Browsing by Author "Ngubelanga, Sifiso Allan."

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Exact solutions for relativistic models.
(2011) Ngubelanga, Sifiso Allan.; Maharaj, Sunil Dutt.; Ray, Subharthi.
In this thesis we study spherically symmetric spacetimes related to the Einstein field equations. We consider only neutral matter and apply the Einstein field equations with isotropic pressures. Our object is to model relativistic stellar systems. We express the Einstein field equations and the condition of pressure isotropy in terms of Schwarzschild coordinates and isotropic coordinates. For Schwarzschild coordinates we consider the transformations due to Buchdahl (1959), Durgapal and Bannerji (1983), Fodor (2000) and Tewari and Pant (2010). The condition of pressure isotropy is integrated and new exact solutions of the field equations are obtained utilizing the transformations of Buchdahl (1959) and Tewari and Pant (2010). These exact solutions are given in terms of elementary functions. For isotropic coordinates we can express the condition of pressure isotropy as a Riccati equation or a linear equation. An algorithm is developed that produces a new solution if a particular solution is known. The transformations reduce to a nonlinear Bernoulli equation in most instances. There are fundamentally three new classes of solutions to the condition of pressure isotropy.
Models in isotropic coordinates with equation of state.
(2014) Ngubelanga, Sifiso Allan.; Maharaj, Sunil Dutt.; Ray, Subharthi.
In this thesis we consider spacetimes which are static and spherically symmetric related to the Einstein and Einstein-Maxwell system of equations in isotropic coordinates. We study both neutral and charged matter distributions with isotropic and anisotropic pressures, respectively. Our aim is to model relativistic stellar models. A known transformation that has been utilised by other researchers is applied to rewrite the field equations in equivalent forms. We produce new models to the Einstein system of equations with isotropic pressures by developing an algorithm that generates new classes of exact solutions if a particular seed solution is known. By applying the algorithm to the field equations and the condition of pressure isotropy we obtain a nonlinear Bernoulli equation which can be integrated. We also consider charged matter distributions with anisotropic pressures by introducing barotropic equations of state. Both linear and quadratic equations of state are considered and new exact solutions of the Einstein-Maxwell system are found. This is achieved by specifying a particular form for one of the metric functions and the electric field intensity. We select particular parameter values to regain the masses of known stars. For the linear equation of state we regain masses of the stars PSR J1614-2230, Vela X-1, PSR J1903+327, 4U 1820-30 and SAX J1808.4-3658. The masses for the stars PSR J1614-2230, 4U 1608-52, PSR J1903+327, EXO 1745-248 and SAX J1808.4-3658 are generated when a quadratic equation of state is imposed. Extensive physical analyses for the stars PSR J1614-2230 and PSR J1903+327 indicate that our models are well behaved.