An efficient integrator for stellar dynamics in effective gravity fields based on the isochrone potential

Abstract: Context. Integrating the motion of stars in a smoothed potential is necessary in many stellar and galactic studies. Previous works have often used numerical integrators that alternate between linear drifts and velocity kicks (such as the Leapfrog scheme). This approach contrasts with the sophisticated methods developed in planetary dynamics, for which integrators alternate between Keplerian drifts and velocity kicks. Aims. Inspired by the splitting methods used in planetary dynamics, we aim to build an integration scheme dedicated to stellar and galactic dynamics. Methods. We took advantage of the properties of H\'enon's isochrone potential to design a symplectic splitting scheme that can be used to integrate the motion of stars in any gravitational potential. This scheme alternates between isochrone drifts and velocity kicks. As a first application, we consider the motion of a star in a Plummer potential -- an essential constituent of galactic potentials -- and determine integration parameters that provide the best efficiency (i.e. best conservation of energy for lowest computational cost). Results. We derive the analytical solution for all kinds of orbits in H\'enon's isochrone potential (bound and unbound trajectories) as needed in our integration scheme. Our experiments for stars in a Plummer potential show excellent performances in the inner and outer parts of the gravity field, that is, where the motion of stars is well approximated by isochrone trajectories (with perturbations of order $10^{-3}$ or less). For highly elongated orbits that cross the characteristic length of the Plummer potential, the performance is equivalent to that of previous methods. Conclusions. The splitting scheme presented here is a good alternative to previous methods: it performs at least as well, and up to orders of magnitude better, depending on the dynamical regime of the star.