Tidal tails of nearby open clusters

II. A review of simulated properties and the reliability of observational catalogues

¹ Helmholtz-Institut für Strahlen- und Kernphysik, Universität Bonn, Nussallee 14-16, 53115 Bonn, Germany
² Astronomical Institute, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, 180 00 Praha 8, Czech Republic

^★ Corresponding authors: vikrant-vinayak.jadhav@matfyz.cuni.cz; pkroupa@uni-bonn.de

Received: 6 June 2025
Accepted: 20 August 2025

Abstract

Context. Recent studies using Gaia data have reported tidal tail detections for tens to hundreds of open clusters. However, a comprehensive assessment of the reliability and completeness of these detections is lacking.

Aims. This work aims to summarise the expected properties of tidal tails based on N-body simulations, review the reliability of tidal tail detections in the literature, and grade them according to a set of diagnostic tests. We also provide an overview of the general characteristics of tidal tails available in the literature.

Methods. We used a grid of 68–20 000 M_☉ simulated clusters and analysed the formation and evolution of the tidal tails. We compiled 122 catalogues from the recent literature, encompassing 58 unique clusters within 500 pc of the Sun. We employed various tests based on photometric, morphological, and dynamical signatures and comparisons with simulated clusters to grade the tidal tails as gold, silver, and bronze. One of the primary tests was to measure apparent torsion in the Galactocentric XY plane.

Results. Based on the simulations, we analysed the complex morphology of the tidal tails and their properties (such as their size, span, stellar types, number density, and mass function) at various cluster masses and ages. During the first 100−200 Myr of evolution, the tails typically form a characteristic Ƨ shape, with an amplitude that scales with cluster mass. The tail span increases at a rate of ≈ 4 times the initial velocity dispersion, and the near-tail (within 100 pc of the cluster) is predominantly populated by recent escapees. In evaluating 122 published tidal tail catalogues, we found that 15 gold-quality catalogues and 55 silver-quality catalogues passed the majority of the tests. The remaining 51 catalogues were graded as bronze; care should be taken before using these catalogues for further analysis. The age, metallicity, binary fraction, and mass function of stars in the tails were generally consistent with those of their parent clusters.

Conclusions. The simulations presented here provide first-order approximations of the structure and evolution of the tidal tails. The gold and silver-grade catalogues (69 catalogues of 40 clusters) represent reliable samples for detailed analyses of tidal tails. Future spectroscopic and astrometric data from large-scale surveys will be essential for further validation and for leveraging tidal tails as tracers of cluster dissolution and the Galactic potential.