UV and optical studies of the physical properties of DAO white dwarfs
An investigation into the physical properties and evolutionary status of DAO white dwarfs has been conducted, using optical and far-UV observations. Out of the 22 objects for which optical data were obtained, the spectrum of only one is best fitted by a stratified H+He composition model; this may be an object transferring between the He- and H-rich cooling sequences.;Homogeneous models are preferred for the remainder, but with 90% confidence in less than half the objects. The DAOs fall into two groups: The low Teff, high log g objects have temperatures and gravities that are similar to the majority of the DAs and mostly consist of DAO+dM binaries. The high T eff, low log g objects, which make up approximately two thirds of the sample, have relatively high luminosities for white dwarfs, and the most likely mechanism that is countering the gravitational settling for these objects is mass loss. When this ceases these objects will become low-mass DAs.;A search for evidence of binarity in the far-UV FUSE observations of 22 objects yielded positive results for only 5, suggesting that the majority of DAOs are descended from isolated stars. As seen in the DAs, the Teff derived from Balmer and Lyman lines for objects hotter than ∼50,000 K were found to be different, with the latter yielding the higher temperature. For 7 objects, the Lyman lines are so shallow that a model with Teff > 120,000 K is required to reproduce them. An investigation of the strength of photospheric absorption lines showed that the low log g objects have high heavy element abundances compared to DAs, whereas the DAO+dM binaries have slightly elevated lighter element abundances, with the heavier elements abundances similar to those seen in the DAs. This is qualitatively consistent with mass loss and accretion from a companions wind respectively, as the explanation for the helium that is observed in the spectra of the stars.