Abstract

As star clusters are expected to form with low star formation efficiencies, the gas in the cluster is expelled quickly and early in their development: the star cluster 'pops'. This leads to an unbound stellar system, evolving in the Galactic potential. Previous N-body simulations have demonstrated the existence of a stepped number density distribution of cluster stars after popping, both in vertical position and vertical velocity, with a passing resemblance to a Christmas tree. Using numerical and analytical methods, we investigate the source of this structure, which arises due to the phase mixing of the out-of-equilibrium stellar system, determined entirely by the background analytic potential. Considering only the vertical motions, we construct a theoretical model to describe the time evolution of the phase space distribution of stars in a Miyamoto-Nagai disc potential and a full Milky Way-type potential comprising bulge, halo and disc components, which is then compared with N-body simulations. Using our theoretical model, we investigate the possible observational signatures and the feasibility of detection.