Spectroscopic Analysis of Double-Lined Double Degenerate Systems and the Interstellar Medium Towards White Dwarfs

Double degenerates in close binaries make interesting objects of research. Firstly, they are the products of common envelope evolution, which has yet to be observed explicitly. Secondly, their small orbital periods often mean that a merger will occur within a Hubble time. The product of such a merger depends on the system's mass, and, therefore, these double degenerate systems could be the progenitors of type Ia supernovae. To date, there are ~100 known white dwarf-white dwarf binaries with orbital periods below 1 day, as well as ~50 white dwarf-hot subdwarf binaries. However, there are only a handful of proposed double hot subdwarf/pre-white dwarf systems. The latter have more recently shed their common envelopes, and the particulars of their formation are still under discussion.

In the first two chapters of this thesis I undertake the first spectroscopic analysis of two such systems. The first contains binary central stars within a planetary nebula, whose spectra resemble those of hot subdwarf stars. These stars were recently reported to have a low orbital period and high system mass, making this system a supernova type Ia candidate. The second system is the first eclipsing double hot subdwarf system to be discovered. However, the analysis is limited in the first system by interstellar reddening, and in the second by the faintness of the stars. In the last chapter, I analyse the interstellar medium absorption lines present in the spectra of three white dwarf stars. These stars display highly ionised absorption lines not originating from the stellar photospheres. An analysis of the lines' profiles can reveal the temperature and turbulence of the source. However, this analysis is limited by the number of elements present in a given component.