The effect of photospheric heavy elements on the hot DA white dwarf temperature scale

Using the latest non-local thermodynamic equilibrium (non-LTE) synthetic spectra and stellar model calculations, we have evaluated the potential effect of the presence of heavy elements in the photospheres of hot H-rich DA white dwarfs. In particular, we have examined their influence on the effective temperature and surface gravity perceived from analysis of the Balmer line profiles. It is apparent that both the inclusion of non-LTE effects in the models and significant quantities of heavy elements act independently to lower the value of T(eff) determined from a particular spectrum. Hence, the true effective temperatures of the heavy element-rich DA white dwarfs, currently estimated to be above 55000 K, are apparently lower than previously reported from pure-H LTE analyses, by some 4000-7000 K. We do not see any similar influence on measurements of log g. This work concentrates on a group of relatively bright well-studied objects, for which heavy element abundances are known. As a consequence of this, establishment of correct temperatures for all other hot white dwarfs will require a programme of far-UV spectroscopy in order to obtain the essential compositional information. Since only stars with effective temperatures lying notionally in the range from approximate to 55000 to 70000 K (52000-62000 K when the non-LTE effects and heavy elements are taken into account) have been considered here, important questions remain regarding the magnitude of any similar effects in even hotter white dwarfs and pre-white dwarfs. The resulting implications for the plausibility of the evolutionary link between the main hot DA population and their proposed precursors, the II-rich central stars of planetary nebulae, need to be investigated.