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 one-dimensional studies of spherical clusters of main-sequence stars. The two-dimensional 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 individual stars and a bulk stellar bar perturbation in the system's gravitational potential. The 3 Myr main-sequence lifetime for large stars provides an upper limit on the allowed simulation times. Two general classes of initial systems are studied: Plummer spheres, which represent stellar clusters, and "γ = 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 main-sequence 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 (IMF) is found to play a crucial role. When using an IMF similar to that observed for dense stellar clusters (weighted towards high masses but with a high-mass cutoff of [special characters omitted]) the simulations presented here show, in all cases, that the stellar system forms massive [special characters omitted] objects by collisional mergers of lower-mass stars; 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.
Possible extensions of the work presented here include continuing to track stellar populations after they evolve off the main sequence, and allowing for a (possibly changing) nonspherical component to the overall system potential.
|Advisor:||Field, George B.|
|School Location:||United States -- Massachusetts|
|Source:||DAI-B 71/07, Dissertation Abstracts International|
|Keywords:||Bar perturbation, Black holes, Fokker-Planck method, Quasars, Stellar mergers|
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