This thesis investigates two problems concerned with contact binary stars: the mechanism of energy transfer and the evolutionary states in the W UMa-type contact binary systems. The observational and theoretical background is first reviewed. This highlights the importance of a proper treatment of energy transfer if W UMa stars are to be understood. Expressions for the ratio of the transferred luminosity to the primary's radiated and centrally generated luminosities are derived. An estimate of the size of the transfer rate in observed unevolved systems is determined. The implications of accurate mass-ratios are briefly discussed. The generalized transport equation is shown to contradict the requirements of the TRO theory. A more general formula is put forward. The sideways convection model is investigated critically and is found to work only in very deep A-type systems. The implications of this result on contact binaries' structure, stability and evolution are discussed. A variety of different energy transfer models are explored. Equilibrium circulation and turbulent conduction models require moderately deep A-type systems. The small- and large-scale models can, respectively, transfer enough energy in the shallow and moderately shallow W-type systems. Webbink's ideas of energy transfer are used to construct numerical contact binary models which for the first time use an explicit mechanism to represent the energy transfer process. The added complications caused by the implementation of such a mechanism rendered the procedure used to build numerical models ineffective. An alternative procedure is briefly discussed. Hazlehurst's dissipation mechanism is simulated numerically. We argue on the basis of our results that this mechanism is unworkable in its present form. We attribute this to a fundamental error in the formulation of the model and sketch a possible reformulation of the model in terms of the theory of thermals. The consequences of strict hydrostatic equilibrium for the overall structure of contact binaries are discussed. It is shown that imposition of strict hydrostatic equilibrium in the envelope requires (1) the observed anomalous M-L relation hold and (2) the contact phenomenon be considered as a single (severely distorted) configuration. A model is proposed which deviates markedly from hydrostatic equilibrium just below the envelope, with accelerating flow through the L, -point and a shock in the secondary component. A substantial amount of energy can be transferred, with conditions in the envelope remaining very close to hydrostatic equilibrium. The model works independently of the overlying envelope structure. We approach the second of our topics by investigating the evolutionary states in 9 A-type systems. Depending on their observed properties the systems are divided into two distinct groups. Group I members are evolved. Group II members belong to a distinct, still problematic category. The W-type system TX Cnc in Praesepe may be a pre-main sequence contact binary system. The implications of these results on the transfer mechanism are discussed.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:543317 |
| Date | January 1988 |
| Creators | Sinjab, Issam Musleh |
| Publisher | University of Sussex |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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