Volume 541, May 2012
|Number of page(s)||12|
|Section||Interstellar and circumstellar matter|
|Published online||07 May 2012|
Probing the anomalous extinction of four young star clusters: the use of colour-excess, main-sequence fitting and fractal analysis⋆
1 Universidade de São Paulo, IAG, rua do Matão 1226, 05508-900 São Paulo, Brazil
2 Universidade Federal do ABC, CECS, rua Santa Adélia 166, 09210-170 Santo André, SP, Brazil
Received: 10 July 2011
Accepted: 22 February 2012
Aims. We studied four young star clusters to characterise their anomalous extinction or variable reddening and asses whether they could be due to contamination by either dense clouds or circumstellar effects.
Methods. We evaluated the extinction law (RV) 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 the observed colour distribution was compared to the normal distribution. An algorithm to reproduce the zero-age main-sequence (ZAMS) reddened colours was developed to derive the average visual extinction (AV) that provides the closest fit to the observational data. The structure of the clouds was evaluated by means of a statistical fractal analysis, designed to compare their geometric structure with the spatial distribution of the cluster members.
Results. The cluster NGC 6530 is the only object of our sample affected by anomalous extinction. On average, the other clusters suffer normal extinction, but several of their members, mainly in NGC 2264, seem to have high RV, probably because of 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 differs from the substructures found in the density distribution of the cloud projected in the AV map, suggesting that the original cloud was changed by the cluster formation. However, the fractal dimension and statistical parameters of Berkeley 86, NGC 2244, and NGC 2264 indicate that there is a good cloud-cluster correlation, when compared to other works based on an artificial distribution of points.
Key words: dust, extinction / stars: pre-main sequence / methods: statistical
Appendix is available in electronic form at http://www.aanda.org
© ESO, 2012
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.