Far-ultraviolet spectroscopy of the hot DA white dwarf WD 2218+706 (DeHt5) with STIS

We report a study of the photospheric composition of the hot DA white dwarf WD 2218+706, which is also the central star of the old planetary nebula DeHt5. Helium is detected in the far-UV spectrum. In addition, the star clearly contains significant quantities of elements heavier than He at abundances generally a factor of 2 to 10 higher than those found in the archetypal heavy element-rich DA G191-B2B. This is the first detection of trace He using the He II gimel 1640 line in an isolated DA white dwarf, but the low surface gravity is more indicative of a binary evolution route from the red giant branch rather than a path along the asymptotic giant branch (AGB) as a single star. However, the absence of any evidence for a companion star and the uncertainty in the measured mass for WD 2218+706 still allow the possibility of an origin along an AGB evolutionary track. We reanalyse the existing optical spectra of WD 2218+706 using our latest pure H and heavy element-rich model atmospheres, obtaining a good match between the observed and synthetic spectra with either set of models. We find little evidence of any inconsistency in the temperature required to fit individual Balmer lines, as reported elsewhere for this star. Any discrepancies we see are confined to the Ha line and the core of HG but they do not compromise our analysis.