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

TL;DR

This paper investigates the stability of ultra-rotating, scalarized neutron stars in massive scalar-tensor theory, demonstrating that the turning-point criterion remains a sufficient condition for their axisymmetric stability through numerical evolution.

Contribution

It provides the first numerical analysis of the dynamical stability of these extreme neutron star solutions in scalar-tensor gravity.

Findings

Turning-point criterion remains a sufficient condition for stability.

Scalarized neutron stars can possess angular momentum an order of magnitude higher than in GR.

Potential formation of ultra-highly rotating remnants in neutron star mergers.

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.