Scalar gravitational waves from relativistic stars in scalar-tensor gravity

Unlike general relativity, scalar gravitational waves can be excited due to the radial oscillations in scalar-tensor gravity. To examine scalar gravitational waves in scalar-tensor gravity, we derive the evolution equations of the radial oscillations of neutron stars and determine the specific oscillation frequencies of the matter oscillations and scalar gravitational waves, where we adopt two different numerical approaches, i.e., the mode analysis and direct time evolution. As a result, we observe the spontaneous scalarization even in the radial oscillations. Depending on the background scalar field and coupling constant, the total energy radiated by the scalar gravitational waves dramatically changes, where the specific oscillation frequencies are completely the same as the matter oscillations. That is, via the direct observations of scalar gravitational waves, one cannot only reveal the gravitational theory but also extract the radial oscillations of neutron stars.