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Isolating the pre-main sequence in Collinder34, NGC3293, NGC3766 and NG6231

We employed field star decontaminated 2MASS photometry to study four nearby optical embedded clusters -- Collinder34, NGC3293, NGC3766 and NGC6231 -- obtaining deep colour-magnitude diagrams and stellar radial density profiles. We found what seems 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 NGC3766 and NGC6231 might be evolving to OB associations.