An empirical view on the formation of stellar clusters: self-triggering

Star formation configures our Universe and observations suggest most
stars originate in clusters.  Because of the large physical scales
involved we do not have a working theory of star formation, one that
could explain, for example, why the stellar product (the IMF) is
"universal" nor why is the clustered mode favored by nature. In this
talk I will review briefly the current cluster formation scenarios,
their successes and shortcomings, and based on observational evidence
I will suggest a new scenario for cluster formation. In this new
scenario massive stars form first and play a critical role on the
formation of a cluster, effectively synchronizing the collapse of an
ensemble of dense cores in a molecular cloud clump. "It is not that
massive stars are found in clusters; it's clusters that are found
around massive stars". This scenario has the potential to provide a
natural explanation for an "universal" IMF under very different
physical conditions. Observational evidence for and against this
scenario will be discussed.