Publication date: Aug 2013
Abstract:
We present results for models of neutron stars and strange stars
constructed using the Hartle-Thorne slow-rotation method with a wide
range of equations of state, focusing on the values obtained for the
angular momentum J and the quadrupole moment Q, when the gravitational
mass M and the rotational frequency Ω are specified. Building on
previous work, which showed surprising uniformity in the behaviour of
the moment of inertia for neutron-star models constructed with widely
different equations of state, we find similar uniformity for the
quadrupole moment. These two quantities, together with the mass, are
fundamental for determining the vacuum space-time outside neutron stars.
We study particularly the dimensionless combination of parameters
QM/J2 (using units for which c = G = 1). This quantity goes
to 1 in the case of a Kerr-metric black hole and deviations away from 1
then characterize the difference between neutron-star and black hole
space-time. It is found that QM/J2 for both neutron stars and
strange stars decreases with increasing mass, for a given equation of
state, reaching a value of around 2 (or even less) for maximum-mass
models, meaning that their external space-time is then not very far from
that of the Kerr metric. If QM/J2 is plotted against R/2M
(where R is the radius), it is found that the relationship is nearly
unique for neutron-star models, independent of the equation of state,
while it is significantly different for strange stars. This gives a new
way of possibly distinguishing between them.
Authors:
Urbanec, M.; Miller, J. C.; Stuchlík, Z.;