Structure and mass function of five intermediate/old open clusters

Abstract

The UBVRI data of five northern open star clusters are used to study the mass function (MF), structure and dynamical state of these clusters. The clusters under discussion have ages ranging from ∼0.6 to 5 Gyr. A comparison of the density profiles indicates that the evolution of the core of these clusters is almost the same, whereas the corona of the clusters are probably affected by the external environment and dynamical evolution. For the entire cluster region, the slope of the MF of three clusters (Be 64, Be 69 and King 5) has a value that agrees within the error with the Salpeter value, whereas King 7 and Be 20 show a steeper (Γ = −2.02 ± 0.24)

and almost a flat MF respectively. We find that the slope of the MF of two clusters (King 5 and King 7) changes

significantly from the inner region to the outer region, becoming steeper at larger radii. The dynamical relaxation

time of the clusters under discussion is less than the age of the clusters, which indicates that all of these clusters

are dynamically relaxed. Thus the observed mass segregation in three clusters can be attributed to the dynamical

evolution of the clusters. The ratio of the clusters’ present radius to the limiting radius (determined from the relation given by King 1962) can be representated by an exponent law.