  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 quasi-static configurations. Here, we
test the validity of chameleon screening in strong-field and highly
relativistic/dynamical conditions, by performing fully non-linear 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 comment on their detectability with future gravitational-wave
interferometers.
