White dwarfs in close binaries: Gravitational waves, X-rays and type Ia supernova progenitors

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 Laser Interferometer Space Antenna ( LISA ) gravitational wave observatory. Detailed observations of ~1000 double white dwarfs with 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 chemical evolution of galaxies, the nature of the progenitor(s) of SNe Ia remains unknown. 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.

It is found that (i) 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 found that the halo population of double white dwarfs will have no significant effect on the resulting gravitational wave foreground. (ii) It is indeed possible to explain the faint, hard X-ray population of point sources in the Galactic center (observed with Chandra ) as being magnetic cataclysmic variables (Intermediate polars). (iii) Within the current framework of the most updated population synthesis methods, only the rates of merging carbon-oxygen white dwarfs (with M tot > 1.4 [Special characters omitted.] ) are able to match the observed rates of SN Ia explosions in Milky Way-like galaxies. A detailed overview of these astrophysical questions and the implications of the results will be discussed in this dissertation.