Massive black hole binaries from runaway collisions: the impact of
  metallicity
  The runaway collision scenario is one of the most promising mechanisms to
explain the formation of intermediate-mass black holes (IMBHs) in young dense
star clusters. On the other hand, the massive stars that participate in the
runaway collisions lose mass by stellar winds. In this paper, we discuss new
N-body simulations of massive (6.5x10^4 Msun) star clusters, in which we added
upgraded recipes for stellar winds and supernova explosion at different
metallicity. We follow the evolution of the principal collision product (PCP),
through dynamics and stellar evolution, till it forms a stellar remnant. At
solar metallicity, the mass of the final merger product spans from few solar
masses up to ~30 Msun. At low metallicity (0.01-0.1 Zsun) the maximum remnant
mass is ~250 Msun, in the range of IMBHs. A large fraction (~0.6) of the PCPs
are not ejected from the parent star cluster and acquire stellar or black hole
(BH) companions. Most of the long-lived binaries hosting a PCP are BH-BH
binaries. We discuss the importance of this result for gravitational wave
detection.
