Self-Gravitating Magnetic Monopoles and Dyons in String-Inspired Models: Structure and Stability

Abstract

We present classical magnetic--monopole (MM) and dyon solutions induced by global monopoles in string-inspired gravitational models. Although several ingredients of the construction have appeared previously, we examine a new synthesis via their organised use in a self-gravitating finite-energy monopole/dyon system with explicit mass, charge, core and stability diagnostics. The global monopole provides the topological seed, while the dilaton, Kalb--Ramond (KR) axion and Born--Infeld (BI) electromagnetic (EM) sector supply the string-inspired dressing. Two related branches are analysed: (i) a dilaton--BI magnetic branch, distinct from earlier constructions, and (ii) a dyonic extension in which the KR axion couples electric and magnetic sectors through an axion-Pontryagin-anomaly interaction. In both cases BI dynamics regularises the EM self-energy within a self-gravitating global-monopole construction, and the solutions possess a de Sitter core, a positive ADM-mass parameter range and satisfy the standard energy conditions. A further novel feature is a limiting branch in which the ADM mass tends to zero while the magnetic charge remains finite, the configuration being supported by magnetic charge and de Sitter core pressure.We also examine stability diagnostics. The stress--energy tensor gives finite force components and an outward-pointing radial force, indicating absence of core collapse within the mechanical stability criterion. In the exterior analytic region we analyse gauge-invariant EM perturbations using the Gervalle-Volkov framework. The helicity modes $\ddot{I}{ˆ}_{\pm }$ obey a self-adjoint Sturm--Liouville system. MM helicities decouple, while dyons exhibit axion-induced helicity mixing and birefringent polarisation structure. Positivity of the EM helicity block provides a necessary long-range stability diagnostic; the full coupled Einstein-matter spectral problem is discussed

34 pages revtex, 7 pdf figures incorporated; modified abstract to highlight novel aspects of solutions; added discussion on linear stability, identifying criteria for decoupling of electromagnetic-sector perturbations from those in the other sectors, so current study suffices as a stability diagnostic to leading order; addition of Appendix B, sketching linear-stability analysis of the full system