Cataclysmic variables (CVs) are short–period (' 80 min to ' day) binaries in which a white dwarf accretes from a low–mass companion. Since both stellar components are structurally simple and there is a sufficiently large number of relatively bright CVs known, it is possible to carry out detailed observational population studies. Therefore CVs are one of the best–suited classes to test, constrain, and further develop our understanding of the evolution of all interacting compact binaries, such as black hole binaries, X-ray transients, milli–second pulsars and, more importantly, Type Ia Supernova (SN Ia), our yardsticks for measuring distances on cosmological distance scales. In this thesis, I present the study of a large sample of CV white dwarfs. Their effective temperatures are determined from the analysis of their ultraviolet Hubble Space Telescope (HST ) spectra and provide a test for the current models of CV evolution. Our results highlight the presence of a number of discrepancies between current population models and observations, particularly the lack of period bounce systems, i.e. highly evolved CVs with brown dwarf companions, which are predicted to make up for ' 40 . 70 % of the present day Galactic CV population. The combination of the HST data with optical phase–resolved X–shooter observations is a powerful tool to identify period bouncers. We study the spectral energy distribution of QZ Lib, in which we identify the coolest white dwarf at the longest orbital period below the period gap and we spectroscopically confirm the presence of a brown dwarf donor. These characteristics make QZ Lib the strongest period bouncer candidate identified so far. Although successful, this method requires the use of expensive space–based and large ground–based facilities, indicating the need for a more efficient observing strategy. We present here CHiCaS, the Compact binary HIgh CAdence Survey which represents the first systematic attempt to identify, via the detection of their eclipses, the elusive population of period bouncers. By the end of next year, this program will deliver one minute cadence lightcurves for ' 2:5 million objects as faint as g ' 21:5, along with full colour information. CHiCaS will also provide a complete and unbiased view into the short term variability of thousands of binaries, eclipsing systems, pulsating stars and CVs in the period gap. In our HST sample, we identify another remarkable CV: SDSS J123813.73–033932.9. Its lightcurve shows sudden increases in brightness, up to ' 0:45 mag, occurring quasi–periodically every 8–12 hours (the “brightenings”) and a nearly sinusoidal variation at half the orbital period (the “double–humps”). The HST observations show that these phenomena arise from the heating and cooling of a fraction of the white dwarf, which we conclude being caused by a modulation in the accretion rate owing to spiral density waves and thermal instabilities in the accretion disc.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:742280 |
| Date | January 2018 |
| Creators | Pala, Anna Francesca |
| Publisher | University of Warwick |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://wrap.warwick.ac.uk/102613/ |
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