More on the structure of tidal tails

TL;DR

This paper explores the dynamics of stars escaping from star clusters, revealing how epicyclic motion shapes tidal tail structures and identifying key radii influencing star escape, with implications for interpreting observed substructures.

Contribution

It introduces a method using streaklines adjusted to N-body simulations to accurately reproduce tidal tail substructures and clarifies the roles of tidal and edge radii in star escape processes.

Findings

Epicyclic motion causes over- and underdensities in tidal tails.

Adjusted streaklines can replicate observed substructures.

Edge radius is larger than the tidal radius and correlates with King radius.

Abstract

We investigate the epicyclic motion of stars escaping from star clusters. Using streaklines, we visualise the path of escaping stars and show how epicyclic motion leads to over- and underdensities in tidal tails of star clusters moving on circular and eccentric orbits about a galaxy. Additionally, we investigate the effect of the cluster mass on the tidal tails, by showing that their structure is better matched when the perturbing effect of the cluster mass is included. By adjusting streaklines to results of N-body computations we can accurately and quickly reproduce all observed substructure, especially the streaky features often found in simulations which may be interpreted in observations as multiple tidal tails. Hence, we can rule out tidal shocks as the origin of such substructures. Finally, from the adjusted streakline parameters we can verify that for the star clusters we studied escape mainly happens from the tidal radius of the cluster, given by x_L = (GM/(\Omega^2-\partial^2\Phi/\partial R^2))^{1/3}. We find, however, that there is another limiting radius, the "edge" radius, which gives the smallest radius from which a star can escape during one cluster orbit about the galaxy. For eccentric cluster orbits the edge radius shrinks with increasing orbital eccentricity (for fixed apocentric distance) but is always significantly larger than the respective perigalactic tidal radius. In fact, the edge radii of the clusters we investigated, which are extended and tidally filling, agree well with their (fitted) King radii, which may indicate a fundamental connection between these two quantities.