The spin parameter of uniformly rotating compact stars

TL;DR

This study investigates the maximum spin parameter of rotating neutron and quark stars, revealing a universal upper bound for neutron stars and variable bounds for quark stars, with implications for astrophysics.

Contribution

The paper provides the first numerical analysis of the maximum spin parameter for neutron and quark stars, highlighting a universal limit for neutron stars and variability for quark stars.

Findings

Maximum spin parameter for neutron stars is about 0.7.

Quark stars can have spin parameters exceeding unity.

The spin parameter upper bound for neutron stars is insensitive to mass above 1 solar mass.

Abstract

We study the dimensionless spin parameter $j (= c J/ (G M^2))$ of uniformly rotating neutron stars and quark stars in general relativity. We show numerically that the maximum value of the spin parameter of a neutron star rotating at the Keplerian frequency is $j_{\rm max} \sim 0.7$ for a wide class of realistic equations of state. This upper bound is insensitive to the mass of the neutron star if the mass of the star is larger than about $1 M_\odot$. On the other hand, the spin parameter of a quark star modeled by the MIT bag model can be larger than unity and does not have a universal upper bound. Its value also depends strongly on the bag constant and the mass of the star. Astrophysical implications of our finding will be discussed.