  Young and massive star clusters above a critical mass form thermally unstable
clumps reducing locally the temperature and pressure of the hot 10$^{7}$~K
cluster wind. The matter reinserted by stars, and mass loaded in interactions
with pristine gas and from evaporating circumstellar disks, accumulate on
clumps that are ionized with photons produced by massive stars. We discuss if
they may become self-shielded when they reach the central part of the cluster,
or even before it, during their free fall to the cluster center. Here we
explore the importance of heating efficiency of stellar winds.
