Present-day Mass Function of Six Small Magellanic Cloud Intermediate-age and Old Star Clusters

Abstract

We determined the present-day mass functions (PDMFs) of the five intermediate-age star clusters Lindsay 1, Kron 3, NGC339, NGC416, and Lindsay 38 and the old star cluster NGC121 in the Small Magellanic Cloud (SMC) based on observations with the Hubble Space Telescope Advanced Camera for Surveys. The global PDMFs are well matched by Salpeter-like power laws from their main-sequence turnoffs to 0.6 M with a power-law exponent α ranging from 1.51 0.11 (Lindsay 1) to 2.29 0.15 (NGC339). We derive total stellar masses of 105 M , except for Lindsay 38, whose mass is of the order of 104 M. Differences between the PDMFs most likely reflect the varying stages of dynamical evolution of the clusters. These SMC clusters do not follow the α versus concentration parameter c correlation as found for Galactic globular clusters of similar mass. This might be an age effect or due to their location in a galaxy where bulge and disk crossings do not play a role. No correlation is found between α and the cluster core and tidal radii (rc and rt , respectively), the half-light radii rh , age, central surface brightness, metallicity, and galactocentric radius rgc. All six clusters mass-segregated to different degrees. The two clusters Lindsay 1 and Kron 3 barely show signs for mass segregation, but have low-mass star deficient global PDMFs and might be the remnants of star clusters whose outer parts were stripped. A trend exists between the degree of mass segregation and the ratio age/relaxation time tr, h, which indicates the stage of dynamical evolution for a cluster. Our data thus suggest that the SMC clusters in the present sample had a range of initial densities and presumably different amounts of mass loss that led to different rates of dynamical evolution. The clusters' positions in the rh, m/rt versus r0/rh, m plane imply that all of the clusters are tidally filled. Our SMC clusters with projected distances larger than 3kpc from the SMC center should have Jacobi radii significantly larger than their observed King tidal radii. The clusters also have higher mean densities than the estimated central density of the SMC. It is possible that these clusters formed in a denser overall environment of the younger SMC, or that the cluster structures were unusually strongly influenced by encounters with giant molecular clouds.