Formation of proto-cluster: a virialized structure from gravo-turbulent
  collapse I. Simulation of cluster formation in collapsing molecular cloud
  Stars are often observed to form in clusters. It is therefore important to
understand how such a region of concentrated mass is assembled out of the
diffuse medium and its properties eventually prescribe the important physical
mechanisms and determine the characteristics of the stellar cluster. We study
the formation of a gaseous proto-cluster inside a molecular cloud by performing
high resolution MHD simulations and associate its internal properties to those
of the parent cloud by varying the level of the initial turbulence of the
cloud, with a view to better characterize the subsequent stellar cluster
formation. The gaseous proto-cluster is formed out of global collapse of a
molecular cloud, and has non-negligible rotation due to angular momentum
conservation during the collapse of the object. Most of the star formation
occurs in this region which occupies only a small volume fraction of the whole
cloud. We identify such regions in simulations and compare the gas and sink
particles to observations. The gaseous proto-cluster inferred from simulation
results present a mass-size relation that is compatible with observations. We
stress that the stellar cluster radius, although clearly correlated with the
gas cluster radius, depends sensitively on its definition. Energy analysis is
performed to confirm that the gaseous proto-cluster is a product of
gravo-turbulent reprocessing and that the support of turbulent and rotational
energy against self-gravity yields a state of global virial equilibrium
although collapse is occurring at smaller scale and the cluster is forming
stars actively. This object then serves as the antecedent of the stellar
cluster, to which the energy properties are passed on.
