Hierarchical Cluster Assembly in Globally Collapsing Clouds
  We discuss the mechanism of cluster formation in a numerical simulation of a
molecular cloud (MC) undergoing global hierarchical collapse (GHC). The global
nature of the collapse implies that the SFR increases over time. The
hierarchical nature of the collapse consists of small-scale collapses within
larger-scale ones. The large-scale collapses culminate a few Myr later than the
small-scale ones and consist of filamentary flows that accrete onto massive
central clumps. The small-scale collapses form clumps that are embedded in the
filaments and falling onto the large-scale collapse centers. The stars formed
in the early, small-scale collapses share the infall motion of their parent
clumps. Thus, the filaments feed both gaseous and stellar material to the
massive central clump. This leads to the presence of a few older stars in a
region where new protostars are forming, and also to a self-similar structure,
in which each unit is composed of smaller-scale sub-units that approach each
other and may merge. Because the older stars formed in the filaments share the
infall motion of the gas onto the central clump, they tend to have larger
velocities and to be distributed over larger areas than the younger stars
formed in the central clump. Finally, interpreting the IMF at face-value as a
probability distribution implies that massive stars only form once the {\it
local} SFR is large enough to sample the IMF up to high masses. In combination
with the increase of the SFR, this implies that massive stars tend to appear
late in the evolution of the MC, and only in the central massive clumps. We
discuss the correspondence of these features with observed properties of young
stellar clusters, finding very good qualitative agreement, thus providing
support to the scenario of global, hierarchical collapse of MCs, while
explaining the origin of the observed cluster structure.
