The presence of helium in hot DA white dwarfs - The role of radiative levitation and the case for stratifies atmospheres

Details The presence of helium in hot DA white dwarfs - The role of radiative levitation and the case for stratifies atmospheres The presence of helium in hot DA white dwarfs - The role of radiative levitation and the case for stratifies atmospheres

Aug 1988

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988apj...331..876v&link_type=abstract

Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 331, Aug. 15, 1988, p. 876-897. Research supported by the Ministere de l'Ed

Other

96

Abundance, Hot Stars, Stellar Atmospheres, Stellar Radiation, White Dwarf Stars, X Rays, Exosat Satellite, Helium, Levitation, Photosphere

Scientific paper

The role of helium radiative levitation in the envelopes of hot DA white dwarfs is examined. Explicit time-dependent calculations of helium diffusion in the presence of radiative support carried out in evolving DA models show that the helium distribution in the outermost layers of the models relaxes rapidly to a configuration corresponding to diffusive equilibrium. Observations of these regions show only equilbrium helium abundances and are totally decoupled from the evolving internal helium profile. It is shown that the helium abundances supported by radiative forces in the atmospheric layers of these stars are too small to account for the observations. It is suggested that a mechanism other than radiative levitation must be used to compete against gravitational settling in these stars. A number of mechanisms are discussed and it is concluded that an equilibrium between ordinary diffusion driven by a composition gradient and settling may be the best possibility. The implications of this model are discussed in detail.

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