White dwarfs have been known for slightly more than fifty years; with their planetlike diameters, and stellar-like masses their densities are enormously much higher than those of ‘normal’ stars. They thus appear to represent a state of matter totally unknown and possibly unattainable on earth.

(UBVRI) observations of known subluminous stars were published in a previous discussion (Eggen, 1970a) where the bulk of the objects, both red (RSL) and blue (BSL) populated a sequence extending for M(I) near + 11.m5 at (R – I) near 0.m0 to M(I) of + 15m near R – I = +0.m3. However, four RSL stars appeared to form a separate sequence near M(I)= +11m and R–I between +0.m3 and +0.m7. These four stars, plus an additional object (LTT 2236) are listed in Table I together with the source of luminosity; the weights of the trigonometric parallaxes are given in parentheses following the parallax. These five stars are represented in Figure 1 by crosses; the top of the steep sequence of subluminous stars is shown as the hatched region in the figure. For reference, Figure 1 also contains (1) the Hyades main sequence (Eggen, 1969b), indicated by a broken curve, (2) the evolved main sequence of the old disk population (Eggen, 1970b), shown by open circles, and (3) the subdwarfs, which are listed in Table II. In addition to the photometric results, Table II contains the Yale Parallax Catalogue number, the mean trigonometric parallax and its weight, in parentheses, as well as the resulting values of M (I).

The U.S. Naval Observatory program on trigonometric stellar parallaxes with the 61-inch Astrometric Reflector has been in progress since 1964.

Radcliffe spectra of this star in 1961 confirmed Luyten's suspicion on the basis of colour and proper motion that this object is a bright white dwarf. The profile of Hγ was found to be closely similar to that of o2 Eri B. Hill and Hill have published photometry yielding V = 11.40. The only known white dwarfs brighter than this are Sirius B, o2 Eri B, Procyon B, Feige 34 and CD −38° 10980.

During the course of the Lowell proper motion survey that has been in progress since 1957, an effort has been made to identify the very blue stars and more recently the extremely red ones. The proper motions are detected and measured directly by projection on the calibrated grid of the blink microscope. The regular program catalogs all motions found >0″.26/year and fainter than the eighth magnitude. From this portion of the program, we have identified 179 white dwarf suspects, 48 of color class– 1 and 131 of color class 0, which have been published in the Lowell Bulletins under the title of ‘Lowell Proper Motions II through XV’. These lists cover the entire northern hemisphere, and a few regions down as far as −;10° South, and contain 10382 different individual stars. Finding charts are provided for each object.

Van Maanen's star (1950: 0h46.m5, + 5°.09′) is a white dwarf (visual magnitude 12.4, spectrum DF) with a proper motion of 2″.95 in 155°.5 and parallax 0″.234. An early determination of radial velocity yielded +238 km/sec (Adams and Joy, 1926). It was suggested by Russell and Atkinson (1931) that van Maanen's star might show a very large Einstein shift of at least +700 km/sec. Oort (1932) pointed out that the observed value of +238 km/sec is incompatible with the theory of galactic rotation: this would support the Einstein shift interpretation.

The Catalogue of Nearby Stars (Gliese, 1969) contains 1529 single stars and systems with altogether 1890 components. It includes 54 white dwarfs with apparent magnitudes from 8.5 to 17; namely 38 single stars and 16 degenerate stars which are components of binaries or triples. Only 48 of these objects have parallaxes larger than 0″.050. But our knowledge is very incomplete.

Time-series data for 14 stars in the list of Eggen and Greenstein have been used to compute their power spectra, which confirm previously found quiescency in the 4 to 700 sec period range. Additionally, characteristics of the continuous power spectra are considered.

Determination of temperature and surface gravity by colors and hydrogen-line profiles have been carried out for hot halo stars. A narrow horizontal branch is found stretching to above 40000 K; the hot O subdwarfs show a nearly vertical sequence, dropping towards the hot white dwarfs.

Spectra for 285 white dwarf stars have been obtained, and the classification scheme is reviewed. Theoretical problems of these spectra remain, largely, unsolved.

The red subluminous stars found by Eggen were studied spectroscopically; among 68 stars only one new red degenerate star was found. The others are very metal-poor, high-velocity stars with large ultraviolet excess.

The spectroscopic criteria for identifying strong-lined DA-type stars on low-dispersion objective prism plates are reviewed, as are the objective prism surveys made to date with discovery of white dwarfs a partial or major objective (Table I). It is claimed that there may be upwards of 100 undiscovered white dwarfs brighter than B magnitude 14.0. Some data for all the white dwarfs so far discovered by objective prism are collected in Table II; new identification charts are given for two of these.

White dwarf stars are among the most challenging and interesting objects which can be studied. Because they represent the interiors of highly-evolved stars, the chemical composition can be enormously variable from object to object. Furthermore, because of the very large gravities, the composition of the atmosphere may be very different from that in the interior. The theory of the degenerate interior provides a relation among mass, radius and chemical composition. Since temperatures, effective gravities, and redshifts can, for certain stars, provide further relations between mass and radius, one can hope to make checks on the theory which are not possible with ordinary stars.

Stellar evolution is characterized by fast and slow phases. Usually the periods of rapid change are difficult to follow observationally; but, this does not seem to be the case when passing through the planetary nebula stage. Because of their high intrinsic luminosities and easy identification, it is possible to identify and study these objects and their central stars rather completely. It is quite relevant to discuss these objects at a symposium on white dwarfs since the central stars may be in the immediate progenitor stage before white dwarfs. The actual picture of the evolution of the nuclei has changed rather little in the past few years and is the subject of an earlier review article (O'Dell, 1968) to which the reader is referred.

During the course of two observational programs to search for magnetic fields in white dwarfs (Angel and Landstreet, 1970a; Kemp, 1970), it was discovered by Kemp and Swedlund (Kemp et al., 1970b) that the continuum optical radiation from the λ4135 type white dwarf Grw + 70° 8247 is circularly polarized. This circular polarization was measured in a broad band from about 4000 to 7000 Å a number of times; in this band the circular polarization does not appear to vary from its mean value of 3.29% with an amplitude of more than about 0.1% (or 3% of the measured mean polarization) on any time scale from 24 seconds to 2 weeks (Angel and Landstreet, 1970b).

We first consider the general information scheme for the interpretation of observational data (Figure 1). From the relations plotted it is evident that (in going from left to right) this scheme can only be solved if distances are known and if we are able to determine the atmospheric parameters: effective temperature, Teff, surface gravity, g, and chemical composition from observations of colors and spectra – which is the genuine task of the theory of stellar atmospheres.

A knowledge of the precise relationship between the age and luminosity of a white dwarf can in principle be used to determine the compositions of the white dwarfs in galactic clusters. To this end the assumptions in Mestel's theory of white dwarf cooling are critically reviewed, and the results of recent work aimed at relaxing these restrictions are briefly summarized. It is concluded that on the basis of current knowledge an accuracy of the order of 10 or 20% in the age-luminosity relation should be attainable.

Model atmospheres with temperatures in the range 10000 ≤ Teff ≤ 25000 K and with gravities 6 ≤ log(g) ≤ 9 have been constructed for various helium abundances with a view to understanding the spectra of the hotter white dwarfs. It is shown that the DB stars are confined to the temperature range 15000 ≤ Teff ≤ 18000 K in which convection also becomes important in the outer layers of helium rich stars. The DA stars seem to avoid this temperature range. The hydrogen and metal abundances in DB atmospheres are shown to be reduced by factors 105 and 103 respectively compared to solar values. Possible explanations of DB and DC spectra are discussed.

Model evolutionary calculations have been made for population I stars (X = 0.7, Z = 0.03) with masses of 0.8, 1.5, 3, 5, 7, 10, and 15 M⊙ (Paczyński, 1970). Neutrino losses were taken into account. All the models were evolved up to the red supergiant phase with the hydrogen and helium burning shell sources. In this phase of evolution the hydrogen rich envelopes of 0.8, 1.5, and 3 M⊙ stars became dynamically unstable due to the large depth of the hydrogen and helium ionization zones. It was assumed that almost entire envelopes were lost as a result of that instability. The degenerate carbon-oxygen cores of 0.6, 0.8, and 1.2 M⊙ were left with the hydrogen rich envelopes of a small mass. Almost all the hydrogen that was left was subsequently burnt in the shell source. Finally, the hydrogen and helium burning shell sources disappeared as the nuclear fuel was exhausted. The models were cooling down to the white dwarf phase and they had a small amount of hydrogen left close to their surfaces. It is estimated that population I stars with masses up to 3.5 M⊙ may produce white dwarfs with masses up to 1.37 M⊙. The hydrogen rich envelopes that were lost as a result of dynamical instability could form planetary nebulae with masses up to 2 M⊙. Further details are published in the paper referred to above.

The determination of atmospheric parameters for non-DA white dwarfs is investigated with the computed helium-rich model atmospheres by Bues (1970). Only poor predictions are possible from UBV colors alone for DB and DC stars. From uvby colors a determination of effective temperature is possible within 1000 K. Profiles of lines in different parts of the spectrum are necessary for better results.

A deficiency of metal abundances for the cooler non-DA stars is obtained.