Far and Extreme Ultraviolet Spectroscopy of Hot White Dwarf Stars: A Laboratory for Element Diffusion

AbstractEvidence is now compelling that most externally-polluted white dwarfs derive their heavy atoms by accretion from asteroids – the building blocks of rocky planets. Optical and ultraviolet spectroscopy of a small sample of suitable white dwarf stars shows that to zeroth order, the accreted extrasolar parent bodies compositionally resemble bulk Earth. (1) Extrasolar planetesimals are at least 85% by mass composed of O, Mg, Si and Fe. (2) Compared to the Sun, C is often deficient, usually by at least a factor of 10 and therefore comprises less than 1% of an extrasolar planetesimal's mass. At least to-date, C has never been found to be enhanced as would be expected if carbon-rich planetesimals have formed. (3) While there may be individual exceptions, considered as a whole, the population of extrasolar asteroids accreted onto a well-defined sample of local white dwarf stars is less than 1% water by mass.

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Spectra of White-Dwarf Stars at Wavelengths < 3000 A: A Theoretical Perspective

International Astronomical Union Colloquium ◽

10.1017/s025292110007576x ◽

1979 ◽

Vol 53 ◽

pp. 86-106 ◽

Cited By ~ 1

Author(s):

Harry L. Shipman

Keyword(s):

Solar System ◽

White Dwarf ◽

Extreme Ultraviolet ◽

Theoretical Perspective ◽

Electromagnetic Spectrum ◽

Dwarf Stars ◽

X Ray ◽

Satellite Technology ◽

White Dwarf Stars ◽

Current State

Astronomers studying objects outside the solar system first used the ultraviolet, extreme ultraviolet, and x-ray regions of the electromagnetic spectrum in the 1970s. The exploration of these wavelength regions has produced considerable improvements in our understanding of these objects. The achievements of x-ray astronomy are perhaps the best known. With the advance of satellite technology, other wavelength regions begin to play a role, and x-ray astronomy moves into the luminosity domain where quiescent as well as violent astrophysical processes can produce detectable amounts of radiation. This paper reviews the current state of our interpretation of white-dwarf stars at wavelengths less than 3000 A.

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The impact of element diffusion on the formation and evolution of helium white dwarf stars

Monthly Notices of the Royal Astronomical Society ◽

10.1046/j.1365-8711.2001.04324.x ◽

2001 ◽

Vol 324 (3) ◽

pp. 617-622 ◽

Cited By ~ 34

Author(s):

L. G. Althaus ◽

A. M. Serenelli ◽

O. G. Benvenuto

Keyword(s):

White Dwarf ◽

Dwarf Stars ◽

White Dwarf Stars ◽

Element Diffusion ◽

Formation And Evolution ◽

The Impact

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The Hot White Dwarf Population in the EUVE Survey

International Astronomical Union Colloquium ◽

10.1017/s0252921100035971 ◽

1996 ◽

Vol 152 ◽

pp. 193-202

Author(s):

Stéphane Vennes

Keyword(s):

Stellar Evolution ◽

White Dwarf ◽

Extreme Ultraviolet ◽

Model Atmosphere ◽

Asymptotic Giant Branch ◽

Population Characteristics ◽

Dwarf Stars ◽

Origin And Evolution ◽

White Dwarf Stars ◽

Main Population

The processes leading to the formation of white dwarf stars are known only in their most general principles; post-asymptotic giant branch evolution, leading to the formation of C-O degenerate cores, is possibly the main formation channel of white dwarf stars. In contrast, observations of hot white dwarf stars and studies of their main population characteristics offer detailed insights into the origin and evolution of these objects. We examine some new facts uncovered in the study of the survey of hot white dwarf stars at extreme ultraviolet (EUV) wavelengths. We describe model atmosphere techniques required to interpret these observations and discuss some implications of our findings for stellar evolution theory.

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TheEUVEOptical Identification Campaign II: Late-Type and White Dwarf Stars

International Astronomical Union Colloquium ◽

10.1017/s0252921100036447 ◽

1996 ◽

Vol 152 ◽

pp. 491-496

Author(s):

N. Craig ◽

A. Fruscione ◽

J. Dupuis ◽

M. Mathioudakis ◽

J.J. Drake ◽

...

Keyword(s):

White Dwarf ◽

Extreme Ultraviolet ◽

Late Type ◽

Dwarf Stars ◽

White Dwarf Stars ◽

Sky Survey ◽

Deep Survey ◽

Balmer Lines ◽

Optical Identifications ◽

Error Circle

We present optical identifications of nine previously unidentified extreme ultraviolet (EUV) sources discovered during theExtreme Ultraviolet Explorer(EUVE) satellite surveys. The all-sky survey detected four of the sources and the more sensitive deep survey detected the other five sources. Three of the four all-sky survey sources, EUVE_J1918+59.9, EUVE_J2249+58.5, and EUVE_J2329+41.4, are listed in present catalogs as having possible associations with optical counterparts but without spectral class. The first two of these sources are hot DA white dwarfs showing an optical spectrum with broad Balmer lines. The source EUVE_J2329+41.4 is listed as having a possible association with an unclassified M star. We show that a pair of dMe stars are actually optical counterparts located within the error circle of theEUVEsource position. The EUVE_J2114+503 remains unidentified even though all the possible candidates have been studied. Based on the count rates we predict a fainter white dwarf or a cataclysmic variable counterpart for this candidate. All five sources discovered with theEUVEdeep survey, EUVE_J0318+184, EUVE_J0419+217, EUVE_J2053−175, EUVE_J2056−171 and EUVE_J2233−096, have been identified as late-type stars. The spectral classes, distances, visual magnitudes, and estimated hydrogen column densities for theseEUVEsources are presented.

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The Time Dependence of the Phases of the Harmonics Relative to the 1490 sec Fundamental in PG1346+082

International Astronomical Union Colloquium ◽

10.1017/s0252921100099759 ◽

1989 ◽

Vol 114 ◽

pp. 296-299

Author(s):

J. L. Provencal ◽

J. C. Clemens ◽

G. Henry ◽

B. P. Hine ◽

R. E. Nather ◽

...

Keyword(s):

White Dwarf ◽

Compact Object ◽

Detailed Knowledge ◽

Line Profiles ◽

Pressure Broadening ◽

Dwarf Stars ◽

White Dwarf Stars ◽

Adequate Understanding ◽

Magnitude Variation ◽

Hydrogen Lines

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.

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