The Composition and Structure of White Dwarf Atmospheres Revealed by Extreme Ultraviolet Spectroscopy

The ROSAT and EUVE all-sky surveys have resulted in an important change in our understanding of the general composition of hydrogen-rich DA white dwarf atmospheres, with the photospheric opacity dominated by heavy elements rather than helium in the hottest stars (T > 40, 000 K). Most stars cooler than 40,000 K have more or less pure H atmospheres. However, one question, which has not been resolved, concerned the specific nature of the heavy elements and the role of helium in the hottest white dwarfs. One view of white dwarf evolution requires that H-rich DA stars form by gravitational settling of He from either DAO or He-rich central stars of planetary nebulae. In this case, the youngest (hottest) DA white dwarfs may still contain visible traces of He. Spectroscopic observations now available with EUVE provide a crucial test of these ideas. Analysis of data from the EUVE Guest Observer programme and EUVE public archive allows quantitative consideration of the sources of EUV opacity and places limits on the abundance of He which may be present.

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Diffusion and Metal Abundances in Hot White Dwarfs Gerard Vauclair

International Astronomical Union Colloquium ◽

10.1017/s0252921100099541 ◽

1989 ◽

Vol 114 ◽

pp. 176-187 ◽

Cited By ~ 1

Author(s):

Gérard Vauclair

Keyword(s):

Time Scales ◽

White Dwarf ◽

White Dwarfs ◽

Dwarf Stars ◽

White Dwarf Stars ◽

Metal Abundances ◽

Cool White Dwarfs ◽

First Time ◽

Radiative Acceleration

While the efficiency of gravitational settling to produce chemically pure atmospheres in white dwarf stars was outlined for the first time 30 years ago (Schatzman 1958), the competing role of the radiation flux in the hot white dwarfs was considered only 10 years ago (Fontaine and Michaud 1979; Vauclair, Vauclair and Greenstein 1979). At that time, there was more motivation to understand how metals could reappear in the long lived cool non DA white dwarfs, where diffusion time scales are shorter by orders of magnitude than evolutionary time scales. Various processes were invoked to help restore some metal content in the white dwarf atmospheres: convection mixing and dredge up, accretion of interstellar matter. In cool white dwarfs, the radiative acceleration is negligeable in the diffusion process; this is not the case at the hot end of the sequence where radiation may balance gravity. The short lived hot white dwarfs just started to become exciting with the contemporary discoveries that i) some show metallic lines in their spectra, both hydrogen rich and hydrogen poor; ii) some of these are pulsating. In the following years, the number of hot white dwarfs revealing trace abundance of metals has increased, mainly owing to IUE observations.

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White Dwarfs and Planetary Central Stars

Symposium - International Astronomical Union ◽

10.1017/s0074180900139166 ◽

1989 ◽

Vol 131 ◽

pp. 545-554

Author(s):

James Liebert

Keyword(s):

White Dwarf ◽

White Dwarfs ◽

High Surface ◽

Planetary Nebulae ◽

Hydrogen Atmosphere ◽

Surface Gravity ◽

Surface Hydrogen ◽

Central Stars

Studies of hot white dwarf samples constrain the properties and evolution of planetary nuclei and the nebulae. In particular, the white dwarf and planetary nebulae formation rates are compared. I discuss the overlap of the sequences of white dwarfs having hydrogen (DA) and helium-rich (DO) atmospheres with known central stars of high surface gravity. There is evidence that the hydrogen atmosphere nuclei have “thick” outer hydrogen layers (≳ 10−4 M⊙), but that DA white dwarfs may have surface hydrogen layers orders of magnitude thinner. Finally, a DA planetary nucleus is discussed (0950+139) which has undergone a late nebular ejection; this object may be demonstrating that a hydrogen layer can be lost even after the star has entered the white dwarf cooling sequence.

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ROSAT Studies of the Composition and Structure of DA White Dwarf Atmospheres

Symposium - International Astronomical Union ◽

10.1017/s0074180900172304 ◽

1993 ◽

Vol 155 ◽

pp. 497-497

Author(s):

C.J. Diamond ◽

M.A. Barstow ◽

A.E. Sansom ◽

M.C. Marsh ◽

S.R. Rosen ◽

...

Keyword(s):

White Dwarf ◽

White Dwarfs ◽

X Ray ◽

Composition And Structure ◽

Sky Survey

We have made a detailed study of a sample of 28 hot DA white dwarfs detected in the ROSAT EUV and soft X-ray all-sky-survey.

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Central stars of planetary nebulae: The white dwarf connection

Proceedings of the International Astronomical Union ◽

10.1017/s1743921312010952 ◽

2011 ◽

Vol 7 (S283) ◽

pp. 196-203 ◽

Cited By ~ 3

Author(s):

Klaus Werner

Keyword(s):

Iron Deficiency ◽

Effective Temperature ◽

White Dwarf ◽

White Dwarfs ◽

Spectral Classification ◽

Present Evidence ◽

The Common ◽

Central Stars ◽

Evolution Sequence

AbstractThis paper is focused on the transition phase between central stars and white dwarfs, i.e. objects in the effective temperature range 100 000 – 200 000 K. We confine our review to hydrogen-deficient stars because the common H-rich objects are subject of the paper by Ziegler et al. in these proceedings. We address the claimed iron-deficiency in PG1159 stars and [WC] central stars. The discovery of new Ne vii and Ne viii lines in PG1159 stars suggests that the identification of O vii and O viii lines that are used for spectral classification of [WCE] stars is wrong. We then present evidence for two distinct post-AGB evolutionary sequences for H-deficient stars based on abundance analyses of the He-dominated O(He) stars and the hot DO white dwarf KPD 0005+5106. Finally, we report on evidence for an H-deficient post-super AGB evolution sequence represented by the hottest known, carbon/oxygen-atmosphere white dwarf H 1504+65 and the recently discovered carbon-atmosphere “hot DQ” white dwarfs.

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Implications of White-Dwarf Crystallization for the Chemical Composition of the Planetary Nuclei

Symposium - International Astronomical Union ◽

10.1017/s0074180900020970 ◽

1968 ◽

Vol 34 ◽

pp. 425-427

Author(s):

H.M. Van Horn

Keyword(s):

Chemical Composition ◽

White Dwarf ◽

White Dwarfs ◽

Planetary Nebulae ◽

Considerable Importance ◽

Central Stars

It now seems to be reasonably well-established that the central stars of planetary nebulae evolve directly into white dwarfs. Evidently a knowledge of the chemical composition of the white dwarfs would therefore be of considerable importance in helping to identify the point in the evolution at which the mechanism responsible for expulsion of the nebular shell becomes operative. For this reason I would like to present some evidence which provides a direct suggestion for the internal composition of some of the white dwarfs and to examine briefly the implications of this suggestion for the relation between the planetary nuclei and the white dwarfs.

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Equilibrium Abundances of Heavy Elements Supported by Radiative Levitation in the Atmospheres of Hot DA White Dwarfs

International Astronomical Union Colloquium ◽

10.1017/s0252921100035995 ◽

1996 ◽

Vol 152 ◽

pp. 211-215

Author(s):

P. Chayer ◽

S. Vennes ◽

A.K. Pradhan ◽

P. Thejll ◽

A. Beauchamp ◽

...

Keyword(s):

Line Profile ◽

White Dwarfs ◽

Heavy Elements ◽

Background Plasma ◽

Pressure Broadening ◽

Opaque Material ◽

Trace Pollutants ◽

Flux Redistribution

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.

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Diffusion Coefficients in Dense Plasmas

International Astronomical Union Colloquium ◽

10.1017/s025292110007593x ◽

1979 ◽

Vol 53 ◽

pp. 192-196

Author(s):

G. Fontaine ◽

G. Michaud

Keyword(s):

Diffusion Model ◽

Time Scales ◽

White Dwarf ◽

Diffusion Coefficients ◽

White Dwarfs ◽

Diffusion Time ◽

Heavy Elements ◽

Spectral Evolution ◽

Dense Plasmas ◽

Quantitative Estimates

It has been known for some time (Schatzman 1958) that diffusion could play an important role in the spectral evolution of white dwarfs. However, it is only recently that quantitative estimates have become available, making use of detailed envelope models (Fontaine and Michaud 1979, hereafter referred to as FM; Vauclair, Vauclair, and Greenstein 1979; Alcock and Illarionov 1979). These studies suggest that the observed monoelemental character of white dwarf spectra can qualitatively be explained in terms of the diffusion model. Moreover, it appears that the diffusion time scales are so short compared to evolutionary times that competing mechanisms, such as accretion, must be invoked to explain the small but measurable abundances of heavy elements in the spectra of cooler white dwarfs.

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IUE Observations of Central Stars

Symposium - International Astronomical Union ◽

10.1017/s007418090009392x ◽

1983 ◽

Vol 103 ◽

pp. 375-390

Author(s):

Sara R. Heap

Keyword(s):

White Dwarf ◽

Spectral Properties ◽

White Dwarfs ◽

Planetary Nebulae ◽

Hot Stars ◽

Gravitational Settling ◽

Ultimate Fate ◽

Central Stars

Despite similar evolutionary histories and a common ultimate fate as white dwarfs, central stars of planetary nebulae have surprisingly diverse spectral properties. Their visual spectral types encompass all varieties known for hot stars, including Wolf-Rayet, O and Of, subdwarf O, white-dwarf, and continuous (Aller 1968, 1976), and O VI-emission types (Smith and Aller 1969, Heap 1982). Their spectroscopic temperatures range from less than 30,000°K (e.g. He 2-138, Mendez and Niemela 1979; the WC 11 stars, Houziaux and Heck 1982) to upwards to 150,000°K or more (e.g. NGC 246, Heap 1975; Abell 30, Greenstein 1981). Their atmospheres range from demonstrably helium- and carbon-rich (e.g. the WR stars, Barlow and Hummer 1982, Benvenutı et al. 1982) to apparently normal (e.g. the Of stars, Heap 1977a,b), to helium-poor (e.g. the nascent white dwarfs in Abell 7 and NGC 7293, where gravitational settling appears to have already taken effect, Mendez et al. 1981).

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