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Four young star clusters were studied in order to characterize theiranomalous extinction or variable reddening that could be due to a possiblecontamination by dense clouds or circumstellar effects. The extinction law (Rv)was evaluated by adopting two methods: (i) the use of theoretical expressionsbased on the colour-excess of stars with known spectral type, and (ii) theanalysis of two-colour diagrams, where the slope of observed coloursdistribution is compared to the normal distribution. An algorithm to reproducethe zero age main sequence (ZAMS) reddened colours was developed in order toderive the average visual extinction (Av) that provides the best fitting of theobservational data. The structure of the clouds was evaluated by means ofstatistical fractal analysis, aiming to compare their geometric structure withthe spatial distribution of the cluster members. The cluster NGC 6530 is theonly object of our sample showing anomalous extinction. In average, the otherclusters are suffering normal extinction, but several of their members, mainlyin NGC 2264, seem to have high Rv, probably due to circumstellar effects. TheZAMS fitting provides Av values that are in good agreement with those found inthe literature. The fractal analysis shows that NGC 6530 has a centrallyconcentrated distribution of stars that is different of the sub-structuresfound in the density distribution of the cloud projected in the Av map,suggesting that the original cloud has been changed with the cluster formation.On the other hand, the fractal dimension and the statistical parameters ofBerkeley 86, NGC 2244, and NGC 2264 indicate a good cloud-cluster correlation,when compared to other works based on artificial distribution of points.

Probing the anomalous extinction of four young star clusters: the use of colour-excess, main sequence fitting and fractal analysis