THE SPIN PARAMETER OF UNIFORMLY ROTATING COMPACT STARS

We study the dimensionless spin parameter j (= cJ/(GM(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(max) similar to 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 1M(circle dot). 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.