Evidence for the stratification of Fe in the photosphere of G191-B2B

The presence of heavy elements in the atmospheres of the hottest H-rich DA white dwarfs has been the subject of considerable interest. While theoretic al calculations can demonstrate that radiative forces, counteracting the ef fects of gravitational settling, can explain the detections of individual s pecies, the predicted abundances do not accord well with observation. Howev er, accurate abundance measurements can only be based on a thorough underst anding of the physical structure of the white dwarf photospheres, which has proved elusive. Recently, the availability of new non-local thermodynamic equilibrium model atmospheres with improved atomic data has allowed self-co nsistent analysis of the extreme ultraviolet (EUV), far UV and optical spec tra of the prototypical object G191-B2B, Even so, the predicted and observe d stellar fluxes remain in serious disagreement at the shortest wavelengths (below approximate to 190 Angstrom), while the inferred abundances remain largely unaltered. We show here that the complete spectrum of G191-B2B can be explained by a model atmosphere where Fe is stratified, with increasing abundance at greater depth. This abundance profile may explain the difficul ties in matching observed photospheric abundances, usually obtained by anal yses using homogeneous model atmospheres, to the detailed radiative levitat ion predictions, particularly as the latter are only strictly valid for reg ions deeper than where the EUV/far UV lines and continua are formed. Furthe rmore, the relative depletion of Fe in the outer layers of the atmosphere m ay be evidence for radiatively driven mass-loss in G191-B2B.