Fitting isochrones to open cluster photometric data

Abstract

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.

Radial velocities and metallicities from infrared Ca II triplet spectroscopy of open clusters

“…It is the youngest cluster of our sample with an age similar to that of the Hyades, (0.7 ± 0.1 Gyr; Kaluzny & Mazur 1991;Salaris et al 2004;Monteiro et al 2010;Dias et al 2012), although some authors have obtained a slightly older age (1.2 ± 0.1 Gyr; Maciejewski & Niedzielski 2007;Maciejewski et al 2008). Its color-magnitude diagram has a populated mainsequence, especially in the turn-off area, and a well-defined redclump located at V ∼ 13.7 with a few stars above it (Fig.…”

Proper motions of the optically visible open clusters based on the UCAC4 catalog

“…We refer the reader to the A79, page 2 of 8 papers of Rubinstein (1997), Rubinstein (1999), and Kroese & Rubinstein (2006) for their very complete discussion of the technique and the series of papers of our group (Monteiro et al 2010;Monteiro & Dias 2011;Dias et al 2012;and Oliveira et al 2013) for an application of the method to the determine fundamental parameters of open clusters. The main goal of the CE continuous multi-extremal optimization method is to find a set of parameters for which the model provides the best description of the data as per maximum likelihood definition.…”

ASteCA: Automated Stellar Cluster Analysis

“…This can be seen for example in the series of articles by Kerber et al (Kerber et al 2002;Kerber & Santiago 2005) where an estimate of the density of field stars in the cluster region is used to implement a field star removal process together with a cluster CMD modeling strategy that selects the best observed vs. artificial fit via a statistical tool; the white dwarf based Bayesian CMD inversion technique developed in von Hippel et al (2006) expanded and coupled with a basic field star cleaning process in van Dyk et al (2009); the synthetic cluster fitting method introduced in Monteiro et al (2010;MDC10, further developed in the articles Dias et al 2012;Oliveira et al 2013), which includes a likelihood-based decontamination algorithm; the work by Pavani et al (2011) where CMD density-based membership probabilities are given to stars within the cluster region to later apply a very basic isochrone fitting process that makes use of stars close to a given isochrone in CMD space; and the articles by Alves et al (2012) and Dias et al (2014), who employ the same membership probability assignment method used in Pavani et al (2011) coupled with a slightly improved isochrone fitting algorithm based on the one developed by MDC10, but applied to a Hess diagram of the CMD instead of the full CMD. In Buckner & Froebrich (2013) the membership assignment method presented in Froebrich et al (2010), a variation of the Bonatto & Bica (2007) algorithm, is used in conjunction with the Besançon model of the galaxy 16 to derive distances to OCs, based on foreground stars density estimations.…”