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

The Astrophysical Journal1988Vol. 331, pp. 876–876

Citations Over TimeTop 10% of 1988 papers

S. Vennes, C. Pelletier, G. Fontaine, F. Wesemaël

Abstract

The role of helium radiative levitation in the envelopes of hot DA white dwarfs is investigated. This is motivated by the presence of photospheric traces of helium inferred from short-wavelength observations of several hot DA white dwarfs or measured from optical observations of even hotter DAO stars. Using a simple prescription for the computation of the radiative acceleration, explicit time-dependent calculations of helium diffusion in presence of radiative support are carried out in evolving DA models. These computations firmly establish that the helium distribution in the outermost layers of the models relaxes extremely rapidly to a configuration corresponding to diffusive equilibrium. Thus, the regions accessible to direct observation show only equilibrium helium abundances and are totally decoupled from the evolving internal helium profile. This welcome decoupling permits to determine more quantitatively the equilibrium abundances of helium in these regions with the help of inexpensive static envelope models. To this end, detailed calculations of the selective acceleration on helium embedded in a hydrogen plasma are carried out for a wide range of physical conditions. It is shown that the abundances of helium supported by radiative forces in the atmospheric layers of hot DA white dwarfs are too small by at least two orders of magnitude to account for the observations. Hence, a more efficient mechanism than radiative levitation must be invoked to compete against gravitational settling in these stars. A number of mechanisms are discussed, and it is suggested that an equilibrium between ordinary diffusion (driven by a composition gradient) and settling may be a most attractive possibility. In this model, hot DA white dwarfs can be interpreted as stratified objects with an outer layer of hydrogen which is sufficiently thick that radiation in the visible escapes only from hydrogen-rich regions, and yet sufficiently thin that the EUV/soft X-ray radiation escapes from deeper layers, polluted by the tail of the helium equilibrium distribution which extends upward. A simple opacity effect leads naturally to a positive correlation between helium abundance and effective temperature: as cooling proceeds, the opacity of the atmospheric layers increases and the EUV/soft X-ray photosphere recedes to the surface, to regions of smaller helium abundance. However, the observed correlation is steeper than that obtained for an evolving model in which the mass of the outer layer of hydrogen is kept constant. The observations can be counted for if the mass and thickness of the hydrogen layer increase with decreasing effective temperature. For example, a value of H/He = 10-2.5 (which is typical of DAO stars) is present at the soft X-ray photosphere of a stratified 60,000 K, 0.6 M_⊙ DA model if the hydrogen layer contains only a fractional mass ~10-14.7. To account for the observed value He/H ~ 10-5 (more typical of ~30,000 K DA stars), the model requires a hydrogen layer containing a fractional mass ~ 10-13.3. These numbers imply that hydrogen-helium separation still goes on in hot DA white dwarfs and argue against the existence of strong winds and substantial residual burning in such stars. The suggestion that hot DA white dwarfs have thin hydrogen-rich layers further adds to mounting evidence in favor of thin hydrogen and helium layers in white dwarfs in general. This is in strong contradiction with the predictions of standard evolution theory.

Citations Over TimeTop 10% of 1988 papers

Abstract

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