Neutron star transition to strong-scalar-field state in tensor scalar gravity

Abstract

Spherical neutron star models are studied within tensor-scalar theories of gravity. Particularly, it is shown that, under some conditions on the second derivative of the coupling function and on star's mass, for a given star there exist two strong-scalar-field solutions as well as the usual weak-field one. This last solution happens to be unstable and a star, becoming massive enough to allow for all three solutions, evolves to reach one of the strong field configurations. This transition is dynamically computed and it appears that the star radiates away the difference in energy between both states (a few $10^{-3} M_\odot c^2$) as gravitational radiation. Since part of the energy ($\sim 10^{-5} M_\odot c^2$) is injected into the star as kinetic energy, the velocity of star's surface can reach up to $10^{-2} c$. The waveform of this monopolar radiation is shown as well as the oscillations undergone by the star. These oscillations are also studied within the slowly-rotating approximation, in order to estimate an order of magnitude of the resulting quadrupolar radiation.