Tidal Tails of Nearby Open Clusters. II. A Review of Simulated Properties and the Reliability of Observational Catalogues

TL;DR

This study uses N-body simulations to establish baseline properties of tidal tails around open clusters and to critically assess the reliability of observed tails in Gaia data. By matching simulated tails to a literature survey of 122 catalogues for 58 nearby clusters, the authors develop a six-flag diagnostic framework, yielding 15 gold and 55 silver catalogues as reliable samples, with 51 bronze catalogues treated cautiously. They reveal a characteristic S-shaped near-tail, mass-dependent tail amplitudes, and a linear growth in tail span, along with features like epicyclic overdensities and 1D density variations, while highlighting biases and incompleteness in current detections. The work provides a practical, model-informed path to leverage tidal tails as tracers of cluster dissolution and the Galactic potential, and emphasizes the need for upcoming wide-area spectro-photometric surveys to improve validation and discovery.

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 & 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. Methods: We used a grid of 68-20000 Msun simulated clusters & analysed the formation & evolution of the tails. We compiled 122 catalogues (58 unique clusters) from literature, within 500 pc of the Sun. We employed tests based on photometric, morphological & dynamical signatures, & comparisons with simulations, to grade their tails. Results: Based on the simulations, we analysed the complex morphology of the tails & their properties (e.g., size, span, stellar types, number density, & mass function) at various cluster masses & ages. During the first 200 Myr of evolution, the tails typically form a characteristic S shape, with an amplitude that scales with cluster mass. The tail span increases at a rate of 4 times the initial velocity dispersion, & the near-tail is predominantly populated by recent escapees. Conclusions: In the 122 published tail catalogues, we found that 15 gold & 55 silver catalogues passed the majority of the tests. The remaining 51 were graded as bronze; care should be taken before using these for further analysis. The age, metallicity, binary fraction, & mass function of stars in the tails were generally consistent with those of their parent clusters. The gold/silver catalogues (69 catalogues of 40 clusters) represent reliable samples for detailed analyses of tails. Future data will be essential for further validation & for leveraging tidal tails as tracers of cluster dissolution & the Galactic potential.

Figures (46)

• Figure 1: Evolution of tidal tail morphology. The top left (blue) models: M20000 (on circular orbit). The bottom left (green) models: M20000_e (on eccentric orbit). The right (purple) models: Grid of M20000, M4641, M1000, M464, and M100 clusters on circular orbits. The different columns show the clusters at the age written above each panel. For the 1857 and 4381 Myr models, the whole simulation (middle right), a zoomed-in section of the cluster (middle left), and arbitrary zoomed-in sections along the leading (top right) and trailing (bottom right) tail are shown to compare the position of the tidal tail and the orbit at large distances. The cluster orbit (orange) and the Galactic centre position (black) are shown for reference. All clusters are rotated and placed on the X-axis for easier comparison.
• Figure 2: Plots regarding the amplitude of the S shape (a--b), average mass along the tail (c), and kinematics of the tail (d--f). (a) $r_{galactocentric}$ histogram for the M20000, M4641 and M1000 models at an age of 500 Myr. The leading (solid lines) and trailing (dashed lines) are shown separately. (b) The distribution of $distance\_along\_orbit$ with $r_{galactocentric}$ for the same models in (a). The central 2$R_{tidal}$ region is omitted while plotting the distributions in (a) and (b). (c) Average stellar mass as a function of distance_along_orbit in 200 pc bins for the models M20000, M4641 and M1000 (at an age of 1000 Myr). The shaded regions indicate 5--95 percentile values of the stellar masses. The dashed lines show the average mass of all stars in each simulation. (d--f) The variation of $distance\_along\_orbit$ with $v_{galactocentric}$ (d) total specific energy (e) and the time since escape (f) for M20000, M4641, and M1778 for their oldest snapshot. The orange dashed lines in panel (f) represent stars moving with a speed of 1.5 pc Myr$^{-1}$ (= 1.47 km s$^{-1}$) away from the cluster. The top row shows the results for M20000, M4641, and M1778. The bottom row shows the results for the M20000_e model.
• Figure 3: Evolution of tail and cluster parameters for various models. (a) The tidal radius (solid curve) and half mass radius (dotted curve). (b) Number of stars in the tail normalised by the total number of stars at the beginning. (c) Number of stars in the cluster. (d) Mass of the tail normalised by the total mass at the beginning. (e) Mass of the cluster. (f) Number of averaged escapees per Myr. (g) Span of the leading (solid curves) and trailing (dashed curves) tail. (h) Variation of the rate of increase in the total tail span ($span_{total}$) with the initial velocity dispersion ($\sigma_{velocity, 0}$) of the clusters, coloured according to the initial mass ($M_0$). See Figure \ref{['fig:number_escapee_span_appendix']} for comparison with the analytical formulae.
• Figure 4: Evolution of the MF for the cluster, near-tail and far-tail for the M20000 (top row), M4641 (middle row) and M1000 (bottom row) models. (a) Evolution of the MF of the cluster. The colours represent the cluster age (blue: younger, red: older). The solid lines show the MFs of the non-degenerate (ignoring white dwarfs, neutron stars, and black holes) population. (b) Evolution of the MF within the observationally detectable near-tail region. The non-degenerate MF is shown using dash-dotted lines. (c) Evolution of the MF in the far-tails region. The non-degenerate MF is shown using dashed lines. (d) Normalised (scaled by the total number of stars) MF of the cluster (solid lines) and near-tail (dash-dotted lines). The dotted lines show the MFs of degenerate stars in (a)--(c) panels. Grey lines with the Salpeter1955ApJ...121..161S IMF slope ($\Gamma=-1.35$) are shown as for reference.
• Figure 5: Linear number density (N/pc) within the cluster tails as calculated by binning $distance\_along\_orbit$ every 10 pc. (a) Frequency of the number density in all bins coloured according to the cluster age. The frequency of empty bins is shown by crosses at the left end of the plot. (b) The number density for M20000 (solid lines) and M4641 (dotted lines) at various ages. (c) Evolution of the tail density in M20000 as a function of $distance\_along\_orbit$ (as a multiple of $R_{tidal}$). (d) Density variation in M20000 as a function of the $distance\_along\_orbit$ (in pc). The grey colour denotes the region with $<3$ particles per bin. The $\nu_{relax}$, $R_{1/2}$, $R_{tidal}$, $N_{1/2}$, and $N_{tidal}$ are indicated for reference. The 20, 40, and 60 $R_{tidal}$ are marked by grey triangles as visual guides near overdensities. The first, second, third, and fourth rows show the results for M20000, M4641, M2154, and M20000_e models, respectively.