We used the deep colour-magnitude diagrams ( CMDs ) of five rich LMC clusters ( NGC 1805 , NGC 1818 , NGC 1831 , 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 .
The PDMF was parameterized as a power law in the available main-sequence mass range of each cluster , typically 0.9 \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } m / M _ { \odot } \lower 2.15 pt% \hbox { $ \buildrel < \over { \sim } $ } 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 \alpha ( 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 , \alpha 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 ( f _ { bin } = 100 % ) with random pairing of masses from the same PDMF .
In all clusters , significant mass segregation is found from the positional dependence of the PDMF slope : \alpha \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } 1.8 for R \leq 1.0 R _ { core } and \alpha \sim Salpeter inside R = 2 \sim 3 R _ { core } ( except for Hodge 14 , where \alpha \sim Salpeter for R \sim 4 R _ { core } ) .
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 \alpha 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 .