White Dwarfs in Close Binaries: Gravitational Waves, X-rays and Type Ia Supernova Progenitors

Ashley Ruiter

White dwarfs in close binary systems play a crucial role in astrophysics. First, binaries consisting of two white dwarfs are predicted to be numerous in the Milky Way, and are a guaranteed source of gravitational waves for the future gravitational wave observatory LISA. Detailed observations of ~1000 double white dwarfs with the Laser Interferometer Space Antenna (LISA) will enable an unprecedented understanding of white dwarf physics and close binary evolution. Second, close binaries involving white dwarfs account for the vast majority of X-ray binary stars in the universe (e.g., magnetic cataclysmic variables and supersoft X-ray sources), a number of which are precursors to explosive events such as novae and possibly Type Ia supernovae (SNe Ia). Third, binary systems involving one (Single Degenerate) or two (Double Degenerate) white dwarfs are likely to be the progenitors of SNe Ia; the most important standard candles used on cosmological distance scales. Despite the great relevance of SNe Ia to cosmology and the chemical evolution of galaxies, the nature of the progenitor(s) of SNe Ia remains unknown.

It is found that:

1. The double white dwarf population of the Galaxy may produce a non-negligible confusion foreground noise for LISA due to the presence of mass-transferring white dwarf binaries. In contrast to previous studies, it is also found that the halo population of double white dwarfs will have no significant effect on the resulting gravitational wave foreground noise.

2. It is indeed possible to explain the faint, Chandra population of hard X-ray point sources in the Galactic center as being magnetic cataclysmic variables (Intermediate polars).

3. Within the current framework of the most updated population synthesis methods, it is found that only merging carbon-oxygen white dwarfs with a total mass exceeding 1.4 Msun are able to explain the observed rates of SN Ia explosions in Milky Way-like spiral galaxies, and are also the dominant evolutionary pathway leading to SNe Ia among old stellar populations.

With the binary evolution population synthesis code StarTrack, important questions pertaining to all three of these aforementioned areas of study are addressed in my dissertation work.