Cluster assembly and the origin of mass segregation in the STARFORGE
  simulations
  Stars form in dense, clustered environments, where feedback from newly formed
stars eventually ejects the gas, terminating star formation and leaving behind
one or more star clusters. Using the STARFORGE simulations, for the first time
it is possible to simulate this process in its entirety within a molecular
cloud, while explicitly evolving the gas radiation and magnetic fields and
following the formation of individual, low-mass stars. We find that individual
star-formation sites merge to form ever larger structures, while still
accreting gas. Thus clusters are assembled through a series of mergers. During
the cluster assembly process a small fraction of stars are ejected from their
clusters; we find no significant difference between the mass distribution of
the ejected stellar population and that of stars inside clusters. The
star-formation sites that are the building blocks of clusters start out mass
segregated with one or a few massive stars at their center. As they merge the
newly formed clusters maintain this feature, causing them to have
mass-segregated substructures without themselves being centrally condensed. The
cluster relaxes through dynamical interactions to a centralized configuration,
but this process does not finish before feedback expels the remaining gas from
the cluster. The gas-free clusters then become unbound and break up. We find
that turbulent driving and a periodic cloud geometry can significantly reduce
clustering and prevent gas expulsion. Meanwhile, the initial surface density
and level of turbulence have little qualitative effect on cluster evolution,
despite the significantly different star formation histories.
