Dynamics of compact objects in dense stellar clusters and the implications for gravitational wave detections

Barber, Jordan 2025. Dynamics of compact objects in dense stellar clusters and the implications for gravitational wave detections. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Dense stellar clusters, such as globular clusters, are gravitationally bound groups of stars that evolve through numerous few-body interactions. The evolution of these clusters is closely linked to the dynamics of the black holes within them, leading to the formation of binary black hole systems and black hole-star systems; as well as driving binary black holes to merger. To better understand these dynamics, it is essential to use sophisticated simulation codes that accurately model the stellar evolution, setting the black hole mass distribution, and the gravitational interactions within clusters. Chapter 1 introduces some of the physics surrounding black hole formation and subsequent evolution, both from single stars, and stellar binaries. We then discuss how the dynamics of black hole interactions couples with the evolution of the host stellar cluster; highlighting the effect of dynamics on the binary black hole orbital properties. The second half of the chapter reviews the current state of cluster simulation codes, with a particular focus on N-body methods, which are central to the work presented in this thesis. In Chapter 2 we examine the role of primordial binaries within stellar clusters. We begin by simulating the isolated formation of a population of binary black holes using the rapid population synthesis code COMPAS. Using theoretical arguments, we then make predictions for the subsequent evolution of these binaries within a star cluster and compare the results to those from N-body simulations. We conclude this chapter with an analysis of the types of interactions expected within clusters, concluding that binary-binary encounters are likely the dominant form of interactions in clusters with escape velocity < 100 km/s and large primordial binary fractions. Chapter 3 presents a detailed analysis of data from 34 N-body cluster simulations produced using the new N-body code PeTar. These simulations cover a range of initial cluster masses, half-mass densities and metallicities, and include models both with and without primordial binaries. We investigate the orbital properties of merging binary black holes, their system multiplicity, and track the evolutionary history of particularly massive black holes. For each cluster, we compute both the number of mergers and the cluster’s merger efficiency, comparing these against results from more approximate methods. Building on this, Chapter 4 focuses on black hole-star binaries. We analyse their presence in our cluster models, explore their orbital properties, and search for systems that could resemble the three Gaia black hole systems that have recently been observed. Finally Chapter 5 offers concluding remarks and a summary of the key findings from the thesis, along with discussions for the individual works presented throughout.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Schools > Physics and Astronomy
Subjects:	Q Science > QC Physics
Uncontrolled Keywords:	astrophysics, N-body simulations, stellar evolution, black hole physics, binary black hole mergers, star cluster evolution, gravitational waves.
Funders:	STFC
Date of First Compliant Deposit:	15 September 2025
Last Modified:	15 Sep 2025 09:22
URI:	https://orca.cardiff.ac.uk/id/eprint/181082

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