Dilaton gravity, (quasi-) black holes, and scalar charge

Abstract

We consider static electrically charged dust configurations in the framework of Einstein-Maxwell-dilaton gravity with the interaction term P(\chi) F_{mn} F^{mn} in the Lagrangian, where P(\chi) is an arbitrary function of the dilaton field \chi, and the latter is allowed to be normal or phantom. It is shown that, for any regular P(\chi), static configurations are possible with arbitrary functions g_{00} = e^{2\gamma(x^i)} (i=1,2,3) and \chi = \chi(\gamma), without any assumption of spatial symmetry. The corresponding matter, electric charge and scalar charge densities are found from the field equations. Meanwhile, configurations with nontrivial \chi(x^i) generically require a nonzero scalar charge density distribution. The classical Majumdar-Papapetrou (MP) system is obtained as a special case where \chi = const; there is its scalar analogue in the case F_{mn} = 0, but only with a phantom \chi field. Among possible solutions are black-hole (BH) and quasi-black-hole (QBH) ones. Some general results on QBH properties obtained previously for the MP system are here extended to systems with the dilaton. Particular examples of asymptotically flat spherically symmetric BH and QBH solutions are found, some of them being phantom-free, that is, exist with positive energy densities of matter and both scalar and electromagnetic fields.