Abstrakt
Context. Core collapse is a prominent evolutionary stage of self-gravitating systems.
In an idealised collisionless approximation, the region around the cluster core evolves in a self-similar way prior to the core collapse. Thus, its radial density profile outside the core can be described by a power law, p alpha r(-alpha).
Aims. We aim to find the characteristics of core collapse in N-body models.
In such systems, a complete collapse is prevented by transferring the binding energy of the cluster to binary stars. The contraction is, therefore, more difficult to identify.
Methods. We developed a method that identifies the core collapse in N-body models of star clusters based on the assumption of their homologous evolution.
Results. We analysed different models (equal- and multi-mass), most of which exhibit patterns of homologous evolution, yet with significantly different values of alpha : the equal-mass models have alpha approximate to 2.3, which agrees with theoretical expectations, the multi-mass models have alpha approximate to 1.5 (yet with larger uncertainty).
Furthermore, most models usually show sequences of separated homologous collapses with similar properties. Finally, we investigated a correlation between the time of core collapse and the time of formation of the first hard binary star.
The binding energy of such a binary usually depends on the depth of the collapse in which it forms, for example from 100 kT to 10(4 )kT in the smallest equal-mass to the largest multi-mass model, respectively. However, not all major hardenings of binaries happened during the core collapse.
In the multi-mass models, we see large transfers of binding energy of similar to 10(4) kT to binaries that occur on the crossing timescale and outside of the periods of the homologous collapses.