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

Four young star clusters were studied in order to characterize their anomalous extinction or variable reddening that could be due to a possible contamination by dense clouds or circumstellar effects. The extinction law (Rv) was evaluated by adopting two methods: (i) the use of theoretical expressions based on the colour-excess of stars with known spectral type, and (ii) the analysis of two-colour diagrams, where the slope of observed colours distribution is compared to the normal distribution. An algorithm to reproduce the zero age main sequence (ZAMS) reddened colours was developed in order to derive the average visual extinction (Av) that provides the best fitting of the observational data. The structure of the clouds was evaluated by means of statistical fractal analysis, aiming to compare their geometric structure with the spatial distribution of the cluster members. The cluster NGC 6530 is the only object of our sample showing anomalous extinction. In average, the other clusters are suffering normal extinction, but several of their members, mainly in NGC 2264, seem to have high Rv, probably due to circumstellar effects. The ZAMS fitting provides Av values that are in good agreement with those found in the literature. The fractal analysis shows that NGC 6530 has a centrally concentrated distribution of stars that is different of the sub-structures found 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 of Berkeley 86, NGC 2244, and NGC 2264 indicate a good cloud-cluster correlation, when compared to other works based on artificial distribution of points.