Proca stars with nonminimal coupling to the Einstein tensor

Abstract

We investigate Proca star (PS) solutions, namely boson star (BS) type solutions for a complex vector field with mass and nonminimal coupling to the Einstein tensor. Irrespective of the existence of nonminimal coupling, PS solutions are mini-BS type, but the inclusion of it changes properties. For positive nonminimal coupling parameters, PS solutions do not exist for central amplitudes above some certain value due to the singular behavior of the evolution equations. For negative nonminimal coupling parameters, there is no such singular behavior but sufficiently enhanced numerical resolutions are requested for larger amplitudes. Irrespective of the sign of the nonzero nonminimal coupling parameter, PSs with the maximal Arnowitt-Deser-Misner mass and Noether charge are gravitationally bound. Properties of PSs are very similar to those of BSs in scalar-tensor theories including healthy higher-derivative terms.