A key factor to the spin parameter of uniformly rotating compact stars: crust structure

We study the dimensionless spin parameter j ≡ cJ/(GM 2 ) of different kinds of uniformly rotating compact stars, including traditional neutron stars, hyperonic neutron stars and hybrid stars, based on relativistic mean field theory and the MIT bag model. It is found that j max ~ 0.7, which had been suggested in traditional neutron stars, is sustained for hyperonic neutron stars and hybrid stars with M > 0.5 M ⊙ . Not the interior but rather the crust structure of the stars is a key factor to determine j max for three kinds of selected compact stars. Furthermore, a universal formula j = 0.63(f /f K ) − 0.42(f /f K ) 2 + 0.48(f /f K ) 3 is suggested to determine the spin parameter at any rotational frequency f smaller than the Keplerian frequency f K .