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Title:
The chemical evolution of white dwarf atmospheres - Diffusion and accretion
Authors:
Vauclair, G.; Vauclair, S.; Greenstein, J. L.
Affiliation:
AA(Paris, Observatoire, Meudon, Hauts-de-Seine, France; California Institute of Technology, Pasadena, Calif.), AB(Paris, Observatoire, Meudon, Hauts-de-Seine, France; California Institute of Technology, Pasadena, Calif.), AC(Hale Observatories, Pasadena, Calif.)
Publication:
Astronomy and Astrophysics, vol. 80, no. 1, Nov. 1979, p. 79-96. Research supported by the Lee A. DuBridge Professorship Fund. (A&A Homepage)
Publication Date:
11/1979
Category:
Astrophysics
Origin:
STI
NASA/STI Keywords:
ATMOSPHERIC DIFFUSION, CHEMICAL EVOLUTION, STELLAR ATMOSPHERES, STELLAR EVOLUTION, STELLAR MASS ACCRETION, WHITE DWARF STARS, ABUNDANCE, CARBON, CHEMICAL COMPOSITION, METALS, NITROGEN, OXYGEN, RADIATIVE TRANSFER, STELLAR ENVELOPES
Comment:
A&AA ID. AAA026.126.025
Bibliographic Code:
1979A&A....80...79V

Abstract

A study of diffusion processes in white dwarfs is presented, with particular emphasis on the estimate of the diffusion time scales for C, N, O, Mg, and Ca along the cooling sequence. The effect of radiative acceleration is important in hot white dwarfs while in cooler ones thermal diffusion dominates the gravitational setting. In hot white dwarfs, there should be an observable amount of CNO elements unless they have previously left the stars by a selective wind. Observational tests of this result are discussed. The diffusion time scales are always short compared to the evolutionary time scales. It is shown that in both hydrogen and helium envelopes, the convection zone, even at its maximum depth, is not able to bring back to the stellar surface the metals which have previously diffused downwards. The diffusion alone predicts a complete absence of metals in white dwarf atmospheres and envelopes. As metals are observed in white dwarfs, at least at effective temperatures lower than 15,000 K, there must be some mechanism competing with diffusion. The competition between diffusion and accretion is investigated and a general scheme for the chemical evolution of white dwarf atmospheres along the cooling sequence is proposed

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