Metastability in spontaneously scalarized neutron stars

Spontaneous scalarization is a scenario in scalar-tensor theories where certain neutron stars can be endowed with stable scalar clouds. The general relativity (GR) solutions are still valid in this case, but some of them become unstable due to a tachyonic degree of freedom. Thus, one expects to astrophysically observe the scalarized objects rather than their GR counterparts whenever the instability is incited. We will present exceptions to this general picture, where some GR solutions with no instability have a scalarized counterpart with the same stellar mass. If we observe a star at such a mass value, will it be scalarized or not? We show that the scalarized configurations are favored in terms of the gravitational binding energy, but we also expect the GR solutions to be potentially viable. That is, the GR solution would prefer to mutate into its scalarized version if there is a large enough perturbation, but it can also live on practically indefinitely unless there is a violent astrophysical event. We quantify how common such metastable GR solutions are. We also discuss the observational signatures of a violent transition from the GR solution to the scalarized one if that indeed occurs.