The Metal Content of Hot DA White Dwarf Spectra

In this thesis, a study of the high ionisation-stage metal absorption features in the spectra of hot DA white dwarfs is presented. Metals are present in the photospheres of such stars due to radiative levitation (Chayer et al. 1994, 1995; Chayer Fontaine & Wesemael 1995). However, studies of the patterns between metal abundance and Teff show that, though the broad patterns predicted are seen, individual abundance measurements often do not reflect the predictions of radiative levitation theory (e.g. Barstow et al. 2003b). In this thesis, an analysis of the nitrogen abundance in three stars is performed, where a highly abundant layer of nitrogen was thought to reside at the top of the photospheres of the stars. The nitrogen abundance and distribution in these DAs is found to be homogeneous and of an abundance in keeping with stars of higher Teff. The accretion of metals from circumstellar discs has been shown to be the source of photospheric metals in DAs with Teff < 25,000 K (e.g. Zuckerman et al. 2003), where gravitational diffusion dominates (Koester & Wilken, 2006). In some cases, gaseous components are seen at such discs (e.g. SDSS 122859.93+104032.9; Gänsicke et al. 2006). A survey is made of a sample of hot (19,000 K < Teff < 51,000 K) DAs, where similar accretion may explain the inability of radiative levitation theory alone to account for the detected photospheric metal abundances. No circumstellar gas discs are found, though accretion from as yet undetected circumstellar sources remains an attractive explanation of the photospheric abundances of the stars. Circumstellar absorption is seen in the UV spectra of some hot DA stars (Holberg et al. 1998; Bannister et al. 2003). Sources suggested for this material include circumstellar discs, the ionisation of the ISM, stellar mass loss and planetary nebulae. A re-analysis of this absorption is presented, using a technique that for the first time allows proper modelling of the circumstellar absorption features, and provides column densities for all components. The ionisation of circumstellar discs or planetesimals, the ionisation of the ISM and the ionisation of mass lost by binary companions are put forward as the origin for this circumstellar material.