Isolating the pre-main sequence in Collinder 34, NGC 3293, NGC 3766 and NGC 6231

We employed field star decontaminated Two Micron All Sky Survey photometry to study four nearby optical embedded clusters - Collinder 34, NGC 3293, NGC 3766 and NGC 6231 - obtaining deep colour-magnitude diagrams and stellar radial density profiles. We found what seem to be pre-main sequences detached in different amounts from main sequences in these diagrams. The structural analysis of each cluster revealed different radial distributions for these two sequences. We argued that the detached evolutionary sequences in our sample may be evidence of sequential star formation. Finally, we compared the sample cluster parameters with those of other young clusters in the literature and point out evidence that NGC 3766 and NGC 6231 might be evolving to OB associations.