Magnetic monopole solutions in a modified Einstein-Yang-Mills-Higgs system

We study the Yang-Mills-Higgs system within the framework of general relativity with an unconventional coupling of the scalar field to gravity. In the static situation, using a Bogomol'nyi-type analysis, we derive a positive-definite energy functional with a lower bound that is attained when the Bogomolnyi conditions are satisfied. Specializing to the gauge group SU(2) and the 't Hooft--Polyakov ansatz for the gauge and Higgs fields, we seek static, spherically symmetric solutions to the coupled system of equations together with Bogomol'nyi conditions. In both the isotropic and standard coordinate systems, in the spontaneously broken symmetry situation, we find great simplifications reducing the solutions of the coupled system to the solution of a single nonlinear differential equation, different one in each case, but well known in other contexts of physics. We find Abelian and non-Abelian monopole solutions with gravitational fields playing the role of Higgs fields in providing attraction that balances the repulsion due to the gague fields. These solutions in general have naked singularities at the origin. But as solutions we also find extreme Reissner-Nordstr\"om black holes as well as a new non-Abelian monopole solution that has a horizon enclosing the singularity.