Generation of scalar-tensor gravity effects in equilibrium state boson stars

Boson stars in zero-, one- and two-node equilibrium states are modelled numerically within the framework of scalar-tensor gravity. The complex scalar field is taken to be both massive and self-interacting. Configurations are formed in the case of a linear gravitational scalar coupling (the Brans-Dicke case) and a quadratic coupling which has been used previously in a cosmological context. The coupling parameters and asymptotic value for the gravitational scalar field are chosen so that the known observational constraints on scalar-tensor gravity are satisfied. It is found that the constraints are so restrictive that the field equations of general relativity and scalar-tensor gravity yield virtually identical solutions. We then use catastrophe theory to determine the dynamically stable configurations. It is found that the maximum mass allowed for a stable state in scalar-tensor gravity in the present cosmological era is essentially unchanged from that of general relativity. We also construct boson star configurations appropriate to earlier cosmological eras and find that the maximum mass for stable states is smaller than that predicted by general relativity, and the more so for earlier eras. However, our results also show that if the cosmological era is early enough then only states with positive binding energy can be constructed.