The nature of orbits in a galactic model with a triaxial halo

Abstract

In the present work, we have used a galactic model with a triaxial dark matter halo, a disk and a spherical central nucleus in order to examine the nature of orbits of the stars moving near the disk plane. We study in detail the proportion of the chaotic and regular motion due to each contribution of the potential, as well as the dynamical effcts of the prolate and oblate haloes on the stability of trajectories. We apply the classical method of Poincar e phase plane, in order to distinguish between stable and chaotic orbits. The Lyapunov exponents are used to estimate quantitatively the chaos. Our results indicate that the presence of a triaxial dark matter halo increases the chaotic motion, being the fraction of unstable orbits larger for oblate models. The thickness of the disk introduces stochasticity to the trajectories, producing vertical instability. The mass of the central nucleus crucially affects the nature of orbits, as the proportion of ordered trajectories shrinks very rapidly with increasing central mass.