Research On Sound Speed Of Dense Matter And Properties Of Neutron Stars

The internal structure and macroscopic properties of neutron stars(including the observable mass M and radius R)are strongly correlated with the equation of state(EOS)of dense matter,however,the EOS remains exceedingly uncertain especially at high densities due to the lack of cognition on their microphysical interaction.Those make it difficult to directly study the properties of the neutron stars through the information of the EOS.To another aspect,the upper limit of sound speed,as a crucial quantity to describe the stiffness and incompressibility of EOS for dense matter,has significant influence on neutron-star maximum mass.Based on the astronomical observations,it could impose further constraints on EOSs.In fact,the research on sound speed to understand the nature of the neutron star is always a hot topic in both nuclear theory and astrophysics related compact stars.In this paper,we partly modify the EOS and explore the corresponding effect on the stellar properties,especially on the radius of the typical neutron star(1.4M_?)and the maximum stellar mass of NS sequence for a given EOS.We also investigate the dependence of the mass-radius curve and its slope(dM/dR)on the EOS.It is found that the slope is mainly determined by the EOS above the saturation nuclear density.To determine the lowest quantity for the sound-speed limit,we consider the observational restriction on maximum mass(2.01±0.04M_?).By employing a set of heterogeneous EOSs and through a constructed EOS model to describe the high-density segments,we conclude that the upper limit of sound speed can be as low as 0.538c(where c is the speed of light).Furthermore,in order to eliminate the effect of the model dependence for the EOSs,we adopt a set of parameterized EOSs to inspect the validity,and as a result,a lower bound 0.574c of the upper limit of sound speed is obtained.In addition,we semi-empirically analyze the characteristics of the profile of neutron star M-R relation and find out that the profiles of M-R relations mainly depend on the sound speed of the central density of neutron star sequences in the mass range around 0.3-1M_?.