Dynamics of Dense Stellar Systems Including the Effects of Stellar Collisions

Abstract

Dynamical evolution of dense stellar systems is followed by integrating Fokker-Planck equation including successive mergers between stars. We have assumed that all the tidal captures lead to mergers. The initial cluster is assumed to be a single component Plummer model with mass of individual stars being 0.7 M⊙. The highest mass that is allowed to form through successive mergers in our model is 32 × 0.7M⊙ =22.4 M⊙. The stellar evolution is simulated by removing stars from the cluster assuming that the stellar material escapes as the star finishes the evolution. We have estimated the number of stars that evolve off per unit time using “evolution function” that depends on the mean age and lifetime. The mean ages are estimated from the rates of formation of stars (through merger). While the formation of massive stars leads to the acceleration of core collapse through mass segregation, the indirect heating effect due to stellar evolution makes the core collapse slow. The net effect depends on the initial conditions. The number of high mass stars depends sensitively on the cluster parameters. Core collapse is usually found to be terminated by the indirect heating effect of evolution of moderate mass stars (mainly 2.8 or 5.6 M⊙ in our models). The maximum number stars in highest mass bin (i.e., M=22.4 M⊙) varies from unity to an order of 101 depending on exact initial conditions for the models appropriate for the Galactic center. These numbers are too small to explain the observed HeI emission line stars which are interpreted as envelope stripped high mass (20~ 40 M⊙) stars. However, if the ejected material from the final stage of stellar evolution can form stars efficiently instead of escaping from the potential well of the stellar system, more high mass stars can be found. The presence of massive black hole in the center would also boost the merger rates.