Ultraviolet spectra of DA white dwarfs

Using the International Ultraviolet Explorer (IUE) satellite, observations of cool to moderately warm DA white dwarfs showed the presence of broad absorption features at lambda 1400 and 1600. The lambda 1600 feature is prominent for T/sub eff/ < 13,0000K and the lambda 1400 feature is visible up to about T/sub eff/ = 18,0000K. Proposed mechanisms for absorption at lambda 1400 have included a Si V doublet, H2 Lyman bands, and the Call ionization edge at lambda 1420. It was recently suggested that the lambda 1600 feature is due to the photoionization edge of Mg I at lambda 1625. None of these has been able to explain all of the observations without invoking some quite unconventional circumstances. On the basis of computer models, this thesis proposes that the absorption in both cases is due to perturbations of the energy level structure of neutral hydrogen atoms undergoing collisions. The lambda 1600 feature is due to absorption by the hydrogen quasi-molecule while that at lambda 1400 arises from a ground state transition of the hydrogen quasi-molecule ion