Scalar field induced oscillations of relativistic stars and gravitational collapse

We study the interaction of massless scalar fields with relativistic stars by means of fully dynamic numerical simulations of the Einstein-Klein-Gordon perfect fluid system. Our investigation is restricted to spherical symmetry and the stars are approximated by relativistic polytropes. Studying the nonlinear dynamics of isolated compact objects is very effectively performed within the characteristic formulation of general relativity, in which the spacetime is foliated by a family of outgoing light cones. We are able to compactify the entire spacetime on a computational grid and simultaneously impose natural radiative boundary conditions and extract accurate radiative signals. We study the transfer of energy from the scalar field to the fluid star. We find, in particular, that depending on the compactness of the stellar model, the scalar wave forces the star either to oscillate in its radial modes of pulsation or to undergo gravitational collapse to a black hole on a dynamical time scale. The radiative signal, read off at future null infinity, shows quasinormal oscillations before the setting of a late time power-law tail.