The complex kinematics of rotating star clusters in a tidal field

TL;DR

This paper explores the complex three-dimensional kinematic evolution of rotating star clusters in tidal fields, revealing features like precession, nutation, and counter-rotation that inform observational interpretations.

Contribution

It relaxes traditional assumptions on initial conditions and demonstrates how internal dynamics and external tidal interactions produce complex kinematic features.

Findings

Development of precession and nutation in rotation axes

Radial variation leading to counter-rotation in outer regions

Non-monotonic ellipticity and isodensity contour twisting

Abstract

We broaden the investigation of the dynamical properties of tidally perturbed, rotating star clusters by relaxing the traditional assumptions of coplanarity, alignment, and synchronicity between the internal and orbital angular velocity vector of their initial conditions. We show that the interplay between the internal evolution of these systems and their interaction with the external tidal field naturally leads to the development of a number of evolutionary features in their three-dimensional velocity space, including a precession and nutation of the global rotation axis and a variation of its orientation with the distance from the cluster centre. In some cases, such a radial variation may manifest itself as a counter-rotation of the outermost regions relative to the inner ones. The projected morphology of these systems is characterized by a non-monotonic ellipticity profile and, depending on the initial inclination of the rotation axis, it may also show a twisting of the projected isodensity contours. These results provide guidance in the identification of non-trivial features which may emerge in upcoming investigations of star cluster kinematics and a dynamical framework to understand some of the complexities already hinted by recent observational studies.