Mass functions and photometric binaries in nine open clusters

Abstract

Using homogeneous CCD photometric data from the 105 cm Kiso Schmidt telescope covering a 50' x 50' field, we study the mass functions (MFs) of nine open clusters. The ages and Galactocentric distances of the target clusters vary from 16-2000 Myr and 9-10.8 kpc, respectively. The values of MF slopes vary from -1.1 to -2.1. The classical value derived by Salpeter in 1955 for the slope of the initial mass function (IMF) is Γ = -1.35. The MFs in the outer regions of the clusters are found to be steeper than in the inner regions, indicating the presence of mass segregation in the clusters. The MF slopes (in the outer region as well as the whole cluster) undergo an exponential decay with the evolutionary parameter τ (=age/relaxation time). It seems that the evaporation of low-mass members from outer regions of the clusters is not significant at larger Galactocentric distances. It is concluded that IMF in the anti-center direction of the Galaxy might have been steeper than the IMF in the opposite direction. A comparison of the observed color-magnitude diagrams (CMDs) of the clusters with synthetic CMDs gives a photometric binary content of ~40%.