N. Spyrou
Articles written in Journal of Astrophysics and Astronomy
Volume 8 Issue 1 March 1987 pp 1-16
Stellar orbits in elliptical galaxies with relativistically active nuclei
In a model galaxy composed of a relativistically active nucleus, a main body, and a halo, all three components considered as homogeneous prolate ellipsoids, we explore the probable association of the internal characteristics of the nucleus and the observed orbits of the stars near the surface of the main body. Using the authors’ theoretical framework of post-Newtonian general relativistic galactic dynamics, proposed earlier, we prove that a fast-rotating and possibly expanding or contracting nucleus affects the distribution of the box-type orbits near the surface of the main body resulting in a flattening of the main body. The nuclear rotation always results in a flattening, and the contraction contributes less to the flattening than the expansion. However, the contributions of a rotating and changing nucleus are not additive. The study of the post-Newtonian effects in the nucleus on the stellar orbits in the main body, and the consequent modifications of the corresponding non-relativistic results, could in principle provide useful information concerning the kinematical and dynamical characteristics of the nuclei of the elliptical galaxies. The explanation (of at least the post-Newtonian part) of the flattening of elliptical galaxies attempted here seems to be the first theoretical one proposed in the literature.
Volume 9 Issue 1 March 1988 pp 25-39
Accretion effects on compact members of binary stars
We explore the change in the period of axial rotation and in the radius of a magnetized compact star in a binary system, induced by the accretion on it of mass with angular momentum from the surface of its non-compact companion. No specific assumption is made concerning the accretion model, and the primary’s interior is described by the Fermi-Dirac statistics for degenerate matter. The rate of change with time of the period and radius is expressed in terms of the compact primary’s physical parameters and total absolute luminosity. The conditions are fully derived under which the above changes can be positive, negative or even vanish. In the case of the millisecond pulsars in binary X-ray sources the predicted values of the period time derivative, depending on the values of the accretion rate and. the absolute luminosity, can be positive or negative—if not vanishing—and they fall absolutely in the range 10^{−21} −10^{−17} ss^{−1}, in good agreement with current observational data. The corresponding rate of change of radius, either positive or negative, fall in the range of 10^{−3} −10^{−1} cm y^{−1}. Finally, it is proved that the well-known bursters can be explained by thermonuclear flash due to gravitational instability in the accreted matter, but their explanation as a result of direct contraction could be possible only for quite high accretion rates (>10^{−7}
Volume 13 Issue 4 December 1992 pp 327-347
Physical identification of static and axially-symmetric vacuum metric tensors
We present a method of a possible physical identification of the static and axially symmetric Weyl-type vacuum γ and nγ metrics. This method, in which no interior solutions of any kind are involved, is based on the comparison of the far-field forms of the γ and nγ metrics, and of the far-field form of the metric tensor due to a bounded gravitating perfect-fluid source given correctly to post-Newtonian accuracy. The parameters of the vacuum solutions are expressed in terms of physical parameters of a prolate fluid source, namely total mass-energy, semiaxes and eccentricity, defined consistently to post-Newtonian accuracy. The results, based on the otherwise arbitrary fluid source, appear physically general. Possible astrophysical candidates for the far-field γ and
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