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

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, the authors' group has developed a nonparametric method that estimates the membership likelihood for apparent cluster stars. 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, the photometric errors, and the limiting magnitude of observations. Using this likelihood estimation as a weight in the fitting procedure, they 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.