IMPROVED CALCULATIONS OF THE EQUILIBRIUM ABUNDANCES OF HEAVY-ELEMENTS SUPPORTED BY RADIATIVE LEVITATION IN THE ATMOSPHERES OF HOT DA WHITE-DWARFS

We present revised estimates of the equilibrium abundances of heavy elements supported by radiative levitation in the atmospheres of hot DA white dwarfs. We emphasize, in particular, the role of trace pollutants that may be present in the background plasma, an effect which has been heretofore neglected. We take advantage of the availability of a table of detailed monochromatic opacities calculated for a plasma made of H and containing small amounts of C, N, O, and Fe to illustrate how the equilibrium abundances of levitating elements react to the flux redistribution caused by the addition of these small traces of opaque material. We address this problem through a model-atmosphere approach, which gives an exact description of the flux redistribution. We also consider two other improvements: a more sophisticated treatment of the momentum redistribution process an ion experiences following a photoexcitation and use of an upgraded value for the line profile width associated with pressure broadening. These improvements bring the present results to the state-of-the-art level in the context of equilibrium radiative levitation theory.