Eccentric von Zeipel–Lidov–Kozai effects under mildly hierarchical triple systems: influence of Brown corrections upon orbit flipping

ABSTRACT Mildly hierarchical three-body systems are widespread in the Universe, exemplified by planets in stellar binaries and stars in black-hole binaries. In such systems, Brown Hamiltonian corrections play a crucial role in governing the long-term dynamical evolution. In this work, we extend Brown corrections to include octupole-order coupling terms, thereby formulating a more accurate dynamical model for predicting long-term dynamical behaviours. The utilization of the gauge freedom in canonical transformation shows that the quadrupole–octupole coupling term vanishes and the octupole–octupole coupling term is axisymmetric. Under triple systems with different levels of hierarchies, we systematically investigate the impact of Brown corrections on orbital flipping induced by the eccentric von Zeipel–Lidov–Kozai mechanism. Our analysis reveals that, as the hierarchy of triple systems becomes lower, the asymmetry in the flipping regions becomes more significant. The asymmetric structures are examined in detail using Poincaré sections and perturbative techniques, showing that Brown corrections are the key factor responsible for breaking the symmetry of flipping regions. Finally, we extend the classical pendulum approximation to our refined model and demonstrate that its analytical predictions agree remarkably well with those derived from perturbative methods, particularly in the high-eccentricity regime.