Hervy Element Abundances Predicted by Radiative Support Theory in the Atmospheres of Hot White Dwarfs

Abstract

The surface composition of a white dwarf evolves as a result of the interaction of several mechanisms, the most important of which being gravitational settling. In the early phases of the evolution, theory shows that selective radiative levitation can occasionally defeat settling and, thus, prevent the formation of a pristine pure hydrogen (helium) atmospheric layer in a hot DA (non-DA) white dwarf (Fontaine and Michaud 1979; Vauclair, Vauclair, and Greenstein 1979). The exciting discovery of sharp metallic features in the ultraviolet spectra of several hot DA and non-DA stars alike resulting from the work of several investigators has provided the essential motivation for further theoretical investigations of radiative levitation in the atmospheres of white dwarfs. Additionnal impetus comes from the continuing investigations of hot DA white dwarfs carried out by Bruhweiler and Kondo which have already revealed a most interesting observational pattern of heavy elements in these stars (Bruhweiler 1985). Moreover the recent availability of theoretical equivalent widths of selected astrophysically important ultraviolet metal lines in hot DA white dwarfs (Henry, Shipman, and Wesemael 1985) makes a comparison between theory and observations -in at least this type of stars- a timely and useful exercise.