The orbital period distribution of cataclysmic variables from the Sloan Digital Sky Survey (SDSS)

Doctoral thesis English OPEN
Dillon, Monihar
  • Subject: QB
    arxiv: Astrophysics::Galaxy Astrophysics | Astrophysics::Cosmology and Extragalactic Astrophysics | Astrophysics::High Energy Astrophysical Phenomena | Astrophysics::Solar and Stellar Astrophysics

Over the years, substantial effort has gone into calculating models of the intrinsic population of galactic Cataclysmic Variables (CVs). However, comparison with the observed properties of the known CV sample has consistently failed to match the predictions put forward by the ‘disrupted magnetic braking’ model of CV evolution. Some of the discrepancies have been blamed on the heterogeneous set of known CVs, which are not well-suited for a quantitative test of the population models.\ud \ud The Sloan Digital Sky Survey (SDSS) has dramatically improved the observational side of CV population studies. Sampling a large volume in ugriz colour space and extending deeper than any previous large-scale survey, SDSS provides the most homogeneous and complete sample of CVs to date.\ud \ud At the time of writing, the sample of SDSS CVs contains 213 systems, of which 177 are new discoveries. Establishing the detailed properties of these CV is a major task. The work presented in this thesis is part of a larger effort to derive the orbital period distribution of SDSS CV sample.\ud \ud In this thesis, I present my contribution and results of time-series photometric and spectroscopic observations of a total of 29 SDSS CVs. I provide a discussion of their properties and determine orbital periods for 20 systems. I also provide a quantitative analysis of the new sample of CVs from SDSS and compare their intrinsic qualities with the previously known population of CVs.\ud \ud The results show that the period distribution of the SDSS CVs differs from that of the previously known CVs at a 3-σ level. A substantially larger fraction of below-the-gap to above the-gap systems is observed, accompanied by a prominent accumulation of CVs at the orbital period minimum. This result is important, as the accumulation of systems at short orbital periods has been long theorised but never proved observationally. More specifically, the origin of the 80–86min period spike is entirely due to the new CVs identified in SDSS. The systems in the period spike also differ in spectral morphology and accretion activity from the longer-period CVs. Indeed ∼ 20% of the CVs identified by SDSS have white-dwarf dominated spectra indicating low mass transfer rates and late spectral type donors, the majority of which have been found close to the period minimum.\ud \ud A homogeneously selected sample of CVs, with well defined parameters has been long overdue and the results from SDSS are very promising. With this improved observational evidence, theoretical models can be modified to re-address the discrepancies in the current theories that have long plagued our understanding of CV evolution.
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