Equipartition and Mass Segregation

Simulations of star clusters with two mass components


The dynamical evolution of an isolated self-gravitating system is studied under the aspects of mass segregation and equipartition processes. We analyse the idealized case of a cluster with two distinct mass components. The individual ratio of the heavy to light bodies, \mu = m_2/m_1, is varied from 1.25 to 50.0 and the fraction of the total heavy mass q is altered from 5% to 40% of the whole cluster. We also examine the cluster properties like the core collapse, the evolution of the central potential, the radial stratification of masses as well as escapers. We present new, high-accuracy collisional N-body simulations, using the high-order integrator NBODY6++. We modelled up to N = 20,000 particles on the parallel supercomputer CRAY T3E, and the statistical significancy of the lower-N simulations was improved by ensemble averages. Our objective is to check the various theories on early star formation and dynamical evolution. We find significant deviations of the evolutionary time scale in the regime \mu —> 1. Equipartition slows down the gravothermal contraction of the core slightly. Beyond a critical value of \mu ~ 2, no equipartition can be achieved between the different masses, a phenomenon known as the Spitzer equipartition instability; the heavy component decouples and collapses under the influence of the gravothermal instability of its own subsystem. For the first time the critical boundary between Spitzer-stable and -unstable systems is demonstrated in direct N-body models.

Border of Equipartition Stability

Parameter space of the models examined in this work. The position of the Roman letters indicate a model determined by its q and mu. The lines are the borders for equipartition stability as proposed by various theories (Fig. 8.1).

A very short summary of notable results.
Refereed Journal Article in the ADS abstract service (2007).
Preliminary Results in the ASP Conference Series (2001).
Complete thesis (9 MB, pdf) at the University of Heidelberg (2003).
Introduction to the Dynamics of Stellar Clusters at popular science (2001).

Last modified: 2015, Apr 18