Mass segregation in rich LMC clusters from modelling of deep HST colour-magnitude diagrams

Abstract

Aims. We used the deep colour–magnitude diagrams (CMDs) of five rich LMC clusters (NGC1805, NGC1818, NGC1831, NGC 1868, and Hodge 14) observed with HST/WFPC2 to derive their present day mass function (PDMF) and its variation with position within the cluster. Methods. The PDMF was parameterized as a power law in the available main-sequence mass range of each cluster, typically 0.9 <∼ m/M <∼ 2.5; its slope was determined at different positions spanning from the very centre out to several core radii. The CMDs in the central regions of the clusters were carefully studied earlier, resulting in accurate age, metallicity, distance modulus, and reddening values. The slope α (where Salpeter is 2.35) was determined in annuli by following two distinct methods: 1) a power law fit to the PDMF obtained from the systemic luminosity function (LF); 2) a statistical comparison between observed and model CMDs. In the second case, α is a free input parameter in the CMD modelling process where we incorporate photometric errors and the effect of binarity as a fraction of unresolved binaries ( fbin = 100%) with random pairing of masses from the same PDMF. Results. In all clusters, significant mass segregation is found from the positional dependence of the PDMF slope: α <∼ 1.8 for R ≤ 1.0 Rcore and α ∼ Salpeter inside R = 2 ∼ 3 Rcore (except for Hodge 14, where α ∼ Salpeter for R ∼ 4 Rcore). The results are robust in the sense that they hold true for both methods used. The CMD method reveals that unresolved binaries flatten the PDMF obtained form the systemic LF, but this effect is smaller than the uncertainties in the α determination. For each cluster we estimated dynamical ages inside the core and for the entire system. In both cases we found a trend in the sense that older clusters have flatter PDMF, consistent with a dynamical mass segregation and stellar evaporation.