Computer Science
Scientific paper
Jan 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996phdt.........4s&link_type=abstract
Thesis (PH.D.)--THE UNIVERSITY OF WISCONSIN - MILWAUKEE, 1996.Source: Dissertation Abstracts International, Volume: 57-04, Sect
Computer Science
9
Neutron Stars
Scientific paper
We conduct the first direct comparison of codes based on two different numerical methods for constructing rapidly rotating relativistic stars and find good agreement between corresponding models. For representative equations of state we construct 2-dimensional surfaces of equilibrium configurations in the 3-dimensional space of angular momentum, mass and central density. The maximum mass, baryon mass, angular momentum and angular velocity configurations are shown to be, generically, different, while for some equations of state all four maximum stable configurations coincide. We investigate the upper limit on rotation of relativistic stars set by causality and compute the minimum period as a function of the maximum observed mass. No gravitationally bound star can rotate faster than 0.29(M max/1.442rm M_odot) ms. The equation of state yielding the minimum period set by causality consists of a high-density region at the causal limit, which is matched to the known low density equation of state by an intermediate constant pressure region. This results in stars with stiff enough cores to sustain a certain mass and the softest possible exterior to allow for the fastest rotation. Finally, we study nonaxisymmetric perturbations of rotating relativistic stars. The neutral (zero-frequency), fundamental l=m modes are computed for several polytropic equations of state in the context of general relativity. These modes participate in setting the upper limit on the rotation of rapidly rotating neutron stars (created by the accretion-induced collapse of white dwarfs) by making the stars unstable to the emission of gravitational waves. We find that general relativity causes the nonaxisymmetric instability to set in at a significantly lower rotation rate than Newtonian theory suggests. Most strikingly, the m = 2 bar mode can become unstable even for soft polytropes of index N<=q1.3, while in Newtonian theory it becomes unstable only for stiff polytropes of index N<=q0.808. Thus, the classical result for the critical adiabatic index Gammacrit=2.237 for the onset of the m=2 instability in Newtonian theory, becomes Gammacrit<=q1.77 in the context of general relativity.
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