Actions of highly eccentric orbits

TL;DR

This paper discusses methods for computing actions of highly eccentric orbits in axisymmetric potentials, distinguishing between box and loop orbits, and introduces algorithms and modifications to improve calculations.

Contribution

It presents new algorithms for determining critical actions and addresses challenges in computing orbit actions near critical values using the Staeckel Fudge.

Findings

Identified a critical value I_{3crit}(E) for orbit classification.

Developed algorithms to determine I_{3crit}(E) and Jzcrit.

Proposed modifications to the Staeckel Fudge to improve accuracy.

Abstract

The challenge presented by computing actions for eccentric orbits in axisymmetric potentials is discussed. In the limit of vanishing angular momentum about the potential's symmetry axis, there is a clean distinction between box and loop orbits. We show that this distinction persists into the regime of non-zero angular momentum. In the case of a Staeckel potential, there is a critical value I_{3crit}(E) of the third integral I_3 below which I_3 does not contribute to the centrifugal barrier. An orbit is of box or loop type according as its value of I_3 is smaller or greater than I_{3crit}. We give algorithms for determining I_{3crit}(E) and the critical action Jzcrit below which orbits in any given potential are boxes. It is hard to compute the actions and especially the frequencies of orbits that have Jz ~ Jzcrit using the Staeckel Fudge. A modification of the Fudge that alleviates the problem is described.