How inclusion of binaries affects the fitting of giant branches and determination of fundamental parameters of star clusters

Open clusters (OCs) play an important role in the study of star formation and the structural evolution of the Milky Way. This paper fits the color-magnitude diagrams (CMDs) of 17 OCs using a tool called Powerful CMD, with the aim of studying the effect of inclusion of binary stars on the fitting of cluster CMDs with clear giant branches. The fitting results of these star clusters were analyzed and compared under two scenarios, i.e., single-star isochrone fitting and binary-star isochrone fitting. In the binary-star isochrone fitting, the binary fraction is greater than 0 and no larger than 1, which is determined by Powerful CMD based on the goodness of fit to the CMD. The research findings indicate that binaries enhance the goodness of fit for all sample star clusters. In particular, the fit to giant branches becomes better when binary stars are considered. Furthermore, binaries contribute to improving accuracy in determining the fundamental parameters such as age, color excess, and distance modulus for star clusters, because the parameter uncertainties become smaller when binary stars are included in theoretical models. Moreover, the distance modulus and age of clusters become larger when binary-star isochrones are used instead single-star isochrones. The results suggest that one should not neglect the influence of binaries on the shape of giant branches and their role in determining fundamental parameters, in the CMD studies of star clusters.