Hyperon Effects on the Spin Parameter of Rotating Neutron Stars**Supported by the National Natural Science Foundation of China under Grant Nos 11175108, U1432119, 1146114100, 11205075, 11375076 and 11475104, the Shandong Natural Science Foundation under Grant No ZR2014AQ012, and the Foundation of Shandong University under Grant No 2015WHWLJH01.

Abstract

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 jmax of a neutron star rotating at the Keplerian frequency fK is jmax ∼ 0.7 when the star mass M > 0.5M⊙, which is sustained for various versions of equations of state without and with hyperons. The relationship between j and the scaled rotation frequency f/fK 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.