THE EFFECT OF PHOTOSPHERIC HEAVY-ELEMENTS ON THE HOT DA WHITE-DWARF TEMPERATURE SCALE

Using the latest non-local thermodynamic equilibrium (non-LTE) synthet ic spectra and stellar model calculations, we have evaluated the poten tial effect of the presence of heavy elements in the photospheres of h ot H-rich DA white dwarfs. In particular, we have examined their influ ence on the effective temperature and surface gravity perceived from a nalysis of the Balmer line profiles. It is apparent that both the incl usion of non-LTE effects in the models and significant quantities of h eavy 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 abo ve 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 relati vely bright well-studied objects, for which heavy element abundances a re known. As a consequence of this, establishment of correct temperatu res for all other hot white dwarfs will require a programme of far-UV spectroscopy in order to obtain the essential compositional informatio n. Since only stars with effective temperatures lying notionally in th e range from approximate to 55000 to 70000 K (52000-62000 K when the n on-LTE effects and heavy elements are taken into account) have been co nsidered here, important questions remain regarding the magnitude of a ny similar effects in even hotter white dwarfs and pre-white dwarfs. T he resulting implications for the plausibility of the evolutionary lin k between the main hot DA population and their proposed precursors, th e II-rich central stars of planetary nebulae, need to be investigated.