Binaries in Globular Clusters

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Globular clusters, massive star clusters found within all major galaxies in the local universe, play a pivotal role in different research areas within astrophysics. While historically, globular clusters have been used to constrain the age of the universe or the size of our own Milky Way, nowadays they serve as benchmarks for models of star formation and stellar evolution, help us to understand galaxy assembly, and may even hold the key to interpret the wealth of gravitational wave data expected in the coming decades. In fact, the centers of massive star clusters have been identified as one of the most likely sites for black holes to find each other and merge. However, in order to understand the true potential of globular clusters as "gravitational wave factories", we need understand their internal evolution. Over its lifetime, each cluster member -- may it be a black hole, a neutron star or a luminous star -- will experience a number of close encounters with other members that can drastically alter its fate, yes even eject it from its host cluster. Owing to their large cross sections, binary systems play a crucial role in moderating the encounter rate in star clusters, but their study has been subject to severe challenges so far. Thanks to new observing techniques and the availability of sophisticated dynamical models, we are now in a situation where we can significantly advance our understanding of binary stars inside massive star clusters.Because of the very high stellar densities, spectroscopic studies of representative numbers of stars in globular clusters have long been infeasible. Integral field spectroscopy has changed this fundamentally. In particular, the Multi Unit Spectroscopic Explorer (MUSE) of the European Southern Observatory (ESO) allows spectra of tens of thousands of stars per globular cluster to be analyzed spectroscopically very efficiently. With our guaranteed observing time for MUSE, we have conducted an extensive survey of globular clusters in recent years.The goal of this project is to search for binary stars in our unprecedented survey of globular clusters and also to analyze them in conjunction with deep radio and X-ray observations and dynamical models. We are paying particular attention to those clusters of which we have a large number of observational epochs, so that we can also obtain complete orbit solutions for binary stars, allowing the detection of companion stars to black holes.As a result of this project, we hope to find answers to the following questions:How many stellar black holes are there in globular clusters? What were the initial properties of binary stars in globular clusters? How do exotic objects such as blue straggler, sub-subgiants, and low-mass x-ray binaries form and evolve?