Gas expulsion vs gas retention: what process dominates in young massive
  clusters?
  The ability of young stellar clusters to expel or retain the gas left over
after a first episode of star formation is a central issue in all models aiming
to explain multiple stellar populations and the peculiar light element
abundance patterns in globular clusters. Recent attempts to detect the gas left
over from star formation in present day clusters with masses similar to those
of globular clusters did not reveal a significant amount of gas in the majority
of them, which strongly restricts the scenarios of multiple stellar population
formation. Here the conditions required to retain the gas left over from star
formation within the natal star forming cloud are revised. It is shown that the
usually accepted concept regarding the thermalization of the star cluster
kinetic energy due to nearby stellar winds and SNe ejecta collisions must be
taken with care in the case of very compact and dense star forming clouds where
three star formation regimes are possible if one considers different star
formation efficiencies and mass concentrations. The three possible regimes are
well separated in the half-mass radius and in the natal gas central density vs
pre-stellar cloud mass parameter space. The two gas free clusters in the
Antennae galaxies and the gas rich cluster with a similar mass and age in the
galaxy NGC 5253 appear in different zones in these diagrams. The critical lines
obtained for clusters with a solar and a primordial gas metallicity are
compared.
