Constraints on the equation of state from the stability condition of neutron stars

The stellar equilibrium and collapse, including mainly white dwarfs, neutron stars and super massive stars, is an interplay between general relativistic effects and the equation of state of nuclear matter. In the present work, we use the Chandrasekhar criterion of stellar instability by employing a large number of realistic equations of state (EoSs) of neutron star matter. We mainly focus on the critical point of transition from stable to unstable configuration. This point corresponds to the maximum neutron star mass configuration. We calculate, in each case, the resulting compactness parameter, beta = GM/c(2)R, and the corresponding effective adiabatic index, gamma(cr). We find that there is a modelindependent relation between gamma(cr) and beta. This statement is strongly supported by the large number of EoSs, and it is also corroborated by using analytical solutions of the Einstein field equations. In addition, we present and discuss the relation between the maximum rotation rate and the adiabatic index close to the instability limit. Accurate observational measurements of the upper bound of the neutron star mass and the corresponding radius, in correlation with present predictions, may help to impose constraints on the high density part of the neutron star equation of state.