Element abundances in cool white dwarfs. II. Ultraviolet observations of DZ white dwarfs

摘要

We present a small data base of homogeneously derived photospheric elementabundances of DZ white dwarfs and related objects. Our previous investigationsare supplemented with the analysis of ultraviolet spectra for nine whitedwarfs. Of particular interest is the detection of Lalpha absorption in vanMaanen 2 and a determination of the effective temperature of this star. The newvalue is about 1000K lower than previous results due to the strong ultravioletabsorption by metals which has to be considered consistently. The metalabundances of our sample stars are compatible with the predictions from thetwo-phase accretion model of Dupuis et al. (1992, 1993). Small deviations canbe observed for the abundance ratios in some objects. This could indicatenon-solar metal-to-metal ratios in the accreted material. Hydrogen can bedetected in virtually all of our objects. However, its average accretion ratemust be at least two orders of magnitude lower than the metal accretion rate.