The 800 pc long tidal tails of the Hyades star cluster

Abstract

The tidal tails of stellar clusters provide an important tool for studying the birth conditions of the clusters and their evolution, coupling, and interaction with the Galactic potential. The Gaia satellite, with its high-quality astrometric data, opened this field of study, allowing us to observe large-scale tidal tails. Theoretical models of tidal-tail formation and evolution are available. However, the exact appearance of tidal features as seen in the Gaia catalogue has not yet been studied. Here we present the N-body evolution of a Hyades-like stellar cluster with backward-integrated initial conditions on a realistic 3D orbit in the Milky Way galaxy computed within the AMUSE framework. For the first time, we explore the effect of the initial cluster rotation and the presence of lumps in the Galactic potential on the formation and evolution of tidal tails. For all of our simulations we present Gaia observables and derived parameters in the convergent point (CP) diagram. We show that the tidal tails are not naturally clustered in any coordinate system and that they can span up to 40 km s−1 relative to the cluster centre in proper motions for a cluster age of 600–700 Myr. Models with initial rotation result in significant differences in the cluster mass loss and follow different angular momentum time evolution. Thus the orientation of the tidal tails relative to the motion vector of the cluster and the current cluster angular momentum constrain the initial rotation of the cluster. We highlight the use of the standard CP method in searches for co-moving groups and introduce a new compact CP (CCP) method that accounts for internal kinematics based on an assumed model. Using the CCP method, we are able to recover candidate members of the Hyades tidal tails in the Gaia Data Release 2 and early Data Release 3 (eDR3) reaching a total extent of almost 1 kpc. We confirm the previously noted asymmetry in the detected tidal tails. In the eDR3 data we recovered spatial overdensities in the leading and trailing tails that are kinematically consistent with being epicyclic overdensities and thus would present candidates for the first such detection in an open star cluster. We show that the epicyclic overdensities are able to provide constraints not only on the cluster properties, but also on the Galactic potential. Finally, based on N-body simulations, a close encounter with a massive Galactic lump can explain the observed asymmetry in the tidal tails of the Hyades.