to find significant subject matter not in direct conflict with the specialists’ talks scheduled throughout this Colloquium. I intend, however, to emphasize areas where it is probable that much remains to be done. From the Kiel group come printouts for a large family of models, first for hydrogen and recently for helium-dominated atmospheres, which D. Koester has kindly made available. Hydrogen atmospheres for a wide variety of temperatures by Shipman and McGraw and Shipman exist also in computer printout form. Models of mixed He/H composition are less available, but are being prepared. Some hydrogen-deficient stars are the richest in metals, so that models for a variety of metal/helium ratios will need to be computed, and synthetic spectra. Clearly, further models with differing C/He ratios are needed. The successful observations in the ultraviolet with the IUE also demand computations of Lyman-α profiles using modern broadening theory, and a few now exist (Koester 1979). Present observational capabilities require quite detailed predicted fluxes and He II, He I line profiles.

White Dwarfs (WD) are one of the fields in stellar structure in which the so-called “input physics” is not yet completely clear today. In particular the equation of state (EOS) of WD affects directly one of the eminent problems in present day research of WD, namely is the “paucity” of low luminosity WD real? The reply depends in a sensitive way on the physical properties of the dense matter. For this reason we go in some detail into the physical problems of the “input physics”. We distinguish between the problem of the interior-fully ionized matter and the envelope-partially ionized matter. Unfortunately space-time is not sufficient to cover in any detail the EOS of the envelope although this problem is extremely important to the evolution.

Theoretical helium line spectra in the wavelength region λλ 3000-6000Å are presented and possible spectroscopic gravity indicators are discussed. The feasibility of using spectrum synthesis techniques for the analysis of DB white dwarfs is investigated.

Alcock (1979a,b) has suggested that DB degenerate dwarfs can only be kept observably free of accreted interstellar hydrogen if their masses are not more than about 0.3 M⊙. This is distressing because (a) photometrically determined DB radii (hence masses) are not statistically different from those of DA’s (Shipman 1979), and (b) no one quite knows how to make a degenerate dwarf with this low a mass in the age of the galaxy. Several authors (Vauclair et al. 1979; Michaud and Fontaine 1979) have suggested that accretion is inhibited by effects of coronae and/or magnetic fields. I address here the possibility that the DB’s are a somewhat higher velocity population than the general run of degenerate dwarfs. Although, as we shall see, this may imply somewhat lower than average masses for them, the dominant effect will be reduced accretion due to the high speed of the stars through the interstellar gas.

In the special case of the Maclaurin spheroids, it has been known for some time that the m = 2 barlike modes become secularly unstable for t ≡ T/IWI ≥ 0.1376 where T is the total rotational kinetic energy and W is the total gravitational energy of the spheroid. “Secular” here means that the instability depends on dissipative processes and grows on a long dissipative time scale. In particular, the Dedekind-like bar mode, which has zero eigenfrequency at t = 0.1376 as viewed in the nonrotating frame, is unstable due to gravitational radiation (Chandrasekhar 1970).

Investigation of evolution of a degenerate component (helium or carbon-oxygen dwarf) in a close binary system is rather important. Such dwarfs may be candidates for nova or supernova stars. The first possibility has been studied by several authors (see review paper by Starrfield et al., 1976) and it has been shown that explosive hydrogen burning with enriched CNO abundance in the degenerate envelope of a massive carbon-oxygen dwarf may explain nova phenomena.

The presence of neutron stars in close binary systems, shown by the pulsating X-ray sources, poses the problem of their origin. In the case of the low-mass (M1 + M2 ≤ 5 M⊙) X-ray binaries, the neutron star might have originated from a massive white dwarf, driven over the Chandrasekhar limit by mass transfer (Schatzman 1974). A similar scenario had been put forward by Whelan and Iben (1973) for type I supernovae. To solve the problem of the very low eccentricities observed for the orbits, and to facilitate keeping the system bound after neutron star formation, Canal and Schatzman (1976) suggested a non explosive collapse of the white dwarf to a neutron star. The occurence of this kind of collapse depended on the possibility of avoiding thermonuclear ignition by means of neutronization. Since there is a density interval where the electron captures on carbon go faster than the pycnonuclear reactions, just above the critical density for the beginning of the collapse, there seemed also to be a chance of escape from thermonuclear runaway. A closer examination of this picture leads, however, to significant changes.

In a close binary system with a primary star in the mass range 8-12 M⊙, the primary star leaves a white dwarf composed of 16O, 20Ne, and 24Mg as a result of mass exchange. When the companion star, fills its Roche lobe, overflowing matter accretes onto the white dwarf. We have computed the evolution of such an accreting O-Ne-Mg white dwarf and found that electron captures on 24Mg and 20Ne trigger the collapse when the mass reaches 1.38 M⊙. As a result of the collapse, oxygen begins to deflagrate but the effects of electron captures dominate over the oxygen deflagration. The white dwarf collapses to form a neutron star.

In the course of an exploration of the diffusion processes in white dwarfs, the role of the radiative acceleration has been investigated in detail and found to be quite important in hot white dwarfs (Vauclair, Vauclair and Greenstein, 1979; hereafter referred to as V2G). In view of the increasing interest accorded to hot white dwarfs which become accessible to observations in the ultraviolet owing to satellites like IUE, it appeared quite relevant to obtain theoretical predictions of the effect of the diffusion of metals in hot white dwarfs. This short note is a summary of the results presented in more detail elsewhere (V2G).

The term extreme ultraviolet (EUV or XUV) is employed in upper atmosphere physics and in solar work where it usually denotes the wavelength band between 100 and 1000 Å. Since thermal emission with 30,000 ≲ T ≲ 300,000 K peaks in this band, it might be expected that studies at these wavelengths would be especially useful for objects with effective temperatures in this range. In fact9 few such studies have been carried out. The reason for this anomaly is that very few EUV studies have been made at all, particularly because of unreasonably pessimistic estimates of the opacity of the interstellar medium and partially because of instrumental difficulties encountered at these wavelengths. The first search for extreme ultraviolet emitting objects was carried out in 1975 with instrumentation on the Apollo spacecraft in the Apollo-Soyuz mission. Four of approximately thirty preselected objects were detected with this instrumentation. The objects detected unquestionably are more a reflection of the prejudices of the investigators than they are a sampling of the contents of the universe. Nonetheless, two of the four objects detected were hot white dwarfs: HZ 43 and Feige 24. In addition, upper limits which turned out to be extremely useful were obtained on the Sirius A/B system. These results, plus more recent results obtained on hot white dwarfs will be discussed in this review.

Astronomers studying objects outside the solar system first used the ultraviolet, extreme ultraviolet, and x-ray regions of the electromagnetic spectrum in the 1970s. The exploration of these wavelength regions has produced considerable improvements in our understanding of these objects. The achievements of x-ray astronomy are perhaps the best known. With the advance of satellite technology, other wavelength regions begin to play a role, and x-ray astronomy moves into the luminosity domain where quiescent as well as violent astrophysical processes can produce detectable amounts of radiation. This paper reviews the current state of our interpretation of white-dwarf stars at wavelengths less than 3000 A.

The International Ultraviolet Explorer (IUE) has been used by Greenstein and Oke to obtain low resolution spectra from 1150 to 3200 Å of a selection of white dwarfs and hot subdwarfs.

The observations can be converted to absolute fluxes so that, by including energy distributions measured with the multichannel spectrometer on the Hale telescope, fluxes can be derived between 1150 and 11000 Å. The IUE absolute fluxes show erratic level shifts of up to 20 percent and probably also smaller changes in the slope. These shifts are probably associated with the level of the radiation induced background coupled with uncertainties in the calibration of the non-linear detectors. In many cases, these anomalies in the fluxes are obvious, but they do decrease the accuracy with which temperatures and gravities can be determined.

Several hot subluminous stars were first recognized as such from low-resolution spectra obtained with the ultraviolet objective-prism survey from Skylab, Experiment S-019. About 9 percent of the sky was photographed, 3 percent with unwidened spectra. Two hot subdwarf stars have been reported previously, HDE 283048 (Laget et al. 1978) and the companion to HR 3080 (Parsons et al. 1976a). Papers on two other subdwarfs are in preparation. One previously unrecognized white dwarf star was found, HD 149499 B, and new observations (Wray, Parsons and Henize 1979) show it to be probably the hottest white dwarf known.

The DA white dwarf Sirius B has gained special interest because of the unexplained observed X-rays (Mewe et al. 1975). Coronal emission, accretion and normal photospheric radiation have been discussed as sources.

A DA white dwarf with Teff ∽ 28000K (Savedoff et al. 1976, 1978, Koester 1979) should not have a corona (Böhm and Cassinelli 1971, Fontaine 1973). For accretion we do not see where the accreted mass should come from. Normal photospheric radiation requires an effective temperature around 32000K or higher (Shipman 1976). All possibilities to explain the X-rays therefore encounter severe problems.

The Palomar-Green Survey (1977) contains a list of some 3000 spectroscopically classified stellar objects, chosen on the basis of ultraviolet excess. The survey covers 10,000 square degrees of the sky above |b|=30° with δ≥10° down to an average limiting B=16.2. A significant subset of this sample is a new catalogue of over 700 newly discovered hot degenerates (Green 1979). The combination of selection by color only without regard to motion and the large area covered resulted in the discovery of some rare, luminous degenerates and pre-degenerate objects. We report here on the first observations in a program of more detailed spectroscopic study of these objects, and display representative groups of common spectral or physical properties. These scans at 1.5-2.5 Å resolution were obtained with either the KPNO 2.1-m and intensified image dissector scanner or the Palomar 1.5-m with SIT spectrograph.