Collapse and evolution of flattened star clusters

We investigate the dynamical evolution of point‐mass systems starting from spheroidal geometry. Such structures may arise as a result of violent compression which results from collisions between clouds of interstellar gas. We use N ‐body calculations to seek out traces of the initial conditions in their relaxed structures. For initially cold and thick, or thin but hot, systems, the phase of collapse is well described by adiabatic scaling: we show that the axis ratio of stellar clusters grows ∝ R ‐1/3 during infall, where R is the cluster cylindrical radius. The adiabatic track provides an adequate description of the collapse, provided only that the initial scaleheight of the spheroid, h , is well resolved by the stars: when the mean interparticle distance is larger than h , we find that two‐body effects develop rapidly over a single crossing time. Following the phase of collapse, the clusters `bounce' and quickly establish a core‐‐halo structure. The central region relaxes to near‐spherical shape in a few crossing times; however, the envelope remains more elliptical, since two‐body effects are reduced there. This ellipticity changes only slightly from its value at the time of the bounce over several crossing times of evolution. As cluster morphology at the bounce is deduced from adiabatic invariance, itself fixed by the initial conditions, we relate relaxed configurations to initial conditions through a simple analytic expression.