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Stars form in clusters, while planets form in gaseous disks around young stars. Cluster dissolution occurs on longer time scales than disk dispersal. Planet formation thus typically takes place while the host star is still inside the cluster. We explore how the presence of other stars affects the evolution of circumstellar disks. Our numerical approach requires multi-scale and multi-physics simulations where the relevant components and their interactions are resolved. The simulations start with the collapse of a turbulent cloud, from which stars with disks form, which are able to influence each other. We focus on the effect of extinction due to residual cloud gas on the early evolution of circumstellar disks. We find that this extinction protects circumstellar disks against external photoevaporation, but these disks then become vulnerable to dynamic truncation by passing stars. We conclude that circumstellar disk evolution is heavily affected by the early evolution of the cluster.
Accurately modeling the evolution of a star cluster in a strong tidal field poses unique computational challenges. We present a hybrid code that combines the strengths of two different approaches to computing gravitational forces. The internal, collisional, dynamics of the cluster is followed with a direct N-body integrator, Kira, while the galactic tidal field is modeled with a cosmological code, GADGET, that uses a Barnes-Hut tree to evaluate gravitational forces in O(N log N) time. The quadrupole moment at the center of mass of the cluster is used to compute the external potential and provides a mechanism for mass loss. This forms a robust, bidirectional interaction. The advantages of combining two highly-developed and well-established software packages at such high level are obvious and many; not the least of the these is the ability to include other physical processes, e.g., stellar evolution. One problem to which we applied this technique is the evolution of a dense star cluster near the Galactic Center. We are also using this code to explore the effects of the strong time variation in the tidal field of merging galaxies on the evolution of young star clusters forming during the merger.
The past few years have seen dramatic improvements in the scope and realism of star cluster simulations. Accurate treatments of stellar evolution, coupled with robust descriptions of all phases of binary evolution, have been incorporated self-consistently into several dynamical codes, allowing for the first time detailed study of the interplay between stellar dynamics and stellar physics. The coupling between evolution, dynamics, and the observational appearance of the cluster is particularly strong in young systems and those containing large numbers of primordial binary systems, and important inroads have been made in these areas, particularly in N-body simulations. I discuss some technical aspects of the current generation of N-body integrators, and describe some recent results obtained using these codes.
We describe a fully automated gravitational scattering package capable of determining cross sections and reaction rates for binary-single-star scattering, and present some applications to systems of astrophysical interest.
We examine critically the properties of the large-amplitude oscillations seen in Fokker-Planck simulations of globular clusters, with both continuous and stochastic binary heating, and compare them to the defining characteristics of gravothermal oscillations.
Over the past decade, a very considerable amount of effort in stellar dynamics has gone into the study of interactions between binary systems and other stars. The asymptotic analytic results obtained by Heggie (1975) for binary-single star encounters have been largely confirmed and extended by later numerical experiments (Hills 1975, Hut and Bahcall 1983). Binary-binary interactions have been studied by Mikkola (1983).
We summarize the methods of a new “hybrid” computer code for stellar dynamics. All particles in the inner spatial region are followed exactly via a direct N-body code and all particles in the outer spatial region are treated statistically via a distribution function and Fokker-Planck type methods. An intermediate region, with features of both, allows exchange of particles and energy between the outer and inner regions. We apply our code to the period just before core collapse and just after and summarize the results.
We present here some recent (and very preliminary) findings from a study of the early stages of the post-core-collapse evolution of an isolated cluster of identical point “stars”. The method used to follow the behavior of the system is the unified N-body/statistical treatment described in detail by McMillan and Lightman (1984a) and by Lightman and McMillan elsewhere in this volume. Briefly, the method combines the standard “large-N” and “small-N” approaches to the problem in the régimes where they are appropriate by treating the inner regions (r < rN) exactly with a regularized Aarseth N-body code (Aarseth, 1972), while permitting stars at greater and greater radii to retain less and less of their individual identities, ultimately treating the outer portions of the system (r > KrN) in an almost purely statistical fashion.
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