Quadrupole moments of rotating neutron stars and strange stars
Abstract
We present results for models of neutron stars and strange stars constructed using the HartleThorne slowrotation 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 neutronstar 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 spacetime outside neutron stars. We study particularly the dimensionless combination of parameters QM/J^{2} (using units for which c = G = 1). This quantity goes to 1 in the case of a Kerrmetric black hole and deviations away from 1 then characterize the difference between neutronstar and black hole spacetime. It is found that QM/J^{2} 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 maximummass models, meaning that their external spacetime is then not very far from that of the Kerr metric. If QM/J^{2} is plotted against R/2M (where R is the radius), it is found that the relationship is nearly unique for neutronstar 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.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 August 2013
 DOI:
 10.1093/mnras/stt858
 arXiv:
 arXiv:1301.5925
 Bibcode:
 2013MNRAS.433.1903U
 Keywords:

 stars: neutron;
 stars: rotation;
 Astrophysics  Solar and Stellar Astrophysics;
 Nuclear Theory
 EPrint:
 11 pages, 6 figures. Submitted to MNRAS