Dynamics and evolution of dense stellar systems

Abstract

The research presented in this thesis comprises a theoretical study of several aspects relating to the dynamics and evolution of dense stellar systems such as globular clusters. First, I present the results of a study of mass segregation in two-component star clusters, based on a large number of numerical N-body simulations using our Monte-Carlo code. Heavy objects, which could represent stellar remnants such as neutron stars or black holes, exhibit behavior that is in quantitative agreement with simple analytical arguments. Light objects, which could represent free-floating planets or brown dwarfs, are predominantly lost from the cluster, as expected from simple analytical arguments, but may remain in the halo in larger numbers than expected. Using a recent null detection of planetary-mass microlensing events in M22, I find an upper limit of 25% at the 63% confidence level for the current mass fraction of M22 in the form of very low-mass objects. Turning to more realistic clusters, I present a study of the evolution of clusters containing primordial binaries, based on an enhanced version of the Monte-Carlo code that treats binary interactions via cross sections and analytical prescriptions. All models exhibit a long-lived "binary burning" phase lasting many tens of relaxation times. The structural parameters of the models during this phase match well those of most observed Galactic globular clusters. At the end of this phase, clusters that have survived tidal disruption undergo deep core collapse, followed by gravothermal oscillations.

(cont.) The results clearly show that the presence of even a small fraction of binaries in a cluster is sufficient to support the core against collapse significantly beyond the normal core collapse time predicted without the presence of binaries. For tidally truncated systems, collapse is delayed sufficiently that the cluster will undergo complete tidal disruption before core collapse. Moving a step beyond analytical prescriptions, I incorporate into the Monte-Carlo code an exact treatment of binary-single interactions, and show that the results are in good agreement with those using analytical prescriptions. The direct integration of binary interactions in the Monte-Carlo code requires a reason- ably sophisticated N-body code geared toward small-N dynamics. I present and describe in detail Fewbody, a new, freely available numerical toolkit for simulating small-N gravitational dynamics. Fewbody is a general N-body dynamics code, though it was written for the purpose of performing scattering experiments, and therefore has several features that make it well-suited for this purpose. To validate the method, I compare with several previous binary scattering experiments in the literature and find excellent agreement. As a simple example of the use of Fewbody, I calculate the destruction cross sections and characteristic lifetimes of black hole-pulsar binaries in globular clusters. At present, there should be observable ...