Fitting isochrones to open cluster photometric data II. Nonparametric open cluster membership likelihood estimation and its application in optical and 2MASS near-IR data

Aims. Open clusters are essential tools for understanding Galactic structure, as well as stellar evolution, because they are distributed over the whole Galactic plane, and because their ages, distances, and reddening can be determined. The values of derived cluster fundamental parameters can vary greatly because of the often subjective nature of both the isochrone fitting technique and member star selection. To minimize the subjectivity in the selection of stars and to improve the fitting procedure, our group has developed a nonparametric method that estimates the membership likelihood for apparent cluster stars. Methods. The cluster member selection method is based on the star position relative to the cluster center, the density of stars in the color-magnitude diagram (which can be multidimensional), the photometric errors, and the limiting magnitude of observations. We use this method, together with the global optimization tool developed in our previous articles, to fit theoretical isochrones to open cluster photometric data, making use of UBV and 2MASS data sets. Results. Using this likelihood estimation as a weight in the fitting procedure, we show that the method is robust in that it assigns low weights to most contaminating stars and high weights to the stars that are likely cluster members. Our results show that the fundamental parameters determined using 2MASS data agree with those from UBV data when both are determined from the global optimization fitting method, however, the analysis of the open cluster Dias 6 indicates that a revision of the determined parameters might be required for some cases.