Formation of young massive clusters from turbulent molecular clouds
  Young massive clusters are as young as open clusters but more massive and
compact compared with typical open clusters. The formation process of young
massive clusters is still unclear, and it is an open question whether the
formation process is the same as typical open clusters or not. We perform a
series of $N$-body simulations starting from initial conditions constructed
from the results of hydrodynamical simulations of turbulent molecular clouds.
In our simulations, both open clusters and young massive clusters form when we
assume a density-dependent star formation efficiency. We find that a local star
formation efficiency higher than 50 % is necessary for the formation of young
massive clusters, but open clusters forms from less dense regions with a local
star formation efficiency of $<50$ %. We confirm that the young massive
clusters formed in our simulations have mass, size, and density profile similar
to those of observed young massive clusters such as NGC 3603 and Trumpler 14.
We also find that these simulated clusters evolve via hierarchical mergers of
sub-clusters within a few Myr, as is suggested by recent simulations and
observations. Although we do not assume initial mass segregation, we observe
that the simulated massive clusters show a shallower slope of the mass function
($\Gamma\sim-1$) in the cluster center compared to that of the entire cluster
($\Gamma\sim-1.3$). These values are consistent with those of some young
massive clusters in the Milky Way such as Westerlund 1 and Arches.
