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Globular Cluster Dynamical Evolution

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Abstract

Globular clusters are some of the oldest structures in the universe. They typically contain 105–106 stars and thus they are excellent laboratories in which to explore the effects of dynamical evolution on self-gravitating systems. Two-body scattering between stars is responsible for energy transport within the cluster. Through this mechanism energy flows from stars in the cluster center to stars in the cluster halo. Because a self-gravitating system has a negative heat capacity, when the stars in the cluster core lose energy via two-body scattering, the core contracts and heats up (in the sense that the stellar velocities increase). The flow of energy is then accelerated leading to extremely high central densities; a process known as core collapse. The time taken to reach this stage is a function of the relaxation timescale within the cluster (the timescale required for an accumulated number of distant two-body scatterings to be effective). This timescale is a function of the properties of the cluster mass and radius, and thus clusters observed today have a broad range of central densities. Energy input from binaries within the cluster core can halt core collapse. The cluster will then expand, with energy input continuing to come from binaries in the core. For clusters having a sufficiently large number of stars, the expansion can be unstable leading to so-called gravothermal oscillations. Dynamical processes within the cluster might also produce exotic objects such as intermediate-mass black holes. Clusters do not exist in isolation. The galaxy affects the evolution of a cluster via three mechanisms: (1) dynamical friction will cause clusters at relatively small galactocentric radii to spiral into the galactic center, (2) the galactic tidal field will strip stars from the outer regions of stellar clusters, and (3) the entire cluster will be stirred up as it passes through the galactic disk or close to the bulge which will enhance cluster mass loss. Thus the clusters seen today may only be a small subset of the original population.

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Acknowledgements

I thank the following people for their input and comments on earlier versions of this chapter: Ross Church, Sofia Feltzing, Douglas Heggie, Berry Holl, Lennart Lindegren, Steve McMillan, and Giampaolo Piotto.

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© 2013 Springer Science+Business Media Dordrecht

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DaviesProf., M.B. (2013). Globular Cluster Dynamical Evolution. In: Oswalt, T.D., Gilmore, G. (eds) Planets, Stars and Stellar Systems. Springer, Dordrecht. https://doi.org/10.1007/978-94-007-5612-0_17

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