ON THE PREDICTED ABUNDANCES OF IRON AND NICKEL SUPPORTED BY RADIATIVE LEVITATION IN THE ATMOSPHERES OF HOT DA WHITE-DWARFS

We present the results of detailed radiative forces calculations for iron and nickel levitating in hot DA white dwarf atmospheres. This follows from the determination of the iron abundance in a handful of hot DA stars by a number of authors and, more particularly, from the recent identification and abundance analysis of nickel carried out by Werner and Dreizler for these same stars. Our calculations are based on the extensive atomic data set compiled by Kurucz for the iron group elements. We show that radiative levitation is more than sufficient to account for the abundances of iron and nickel observed in hot DA white dwarfs. However, we also find that the observations exhibit an abundance pattern which cannot be explained in detail by the predictions of radiative levitation theory. This finding adds to mounting evidence that other mechanisms must be at work in the atmospheres of these objects and compete with radiative levitation and gravitational settling.