The mass, temperature and distance of the white dwarf in V471 Tauri

We present an analysis of far-UV observations of V471 Tauri made during the first ORFEUS mission. Combining this spectrum with archival data from IUE allows a first unambiguous measurement of the effective temperature and surface gravity of the white dwarf by matching the observed Lyman line profiles to the predictions of stellar model atmosphere calculations (T-eff = 32 400 +/- 270/-800 K and log g = 8.16 + 0.18/-0.24). However, a mass estimate of the white dwarf based on the Bois et al. mass function provides a more precise value for log g (8.27 +/- 0.07). Using the accurate Hipparcos parallax for the system, from which we obtain a distance of 47 +/- 4 pc, we are able to obtain an independent measurement of the white dwarf radius and, therefore, its mass. Comparison with the theoretical mass-radius relation of Wood, for a star at this temperature, confirms that these evolutionary calculations are a good representation of the white dwarf structure. This lends confidence to spectroscopic mass determinations and estimates of the age of the galactic disc, inferred from the white dwarf luminosity function and cooling ages, based on these models. The white dwarf, now confirmed by the Hipparcos parailax to be a member of the Hyades cluster, is clearly the hottest and possibly the most massive and youngest white dwarf in that cluster. Finally, weak absorption features in the ORFEUS data hint at the possible presence of nitrogen and iron in the photosphere of the white dwarf.