Title : Properties of Black Hole Binaries formed Dynamically in Dense Star Clusters
Abstract : The advanced detectors of gravitational waves detected two black hole binary mergers and one candidate. Such binaries are either formed by evolution of binary systems composed of two massive stars through dynamical processes in dense stellar systems. I will focus on the dynamical scenarios originating from globular clusters. The binaries are predominantly formed via three-body processes and their orbits undergo subsequent evolution toward more compact configuration through interactions with other black holes. Eventually majority of black hole binaries get ejected from the cluster in a few relaxation time. We derive the distribution of semi-major axis as a function of the escape velocity of the host cluster based on the numerical experiments. Some fraction of those ejected binaries will merge within Hubble time. We also considered the clusters with black hole mass function. Because of the mass segregation in early phase of the dynamical evolution the binaries are predominantly formed between black holes with similar mass. Consequently, the ratios of the primary and secondary black holes are close to 1:1 and binaries with mass ratio greater than 2:1 will be very unlikely. Because of the same reason, binaries composed of a black hole and a neutron star will be very rare among dynamical binaries. The estimation of the expected frequency of gravitational merger events of dynamically formed black hole binaries is difficult, but could be as high as ~ several 10s per year per cubic giga parsec.