Precise white dwarf gravitational redshifts can only be obtained utilizing systems of known distance and space velocity. Binaries with known orbits such as 40 Eri B are rare, and although common proper motion pairs have proven highly useful(Wegner 1973: Koester 1987: Wegner & Reid 1987), there are some problems in their interpretation. Another approach is to employ white dwarfs in open clusters; they not only have known systemic velocities, but also provide information on the progenitors of the white dwarfs. Of the nearby galactic clusters, the Hyades currently give the best information for achieving accurate gravitational redshifts: the members are relatively bright and nearby and their kinematics are well known.

Significant progress in our understanding of the nature of the outbursts of the classical novae has occurred over the past two decades (see, e.g., reviews by Truran 1982; Starrfield 1986). Their outbursts are now understood to be driven by thermonuclear runaways proceeding in the accreted hydrogen-rich shells on the white dwarf components of close binary systems. Critical parameters which serve to dictate the varied characteristics of the observed outbursts include the intrinsic white dwarf luminosity, the rate of mass accretion, the composition of the envelope matter prior to runaway, and the white dwarf mass.

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.

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.

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).

Currently, there are disagreements on the theoretical pulsation properties of DA white dwarf models. Winget and his group (cf Winget 1981, Winget and Fontaine 1982, Winget et al. 1981, 1982) use 0.6M ⊙ evolutionary white dwarf models, to find that (Teff )blue is sensitive to the mass of the hydrogen layer and very sensitive to the treatment of convective efficiency. However, (Teff )blue is relatively insensitive to the helium layer mass. Winget and collaborators also find that the hydrogen layer must be restricted to 10−8 > MH /M* > 10−12 for (Teff )blue to occur at ∼ 10,500K in models with ML1 convection (see below). More efficient (ML2) convection pushes the blue edge up to ∼ 12,200K, which is still short of the observed blue edge of ∼ 13,000K.

The ultimate aim in the study of White Dwarf (WD) evolution is to understand properly the observed Luminosity Function (LF) of WDs, that is the number of WDs observed per unit magnitude interval. The complicated route to the interpretation of this scarne quantity (12 fiducial points in the recent update of Liebert et al. 1988) is schematically summarized in figure 1. Clearly, the main input to the LF are the evolutionary (cooling) times, but it is necessary to consider their non trivial dependence on galactic evolutionary inputs, namely the initial mass function of disk stars, their age distribution with time (ultimately: the disk age), and their evolutionary properties. Stellar evolution enters in the problem of cooling by two main routes: first, by determining the mass of the WD as a function of the inital stellar mass and chemistry, second by fixing the internal constitution of the WD remnant for each given mass, and the initial physical conditions at the start of WD evolution (mainly the temperature distribution, which is important for the first phases of evolution). Of course, there is no need of good evolutionary inputs to study “theoretical” WDs. In fact, historically, the first approach in the study of “cooling” (Mestel 1952, Schwarzschild 1958)) has been directly related to the stimulating physical properties of these objects, in which neutrino losses at the beginning (Vila 1966, Savedoff et al. 1969) and, in late stages, liquification and crystallization of the plasma (Brush,Sahlin and Teller 1966, Hansen 1973) long recognized to be dominated by coulomb interactions, (Kirzhnits 1960, Abrikosov 1960, Salpeter 1961), are the main features to be investigated (Mestel and Ruderman 1967, Van Horn 1968, Kovetz and Shaviv 1970).

The most fundamental aspect of white dwarf seismology is the determination of the gravity-mode (g-mode) period structures of models of isolated pulsating white dwarfs. These stars show multiperiodic luminosity variations which result from the superposition of excited pulsation modes. Among the many oscillation modes available in the very rich nonradial g-mode spectra of white dwarfs, the observed modes are selectively chosen by a filtering mechanism. Although the period evolution is strongly tied to the core temperature evolution in a white dwarf, the period structure remains largely specified by the mechanical properties of the star. The most basic structural feature of a white dwarf is its highly degenerate interior, which leads to nearly isothermal and nearly isentropic stratifications in the core region containing more than 99% of the mass of the star. In particular, because the density gradient is almost adiabatic throughout the interior of a white dwarf, the Brünt-Väisälä frequency (see below) is very small there and low-order g-modes cannot propagate. As a result, g-modes are essentially envelope modes in white dwarfs, with large amplitudes occuring only in the non-degenerate outer layers. One can thus expect that g-modes in white dwarfs are extremely sensitive to envelope properties such as compositional stratification and partial ionization mechanisms. Compositional stratification is, in fact, the second structural feature of a white dwarf model which has strong effects on the period structure. Indeed, trapped modes result when a resonance or near-resonance occurs between the local g-mode radial wavelength and the thickness of one of the composition layers. This results in a period structure which strongly bears the signature of compositional stratification in the outer layers. Thus, it has been widely accepted that white dwarf pulsations probe primarily the outer layers of these stars. This point of view has been borne out by detailed pulsation calculations carried out by several independant groups (see Winget 1987 and references therein).

Non-equilibrium heating of white dwarfs during two-step neutronization and formation of small but finite core of the new phase is a main source of energy in these stars when the temperature is sufficiently small and Coulomb crystal becomes degenerate.

Existence of Chandrasekhar mass limit of white dwarfs Mch = 5.83 /μ2 z, μz = A/z, is connected with the prevalence of the relativistic degenerate electrons in pressure. When neutronization is taken into account, the maximal value of mass is smaller and is reached at finite density (Schatzman, 1958). The central density of the white dwarf with limiting mass is larger than threshold density of neutronization. Such star has a small, but finite core of new phase (Seidov, 1967). The neutronization threshold for the iron is equal to ρ = 1.15 109 g/cm3 The neutronization goes through two steps

PG 1707+427 was identified in the Palomar-Green survey (Green Schmidt and Liebert, 1986) as an object with a significant ultraviolet excess. Observations by Bond and Grauer in 1982 showed it to be a pulsating variable (Bond, Grauer, Green and Liebert, 1984). Fourier transforms of the discovery time-series photometric runs revealed power in PG 1707+427’s light curves in two bands with periods near 450-s and 333-s. The longer period peak was observed to have a variable amplitude while the other one appeared relatively constant in strength. Wesemael, Green and Liebert (1985) placed PG 1707+427 in the spectroscopic class of PG 1159–035 (DOV) pulsating variables.

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.

First reported at the IAU Colloquium No. 53 on White Dwarfs (McGraw et al. 1979), PG 1159-035 (GW Vir) is the prototype of a new class of very hot, pulsating, pre-white dwarf stars. It shows complicated, nonradial pulsation modes which have been studied exhaustively, both observationally and theoretically. The effective temperature has been crudely estimated as 100,000 K with log g ~ 7 (Wesemael, Green and Liebert 1985, hereafter WGL).

The hypothesis that Type I supernova explosions originate in binary stars, in which the secondary overflows its Roche lobe and causes a white dwarf primary to pass a critical threshold for dynamical collapse, has gained in popularity over the last decade and a half (Truran and Cameron 1971; Whelan and Iben 1973; Wheeler 1982). During this period attention has also focussed on double-degenerate binary precursors of such systems, in which gravitational radiation of orbital angular momentum causes two white dwarf stars to spiral slowly together until the less massive of the stars begins to overflow its Roche lobe (Tutukov and Yungelson 1979; Webbink 1979; Iben and Tutukov 1984; Paczyński 1985). Cameron and Iben (1986) discussed the stability of such systems as the threshold of Roche lobe overflow was approached and for low mass ratios in which gravitational radiation can maintain a low rate of such overflow. Benz, Bowers, Cameron, and Press (1988, hereafter BBCP) computed the actual merging of a binary system composed of a 1.2M© primary and a 0.9M© secondary using a fully three-dimensional SPH code. Their simulation did not include the effect of nuclear energy release despite the fact that the temperature in the accretion shock reached the carbon ignition threshold, and therefore their conclusions about the structure of the merged object remained uncertain.

The pulsating ZZ Ceti stars cover a narrow range of effective temperatures along the cooling sequence of DA white dwarfs (see, eg., Winget and Fontaine 1982). Fast-photometric searches for pulsating stars in that class have provided strong evidence that the ZZ Ceti phase is an evolutionary phase through which all cooling DA stars will eventually go through (Fontaine et al. 1982). Recent investigations, based on optical or ultraviolet photometry and spectrophotometry, have set the boundaries of the instability strip at temperatures near 10,000-11,000 K and 12,000-13,000 K, respectively (McGraw 1979; Greenstein 1982; Weidemann and Koester 1984; Fontaine et al. 1985; Wesemael, Lamontagne, and Fontaine 1986; Lamontagne, Wesemael, and Fontaine 1987, 1988).

White dwarf stars provide important boundary conditions for the understanding of stellar evolution. An adequate understanding of even these simple stars is impossible without detailed knowledge of their interiors. PG1346+082, an interacting binary white dwarf system, provides a unique opportunity to view the interior of one degenerate as it is brought to light in the accretion disk of the second star as the primary strips material from its less massive companion (see Wood et at. 1987).

PG1346+082 is a photometric variable with a four magnitude variation over a four to five day quasi-period. A fast Fourier transform (FFT) of the light curve shows a complex, time-dependent structure of harmonics. PG1346+082 exhibits flickering – the signature of mass transfer. The optical spectra of the system contain weak emission features during minimum and broad absorption at all other times. This could be attributed to pressure broadening in the atmosphere of a compact object, or to a combination of pressure broadening and doppler broadening in a disk surrounding the compact accretor. No hydrogen lines are observed and the spectra are dominated by neutral helium. The spectra also display variable asymmetric line profiles.

We examine whether or not double white dwarfs are ultimately merging into one body. It has been argued that such a double white dwarf system forms from some intermediate-mass binary stars and will merge due to the gravitational radiation which decreases the separation of binary. After filling the inner critical Roche lobe, the less massive component begins to transfer its mass to the more massive one. When the mass transfer rate exceeds a some critical value, a common envelope is formed. If the common envelope is hydrostatic, the mass transfer is tuned up to be a some value which depends only on the white dwarf mass, radius, and the Roche lobe size. The mass transfer from the less massive to the more massive components leads the separation to increase. On the other hand, the gravitational radiation effect reduces the separation. Which effect wins determines the fate of double white dwarfs, that is, whether merging or not merging. Since the formula of the gravitational radiation effect is well known, we have studied the mass accretion rate in common envelope phase of double white dwarfs assuming the Roche lobe size is as small as 0.03 R⊙ or 0.1 R⊙.

The observed falloff in the white dwarf luminosity function at log (L/L ⊙) ≈ −4.5 (Liebert, Dahn, and Monet 1988) is most easily explained as a result of the finite age of the Galactic disk. Using a simple perturbation approach as our method and a white dwarf evolution code as our tool, we have mapped the sensitivity of the ages of the model sequences in this low-luminosity regime to the uncertainties in the input physics and model parameters. We present here a preliminary overview of what we’ve learned.

We have updated the White Dwarf Evolution Code of Lamb and Van Horn (1975) to include both carbon and oxygen in the core. We interpolate to the mixture composition using our pure-C and pure-O tables and the additive-volume technique (Fontaine, Graboske, and Van Horn 1977). The envelope subroutines calculate stratified H/He/C envelopes of essentially arbitrary layer masses within the range 0 to ∼ 10−2 M *, and treat the composition transition zones as discontinuities. Because the equation of state tables referenced by the envelope routines do not include crystallization, a given sequence ends when the crystallization front reaches the core/envelope boundary.