Models of rapidly rotating neutron stars

Abstract

Interest in rapidly rotating neutron stars has been reinforced by the discovery of the fast pulsar PSR1937 + 214 (ref. 1). Here we report results of the first numerical construction of rapidly rotating relativistic stars based on equations of state (EOS) proposed for neutron-star matter. Of nine EOS considered, none permits uniformly rotating equilibrium configurations for which the ratio T/W of rotational energy to gravitational energy exceeds 0.12. Thus whereas a rotating neutron star spun up by accretion or formed by collapse of a rapidly rotating dwarf can be unstable^2–7 to modes with angular dependence e^imΦ for m = 3 or 4, instability to a bar mode (m = 2) appears very unlikely. For the stiffest EOS and for stellar masses near the Chandrasekhar mass (baryon mass M₀ ≈ 1.4 M_⊙), the upper limiting rotational frequencies imposed by the m = 3 or m = 4 instability are approximately equal to the frequency of the fast pulsar; for the softer EOS, the corresponding limiting frequencies are significantly larger. Uniform rotation increases the maximum gravitational mass of a model by an amount ranging from ∼14% for the softest models to ∼30% for the stiffest models.