The gravitational wave signature of young and dense star clusters

Young star clusters are often born with such high stellar densities that stellar collisions play an important role in their further evolution. In such environments, the same star may participate in several tens to hundreds of collisions ultimately leading to the collapse of the star to a black hole of intermediate mass. At later time, the black hole may acquire a companion star by tidal capture or by dynamical - 3-body - capture. When this companion star evolves, it will fill its Roche-lobe and transfer mass to its accompanying black hole. This then leads to a bright phase of X-ray emission but the binary may also be visible as a source of gravitational wave radiation in the LISA band. If the star captured by the intermediate mass black hole is relatively low mass (≲2 M⊙), the binary will be visible as a bright source in gravitational waves and X-rays for the entire lifetime of the binary. The majority of compact binaries which formed from the population of primordial binaries in young and dense star clusters do not lead to detectable gravitational wave sources.