We study the compact binary population in star clusters, focusing on binaries containing black holes, using a self-consistent Monte Carlo treatment of dynamics and full stellar evolution. We find that the black holes experience strong mass segregation and become centrally concentrated. In the core the black holes interact strongly with each other and black hole-black hole binaries are formed very efficiently. The strong interactions, however, also destroy or eject the black hole-black hole binaries. We find no black hole-black hole mergers within our simulations but produce many hard escapers that will merge in the Galactic field within a Hubble time. We also find several highly eccentric black hole-black hole binaries that are potential Laser Interferometer Space Antenna (LISA) sources, suggesting that star clusters are interesting targets for space-based detectors. We conclude that star clusters must be taken into account when predicting compact binary population statistics.

Galactic globular clusters are old, dense star systems typically containing 104–106 stars. As an old population of stars, globular clusters contain many collapsed and degenerate objects. As a dense population of stars, globular clusters are the scene of many interesting close dynamical interactions between stars. These dynamical interactions can alter the evolution of individual stars and can produce tight binary systems containing one or two compact objects. In this review, we discuss theoretical models of globular cluster evolution and binary evolution, techniques for simulating this evolution that leads to relativistic binaries, and current and possible future observational evidence for this population. Our discussion of globular cluster evolution will focus on the processes that boost the production of tight binary systems and the subsequent interaction of these binaries that can alter the properties of both bodies and can lead to exotic objects. Direct N-body integrations and Fokker-Planck simulations of the evolution of globular clusters that incorporate tidal interactions and lead to predictions of relativistic binary populations are also discussed. We discuss the current observational evidence for cataclysmic variables, millisecond pulsars, and low-mass X-ray binaries as well as possible future detection of relativistic binaries with gravitational radiation.

We use a self‐consistent Monte Carlo treatment of stellar dynamics to investigate black hole binaries that are dynamically ejected from globular clusters in order to determine if they will be gravitational wave sources. We find that many of the ejected binaries initially have short periods and will merge within a Hubble time due to gravitational wave radiation. Thus they are potential sources for ground‐based gravitational wave detectors. We estimate the yearly detection rate for current and advanced ground‐based detectors and find an enhancement over the rate predicted for binaries produced by pure stellar evolution in galactic fields. We conclude that, in agreement with previous studies, including globular cluster populations is essential for calculating the correct merger detection rates for gravitational wave detection.

Blue (metal-poor) globular clusters are observed to have half-light radii that are ∼20 per cent larger than their red (metal-rich) counterparts. The origin of this enhancement is not clear and differences in either the luminosity function or in the actual size of the clusters have been proposed. I analyse a set of dynamically self-consistent Monte Carlo globular cluster simulations to determine the origin of this enhancement. I find that my simulated blue clusters have larger half-light radii due to differences in the luminosity functions of metal-poor and metal-rich stars. I find that the blue clusters can also be physically larger, but only if they have a substantial number of black holes dynamically heating their central regions. For some combinations of metallicity and initial conditions the difference in luminosity function is sufficient to explain the observed effect while for other combinations the size enhancement due to black holes is necessary to match the observations. I conclude that the observed difference in half-light radii between red and blue globular clusters is due to a combination of differences in size and luminosity function. I find an additional corollary: the half-light radius is not a straightforward proxy for cluster size.Key words: methods: numerical -methods: statistical -binaries: close -globular clusters: general -galaxies: star clusters: general. I N T RO D U C T I O NGlobular cluster (GC) systems are common in disc and elliptical galaxies, and in the past decade significant observational progress has been made in understanding them. GCs are frequently used to help understand the formation and evolution of their host galaxies but the observational properties of GCs are not fully understood. A case in point is the half-light radius (r hl ). Several models show that the half-light radius of a cluster should remain fairly constant during its lifetime (Spitzer & Thuan 1972;Aarseth & Heggie 1998) so it is often used to compare the structures of GCs of different ages. However, several observational studies show that metal-poor (blue) GCs have half-light radii that are systematically larger (by ∼20 per cent) than their metal-rich (red) counterparts (Kundu & Whitmore 1998Kundu et al. 1999;Puzia et al. 1999;Larsen et al. 2001a;Larsen, Forbes & Brodie 2001b;Barmby, Holland & Huchra 2002;Harris et al. 2002;Jordán et al. 2004;Harris 2009).The origin of this discrepancy is not clear and it is not even certain that it truly represents a difference in the sizes of the clusters as measured by their half-mass radii (r hm ). Larsen & Brodie (2003) proposed that the observed enhancement in the size of the blue clusters could be due to a projection effect. They note both that blue and red GCs follow different radial distributions and that in the Milky Way there is a relationship between cluster size and galactocentric distance (van den Bergh, Morbey & Pazder 1991). They argue that E-mail: downin@ari.uni-heidelberg.de if such a relationship exists in all galaxies, the differing radial distribut...

Various Galactic globular clusters display abundance anomalies that affect the morphology of their color-magnitude diagrams. In this paper we consider the possibility of helium enhancement in the anomalous horizontal branch of NGC 2808. We examine the dynamics of a self-enrichment scenario in which an initial generation of stars with a topheavy initial mass function enriches the interstellar medium with helium via the low-velocity ejecta of its asymptotic giant branch stars. This enriched medium then produces a second generation of stars which are themselves heliumenriched. We use a direct N-body approach to perform five simulations and conclude that such two-generation clusters are both possible and would not differ significantly from their single-generation counterparts on the basis of dynamics. We find, however, that the stellar populations of such clusters would differ from single-generation clusters with a standard initial mass function and in particular would be enhanced in white dwarf stars. We conclude, at least from the standpoint of dynamics, that two-generation globular clusters are feasible.