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More on the structure of tidal tails


  • Andreas H. W. Küpper,

    Corresponding author
    1. Argelander Institut für Astronomie (AIfA), Auf dem Hügel 71, 53121 Bonn, Germany
    2. European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
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  • Richard R. Lane,

    Corresponding author
    1. Departamento de Astronomía, Universidad de Concepción, Casilla 160 C, Concepción, Chile
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  • Douglas C. Heggie

    Corresponding author
    1. School of Mathematics and Maxwell Institute for Mathematical Sciences, University of Edinburgh, King’s Buildings, Edinburgh EH9 3JZ
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E-mail: (AHWK); (RRL); (DCH)


We investigate the epicyclic motion of stars escaping from star clusters. Using streaklines, we visualize 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 xL= (GM/(Ω2−∂2Φ/∂R2))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.