Heavy element abundances in DAO white dwarfs measured from FUSE data

We present heavy element abundance measurements for 16 DAO white dwarfs, determined from <i>Far-Ultraviolet Spectroscopic Explorer</i> (<i>FUSE</i>) spectra. Evidence of absorption by heavy elements was found in the spectra of all the objects. Measurements were made using models that adopted the temperatures, gravities and helium abundances determined from both optical and <i>FUSE</i> data by Good et al. It was found that, when using the values for the parameters measured from optical data, the carbon abundance measurements follow and extend a similar trend of increasing abundance with temperature for DA white dwarfs, as discovered by Barstow et al. However, when the <i>FUSE</i> measurements are used, the DAO abundances no longer join this trend since the temperatures are higher than the optical measures. Silicon abundances were found to increase with temperature, but no similar trend was identified in the nitrogen, oxygen, iron or nickel abundances, and no dependence on gravity or helium abundances was noted. However, the models were not able to reproduce the observed silicon and iron line strengths satisfactorily in the spectra of half of the objects, and the oxygen features of all but three. Despite the different evolutionary paths that the types of DAO white dwarfs are thought to evolve through, their abundances were not found to vary significantly, apart from for the silicon abundances. Abundances measured when the <i>FUSE</i>-derived values of temperature, gravity and helium abundance were adopted were, in general, a factor 1–10 higher than those determined when the optical measure of those parameters was used. Satisfactory fits to the absorption lines were achieved in an approximately equal number. The models that used the <i>FUSE</i>-determined parameters seemed better at reproducing the strength of the nitrogen and iron lines, while for oxygen the optical parameters were better. For the three objects whose temperature measured from <i>FUSE</i> data exceeds 120 000 K, the carbon, nitrogen and oxygen lines were too weak in the models that used the <i>FUSE</i> parameters. However, the model that used the optical parameters also did not reproduce the strength of all the lines accurately.