Neutron star instabilities in full general relativity using a Γ=2.75 ideal fluid

We present results about the effect of the use of a stiffer equation of state, namely the ideal-fluid Γ=2.75 one, on the dynamical bar-mode instability in rapidly rotating polytropic models of neutron stars in full general relativity. We determine the change on the critical value of the instability parameter β for the emergence of the instability when the adiabatic index Γ is changed from 2 to 2.75 in order to mimic the behavior of a realistic equation of state. In particular, we show that the threshold for the onset of the bar-mode instability is reduced by this change in the stiffness and give a precise quantification of the change in value of the critical parameter βc. We also extend the analysis to lower values of β and show that low-beta shear instabilities are present also in the case of matter described by a simple polytropic equation of state.