The Cluster Birthline and the formation of stellar clusters in M33
  We present a new method to analyze the IMF at its high mass end in young
stellar clusters, which rely on two integrated observables: the cluster
bolometric and Halpha luminosity. Using several cluster samples selected in M33
we show that a stochastically sampled universal IMF is in better agreement with
the data than a truncated IMF whose maximum stellar mass depends on cluster
mass. We also discuss the possibility that a delayed formation of massive stars
is taking place in low density star forming regions as an alternative to a
strong leakage of ionizing photons from HII regions of young luminous clusters.
