Axisymmetric stability of neutron stars as extreme rotators in massive scalar-tensor theory

Abstract

Differentially rotating scalarized neutron stars, mimickers of binary merger remnants, can possess an enormous angular momentum larger than what could possibly be sustained in a neutron star in general relativity by about one order of magnitude. A natural question to ask is whether these solutions are stable and thus can realize in a binary coalescence. With this motivation in mind, we examine the criterion of dynamical stability against axisymmetric perturbations for these ultra-rotators by numerically tracking their nonlinear evolution in an axisymmetric setup. We demonstrate that the turning-point criterion still serves as a sufficient condition for asymmetric (in)stability. Our findings open an interesting question of whether the merger of two scalarized neutron stars can produce (possibly short-lived) ultra-highly rotating merger remnants.