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

We present revised estimates of the equilibrium abundances of heavy el ements supported by radiative levitation in the atmospheres of hot DA white dwarfs. We emphasize, in particular, the role of trace pollutant s that may be present in the background plasma, an effect which has be en heretofore neglected. We take advantage of the availability of a ta ble of detailed monochromatic opacities calculated for a plasma made o f H and containing small amounts of C, N, O, and Fe to illustrate how the equilibrium abundances of levitating elements react to the flux re distribution caused by the addition of these small traces of opaque ma terial. We address this problem through a model-atmosphere approach, w hich gives an exact description of the flux redistribution. We also co nsider two other improvements: a more sophisticated treatment of the m omentum redistribution process an ion experiences following a photoexc itation and use of an upgraded value for the line profile width associ ated with pressure broadening. These improvements bring the present re sults to the state-of-the-art level in the context of equilibrium radi ative levitation theory.