Hyperon Effects on the Spin Parameter of Rotating Neutron Stars

Based on the equations of state from the relativistic mean field theory without and with the inclusion of strangeness-bearing hyperons, we study the dimensionless spin parameter j=cJ/(GM²) of uniformly rotating neutron stars. It is shown that the maximum value of the spin parameter j_max of a neutron star rotating at the Keplerian frequency f_K is j_max～0.7 when the star mass M >0.5 M_?, which is sustained for various versions of equations of state without and with hyperons. The relationship between j and the scaled rotation frequency f/f_K is found to be insensitive to the star mass or the adopted equation of state in the models without hyperons. However, the emergence of hyperons in neutron stars will lead to an uncertainty of the spin parameter j, which in turn could generate a complexity in the theoretical study of the quasi-periodic oscillations observed in disk-accreting compact-star systems.