We present preliminary results from a study of the variations of the optical spectrum of the Earthshine over a period of a year. Our goal is to follow, on several timescales, the spectral changes of the signature of various biomarkers that are potential indicators of habitability and/or biological activity, and of the Vegetation Red Edge, a feature around 700 nm due to photosynthetic organisms.

We present revised estimates of the equilibrium abundances of heavy elements supported by radiative levitation in the atmospheres of hot DA white dwarfs. We emphasize, in particular, the role of trace pollutants that may be present in the background plasma, an effect which has been heretofore neglected. We take advantage of the availability of a table of detailed monochromatic opacities calculated for a plasma made of H containing small amounts of C, N, O, and Fe to illustrate how the equilibrium abundances of levitating elements react to the flux redistribution caused by the addition of these small traces of opaque material. We also consider two other improvements: a more sophisticated treatment of the momentum redistribution process and ion experiences following a photoexcitation, and use of an upgraded value for the line profile width associated with pressure broadening.

The ZZ Ceti stars form a class of variable white dwarfs: the hydrogen dominated atmosphere ones, which do pulsate in an instability strip in the effective temperature range 13000K-11500K. We know 22 such ZZ Ceti white dwarfs. Their variations are caused by nonradial g-mode pulsations with periods are in the range 100-1000 seconds.

A subsample of the ZZ Ceti stars shows amplitude variations on time scales of the order of one month. These variations could be driven by nonlinear phenomena.

Over the last several years, we have developed a comprehensive program aimed at better understanding the properties of pulsating DA white dwarfs (or ZZ Ceti stars). These stars are nonradial pulsators of the g-type, and their study can lead to inferences about their internal structure. For instance, the period spectrum of a white dwarf is most sensitive to its vertical chemical stratification, and one of the major goals of white dwarf seismology is to determine the thickness of the hydrogen layer that sits on top of a star. This can be done, in principle, by comparing in detail theoretical period spectra with the periods of the observed excited modes. Likewise, because the cooling rate of a white dwarf is very sensitive to the specific heat of its core material (and hence to its composition), it is possible to infer the core composition through measurements and interpretations of rates of period change in a pulsator.

It is generally believed that the immediate progenitors of most white dwarfs are nuclei of planetary nebulae, themselves the products of intermediate- and low-mass main sequence evolution. Stars that begin their lifes with masses less than about 7-8 M⊙ (i.e., the vast majority of them) are expected to become white dwarfs. Among those which have already had the time to become white dwarfs since the formation of the Galaxy, a majority have burnt hydrogen and helium in their interiors. Consequently, most of the mass of a typical white dwarf is contained in a core made of the products of helium burning, mostly carbon and oxygen. The exact proportions of C and 0 are unknown because of uncertainties in the nuclear rates of helium burning.

Of all stars in the Hertzprung-Russell diagram, white dwarfs are those for which the clues on past evolution given by photospheric abundances are probably the hardest to decipher. This is because the cooling phase of white dwarfs, a relatively uneventful phase from an evolutionary point of view, is, in contrast, a most active phase for the evolution of the chemical composition of the envelope. Indeed, it is now well established that the often puzzling variety of surface abundances observed in white dwarf stars can be traced to the simultaneous operation, in the outer layers of these stars, of a variety of physical processes which will also erase the abundances present in the photosphere at the onset of cooling.

Downward element diffusion in the intense gravitational field of the degenerate star is perhaps the mechanism which is the most closely identified with white dwarf stars. However, convective mixing, ordinary diffusion, radiative forces, winds, and accretion from the interstellar medium all are equally important processes which, at times, compete efficiently with the rapid element segregation expected in those stars. The various regions, along the cooling sequence of white dwarfs, where individual processes are expected to operate, have been summarized by Fontaine and Wesemael (1987). We illustrate here various combinations of these mechanisms which have been found in white dwarfs, and show how their competition affects the observed abundance patterns. The unity underlying these cases stems from the fact that, in many cases, progress in investigating these complicated situations has come only through the combination of evolutionary calculations with new and powerful numeriques techniques which have been developed at Montréal (Pelletier 1986; Pelletier, Fontaine, and Wesemael 1989).

The spectral evolution of white dwarfs is governed by diffusion processes which enter into competition with mechanisms such as mass loss, convective mixing, and accretion from the interstellar medium in various phases of the evolution. Until recently, our theoretical understanding of the chemical evolution of these stars has been limited by the very severe numerical difficulties which plague a time-dependent description of the problem. Indeed, diffusion problems in white dwarf interiors and envelopes are particularly demanding from a computational standpoint: they involve relative chemical abundances spanning many orders of magnitude, time integration length of a few billion years, and many physical processes operating with greatly different time constants. We have already introduced in the field a robust numerical technique based on an implicit finite difference scheme designed for nonlinear two-point boundary value problems (Pelletier 1986). This method has been used to investigate a number of problems related to the spectral evolution of white dwarfs (Pelletier 1986; Pelletier et al. 1986; Dupuis et al. 1987). As requirements for further progress in the field become more exacting and in the interest of improving the efficiency, we have sought to develop even more powerful numerical techniques. We briefly introduce here an efficient computational approach to diffusion problems in white dwarfs based on a Galerkin finite element method to solve the convective-diffusion equation in an evolving white dwarf model. As an illustrative example, we discuss some sample results of a detailed investigation of the problem of chemical sedimentation (H, He, and C) in the envelopes of hot white dwarfs and the formation of DA stars.

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).

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 optical and UV spectroscopy to determine He abundances and upper limits to He abundances in the photospheres of a selected sample of very hot hydrogen-rich white dwarfs. He abundances in the range log(He/H) -3 to -1.5 are observed in several of these DAs and upper limits of -3 determined for the remainder. In apparent contradiction to the relatively large He abundances inferred from soft X-ray observations for the hot DA G191 B2B, we find no evidence of He in the optical and UV.

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.

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.

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.

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.

The surface composition of a white dwarf evolves as a result of the interaction of several mechanisms, the most important of which being gravitational settling. In the early phases of the evolution, theory shows that selective radiative levitation can occasionally defeat settling and, thus, prevent the formation of a pristine pure hydrogen (helium) atmospheric layer in a hot DA (non-DA) white dwarf (Fontaine and Michaud 1979; Vauclair, Vauclair, and Greenstein 1979). The exciting discovery of sharp metallic features in the ultraviolet spectra of several hot DA and non-DA stars alike resulting from the work of several investigators has provided the essential motivation for further theoretical investigations of radiative levitation in the atmospheres of white dwarfs. Additionnal impetus comes from the continuing investigations of hot DA white dwarfs carried out by Bruhweiler and Kondo which have already revealed a most interesting observational pattern of heavy elements in these stars (Bruhweiler 1985). Moreover the recent availability of theoretical equivalent widths of selected astrophysically important ultraviolet metal lines in hot DA white dwarfs (Henry, Shipman, and Wesemael 1985) makes a comparison between theory and observations -in at least this type of stars- a timely and useful exercise.

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).

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 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).

Ultraviolet energy distributions are analyzed for several hot, helium atmosphere DB white dwarfs, including the four known pulsating stars which define an empirical DB instability strip. Temperatures are derived exclusively from fits to the ultraviolet energy distributions. The blue edge of the empirical DB instability strip lies at 30,000 ± 4,000 K, and the red edge lies near 24,000 ± 2,000 K. The hottest DB star — and the only known one hotter than the instability strip — is PG0112+104 at or above 30,000 K. This leaves no known helium-atmosphere degenerate stars in the interval 30,000 ≤ Te ≤ 45.000K.