Tidal tails of open star clusters as probes to early gas expulsion

Abstract

Aims. We study the formation and evolution of the tidal tail released from a young star Pleiades-like cluster, due to expulsion of primordial gas in a realistic gravitational field of the Galaxy. The tidal tails (as well as clusters) are integrated from their embedded phase for 300 Myr. We vary star formation efficiencies (SFEs) from 33% to 100% and the timescales of gas expulsion as free parameters, and provide predictions for the morphology and kinematics of the evolved tail for each of the models. The resulting tail properties are intended for comparison with anticipated Gaia observations in order to constrain the poorly understood early conditions during the gas phase and gas expulsion. Methods. The simulations are performed with the code NBODY6 including a realistic external gravitational potential of the Galaxy, and an analytical approximation for the natal gaseous potential. Results. Assuming that the Pleiades formed with rapid gas expulsion and an SFE of ≈30%, the current Pleiades are surrounded by a rich tail extending from ≈150 to ≈350 pc from the cluster and containing 0.7× to 2.7× the number of stars in the present-day cluster. If the Pleiades formed with an SFE close to 100%, then the tail is shorter (≲90 pc) and substantially poorer with only ≈0.02× the number of present-day cluster stars. If the Pleiades formed with an SFE of ≈30%, but the gas expulsion was adiabatic, the tail signatures are indistinguishable from the case of the model with 100% SFE. The mass function of the tail stars is close to that of the canonical mass function for the clusters including primordial gas, but it is slightly depleted of stars more massive than ≈1 M⊙ for the cluster with 100% SFE, a difference that is not likely to be observed. The model takes into account the estimated contamination due to the field stars and the Hyades-Pleiades stream, which constitutes a more limiting factor than the accuracy of the Gaia measurements.