Dynamical chameleon neutron stars. Stability, radial oscillations, and scalar radiation in spherical symmetry

Scalar-tensor theories whose phenomenology differs significantly from general relativity on large (e.g., cosmological) scales do not typically pass local experimental tests (e.g., in the Solar System) unless they present a suitable "screening mechanism." An example is provided by chameleon screening, whereby the local general relativistic behavior is recovered in high-density environments, at least in weak-field and quasistatic configurations. Here, we test the validity of chameleon screening in strong-field and highly relativistic/dynamical conditions by performing fully nonlinear simulations of neutron stars subjected to initial perturbations that cause them to oscillate or even collapse to a black hole. We confirm that screened chameleon stars are stable to sufficiently small radial oscillations, but that the frequency spectrum of the latter shows deviations from the general relativistic predictions. We also calculate the scalar fluxes produced during collapse to a black hole, and we comment on their detectability with future gravitational-wave interferometers.