A Fokker-Planck Study of Dense Rotating Stellar Clusters

TL;DR

This study uses a 2D Fokker-Planck model to simulate dense rotating stellar clusters, exploring conditions for supermassive star formation and black hole seeds through stellar collisions, mass segregation, and angular momentum transport.

Contribution

It extends previous 1D models to 2D to include rotation, bar perturbations, and realistic initial mass functions, providing new insights into massive object formation in stellar systems.

Findings

Massive stars form via collisions with a top-heavy IMF.

Stellar bars facilitate core collapse and rapid massive object formation.

Lower-mass IMFs do not produce massive objects under typical conditions.

Abstract

The dynamical evolution of dense stellar systems is simulated using a two-dimensional Fokker-Planck method, with the goal of providing a model for the formation of supermassive stars which could serve as seed objects for the supermassive black holes of quasars. This work follows and expands on earlier 1-D studies of spherical clusters of main-sequence stars. The 2-D approach allows for the study of rotating systems, as would be expected due to cosmological tidal torquing; other physical effects included are collisional mergers of stars and a bulk stellar bar perturbation in the gravitational potential. The 3 Myr main-sequence lifetime for large stars provides an upper limit on simulation times. Two general classes of initial systems are studied: Plummer spheres, which represent stellar clusters, and \gamma=0 spheres, which model galactic spheroids. At the initial densities of the modeled systems, mass segregation and runaway stellar collisions alone are insufficient to induce core collapse within the lifetime limit if no bar perturbation is included. However, core collapse is not a requirement for the formation of a massive object: the choice of stellar initial mass function is found to play a crucial role. When using an IMF similar to that observed for dense stellar clusters the simulations show that the stellar system forms massive (250M_\odot) objects by collisional mergers; in almost all such cases the presence of a stellar bar allows for sufficient additional outward transport of angular momentum that a core-collapse state is reached with corresponding further increase in the rate of formation of massive objects. In contrast, simulations using an IMF similar to that observed for field stars in general (which is weighted more towards lower masses) produce no massive objects, and reach core collapse only for initial models which represent the highest-density galactic spheriods.