The measurement of the rate of change of period with time for the g-mode pulsations in ZZ Ceti stars is a direct measurement of the cooling timescale for a DA white dwarf, which in turn can give a totally independent measurement of the age of the galactic disk. Using asteroseismology, we have obtained a rate of change of the period of the dominant pulsation in the light curve of the ZZ Ceti star G117-B15A of dP/dt = (12.5 ± 5.5) × 10−15s/s, equivalent to a timescale for period change of P/P = (5.5 ± 2.4) × 108 yr, which is consistent with the theoretical value for the cooling timescale of a DA white dwarf around 11,000 K.

Model atmosphere techniques and an experimentally confirmed theory of collision broadening of CaII H and K by helium have been combined to shou that the difference in velocity shifts for the two lines is a sensitive diagnostic for the H/He abundance ratio in cool, metallic line uhite dwarfs.

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.

Spectroscopic observations of hot white dwarfs utilizing the Explorer (IUE) high resolution spectrograph have led to the important discovery of ion absorption features (undetectable at low resolution) which have been ascribed to wind outflow in some cases (cf. Bruhweiler and Kondo 1983) and formation at the photosphere (cf. Sion and Guinan 1983; Bruhweiler and Kondo 1983; Dupree and Raymond 1982) in others. These line features, often weak and sharp, have presented a fundamental challenge to current understanding of the complex interplay of physical processes which control observed surface abundances in white dwarfs: gravitational/thermal diffusion, selective radiative support of ions, mass loss, convective dilution and mixing, and accretion (cf. the review by Vauclair, this volume and references therein).

Gravitational settling is widely accepted as being a fundamental physical process acting upon superficial layers of white dwarfs and resulting in an important alteration of their atmospheric composition. Several investigators have been interested by the problem of gravitational settling in white dwarfs (Fontaine and Michaud 1979; Vauclair, Vauclair, and Greenstein 1979; Alcock and Illarianov 1980; Muchmore 1984; Paquette et al. 1986). As pointed out in Paquette et al. 1986, they all reached the same qualitative conclusion: the gravitational settling time scales of metals in cool white dwarfs are small compared to their evolutionary time scales. These stars should therefore have their photospheres depleted of metals if there is no extrinsic source such as accretion for example. This is consistent with the observational fact that most of the cool white dwarfs spectra just show hydrogen and helium lines while the absence of metallic lines indicates a strong depletion of metals. Although the qualitative agreement between theory and observations is satisfactory, only time-dependent calculations can lead to a thorough understanding of the heavy element abundance patterns in cool white dwarfs. In particular, the predicted abundance of an element within the framework of the accretion-diffusion model does depend explicitly on the results of such calculations. We have already presented some preliminary results of numerical simulation of accretion episodes of heavy elements into white dwarfs (Dupuis et al. 1987). As part of an ongoing detailed investigation of these processes, we focus here exclusively on the mechanism of gravitational settling in white dwarfs in order to clear some confusion which has appeared in the literature.

Nousek et al. (1986) have recently announced the discovery of a remarkable hot compact star, H1504+65, which appears to have an atmosphere devoid of hydrogen and helium. Although the object seems significantly hotter than the related PG1159 stars, its suggested atmospheric composition remains at variance with the He-rich composition inferred for the latter stars (Wesemael, Green, and Liebert 1985). Nousek et al. (1986) have brought forward convincing arguments against a H-dominated atmosphere in H1504+65; it seems that the object is definitely not an extremely hot DA star. On the other hand, the case against a He-rich atmosphere is much weaker and is based on the following circumstantial arguments: (1) no helium lines are detected in the spectrum of H1504+65 (the detection limit is ≈ 0 5 Å), (2) the ultraviolet continuum slope is steeper than that predicted from pure helium modelsaxtrapolated to effective temperatures larger than the upper limits of available grids (Te = 200 000 K for log g = 7 ), and (3) the EXOSAT observations appear to conflict with the He interpretation, although suitable models (with high effective temperatures) are not readily available in this case also. Because of the lack of appropriate models, Nousek et al. (1986) have been careful not to rule out completely the possibility of a He-rich atmosphere for H1504+65. In particular, they admit that He II lines could probably be undetectable if H1504+65 had an effective temperature substantially larger than Te = 150 000 K. Nevertheless, they have found it tempting to hypothesize an atmosphere devoid of both hydrogen and helium. Coupled with the presence of weak C IV and O VI features in the spectrum of H1504+65, this has led to the suggestion that H1504+65 is actually a bare C/O nucleus (cf. Shipman 1987). Although this suggestion is interesting, it has undeservedly remained unchallenged and we have felt it appropriate to reexamine the case against a He-rich atmosphere with the help of further modelling efforts.

Observations of hot DA white dwarfs in the EUV/soft X-ray range have revealed that, in a majority of cases, the detected flux is less than that expected from pure hydrogen atmospheres. This implies an extra opacity source which must be due to the presence of small traces of heavier elements. These elements are generally not spectroscopically detected in hot DA white dwarfs, but the large sensitivity of the EUV/soft X-ray broad-band flux to the presence of extra absorbers can be used with profit to Infer their abundances. For simplicity, it has been assumed that only helium provides the required opacity source in the majority of the analyses carried out so far. In this context, Vennes et al. (1988a) have recently reviewed in details the mechanisms that could be responsible for the presence of small traces of helium in the atmospheres of hot DA white dwarfs. They favor a model in which these stars are interpreted as stratified objects with an outer layer of hydrogen which is sufficiently thick that radiation in the visible escapes only from H-rich regions, and yet sufficiently thin that the EUV/soft X-ray radiation escapes from deeper layers, polluted by the tail of the helium distribution which extends upwards. This model accounts naturally for the positive correlation observed between the inferred helium abundance and the effective temperature in hot DA stars studied at short wavelengths. If the model is correct, hot DA white dwarfs as a class must have very thin outer hydrogen layers with estimated masses in the range–13 > log q(H) = log (M(H)/M) > –15.

Feige 24 is a bright DA white dwarf which has been studied extensively both from ground-based and space-borne observatories. The best determination of its fundamental atmospheric parameters are that of Holberg, Wesemael, and Basile (1986) who have used detailed model atmosphere analyses in conjunction with optical, IUE, and Voyager data. They give log g = 7.23±0.35 and Te(103K) = 55±5. The question of the atmospheric composition is more involved as small traces of heavy elements would not be observable in the optical spectrum of such a hot, hydrogen-dominated atmosphere. If it were isolated, Feige 24 would presumably only show the usual bland optical spectrum of a typical DA white dwarf, i.e. the hydrogen Balmer line series, but the presence of a M dwarf companion complicates its spectrum. On the other hand, Feige 24 belongs to a handful of hot DA white dwarfs sufficiently bright that ultraviolet spectroscopy in the high resolution mode of the IUE has been possible. Following the theoretical expectation of Vauclair, Vauclair, and Greenstein (1979), it was discovered that the photosphere of Feige 24 contains small amounts of C, N, and Si (Dupree and Raymond 1982). Spectral synthesis techniques used by Wesemael, Henry, and Shipman (1984) indicate the following abundances: log(C/H)=−6.4±0.6, log(N/H)=−5.3±1.0, and log(Si/H)=−6.3±0.9. The most plausible explanation to account for these small abundances is the influence of selective radiative forces possibly coupled to a weak wind (Chayer et al. 1987).

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.

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 polarization of radiation emitted by inhomogeneous accretion columns is found to be significantly different from homogeneous columns. A good fit to the V-band circular polarization light curve of EF Eri is obtained using the two-core model with temperature &T=15keV, magnetic field B=26MG, and dimensionless plasma parameter Λ ≤ 106.

The equation of transfer in Thomson scattering atmospheres with magnetic fields is solved using Monte Carlo methods. Two cases, a plane parallel atmosphere with a magnetic field perpendicular to the atmosphere, and a dipole star, are investigated. The wavelength dependence of polarization from a plane-parallel atmosphere is qualitatively similar to that observed in the magnetic white dwarf Grw+70°8247, and the field strength determined by the calculation, 3.2×l08 G, is quantitatively similar to that determined from the line spectrum. The dipole model does not resemble the data as well as the single plane-parallel atmosphere.

Based on observations obtained using the 60-inch telescope at Palomar Observatory which is jointly owned by the California Institute of Technology and the Carnegie Institution of Washington and on observations obtained with the 200-inch Hale telescope which is owned by the California Institute of Technology.

The intial observations of white dwarf stars and their immediate precursors, the hot subdwarfs, suggested that these stars possess the simplest (and, aesthetically, the most pleasing) spectra of any astronomical object. The high gravity leads to the spectrum being dominated, in the most stars, by broad lines of either hydrogen or helium, depending on the composition of the photospheric layers, with a few stars exhibiting lines from both species. However, the more detailed observations of recent years have revealed a higher degree of complexity. In particular, absorption lines of high excitation species (N V, C IV, etc.) have been detected in the ultraviolet spectra of several hot white dwarfs (Bruhweiler & Kondo, 1982) and, most recently, high resolution optical spectra have shown that one of the latter stars, the hottest known DA, G191-B2B, exhibits significant emission in the core of the H-alpha absorption line (Reid &: Wegner, 1988). Following up the latter observation, we have obtained high resolution spectra of a number of hot subdwarfs, with temperatures ranging from ~ 20,000K to more than 60,000 K. Most of these stars also exhibit Baimer emission, at Hβ as well as Hα in at least one case. We suggest the temperature reversal in the stellar atmosphere may be a function of the He/H ratio at the level of the photosphere.

The gravitational redshift is one of Einstein’s three original tests of General Relativity and derives from time’s slowing near a massive body. For velocities well below c, this is represented with sufficient accuracy by:

As detailed by Will (1981), Schiff’s conjecture argues that the gravitational redshift actually tests the principle of equivalence rather than the gravitational field equations. For low redshifts, solar system tests give highest accuracy. LoPresto & Pierce (1986) have shown that the redshift at the Sun’s limb is good to about ±3%. Rocket experiments produce an accuracy of ±0.02% (Vessot et al. 1980), while for 40 Eri B the best white dwarf, the observed and predicted VRS agree to only about ±_5% (Wegner 1980).

We report on a search for short-period binary systems composed of pairs of evolved stars. The search is being carried out concurrently with a program to characterize the kinematical properties of two different samples of stars. Each sample has produced one close binary candidate for which further spectroscopic observations are planned. We also recapitulate the discovery of a close detached binary system composed of two cool DA white dwarfs, and we discuss the null results of Hα observations of the suspected white dwarf/brown dwarf system G 29–38.

Photographic observations of the large proper motion binary, Stein 2051, extended over the period 1966–87, do not support that it is a triple system as previously reported (Strand, 1977). The orbital motion is nearly linear over this interval? however, when results of plates from the Vatican Astrographic Zone from 1908–11 are included, a mass ratio is obtained, leading to a mass of 0.50©for the white dwarf component, given the calculated mass of 0.24© for the red dwarf component.

Surrounding the galaxy,with an aloofness appropriate to their great age,the globular clusters have played a starry role in the development of much of modern astronomy. In particular,they continue to be vital laboratories for testing our ideas of stellar evolution (Renzini and Fusi Pecci 1988). Through the study of cluster color-magnitude diagrams (CMD’s),all the major phases in the life of a common low mass star can be traced,save one. The final sequence, the locus of cooling white dwarfs, remains unexplored. We are forced to glean our knowledge of this evolutionary phase from the study of a heterogeneous sample of relatively nearby stars. Although remarkable progress has been made, the prospect of observing white dwarfs in globular clusters offers the potential for new insights into old problems; e.g., the DA/DB dichotomy, and perhaps a resolution of some of the outstanding issues connected with the clusters themselves.

Connective mixing between the thin superficial hydrogen layer and the more massive and deeper helium layer is generally believed to be responsible for the increased number of non-DA white dwarfs relative to the number of DA below 10000K (see Sion 1984 and references therein). However, because of the spectroscopic invisibility of the helium lines at effective temperatures below 13000K, the true atmospheric composition of these cool stars remains somewhat uncertain. On theoretical grounds, studies of the evolution of white dwarfs on the cooling sequence have shown that if the hydrogen layer is thicker than ~10”6Me, convective mixing does not occur (Tassoul, Fontaine, and Winget 1988). Furthermore, the exact amount of helium pollution is very sensitive to the thickness of the hydrogen layer. It seems therefore imperative to evaluate to what extent DA stars below 13000K truly are hydrogen-rich. In line with our previous efforts geared toward an understanding of the atmospheric properties of the cool DA white dwarfs, we present new insights into the spectroscopic modelling of these cool stars, and also demonstrate, for a particular object, how the helium abundance might be determined.

The recent discovery that the cool DA white dwarf L870-2 (EG11, WD0135-052) is a double-lined spectroscopic binary composed of a detached pair of DA white dwarfs (Saffer, Liebert, and Olszewski 1988, SLO hereafter) has raised some challenging problems for stellar evolution theories of such binary systems. One first important step in the understanding of this short-period system is to establish the atmospheric parameters of each component. SLO have argued from previous determinations of the effective temperature and absolute magnitude of the system, and also from their own study of the composite Hα profile, that the two components should be similar. We wish here to reexamine this assertion by taking a new look at the constraints on the two components brought about by the available observational data.

A zero-age cataclysmic binary (ZACB) we define as a binary system at the onset of interaction as a cataclysmic variable. We present here the results of calculations of the distributions of white dwarf masses and of orbital periods in ZACBs, due to binaries present in a stellar population which has undergone continuous, constant star formation for 1010 years.

Distributions of ZACBs were calculated for binaries formed t years ago, for log t = 7.4 (the youngest age at which viable ZACBs can form) to log t = 10.0 (the assumed age of the Galactic disk), in intervals of log t = 0.1. These distributions were then integrated over time to obtain the ZACB distribution for a constant rate of star formation. To compute the individual distributions for a given t, we require the density of systems forming (number of pre-cataclysmics forming per unit volume of orbital parameter space), n£(t), and the rates at which the radii of the secondary and of its Roche lobe are changing in time, s (t) and L, s (t), respectively. In calculating nf(t), we assume that the distribution of the orbital parameters in primordial (ZAMS) binaries may be written as the product of the distribution of masses of ZAMS stars (Miller and Scalo 1979), the distribution of mass ratios in ZAMS binaries (cf. Popova, et al., 1982), and the distribution of orbital periods in ZAMS binaries (Abt 1983). In transforming the the orbital parameters from progenitor (ZAMS) to offspring (ZACB) binaries, we assume that all of the orbital energy deposited into the envelope during the common envelope phase leading to ZACB formation goes into unbinding that envelope. R.L, s (t) is determined from orbital angular momentum loss rates due to gravitational radiation (Landau and Lifshitz 1951) and magnetic braking (γ = 2 in Rappaport, Verbunt, and Joss 1983). We turn off magnetic braking if the secondary is completely convective.