Frozen-composition effects on rotational failure of neutron-star crusts

Abstract

The elastic response of a neutron-star crust to rotational loading depends on whether weak interactions can restore chemical equilibrium during the deformation. We quantify this effect using unified equations of state and compare chemically equilibrated and frozen-composition responses within the same Newtonian hydro-elastic framework. Frozen composition increases the effective bulk modulus and redistributes the strain throughout the inner crust rather than producing a simple global rescaling. For BSk24, the breaking frequency decreases from $0.4997\Omega_K$ to $0.3912\Omega_K$, a reduction of 21.7 per cent, while the location of first failure moves to higher density. Comparable reductions are obtained for BSk21, whereas SLy4 gives a smaller effect of 16.7 per cent. For BSk24 the relative reduction remains close to 21--22 per cent over the mass range $1.2$--$2.0,M_\odot$. A density-resolved perturbation analysis shows that the result is controlled by the radial overlap between the microphysical frozen-composition stiffening and the mechanical susceptibility of the crust, with most of the linear sensitivity in the BSk models arising from $0.01<n_B<0.03,{\rm fm}^{-3}$. These results show that chemical non-equilibrium can substantially modify both the rotational failure threshold and the layer in which crustal failure begins.