Oblate & Prolate Neutron Stars

Neutron stars are the ultra-dense collapsed cores of massive stars. A typical neutron star measures only several kilometres across, but can pack more mass than the Sun. Neutron stars are generally assumed to be perfectly spherical. However, the presence of extremely strong magnetic fields and/or anisotropic pressure gradients in their cores can deform neutron stars. The deformation can be either oblate or prolate. An oblate object bulges around the equator, whereas a prolate object is elongated along the polar axis.

Figure 1: Artist’s impression of a neutron star.

The oblate or prolate shape of a neutron star is characterised by the deformation parameter “γ”, where γ = 1 denotes a perfect sphere. An oblate neutron star has γ < 1 and a prolate neutron star has γ > 1. Basically, the maximum mass of a neutron star is how massive a neutron star can get before it becomes too massive and collapses into a black hole. The ability to deform results in a range of maximum masses a neutron star can have.

The maximum mass of a neutron star increases with increasing oblateness, but decreases with increasing prolateness. Assuming a perfectly spherical neutron star has a maximum mass of 2.3 times the Sun’s mass. If instead the neutron star has oblateness γ = 0.8, its maximum mass increases to 3.02 times the Sun’s mass. On the contrary, if the neutron star has prolateness γ = 1.2, its maximum mass decreases to 1.81 times the Sun’s mass. 

Figure 2: An oblate neutron star with γ = 0.8 and a maximum mass of 3.02 times the Sun’s mass. Weber et al. (2015).

Figure 3: A prolate neutron star with γ = 1.2 and a maximum mass of 1.81 times the Sun’s mass. Weber et al. (2015).

Reference:
Weber et al. (2015), “Non-Spherical Models of Neutron Stars”, arXiv:1504.03006 [astro-ph.SR]