Escaping Compact Object Binaries from Globular Clusters

Abstract: Since 2015, the LIGO-Virgo-KAGRA scientific collaboration (LVK) has detected gravitational waves from more than 90 merging compact object binaries using ground-based interferometric detectors. Compact objects orbiting in binary systems emit gravitational waves leading to a loss of energy. This results in the decay of the orbit leading to the merger of these inspiralling objects. Around 83 of the merging compact object binaries that have been detected are binary black holes. A couple of binary neutron star mergers and a handful of BH-NS mergers have also been detected. The exact astrophysical origin of these gravitational wave sources is uncertain. The two main formation channels for these compact object binary systems are isolated evolution of massive stars and dynamical formation in dense star clusters. In this thesis project, we explore the latter scenario and investigate compact object binaries that are produced and ejected from globular clusters. We utilise results from around 280 globular cluster simulations to investigate the properties of escaping compact binaries. We find 8765 escaping binary black holes from the simulated star cluster models. More than 80% of these formed due to dynamical exchange encounters. We find that 40% of the escaping binary black holes from our simulations will merge within the age of the Universe by emitting gravitational wave radiation. We find that initially denser cluster models are more likely to produce more merging binary black holes. Binary black holes that form in exchange encounters inside globular clusters can have component masses larger than 50 M_⊙. Dynamical exchange encounters can also produce BBHs with low mass ratios (≲ 0.3) that are difficult to produce through binary evolution of massive stars. We also identify about 124 BH-NS systems. About 50% of these will merge within the age of the Universe and only less than 10% of these formed in dynamical exchange encounters. We find that merging BH-NS binaries with mass ratio less than 0.1 are more likely to have formed dynamically. In summary, we find that dense clusters can be efficient factories for producing compact object binaries that will merge due to gravitational wave radiation. However, if the initial density is too high, this can lead to the formation of an IMBH which then inhibits the formation of stellar-mass BBHs.