Dynamics of binary black holes in low-mass young star clusters
  Young star clusters are dynamically active stellar systems and are a common
birthplace for massive stars. Low-mass star clusters ($\sim{}300-10^3$
M$_\odot$) are more numerous than massive systems and are characterized by a
two-body relaxation time scale of a few Myr: the most massive stars sink to the
cluster core and dynamically interact with each other even before they give
birth to compact objects. Here, we explore the properties of black holes (BHs)
and binary black holes (BBHs) formed in low-mass young star clusters, by means
of a suite of $10^5$ direct $N$-body simulations with a high original binary
fraction (100 % for stars with mass $>5$ M$_\odot$). Most BHs are ejected in
the first $\sim{}20$ Myr by dynamical interactions. Dynamical exchanges are the
main formation channel of BBHs, accounting for $\sim{}40-80$ % of all the
systems. Most BBH mergers in low-mass young star clusters involve primary BHs
with mass $<40$ M$_\odot$ and low mass ratios are extremely more common than in
the field. Comparing our data with those of more massive star clusters
($10^3-3\times{}10^4$ M$_\odot$), we find a strong dependence of the percentage
of exchanged BBHs on the mass of the host star cluster. In contrast, our
results show just a mild correlation between the mass of the host star cluster
and the efficiency of BBH mergers.
